The benefits of using VSDs with motors are well documented, however for some applications the installer must use caution. Using a long VSD cable between the drive and the electric motor can have a major impact on performance and cause premature ageing. For example, long cable runs between the drive, and motor may cause damage due to overvoltage and overcurrent conditions.

Motors connected to PWM VSDs, a combination of fast-switching IGBTs and long motor cable’s can cause a temporary overvoltage at the motor terminals. The main risk to VSDs is a short circuit fault. Disturbances from long cable runs and motor interactions can also disrupt equipment connected to the same distribution network. Moreover, motor overvoltage can occur between two motor windings, damaging the winding insulation causing motor failure.

Users also need to watch for degradation of the motor bearing from common-mode voltage generated by the VSD. Although in the UK, by using 500V standard IEC motors, users are almost immune to these stresses. This is because VSD rated motors are often wound using wire rated for 1440 V to withstand potential burnouts. Despite this, Gambica says a lack of understanding has resulted in users implementing unnecessary and costly countermeasures.

However, if leakage current finds its way back to earth via the bearing roller, arcing damage across the bearing surfaces may result. As a general guide, when using a standard (non-VSD) motor, consider a filter if the output cable is more than 20 metres long.

Long VSD cable length

VSD cable length has an effect because the longer the motor cable, the higher the overvoltage. At longer length’s the rate of voltage increase vs. cable length is also influenced by the switching frequency. For applications with motors running in parallel, the appropriate cable length is the sum of all the cable. For example, if three motors connect to a single VSD with 20-meter cable’s, the total length is 60 meters. Calculating the correct cable length helps protect the VSD from unexpected tripping.

According to Schneider Electric, some planning steps and best practice will avoid harmful problems. Four preventive step’s to safeguard motor equipment include:

1. Using VSDs with preconfigured software for more dependable operation.
2. Output reactors to limit starting current and counter rapid current changes by reducing peak voltage and dV/dt voltage (rise time at the drive output).
3. Output dV/dt filters cut overvoltage and capacitive leakage between phases and phases to earth.
4. Sinusoidal filters suppress the overvoltage effect and reduce EMC disturbance.

For applications that involve long VSD cable, unknown motor insulation levels, or non-standard motors, a dV/dt output filter or sinusoidal filter is the best preventive measure. Selecting preventive measures depends on motor characteristics, cable length, and best practices, including:

– Specifying a motor designed for VSD applications

– Specifying VSDs that integrate voltage reflection superimposition, software suppression

– Minimising the distance between the motor and VSD

– Using unshielded cables if possible. When using shielded cables, calculate them as twice their length

– Reducing the VSD switching/carrier frequency

Finally, remember when retrofitting a VSD to an existing motor with a motor cable over 10 metres, there are many unknowns. The safest approach is to use an inverter output reactor recommended by the drive maker to minimise the risk.

Further reading

Selecting the best solution for connecting motors to VSDs white paper from Schneider Electric

Motor insulation voltage stresses under PWM inverter operation and Motor shaft voltages and bearing currents under PWM inverter operation. Both are available from the UK Trade Association – Gambica