VSD motor lead check

How this is calculated#

A variable speed drive does not send a smooth sine wave down the motor lead. It sends fast pulses at the voltage of its d.c. bus, and each pulse can reflect at the motor. On a long enough lead the reflection nearly doubles the voltage at the motor terminals, which can break down the insulation of a motor not rated for it. This tool works that peak, checks the lead against the drive maker's limit, and gives the current to size the cable for.

The bus voltage is the supply voltage times √2. The critical length is how far a pulse travels in half its rise time; on a lead longer than that the reflection is full and the peak at the motor reaches about twice the bus. On a shorter lead the tool scales it down. A dv/dt filter or an output reactor slows the edge but does not end the reflection, so the tool makes no estimate of its own: you enter the peak at the motor from the filter maker's data for your lead, and the result says it is the maker's figure. Only a sine filter brings the peak down to the bus voltage. The peak is compared with the motor maker's rating. The lead is compared with the drive maker's limit, with or without the filter.

The current is the motor tool's full load current. The rise time, the pulse speed, the limits and the motor's rating are makers' data, entered by you, so the result calls them assumptions. AS/NZS 3000 warns of high frequency current on the protective earth, and notes the cable tables are for 50 Hz.

Worked examples#

• A 22 kW pump on a 60 m lead, standard motor fails: the peak can reach 1,131 V against the motor's 1000 V rating. The cable carries 41.51 A.
• The same pump with a dv/dt filter passes if the filter maker gives a peak of 900 V at the motor for 60 m. The tool prints that 900 V as the maker's figure; it is an example, not an estimate.
• The same pump with an inverter-rated motor passes with a peak of up to 1,131 V on a 1600 V rating.
• A 7.5 kW fan on a 150 m lead fails on length: the drive maker allows 100 m with no filter, though the peak of up to 1,131 V is within the motor's rating.
• A 55 kW conveyor on a 250 m lead with a sine filter passes with a peak near 566 V, inside the 300 m limit with the filter.

Limits#

The peak is a first estimate from the lead length alone. The real peak depends on the cable's and motor's surge impedance, the switching pattern and the drive's own output stage, which the makers test. The tool does not size the cable for harmonics on the drive's supply side, the cable's capacitive charging current, or bearing currents.

See also: Motor, Motor starting and Cable size.