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The VFD Answer Man, Volume 6: Motors, VFD's, and Long Lead Lengths

December 20, 2015 |By: Terry M. Smith

Motors 150 feet (ABB claims 330) or more in cable distance from the output of a VFD are most probably going to suffer from a phenomenon known as voltage doubling. Because of the increased length the waveform being generated by the VFD has a chance to amplify itself at nodes close to the motor leads. This voltage can be very high due to the relationship of the voltage change over time in the VFD (dV/dT). When this ratio is allowed to get high in the case of a long lead length the apparent voltage at the motor can be double that of drives running motors on shorter cable lengths. For a 460V motor this means the apparent voltage looks like 920V. If this were the only part of the problem, however, there would be no winding failures.

With VFD’s the output waveform is based on Pulse Width Modulation of high frequency square wave DC. This means the output voltage is actually closer to 680V DC than 480 V. During a voltage doubling situation the voltage in the motor reaches 1360V due to the doubling. This puts the voltage very close to the insulation rating all motors have of 1500 Volts. Any additional spikes on the incoming line, or feedback from the motor and load can cause this to go higher, exceeding the 1500V insulation rating. This occurs in the first turn of each winding, where resistance causes the voltage to drop to a normal level. Voltage doubling by having a harmonic lead length will increase this voltage to 2720!  

Solutions to winding problems from long lead length:

1. Inverter Duty motors should be considered for all new IGBT drive installations. They offer increased winding slot insulation, increased first turn insulation, and increased phase- to-phase turn insulation. They are more expensive than standard design B motors but are the best motor for the job when it will be controlled by an IGBT variable frequency inverter. The NEMA Standard MG- I (part 3 1) indicates that inverter duty motors shall be designed to withstand 1600 volts peak and rise times of >0.1µsec. Nevertheless, it is wise to confirm the actual motor capability with the manufacturer.

2. Minimize Cable Length between the inverter and motor. Quite often this is somewhat uncontrollable, especially when the application is a deep well submersible pump where the motor is required to be a great distance from the inverter. The longer the cable, the greater the capacitance of the cable, the lower the impedance of the cable and thus a greater mismatch will result between the characteristic line and load impedance's, resulting in higher peak voltage at the motor (load) terminals. Minimize this length whenever possible to avoid problems.

3. Tuned Inductor & Capacitor (LC) Filters are an effective means of taming the output voltage waveform and protecting the motor. An "LC" circuit can result in the best voltage waveform but at a relatively high cost and with some future considerations. Of course these filters are "low pass shunt type filters" tuned for some specific frequency, often in the range of 1 KHz to 2 KHz. Because these filters have essentially zero impedance at their resonant frequency, it is very important that the inverter switching frequency not be set too low. The threat exists that someone may vary the carrier frequency (at a later date) without consideration for the existence of a low pass filter resulting in damage to the inverter or filter. One should be very careful when applying this type of filter on the output of an inverter with variable carrier frequency. LC filters for this purpose cost approximately 3-4 times the cost of a load reactor.

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