Motors & Drives Information

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

Typical Question

I am using VFDs with dv/dt filters installed. Each VFD has eight motors attached. The motors are standard—1/2 HP, 200-230/460V, 3-phase with class B insulation. All motors are started together.

We have had a failure rate of about 10% failure of these motors, mostly on startup. The motors are new and fail on startup or with less than 2 weeks run time.

We are still doing failure analysis on them, but so far there have been bad lead connections, and one burned out motor coil. I have talked with some motor experts, and they say the motors do not show the "typical" signs of VFD compatibility failures. My customer is insisting that we change to VFD rated motor to fix the problem.

Discussion Group Answers

• "Finding inverter-rated motors in small sizes can be difficult. The extra insulation simply takes too much space. The dV/dt filter should be all right. There should be damping resistors parallel to the reactor's to prevent ringing between reactors and cable capacitance.
  
  "This is not the reflected wave phenomenon, but pure LC ringing. The frequency is usually not more than 10 or 20 kHz. It depends on the total cable capacitance and the length of the cables. The ringing not only heats the motors, but also it heats the reactors in the dV/dt filter.
  
  "Try to operate the VFD with the lowest possible carrier (switching) frequency. This will reduce any adverse effects that the switching can have on the motors. Eight motors on one VFD means that you have to run the inverter in scalar (volts per hertz) mode. Motor protection has to be outside the inverter (one thermal for each motor). The motor protection function of the VFD cannot protect the eight motors individually."
  
  
• "It is possible there is too much slip. Measure speed with a stroboscope and compare to the rated speed. If there is too much slip at nominal load, then your voltage/frequency ratio may be on the low side and should be adjusted.
  
  " The thermal protection becomes important when there are several motors connected to one inverter. The inverter has to supply power to all motors and cannot tell if one single motor is working too hard. You need thermal protection for each individual motor."
  
  
• "Marginal interactions among motors should not be ruled out, since each motor is manufactured within certain manufacturing tolerance, which influences the motor parameters. If different motor parameters exist, there will be marginal currents flowing among motors. This is necessary to treat as a system of interacting loads on the VFD output."
  
  
• "The total cable length with 8 motors involved will possibly be quite long. Check this and confirm if you are using screened cable or not. If you are, there are practical limitations to screened motor cable lengths due to the increased high capacitance effect. In some cases, the capacitance effect can cause problems with motors or the actual drive itself. If you have a dv/dt filter then this should minimize the effect."

Clarifications

When dealing with today's IGBT-based PWM VFD, under a specific set of installation conditions, there exists the potential for motor insulation damage. The most significant impact is the possible occurrence of high voltage spikes at the motor terminals that can produce destructive stress of the motor insulation.

Output waveforms consist of repetitive pulses. Motor waveforms can have high peak voltages with amplitudes of 2 to 3 times the DC bus voltage of the VFD. For 460-volt VFDs, the DC bus voltage is about 650 volts. For 230-volt VFDs, the DC bus voltage is about 350 volts.

There is less likelihood of motor insulation stress with 230-volt VFDs. Filters located on the VFD output or at each motor will reduce the effects of the peak voltage seen by the motor, however, these devices will not guarantee that the motor will not suffer an insulation breakdown.

Most motor insulation systems are designed to operate in a utility-supplied voltage environment. The typical utility voltage would be a 460-volt sinewave. A motor insulation system capable of 600 volts would offer suitable protection.

Most motors manufactured today provide a minimum of 1,000 volts insulation protection. Some inverter-rated motors (VFD-rated motors) provide 1,600 volts or 18,000 volts of protection. In almost all cases, the weak insulation point in the motor exists where the motor terminal leads enter the motor frame.

A motor insulation system consists of magnet wire insulation, resin insulation, slot insulation, and coil head insulation. In addition, there are variations in assembly techniques used. An insulation system is much more than the sum of its components.

Several factors could contribute to standard motors failing shortly after being operated. There have been cases where small HP motors began to fail when IEC contactors began replacing NEMA contactors. The faster switching time constant of the IEC contactor placed more stress on the existing motor insulation.

# posted by Medical Information : 3:40 AM