Motors & Drives Information

Typical Question

We have had a relatively high failure rate in one geographic location for Adjustable Frequency Drives of a particular frame (25 to 40hp). In most cases, the users have tried many other brands, all with worse MTBF than ours. Most begin having problems with tripping on overvoltage, even though the supply is well within nominal ranges. Eventually they “blow up.”

Our units lasted longer, probably due to a higher bus voltage tolerance than most, but now we are also succumbing to the problem. This happens in more than one plant, but all within a 100 mile radius and on the same utility.

Here is the interesting part. One of the users hired a consultant on their own since this seems to cross over manufacturers’ lines. This consultant has put forth the theory that the utility has, somewhere in their grid, poorly separated power lines that are occasionally touching each other in the wind, sending very short duration, but repeated, spikes down the lines.

Without much detail, I find this a bit difficult to swallow, but not being a utility guy, I thought I would throw it out here for those who may have heard of this.

Discussion Group Answers

• “Line slap, huh? That does not sound at all like a likely cause to me, especially with the fast acting protective relays around today. I think the best bet is to get a high speed recorder on one of the feeders to get an idea of what kind of voltages you're dealing with. Many utilities will provide this type of device for you to use temporarily. Once the magnitude and duration of the voltage are known, it may help get you in the right direction or at least help identify a short term fix. Discussions with the utility may also shed some light on it.”
  
  
• “I can sympathize with you. Not too long ago, we too experienced repeated failures at three sites within in a common geographical area. At each installation, a number of soft starters and motors had failed over a period of about 12 months, further inspection strongly suggesting transient over-voltage damage in each case.
  
  “The ‘events’ however were not captured by the recording equipment installed by the supply authorities, and for some strange reason, we were discouraged from installing our own. This eliminated any possibility for us to confirm one way or the other.
  
  “On further investigation we found that the HV supply comprised an automatic tap changer that we believed (but could not confirm due to a lack of available information) was open transition by design. Although I cannot confirm the automatic tap changer was the cause of those failures, you might be interested to note (as we were) that the sites have been trouble-free since we outlined our suspicions (10 months ago) to the 3 clients involved.”
  
  
• “I have seen a similar situation happen on a supply system where the supply transformer 22KV/440V was delta primary and wye secondary. The earth neutral link was loose, resulting in a high impedance between the earth and neutral.
  
  “When the power factor correction was switched in and out of circuit, it caused transients on the line. This was capacitively coupled through the transformer, causing the whole secondary to leap above earth momentarily. The 440V three-phase appeared normal phase-to-phase, but there were very high fast transients relative to ground on all phases at the same time.
  
  “This lifted the DC bus away from ground and effectively injected a very fast high voltage transient into the output of the drive (due to the capacitance of the cable and motor to earth). This caused overvoltage trips, instantaneous over current trips, and very occasionally, caused output device failure.”
  
  
• “We had a small Adjustable Frequency Drive tripping frequently on bus overvoltage during run, deceleration, and acceleration. I researched potential causes for the nuisance tripping and went back and re-read the manual word by word. It placed particular emphasis on grounding.
  
  “I had the ground checked at the drive ground terminal to a known good ground. There was 15-19 ohms resistance. We re-grounded the Adjustable Frequency Drive to a known good ground and started the drive...phase-ground fault.
  
  “Meggar checks from the Adjustable Frequency Drive to the field did not pick up anything, even at 1,000VDC on the 480V system. We should have megged from the motor back, but that was a difficult location to get into. The bad ground at the Adjustable Frequency Drive was not allowing the fault to blow through insulation, but it was enough for the drive protection circuitry to pick up and trip on high DC bus voltage.”

Clarifications

Just a little comment on transient recorders. It is equally important to monitor line-to-ground transients as line-to- line. This is particularly important if the substation is using an ungrounded delta secondary.

Theoretically, the “float” voltage can go anywhere until some insulation or protective device conducts, and then if there is power behind the transient, bang! It is even worse to operate continuously with one leg deliberately grounded on a delta secondary substation. Most electronic equipment is not really designed to see full voltage-to-ground on the phase legs and the result is more insulation stress than necessary.

With Adjustable Frequency Drives, manufacturers protect the input by including line-to-ground suppression. Typically, MOVs are used from line-to-line and line-to-ground to limit voltage transients.

The potential problem with this type of protection exists when the supply system impedance is low and the voltage transients exceed the capabilities of the MOVs. Often Adjustable Frequency Drives will first exhibit overvoltage trips indicating the presence of voltage transients, line-to-ground. Then, after many “hits”, the MOVs fail. Since many drives have the MOVs and power rectifiers packaged as modules, a complete rework of the front end of the drive is needed.

Questions concerning various AC drives often arise. Discussions with and answers from various users indicate that the operational behavior of ac drives are still a mystery. This article will clarify some of the concerns raised by typical user’s questions.

Typical Questions

1. When there is a short-circuit in the cable between motor and the AC drive, I always see that the DC bus voltage increases. Why?
2. What is vector control and is it possible to work without motor tuning (measurement of motor equivalent circuit)?
3. The manufacturer of an inverter suggests a maximum cable length of shielded cable between inverter and motor. Why? I believed that the maximum length is valid for unshielded cables only.
4. Every drive has a measurement of the AC output voltage to the motor. Is this the RMS value of the voltage?
5. I’m using AC drives in residential applications, with either 115- or 230-volt single phase input driving a 3-phase motor for pump use. Since water is involved, I’m fairly certain that GFI are required per NFPA 70. But AC drives trip standard GFIs, so it seems a catch-22.

Discussion Group Answers

Answers to question 1:

• The AC drive shuts down when it detects a short. So the DC link is not loaded any more. The voltage you see is mains voltage times sqrt(2).
• The DC bus voltage, when the drive is off, is the peak voltage of the mains: normally the RMS value times sqrt(3). However most input supplies have some distortion, and the resulting DC bus voltage is generally higher than expected. As long as the value is below the overvoltage trip point of the drive, no serious problems should occur. One concern does exist. That is when any stored energy finds its way back to the DC bus from the load. If the DC bus is too close to the design trip point, energy from the load can result in an overvoltage trip.
• When the drive output leads are shorted, virtually all drives will fault out in milliseconds, so I find it unlikely that the DC bus voltage could be read under those conditions.

Answers to question 2:

You can not run a vector control successfully without motor tuning. Magnetizing current and dynamic properties are the first problems—and there will be more. But, you can usually switch to scalar control (U/f) and get up and running without entering motor parameters (you should enter rated power, current, voltage and frequency to get the right operating point for the motor).

Vector control requires the characteristics of the connected load. It is important to remember that the cable is part of the connected load. Entering only motor parameters will allow operation but will not yield optimum performance.

Answers to question 3:

The problem here is that cable capacitance loads the inverter. A shielded cable has a higher capacitance and the length has to be limited or you have to derate the inverter. Reducing the switching (carrier) frequency also helps.

Unshielded cable results in undefined return paths from switching frequency contained within the current waveform. Distance and location of unshielded cable results in unwanted electrical noise in the facilities electrical system. Shielded cable reduces these conditions if correctly installed but will place predictable demands on the AC drive.

Answers to question 4:

The positive work being done is a result of the RMS value of the fundamental frequency. Any other measurement would be totally useless. If you measure the motor voltage with a True RMS meter, you will measure fundamental and high frequency components from the switching action (PWM). That is why you always get a higher reading when you read the motor voltage with an external DMM. Trust the read-out on the inverter.

Answers to question 5:

Personnel GFCI’s are not required (by NEC) for permanently wired circuits.

If you really want to have such protection, you need to use the equipment protective GFCI, which essentially are less sensitive, tripping at 30mA or more, compared to 4-6mA sensitivity for personnel GFCIs required in bathrooms or kitchen.

Application of an isolation transformer should not be ruled out. It is used for electric lawn mowers and similar electrical appliances outdoor. It should also be adequate for electrical pumps. However, the ground fault monitoring is needed.

Questions concerning various AC motors often arise. Discussions with and answers from various users indicate that the operational behavior of ac motors are still a mystery. This article will clarify some of the concerns raised by a typical user question.

Typical Questions

If the motor rated frequency is 60 Hz, and we use the variable frequency drive to run this motor continuously at 5 to 10 Hz frequency, will we damage the motor winding? Will it overheat?

Discussion Group Answers

The answer to that question lies mostly in how much torque the motor will produce at those slow speeds. At very light loads, the motor will be fine. At heavy loads, it may well overheat, since the motor’s fan is not moving enough air to control its temperature.

Another important but smaller factor is the ambient temperature around the motor. If the motor is running in a refrigerated warehouse at 2°C, it will be able to produce more torque without overheating than if it is running in an area that is very warm (near 40°C, for example). A third consideration is motor cleanliness. Clearly, a motor buried in dust, paper pulp, or other dirt will run warmer than a clean motor under the same load conditions.

A motor run a very low speed must either be specifically engineered and designed for such a low speed or a cooling system must be added. Typical cooling systems include external blowers or fans. The difference in price between a TEFC motor and a TEBC motor is rarely negligible. Up to about 30hp, a less expensive option would be a TENV motor. Above that hp level, if you choose TEBC, cost may become a greater issue.

If a Variable Frequency Drive is used, is there any need to use gear box? For example, the motor base speed is 1750 RPM and the required speed is 80 RPM. If Variable Frequency Drive is used, is a gearbox required? Can the motor be connected to the load shaft directly without a gearbox?

If you apply a gearbox to a motor as a means of getting a reduces shaft speed, you will get an increase in torque as the shaft speed is reduced. The gearbox is like a mechanical transformer. Step the speed down and step the torque up. Your motor power rating (horsepower) remains the same.

If you use a variable frequency drive to reduce the speed of the motor shaft, the torque will remain at the rated torque of the motor. This means that, effectively, the motor power rating is reduced. To achieve a power output of 10HP at 10Hz with a 60Hz motor, you would need to use either a 10HP motor and a 6:1 gearbox or a 60HP motor and a variable frequency drive, ignoring efficiencies of gearbox, etc., and ignoring the additional cooling required when running the motor at low speed. In a constant torque application, a variable frequency drive can be used to replace a gearbox. That is not the case in a constant power application.

Clarifications

Your stated requirement to operate at 80 RPM (less than 4% of rated motor speed) would require that you run the motor, without a gearbox, at less than 5HZ. Unless your load is extremely constant, you can expect to experience speed and torque variations.

A better choice would be to use a low ratio gearbox (3:1 or 4:1) and a variable frequency drive to control the motor speed at a higher value. 15 Hz should be considered minimum regarding continuous motor speed.

The decision to use a gearbox is determined by the available torque from the motor at that slow speed and the torque required to drive the load. To find motor rated torque, you use the formula HP = T(ft-lbs) x RPM/5250.

Solving for torque, T(ft-lbs) = 5250 x HP/RPM (nameplate or base speed of the motor), you get the rated torque for the motor. This is the torque available from the motor base speed down to one-third or possibly one-quarter of motor nameplate speed.

For a 4-pole motor (1750 RPM nameplate speed), an approximate value of 3 ft-lbs per horsepower is possible. At 5Hz, it is unlikely that you would be able to get 15% of rated torque continuously unless the motor is located in a very cold place.

If the load requires not more than this 15% level of torque, then no gearbox is required. If it requires more than the 15% level, you need a gearbox to convert some of the available motor speed into load torque.

Below one-quarter of motor nameplate speed, you will have to increase the rated voltage supplied to the motor to achieve rated torque. As long as the motor is fully loaded, the increase in voltage will not create additional losses. If the motor load decreases, the increase in voltage will create additional core losses, causing more heating of the motor.

Questions concerning various ac motors often arise. Discussions with and answers from various users indicate that the operational behavior of ac motors are still a mystery. This article will clarify some of the concerns raised by a typical user question.

Typical Question

I have a basic question regarding wound rotor motors vs squirrel cage motors in high horsepower (4,000 HP+) automobile shredder applications. Automobile shredders, like any large rock crusher, experience very high shock loading. Which type of motor is better suited for this application, and why?

Can I achieve the benefits of a wound rotor motor (high starting torque with lower starting current) along with the added benefits of reduced maintenance by using a squirrel cage motor and an electronic soft starter?

Discussion Group Answers

• A wound rotor motor with an appropriate secondary resistance starter is able to produce a high starting torque from zero speed through to full speed. This will result in a higher acceleration rate than you will achieve with a squirrel cage motor. The starting current will be lower and the motor will be able to start in loaded situations where a standard cage motor will not. The negatives are that both the motor and the starter will require a lot more maintenance than a standard cage motor and the purchased price is higher.
• The soft starter and induction motor will approximately accomplish the similar output to the wound rotor motor. However, the soft starter may be more expensive and more demanding on the service. Also, MTBF may be lower for the soft starter.
• I’m not sure about the MTBF being lower for a soft starter; they seem to be getting more and more reliable these days. But I do not have experience with medium voltage starters in the 1000 HP+ range, either. Are liquid rheostat starters still the current, most reliable technology? If not, are there other types of wound rotor starters out there that are more reliable and less maintenance? If so, what manufacturers?
• The mechanical load profile torque-speed needs to be known to be able to match the motor torque-speed characteristics. Assuming that high starting torque is required, then the squirrel-cage induction motor NEMA Design D may be required. This could be the better solution than the wound-rotor induction motor since the motor may be DOL started, if the power distribution allows it, and it will be simpler to maintain than the wound-rotor induction motor.
• I don’t have knowledge for comprehensive evaluation of your options compared to your application. Considering only the motors themselves (not the other parts of the starting/control): In my experience we have much higher failure rate on wound rotor motors than on our squirrel cage motors (although wound rotor motors are probably used in the more demanding applications).
• If you do not require a high start torque, then the soft starter and cage motor are definitely a very viable option. The reliability of correctly engineered soft start applications is very high. Some installations have been operating for more than twenty years. For this application, it is likely that a wound rotor motor would be required based on the high starting torque requirement.

Clarifications

Wound rotor motors are also squirrel cage motors. A standard squirrel cage motor is normally referred to as a standard induction motor. Wound rotor motors and standard induction motors operating on different principles. The wound rotor motor is a variable % slip motor while the induction motor operates as a fixed % slip motor.

Normally, the supply voltage fed to each motor is fixed frequency (i.e. 60 Hz from the line). Although reduced voltage is sometimes applied, normally, the supply voltage is fixed. With the induction motor, 60 Hz worth of slip occurs forcing the motor to go beyond its pull out torque point (200% to 250% FL). This results in high current (600% to 800% FL) while producing less torque (70% to 120% FL) than the motor is capable of producing.

With the wound rotor motor, increased rotor resistance (higher slip) is initially used and reduced as the motor comes up to operating speed. This method allows the motor to operate without going beyond its pull out torque point. This results in greater starting torque at lower starting currents than achievable using standard induction motors.

When the wound rotor motor is at operating speed, the slip characteristics approach those of the standard induction motor. A wound rotor motor is similar in characteristics of a NEMA D design induction motor.

The use of a electronic soft starter with a standard induction motor does not yield the same performance as a wound rotor motor with a variable rheostat control. Soft start is effectively a reduced voltage start. Since the frequency of the applied voltage is 60Hz, the effect of the reduced voltage is a substantial decrease in starting torque while holding the starting current to some maximum limit.

Questions concerning various AC motors and ac drives often arise. Discussions with and answers from various users indicate that the operational behavior of ac drives and ac motors are still a mystery. This article will clarify some of the concerns raised by a typical user question.

Typical Question

I have been researching AC drives and regenerative braking on three-phase induction motors. Thus far, I have been able to find only basic information on how an AC drive and motor interact to provide regenerative braking.

It seems that AC drives run at a lower frequency than the free-run frequency of the motor causing a reverse current flow in the AC drive and back to the DC bus.

Is the ac drives just supplying the magnetizing current while capturing the torque current supplied by the motor during regeneration? From what I understand, the magnetizing current lags the voltage by 90° and the torque current is in phase with the voltage during normal drive conditions. Under regeneration, I’m not sure what the phase relationships are.

Is an external speed sensor required to keep the AC drive frequency below the motor frequency or can an AC drive subtract the drive signal from the motor current waveform to determine the motor speed?

Discussion Group Answers

Normally we think of an induction motor as supplying shaft power, but it can easily absorb shaft power. This commonly happens, for example, when a crane is lowering a load; the motor is turning one direction but it has to create torque in the opposite direction. For this condition, the motor rotor turns faster than the stator frequency and power is returned to the supply. This is called an over-running condition.

When an ac drive is decelerating an induction motor, the same condition applies, and the rotational energy in the motor rotor/load is returned to the power supply. This energy pumps up the power supply capacitors (DC bus), which is usually dissipated by electronically connecting a resistor across the DC bus as required to keep the DC bus value at some predetermined voltage limit.

There have been a lot of recent advances in regenerative AC drives. Regenerative AC drives work fairly simply. When you want to stop, the AC drive output frequency is driven to a very low level, say 10-15Hz. At that level the excitation is still present but virtually any continued rotation is exceeding the “synchronous” frequency applied to the motor.

Load inertia becomes the prime-mover. With excitation, the AC motor rotating over base speed is an induction generator. The faster it turns, the more negative torque it converts to power. That energy is converted by the reverse diode in the power modules of the AC drive to DC on the bus. Then a separate set of transistors on the front-end of the AC drives reconverts that DC into a fixed frequency voltage applied back to the supply source.

Once the motor speed goes below that threshold output frequency, the AC drive switches over to DC injection braking to finish the task. At low motor speeds, there is usually very little energy left in the load. You don’t need to know the exact rotor speed, but newer versions with Open Loop Vector control maintain a frequency difference for better control.

Some AC drives, in multiple drive applications, are set up to allow their DC link to be tied to the DC links of other AC drives so that the regenerative power from one motor can be used as motoring power for another.

There are companies that make aftermarket regen modules to apply to existing AC drives. They do not track anything, they just monitor the AC drive DC bus voltage, which means you must manually program the AC drive to go to a low frequency output (instead of Off) and DC inject separately from the regen module.

They still work fine, though, because once you establish the desired braking time, you simply set your decel rate a little faster than that. It almost always means the commanded frequency is lower than rotational speed.

If you look closely at the PWM bridge that drives the motor, there are inverse connected diodes that permit regenerated energy to flow back into the DC bus. These are usually part of the PWM power module. Anytime a PWM drive is powering a reactive load such as a motor, the drive needs these to handle reactive current in the motor.

When regenerating, the PWM transistors and the motor inductance act like a boost converter that pushes most of the power flow through the diodes. This is not altogether different than when a PWM drive for a DC motor is reconfigured to act as a boost converter to boost the voltage of the DC motor to match the DC traction power mains during regeneration.

Motors and Drives returns to Rexel's e-zine with a discussion about the differences between NEMA A and NEMA B motors.

Typical Question

I have a 200 hp motor with a 250 hp adjustable frequency drive (AFD) on it. Because the load is sometimes erratic, I have used a dynamic resistor to absorb regen. Through no fault of my own, we ended up using a NEMA A motor.

As the motor reaches synchronous speed, the drive faults out on locked rotor current. The current tries to go through the roof. It only happens when I run towards 60 Hz. If I run at a reduced speed—say 90% of sync speed—it runs fine.

Can anybody help me understand why? Is this symptomatic of NEMA A? Should I be able to tweak the AFD to adjust characteristics to overcome this or am I screwed until I get a NEMA B on there?

Discussion Group Answers

• I have a hard time understanding how your symptoms have anything to do with type A vs B. Yes, a type A has higher locked rotor current. However, you shouldn't be drawing anything near locked rotor current if you ramp up the speed slowly.
  
• As a first approximation, the motor current at breakdown torque is close to locked rotor amps. Thus, if your erratic load is momentarily pushing you toward breakdown torque, it will momentarily push you toward locked rotor current, which is higher for the NEMA A.
  
• Sounds like you should lengthen your acceleration ramp time a little. The NEMA A motor may have a slightly steeper current characteristic as it goes into the short-term overload range, and that may be causing the overcurrent fault on the drive.
  
  Remember that, when operating a motor on an inverter, you never really operate anywhere on the curve except on the "front face," which is the nearly linear section below and just above the nameplate rating point.
  
  We must be seeing slightly different characteristics between the two motors in that zone. What happens to these motors in the breakdown and locked rotor part of the curve is of no consequence because the system never operates there. In my view, a NEMA A motor is about equally suitable as NEMA B on an inverter.
  
  One other thought, maybe you can set up your inverter to hit current limit before the drive faults. That would simply extend the acceleration ramp just enough to avoid the fault if the variable load conditions ran the overload up too high.

Full Story

OK, here is the full story. I have a 1336 II, 250 hp, 575-volt drive powering a 200 hp Siemens, model D-91-056 at 575 volts +/- 10%, with a 180-amp, 1,792 rpm motor. It is used to mix 3,000 lbs of dough, which I have done successfully many times.

As the load is thrown up, the weight can cause regen, hence the extra size on the AFD. Again, nothing new. The acceleration is set to approximately 10 seconds, and I have had it up to 30 seconds. As 60 Hz approaches, it bombs out. When running at 50Hz, it will run forever and a day. Allen Bradley has been in and says that we have this problem because we are using a NEMA A motor.

They say if we change to an impact drive, all out problems will go away. I smell a rat somewhere, something does not ring true. Surely if I disable (not able to get to site easily to test) vector control and de-sensitize the drive, it should not hit this "meltdown point."

Another question: The main's voltage is 575 volts (probably closer to 600), which raises the bus voltage. Is it possible that I am closer to certain limits than at 460 volts and more susceptible to problems?

Full Answer

Your problem is not the motor, but the load. Mixing dough at high speed will increase the power requirements with the speed ratio cubed. That means the horsepower requirements at 1,800 rpm are (1800/1500)^3 = 1.728 times. Some “design B” motors have much higher slip than a “design A” motor, and if you try to overload the motor, the speed drops and the power consumption is not increased that much.

Questions concerning various ac motors often arise. Discussions with and answers from various users indicate that the operational behavior of AC motors is still a mystery. This article will clarify some of the concerns raised by a typical user question.

Typical Question

I have several motors that were manufactured for three-phase 380VAC, 50Hz. I need to run them on a three-phase 380VAC, 60Hz feed. What will be the effect of the configuration? Will the motors overheat? Will increased frequency affect the motors’ lifespan?

Discussion Group Answers

• If the motor winding will take it—usually it will if new, but not if it is old—you need to increase the applied voltage to maintain constant volts per hertz. Since 60 Hz is 120% of 50 Hz, the 60 Hz nominal voltage of the motor would be 456 volts; that is close enough that 480 volts 60 Hz should not hurt it.

• I have numerous motors of all kinds that were once run on 60 Hz, now running on 50 Hz and all the transformers as well. I’m trying to come up with a method and objective tests to quantify how the motors are performing at 50 Hz, and if the transformers are experiencing core saturation at 5 Hz. Have any of you seen a wholesale change of power from 60 Hz to 50 Hz? What would be key things to check in trying to isolate the critical few motors or other loads that should be changed to 50HZ rated equipment?

• Basically, transformers are a lot like motors, except that transformer has no rotating secondary. Transformers have a rated current and rated voltage. If the applied voltage at a given frequency remains constant at rated voltage and frequency, performance should remain the same. If the current stays within the rated value, no significant increase in heating should occur.

• From a pure technical viewpoint, the flux per pole will be reduced based on the equation:
  
  Flux/p = 100 * E / (4.44 * Hz * N * Kp * Kd)

  E = RMS Volts between lines (380 for your case)
  Hz = Frequency in Hertz (50 or 60)
  Kp = Winding pitch factor (constant for each motor)
  Kd = Winding distribution factor (constant for each motor)
  N = Winding turns in series per phase (constant for each motor)
  At original 50 HZ, Flux 50 = 100*380/(4.44*50*N*Kp*Kd)
  At 60 HZ, Flux 60 = 100*380 / ( 4.44*60*N *Kp*Kd)
  

  And the relation of magnetic flux developed in the motor is:
  
  Flux 50/Flux 60 = 50/60 = 0.833 (Only 83.3 % of the 50 HZ flux).
  
  This weakened flux, proportionally produces only 83.3% torque for the same rotor current.
  
  T = B*L*i2

  T = Torque
  B = Air gap Flux density
  i2 = Rotor current

  The speed will be increased following the synchronous flux speed.
  
  RPMs = 120 * Hz / p, where p = number of motor poles (constant for each motor)
  
  The speed increases in the ratio 60/50 = 1.2 ( 120% of original speed at 50 Hz)
  
  The mechanical power developed at the shaft is HP = T * RPM / 5250 (This is the same because the speed increased proportional to the torque reduction).
  
  In conclusion, “The motor will develop the same output power but with lower torque capacity”. This implies problems accelerating heavy inertia or high torque loads.

I have a 60 HP compressor, I don’t need such a large compressor, will an ac drive reduce the HP? Can I fit a ac drive to this thing?

Discussion Group Answers

• An AC drive will reduce motor speed. By reducing the speed, you could adjust the compressor to the desired flow, and the power requirement will be reduced proportionally. Check the motor input current, which approximately follows the power consumption.
• I assume the compressor is a reciprocating (positive displacement) type. If it a screw operated in parallel with other machines, you may need to look more closely.
• It is a screw type compressor. Does that make a difference for the ac drive add-on?
• AC drives on centrifugal compressors for chillers are routinely used…not sure of what happens when speed is reduced for a screw type. It will reduce HP, but I’m not sure what happens to the compression.
• Reducing the HP will reduce CFM of the compressor. In reality, it’s the reduction in the amount of work required (cfm, pressure, etc.) that results in the reduced horsepower.
• That is what ac drives are for! I use them for my compressors and no more blown fuses, tripped breakers, etc. If properly programmed, you cannot kill your motor no matter what happens to your power or compressor. Your motor will run much cooler and use less power even at 60 HZ. The compressor will have to be positively unloaded on start-up with solenoid valves on both stages. They can be controlled with any ac drive.
• I would prefer ac drives for screws rather than reciprocating types. I have been discussing with manufacturers about variable speed reciprocating types but no concrete decision so far. Some drawbacks I presume are 1) Ineffective lubrication, if the oil pump is connected to crankshaft; and 2) Increased vibration due to imbalance at other speeds than designed.
  I saw and used good many refrigeration chillers (screw and centrifugals) with variable speed application. I have reservations for using ac drives with screw air compressors. Proper receiver sizing, minimization of leakage and correct usage pressure with compressor on/off (or load/unload at least) will give better results and at reduced investment. Power consumption reduces if we reduce mass flow rate through a compressor.
• Another thought is that screw compressors do not load up until at least 1/3 speed, so the area that you can control speed may be greatly reduced.
• There are some additional considerations about lubrication. I have found several screw compressors where the mechanically coupled lube pump performance dropped off at a faster rate than the equivalent speed reduction in the compressor, resulting in rapid compressor failure running at less than 70% speed. That reduced the effective speed control range even further, and thus, the energy savings argument. In some cases, we added positive pressure lube systems, but the added cost affected the payback period adversely.

We have a fan driven by a motor 200 KW. There is a throttle valve to control the flow. Most of the time the throttle valve opening is 30%. We would like to know how to calculate the power saving we would reap if we replaced the throttle valve with a variable frequency drive (VFD).

Discussion Group Answers

• It’s hard to calculate the savings from the details provided. I would expect your power consumption to halve, your motor should need less current and so should run cooler. Remember, though, that the motor fan will run slower, so you would have to check it during operation. Some countries allow tax breaks for investment in energy saving schemes so don't forget to investigate.
• 
  
• It would probably depend rather a lot on the characteristics (type) of fan. When you throttle it, does the pressure differential of the fan increase or does it run at a similar pressure ratio with just reduced flow? Throttling flow may increase or decrease drive power. If you plot motor amps at a range of throttled positions, it will give a much better picture of what is actually happening. If motor amps fall as it is throttled more, a variable speed drive will probably not make a huge difference. If motor amps rise sharply with throttled flow, go for the variable speed drive.
• If your throttle valve is having problems, and you are comparing the price of replacing the throttle valve with a VFD, then it is worth investigating. If you are considering replacing the good throttle valve with a VFD, I don’t think you will receive a good payback. When the throttle valve at 30% opening, the fixed speed motor should be at lower load compared to 100% opening. Therefore, changing the fixed speed motor to VFD driven will only reduce the loss of the motor at low load. In my opinion, it would be a good comparison only when a new project is being planned. If you select a VFD, you would save the cost of the throttle valve and some electricity cost due to better efficiency.
• What type of a fan is it and what is your system configuration? Generally, when you reduce the speed of a fan, its pressure-developing capacity also decreases by 1/2nd power of the speed ratio. So you can’t simply use the Q1/Q2 = N1/N2 and P1/P2 = (N1/N2) 3 formulae to assess the power savings. Check the total pressure developed by the fan (with valve wide open) and pressure drop across the fan. The difference between these two readings will give you the required pressure to pump air into the system. Now calculate the required RPM from (N1/N2) 2 = SP1/SP2. Based on this RPM you should be able to calculate the final power consumption.
• Just a reminder here, folks—the performance rules for centrifugal fans in a relatively free-flowing system (air duct with open vanes) are well publicized and understood.
  
  They are 1) flow is proportional to speed; 2) pressure and shaft torque are proportional to the speed squared; and 3) horsepower or kw is proportional to speed cubed. In view of these, we can safely state that the fan, at 30% of full speed will flow 30% of full speed volume, torque and delta p will be .3 x .3 = .09 or 9% of full speed and fan horsepower will be .3 x .3 x .3 = .027 or 2.7% of full speed.
  
  The problem here is that the original question asks for a comparison of energy usage with “throttle valves” , but it doesn't say whether the vanes are on the suction or discharge side of the fan. It makes a lot of difference—from an energy usage standpoint.
• Replacing flow control by a throttle valve with a flow control by using a variable frequency drive will save a lot of power. If this will lead to a cost saving justifying the cost of VFD mainly depends on the time the device is operated at reduced flow. If you operate at reduce flow for one day every year stay with the throttle valve but if you operate with reduced flow for several thousand hours a year, the cost of the VFD will be saved within very short time. Contrary to several “energy saving devices” pushed on the market very aggressively, a VFD in a flow control application is a real “energy saving device”.
• Inlet guide vanes are definitely not an option at the reduced flow rates we are speaking about. They are generally economic if the minimum flow rate is around 80% of maximum flow, below that, efficiency drops to a great extent. The new curve due to inlet control starts from the same shutoff head but with more slope towards maximum flow. So there is no significant difference in performance at lower flow rates. Moreover, it is always possible to run a fan on the locus of BEP by using VFDs. Life of the rotating equipment improves due to low speed but care should be taken to avoid vibration.

• You will run into people that are more than happy to over-specify components to protect their own comfort zone. That’s where you get statements like “you always should specify an inverter-duty motor”, etc. The basic facts about motors on inverters come down to primarily two issues: heat and and insulation integrity.
  
  Heat only matters when a motor with a cooling fan on its rotor shaft is required to develop high torque levels at slow speed. Centrifugal loads—such as most fans and centrifugal pumps—unload as they slow down by the square of the shaft speed. Motors on these types of load rarely have any thermal issues because, even at very slow speed, there is little or no load. These applications can use open drip proof (ODP) motors or totally enclosed fan cooled (TEFC) motors without any extra thermal considerations. In other words, plain, commodity-grade motors are fine.
  
  If your load requires high or full-rated torque at slow speeds, then you need to check with the motor manufacturer for slow speed thermal capacity. Up to about 100hp, most TEFC motors are rated full-torque down to 1/4 nameplate speed. Below that, you will need some auxiliary method of cooling. Within that range, however, ordinary ODP and TEFC motors are fine from a thermal standpoint.
  
  As for the second issue, VFD’s output high voltage pulses to the motor, which can result in premature insulation failure. Roughly, here are guidelines to follow for insulation issues: If your application is 230v or 380v—and your motor leads are not more than 100ft and the drive carrier frequency is 4khz or less—pretty much any old motor will work. If your application is 460V, motor leads are under 60ft, and drive carrier frequency is 4khz or less, any standard Insulation Class F motor is fine.
  
  Insulation Class A or B is absolutely unacceptable. If, at 460V, leads are longer or carrier frequency is higher or you just need a little extra insurance, use a Class F motor with IEEE MG1-Part 31 rating. This is an extra insulation test specifically dealing with VFD-type pulses and gives you a better level of insulation integrity. This extra endorsement is not expensive and is worth specifying on any new motor purchase, whether you need it or not. If your application is 550V or higher, use of inverter rated motors for that specific environment is mandatory. The same is true at 460V if leads are long, carrier frequency must be higher than 4khz, ambient temperatures are at or above 40 degrees C, etc.
  
  As you push the edge on insulation issues, you get into the need for more and more specialized and expensive conditioning equipment while using the better motor grades.

• I’m not sure about the additional cost for inverter-duty motors. It may depend on the size range you are considering. One item that is often mentioned is protection against shaft currents through the bearings that can cause bearing damage. This is a fairly common problem on motors fed through adjustable frequency drives. Most manufacturers provide some form of insulation in one of the bearings to eliminate the shaft currents in their “inverter-duty” motors. I would also recommend specifying some type of embedded temperature detector for any motors 100hp and above on drives.
  
  As for deciding between low voltage (LV) and medium voltage (MV) drives, this is very application specific. The cost of MV drives has come down quite a bit in the past few years. I doubt that specifying two LV systems in place of one MV system will ever be less expensive (at least above about 500hp). In many cases, an inverter-duty motor may not necessary.

• As technology changes, the cost differential between LV and MV drives in many instances may not be very great, for the simple reason that often the smallest MV drive may be far larger than you need. For instance, although a VFD manufacturer may catalog a 500HP MV VFD, in truth they use the same components as in a 2000HP unit because they are the smallest components available at that voltage. Or if you need a 2,500hp drive, you are actually buying a 5,000hp. Of course in that case you really didn’t have an option at LV anyway, but you get my point.

• My personal opinion is to reduce risk of premature failures by “guessing” and installing the motor designed to match the application. Yes, at some hp size, the MV drive will become more costly on a per horsepower basis, no question about it. When I last looked into this, a 1,000hp MV drive and motor was less costly than a 1,000hp LV drive and motor. I’m guessing that the break-over point has dropped in the past few years.
  
• At 1,000hp, I don’t doubt it. 1,000hp LV motors are relatively rare, and as such, so are the VFDs capable of running them (not much point in designing a VFD for a motor that hardly anyone uses). But at 500hp, where both motors and VFDs are plentiful in LV, I would venture to say that 2x500hp LV motors and drives are still less expensive than 1x1000hp MV motor and drive. I don’t know enough about the motor costs, but 500hp 460V VFDs are around US$35K, so US$70K for 2. Last time I checked, 1000hp 4160 VFDs were about US$180K (neither price includes switchgear, harmonics mitigation, etc.). So even if the 2xLV motors cost more than the one MV motor, I doubt it is $110K more. I’d say the breaking point is probably around 700hp because that is where LV motors (and drives) become relatively rare and therefore more expensive, so it helps mitigate the MV premium.

Unlike automobiles, machine tools stand to become more energy-efficient when they move faster. High speed machining offers real potential for reduced energy costs. That’s because, in addition to the direct energy costs of removing material, the machining cycle also has indirect energy costs associated with the machine’s fan motors, pumps, lights and so on. A shorter cycle reduces the amount of time these components spend drawing power. Therefore, technology for faster machining delivers not only productivity, but also energy savings.

Paul Webster of GE Fanuc (Charlottesville, Virginia) says his company has been giving increased attention to the energy impact of machine tool control systems—in part as an extension of GE’s “Ecomagination” campaign. Mr. Webster, a product manager for servo technology, notes various points in the control system where advanced technology can save on indirect costs. Those aspects of the control system include:

* Motor core and power density. The servomotor core can be designed to minimize inertia, he says, while permanent magnet synchronous motors provide high power density and excellent acceleration.
* Motor windings. Dual windings allow one motor to perform like two. A low speed winding lets the machine deliver high torque at lower rpm values, while switching to the high speed winding extends the constant power range to let the machine accelerate to higher speeds more quickly.
*
Control loops. The company’s HRV (High Response Vector) closes the motor control loops at a rapid rate and adjusts the control commands based on the load and speed of the motor. Quickly responding to deviations in the system in this way allows the control to achieve high acceleration without sacrificing accuracy.

The choice of control system technology also affects direct energy costs, Mr. Webster says—specifically by reducing the amount of energy wasted as heat. For example, dynamic control of the motors can optimize the amount of torque produced for a given current. Even more significant is power source regeneration, which lets the decelerating motor act as a generator by putting some of the braking energy back into the system instead of wasting it. Power source regeneration can be the number-one contributor to energy savings in a CNC machine tool’s design.

The machining center shown above combines technologies for both direct and indirect energy savings. Savings such as these are worth considering when a new machine is evaluated, Mr. Webster says. In short, consider how much it will cost to run that machine.

Linear motors account for part of the reason why DaimlerChrysler’s Stuttgart, Germany, manufacturing facility has been able to nearly double the productivity of machining centers producing automotive cylinder heads. The company replaced more conventional machines with horizontal machining centers featuring GE Fanuc (Charlottesville, Virginia) linear motors for X, Y, Z motion, with control coming from a GE Fanuc 161 CNC fast enough to keep up motor position through high speed moves. The high feed rate and high acceleration the linear motors make possible help reduce cycle times. DaimlerChrysler manufacturing personnel Thomas Brandstetter and Ingolf Kurschner analyzed the productivity improvement. They say the linear-motor-equipped machining centers now produce at a rate that would have required 11 traditional machines.
Ingersoll Milling Machine Company (Rockford, Illinois) supplied the six HVM600 linear motor horizontal machining centers for cylinder head production to DaimlerChrysler. The machines have traveled distances of 630 mm by 630 mm by 600 mm (X, Y, Z axes). A hydrostatically mounted main spindle attains outputs of 37.5 kW at speeds of up to 20,000 rpm. Workpieces are clamped to pallets measuring 630 by 630 mm2 on an NC turntable with a load capacity of 1,400 kg.

Four cylinder heads are secured simultaneously in a clamping device. In one continuous work process, the cylinder heads are extensively machined. End faces, screw contact faces and threads are milled. A total of 130 core holes must be drilled and the threads milled on each of four cylinder heads. For example, the main spindle requires 1.2 seconds to mill an M6 thread to a depth of 14.1 mm at a rotary frequency of 20,000 rpm and 700 mm/min. feed rate. The HSC machining center mills screw contact faces of 24 mm diameter with a diamond-coated face mill cutter at speeds of 20,000 rpm (corresponds to 150 m/min. cutting speed) and 5,200 mm/min. feed rate. Forty of these have to be machined on every cylinder head.

Before a toolchange takes place, all similar parts of the four cylinders are machined. During the toolchange, a laser beam verifies that the proper tool is being substituted. With its 32-bit RISC processor and high-cycle frequencies, the CNC system can process NC programming very quickly. Furthermore, it contains the well-developed look-ahead functions that are required for high speed feed.

The high speeds of these six machines allow Daimler Chrysler to produce 300 four-valve cylinder heads for four-cylinder engines daily. In conventional machining centers, 11 machines would have been necessary to produce the same quantity of components, thus requiring more personnel, larger factory floor space, more tools and equipment, maintenance and investment, machine interlinking, lifting gear and additional measuring devices.

The high acceleration power of the hydrostatically mounted main spindle enables the machines to achieve short cutting times. The spindle requires only 1.5 seconds to accelerate to a speed of 20,000 rpm. The linear motors’ high acceleration power and feed force also ensure minimum non-machining time. In rapid motion, they accelerate in the Z axis at 1.5 g (14.5 m/s2) and in the X and Y axes at 1 g (9.8 m/s2) to 76 m/min.

Even at these high speeds, the linear motors position the spindles in the entire work area to a degree of accuracy of approximately 5 microns. To further increase accuracy, glass scales are installed in all axes, and pretensioned roller guides direct the carriages in each axis.

After optimizing some of the para-meters of mechanical engineering, the HSC machining centers with GE Fanuc’s linear motors have proven very successful at shortening production times and increasing accuracy. Cutting figures and productivity levels has exceeded what the experienced machininsts at DaimlerChrysler had envisioned, as the machining centers now achieve uptime levels of more than 95 percent. In addition, the enclosed design of the linear motors protects the equipment.

Whenever a motorcycle part needs to be replaced, there are two options the bike owner has. He can avail himself of original equipment manufacturers (OEM), or of aftermarket parts.

OEM

OEM parts are the components of the motorcycle that come with the vehicle when purchased directly from the vendor; they are brand new and unchanged. This means that replacing a component of your motorcycle with an OEM part is like replacing it with an original part used to setup your motorcycle. OEM parts are outsourced to a third party company that supplies the component needed for the motorcycle. These parts are workable, but tend to be more expensive, since they are considered to be original parts.

Aftermarket Parts

Aftermarket parts are basically non-OEM parts that are manufactured to work in place of original parts. They could be lower, equal, or higher in terms of performance and the same in terms of pricing, although generally if they are meant to match the original part, they are cheaper.

Aside from pricing, an aftermarket motorcycle part may have a substantial advantage over an OEM part because of the high demand for customization of parts. It could simply be a change in the colors of the chassis and the appearance of the rims of the tires or the cables and sheaves inside the motors. The use of aftermarket parts can be for aesthetic values, performance reasons, and safety enhancements.

During "office hours" when there are no changes in your organization, the relevance of knowledge management is minimal. Imagine that your business is involved in selling bikes and this business is prosperous. But than, all of a sudden there is a fall in demand. You verify with the suppliers of the motors and indeed in other areas the demand declines. You are just selling them, and you need to decide what to do.

You need information to understand the cause of the decline in demand. After you have done this, you will find out -- in this case -- that because of the increase of the oil-price, the demand for bicycles has been increasing other the last months. This explains the decline in your sales.

Because of the same information you may also decide to step into the bicycle business. This should solve your problem you think and it will, although the competition is fierce. But that’s not the point. The point is -- knowledge management. What do you need to know to manage your business? Again you need to gather a lot of information about this new market, about the potential clients, information about buying bicycles from the various (new) suppliers. It is all what you should be able to manage. Information management.

Yet, your business will not automatically take of in this new setting. You need experience. "How is the bicycle selling process different?" How do you buy bicycles, from what kind of suppliers, how do you know about quality. A lot of issues that you could translate from the other business. Yet there are also many elements you will not know about (in advance). You have to experience them, these can not be planned, because you do not know them. And in this case your business has made no real transformation. The profile is very much the same. From selling bikes to bicycles. Knowledge management would really be an issue if you switch from selling bikes to trading stocks.

A way to handle this knowledge problem is to contract someone who is experienced in -- who knows about -- selling bicycles. This is a common solution.

General Motors Corporation (GM), the world's largest automotive corporation and vehicle manufacturer has undergone big transformation in regard to ownership of the corporation’s stocks and holdings.

Recently, Capital Research & Management Company, GM’s second biggest investor based in Los Angeles has sold 24 percent of its holdings equivalent to 19.2 million shares. This datum was filed with the Securities and Exchange Commission. Additionally, Brandes Investment Partners LP situated in San Diego, third biggest investor of the corporation, has also sold 4 percent of its holdings equivalent to 2.4 million shares.

When asked about GM’s situation, Brenda Rios, spokesperson of GM merely said, "It's natural for investors to periodically rebalance their holdings.” Nevertheless, she declined to comment any further.

Capital Research & Management Company as well as Brandes Investment Partners LP dismissed inquiries by simply stating that they do not comment on their investments.

On one hand, other investors of the corporation have acquired more shares in GM. Credit Suisse purchased 11.5 million shares. Said investor is now the sixth biggest investor of the corporation. Fidelity Management & Research also purchased 6.8 million shares. Further, according to lionshares.com, Franklin Mutual Advisers LLC also purchased 4.6 million shares.

Craig Fitzgerald, an automotive industry expert, said that the transactions were the result of some investors who bought GM shares at a lower price taking a profit and others seeing signs of progress in GM's restructuring plan. Fitzgerald added, "GM in particular is continuing to do some of the key things they need to be doing. There's no reason to necessarily believe there isn't more upside in the short- and mid-term."

Kirk Kerkorian's Tracinda Corporation, GM’s biggest investor, disclosed that GM is soon to form an alliance with other automotive giants namely Renault SA of France and Nissan Motor Company of Japan. As groundwork to said alliance, teams of employees from the three companies were united to conduct studies regarding its cost and benefits.