How to avoid inverter variable speed drive (VSD) equipment failure
How to avoid inverter variable speed drive (VSD) equipment failure
Connecting motors to variable speed drives (VSDs) via cables delivers flexible control, energy efficiency, and low-cost maintenance. According to Schneider Electric UK however, four precautions are advisable to prevent overvoltage compromising motor lifetime or causing a shutdown.
How cable length can create issues
For motors connected to VSDs, a combination of fast-switching transistors and long motor cables can cause a temporary overvoltage at the motor terminal connection. In extreme cases, high peak voltage can prematurely age the motor winding insulation, causing overall motor failure.
Consequences of overvoltage and overcurrent conditions
Effect on VSD: The main risk of overcurrent in a VSD is a short circuit fault. Capacitive peak current can also cause power transistor temperature to rise, and this above-normal temperature can reduce VSD lifetime.
Effect on the electric cabinet
Disturbances that result from long cable and motor interactions create high frequency circulation current into the ground that can disturb appliances connected on the same network. High frequency currents also generate radiated emission, which can disturb electronic devices around the motor cable.
Effect on motor
Overvoltage at the motor terminal can occur between two motor windings. Depending on the class of winding insulation, partial discharge and insulation ageing can occur, leading to motor failure.
Side effects of a long motor cable: In standard applications, overvoltage occurs whenthe motor cable exceeds 10 meters (32 feet) in length. The longer the motor cable, the higher the overvoltage, an effect that amplifies with a shielded cable. Calculating the cable length correctly helps protect the VSD from any unexpected tripping.
Another side effect is motor bearing degradation, caused by common-mode voltage that the VSD inverter generates, which throws high frequency current into the motor bearings.
Best practices for safeguarding VSDs and motors
– Specify a motor designed for speed drive applications
– Specify VSDs that integrate voltage reflection superimposition software suppression
– Minimise the distance between the motor and VSD
– Use unshielded cables if possible – shielded cables calculate as twice the length
– Reduce the VSD switching frequency
By following these steps, a cable that is 300 meters (984 feet) or shorter will work without any additional options. For applications that involve longer cables, unknown motor insulation levels, or nonstandard motors, a dV/dt output filter or sinus filter is the best preventive measure.
Motor cable length |
Motor conforming
|
Motor NOT conforming
|
| 1 m (66 ft) < Lm < 50 m (164 ft) | Filter not required | dVdt filter |
| 50 m (66 ft) < Lm < 100 m (328 ft) | Filter not required | Sinus filter |
| 100 m (328 ft) < Lm < 300 m (984 ft) | Filter not required | Sinus filter |
| 300 m (328 ft) < Lm < 500 m (984 ft) | dVdt filter | Sinus filter |
| 500 m (328 ft) < Lm < 1000 m (984 ft) | Sinus filter | Sinus filter |
Selecting the right preventive measures depends on motor characteristics and cable length
See more Altivar VSDs from Schneider Electric UK at discounted prices here
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