When a motor larger than a 315 frame size (about 160kW) is to be operated via variable frequency drive, it is recommended to use insulated bearings to eliminate the effects of capacititive leakage currents that flow from the rotor to the stator which arc on the rolling elements and create minute pit holes which cause bearing failure. These currents are the result of the high voltage peaks on the PWM waveform coupled with the stray capacitances within the motor.
If so, consider a cable having a signal pair integrally packaged inside the same outer jacket as the VFD cable. Usually, the VFD manufacturers can provide the specific information needed to make decisions about gauge size and cable run length. Note: if a cable length is too long, it will act like a large capacitor that must be charged up when the system is turned on. After that initial phase, electrical energy continually pumped into the cable from the VFD can surge into the motor, possibly causing motor bearing burn-out or damage to windings.
Another way to do this cheaply would be to put a 500 ohm resistor in the current loop to translate 4-20ma into 2-10 V. Now use this voltage signal into each VFD voltage input. These are high impedance inputs usually so loading should be a big problem. You will have to program the VFDs with 2 volts offset or change the current transmitter to output 0 ma at minimum speed.
Contact person:Terry Huang
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