Much has been discussed of the additional stresses that variable speed drives (VSD’s) can place on motor leads and insulation systems. However, in all but the most sensitive of installations, and assuming the use of properly rated motors, preventive or corrective measures are readily applied, especially during design and engineering stages. While each VSD and motor manufacturer has specific requirements for cabling design and installation practices, there are several elements common to all.
- Limiting motor lead length: by installing drives as close as practical to the motors they control, the magnitude of voltage overshoot is reduced, lessening the risk of over-voltage at the motor terminals (all other things being equal). There is a length, generally referred to as the critical length, beyond which the magnitude of reflected voltage pulses increases to the the extent that motor insulation systems can be affected. Critical length varies based on cable and drive characteristics and is calculated as:
The critical length is a consequence of the rate of travel of the pulse from the […]