Tuesday, September 12, 2006
Motors & Drives - Choose the Proper Motor
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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.
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