VSD motor lead check
Check a drive’s motor lead: peak voltage, length and cable.
Reads AS/NZS 3000:2018. Tables checked 2 Oct 2026.
Try a job 4
Each job fills in example values. Replace them with your values.
- Check a drive motor leadWhether a pump on a drive needs an output filter for its motor lead.
- Check a long fan lead on a VSDA fan motor far from its drive, against the drive maker limit.
- Size a conveyor drive lead with a filterA long conveyor lead on a sine filter and the current for its cable.
- Check an inverter-rated motor on a driveWhether an inverter-rated motor takes the reflected peak without a filter.
Inputs
Worked example with the default inputs. The live calculator replaces it when the page loads.
- Motor power
- 22 kW
- Supply voltage
- 400 V
- Motor power factor
- 0.85
- Motor's rated peak voltage
- 1,000 V
- Motor lead length
- 60 m
- Cable
- Screened, symmetrical
- Output filter
- None
- Maker's lead limit, no filter
- 100 m
- Pulse rise time
- 0.1 µs
- Pulse speed in the cable
- 150 m/µs
Motor lead
Fails
- Peak at the motor, up to
- 1,131V
- Maker's lead limit
- 100m
- Critical lead length
- 7.5m
- Current for the cable size
- 41.51A
- Drive d.c. bus
- 565.7V
This result is a design aid. Verify it against the current standard.
Calculation steps
- Current for the cable size41.51A
- Drive d.c. bus565.7V
- Critical lead length at a 0.1 µs rise time7.5m
- Peak at the motor, up to1,131V
- Motor insulation, rated peak1,000V
- Maker's lead limit, no filter100m
- Motor leadResultFails
About this calculator
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.
| Step | Table or clause | What it decides |
|---|---|---|
| Cable current | The motor tool | The current for the cable size |
| Bus voltage | Supply voltage times √2 | The height of each pulse |
| Critical length | Pulse speed times half the rise time | Where the reflection becomes full |
| Peak at the motor | Up to twice the bus; the filter maker's figure with a dv/dt filter or reactor; the bus with a sine filter | The voltage the motor sees |
| Lead length | The drive maker's limit | Whether the lead needs a filter |
| Earthing and ratings | AS/NZS 3000 Cl 5.4.8 and Cl 4.12.8 Note 1 | The earth current and the 50 Hz basis |
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.
Questions
Each answer describes what this tool calculates. The result is a design aid. Verify against the current standard.
Why does a long motor lead matter on a VSD?
A drive sends fast voltage pulses. On a long lead each pulse reflects at the motor and the voltage there can nearly double. That can break down the insulation of a motor not rated for it.
What is the critical lead length?
The length a pulse travels in half its rise time. Past it the reflection is full. With a rise time of 0.1 µs and a pulse speed of 150 m/µs it is 7.5 m, so most motor leads are past it.
Do I need a dv/dt or sine filter?
Where the lead is past the drive maker's limit, or the peak is past the motor's rating. A dv/dt filter or reactor slows the edge but leaves some overshoot, so the tool takes that peak from the filter maker's data. A sine filter smooths the output nearly to a sine wave.
What cable should a VSD use?
A screened, symmetrical cable with the screen bonded all round at both ends, as the drive maker says. Size it for the motor current, and remember the AS/NZS 3008.1 ratings are for 50 Hz.
Why can a VSD give a shock from earthed metal?
AS/NZS 3000 Cl 5.4.8 warns that a drive can put high frequency current on the protective earth, with touch voltages up to 100 V. A 50 Hz tester may not show it. Earth the drive as its maker says.
Clauses and tables this tool reads
- AS/NZS 3000:2018 Cl 4.12.8Equipment on a converter output
- AS/NZS 3000:2018 Cl 5.4.8Variable frequency devices
- AS/NZS 3000:2018 Cl 5.4.8 and Cl 4.12.8Variable frequency drives
Every clause and table, with how each was verified
Tables last checked on 2 Oct 2026.
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