Unexpected Behaviour in Variable Speed Drives

This page provides an overview of the most common performance issues experienced when using a Variable Speed Drive (VSD) with an AC induction motor. When performance does not match the expected operation but the VSD does not trip, this can be frustrating, as there is no clear indication of a fault. This page covers the most common issues we encounter and how they can be resolved.

If your VSD is not operating as expected but is not tripping, identify the symptom below and go to the relevant section.

Use the list below to navigate, or scroll to browse all topics.

If your VSD is tripping, please see our Common VSD Trips page.

If there is terminology you are unfamiliar with, please see our Know Your VSD page.

If you are installing a VSD and require guidance, please see our Installation and Commissioning page.

VSD is running but the motor is stationary

Useful knowledge:

  • A VSD can run at zero speed, so it may appear stopped when the speed reference is zero.
  • Digital input logic varies between VSDs (PNP or NPN).
  • Some VSDs require a control link to be fitted.

Common causes:

  1. No speed reference signal
  2. Incorrect digital input logic
  3. Missing link on control terminals

Quick check: Check the keypad or display indicator to determine if the VSD is running.

Useful tips:

  • If no speed reference is available, a minimum speed can be set.
  • Logic type can often be changed using a DIP switch or parameter.
  • Ensure the control mode (local or remote) matches the input signals.

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The motor is running in the wrong direction

Useful knowledge:

  • Motor direction is determined by the U, V and W output connections.
  • Reversing contactors should never be used on a VSD output.

Common causes:

  1. Incorrect phase orientation (swap any two motor output phases at the VSD output)
  2. Reverse command selected instead of forward

Quick check: Check the keypad or display to confirm if reverse direction is selected.

Useful tips:

  • Run the motor at low speed during commissioning to verify direction.
  • Do not change control logic to compensate for incorrect wiring.

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The deceleration time is longer than expected

Useful knowledge:

  • A VSD may extend deceleration time automatically to prevent overvoltage trips.
  • Stall protection is a VSD software feature that may influence deceleration behaviour.

Common causes:

  1. Deceleration time too short, causing automatic extension
  2. Braking function not enabled
  3. Incorrect deceleration parameter

Quick check: Check whether a braking resistor is fitted.

Useful tips:

  • Check the value of the braking resistor and ensure it matches the VSD requirements.
  • Some VSDs have multiple deceleration settings.
  • Fast ramp settings may override standard deceleration.
  • Disable the stall protection feature if safe to do so.

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The motor does not accelerate in the expected time

Useful knowledge:

  • A VSD may extend acceleration time to prevent overload.
  • Acceleration time is typically defined from 0% to maximum speed.

Common causes:

  1. Acceleration time too short
  2. Incorrect acceleration parameter
  3. Incorrect V/F setting (for example, squared instead of linear)

Quick check: Ensure the acceleration time is appropriate for the load.

Useful tips:

  • Large fans require longer acceleration times.
  • Acceleration time also applies when changing speed.
  • High acceleration demand may exceed the VSD rating.

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An unloaded motor is taking too much current

Useful knowledge:

  • Incorrect motor parameters can cause excessive current even with no load.
  • Default VSD motor parameters should always be reviewed to match the motor nameplate.

Common causes:

  1. Motor parameters not changed from default
  2. Incorrect motor parameters entered
  3. Motor wired in Star instead of Delta for 230V operation

Quick check: Check motor terminal box connections.

Useful tips:

  • Ensure the correct section of the motor nameplate is used.
  • Ignore 60Hz data for UK/EU (50Hz) installations.
  • Check motor pole configuration.

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The motor is making a high-pitched sound

Useful knowledge:

  • VSD-fed motors often produce a high-pitched sound due to switching frequency.
  • Noise can be reduced, but this may increase drive losses.

Common causes:

  1. Switching frequency too low
  2. Poor motor construction
  3. Incorrect installation (earthing or mounting)

Quick check: Determine if the noise occurs across the full speed range or at a specific speed.

Useful tips:

  • Check VSD derating before increasing switching frequency.
  • Ensure proper earthing.
  • Ensure the motor is securely mounted.

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On a conveyor, the motor is stopping and starting abruptly

Useful knowledge:

  • Linear ramps can cause abrupt changes in speed.
  • S-curve ramps can smooth acceleration and deceleration.

Common causes:

  1. Linear ramp selected
  2. Insufficient S-ramp
  3. Acceleration/deceleration too short

Quick check: Check if S-ramp is available.

Useful tips:

  • Adjust ramp profile for smoother operation.
  • Review deceleration when modifying ramp type.
  • S-ramp may not apply to emergency stop functions.

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On a fan, there is a loud vibrating sound at one speed

Useful knowledge:

  • Fan systems often have natural resonant frequencies.
  • Resonant frequencies can be avoided by using a skip frequency function in the VSD.

Common causes:

  1. Mechanical resonance
  2. Loose components

Quick check: Identify the speed(s) where resonance occurs.

Useful tips:

  • Set skip frequencies in the VSD.
  • Avoid overspeeding.
  • Check rotation direction.

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I want to operate my speed potentiometer over a small speed range

Useful knowledge:

  • Default settings typically scale 0–10V to 0–50Hz.
  • This range can be adjusted to suit the application requirements.

Common causes:

  1. Default parameter settings

Quick check: Refer to our Easy Start Guide for the VSD, as this gives clear instructions on how to do this.

Useful tips:

  • Use minimum and maximum speed limits.
  • Do not change motor base frequency.
  • Do not adjust the analogue input scaling parameters unless you specifically need to change the input signal scaling.

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The operator can select reverse, which may damage the machine

Useful knowledge:

  • VSDs allow forward and reverse operation by default.
  • Reverse operation can often be disabled.

Common causes:

  1. Default parameter settings
  2. Reverse key on the keypad

Quick check: Check local/remote control settings.

Useful tips:

  • Use the VSD’s reverse inhibit feature, if available.
  • Avoid relying on passwords to restrict access to the keypad.

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The motor will not start automatically when powered up

Useful knowledge:

  • Some VSD inputs require a rising edge signal on the digital input.
  • Power-up start is a feature that is often disabled for safety reasons.

Common causes:

  1. Edge-detected input
  2. Power-up start not enabled

Quick check: Check if it possible to install a start/stop switch as a safer alternative.

Useful tips:

  • Power-up start can be dangerous and should be used with caution.
  • Avoid unnecessary power cycling.
  • Follow manufacturer safety guidance.

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A 230V three-phase motor has poor torque performance

Useful knowledge:

  • Correct V/F ratio is required for full torque.
  • The motor must be correctly configured (Star/Delta).

Common causes:

  1. Motor connected in Star instead of Delta
  2. Motor not suitable for 230V operation
  3. Incorrect motor parameters

Quick check: Check the motor nameplate and connections.

Useful tips:

  • Most small motors are dual-rated.
  • Ensure correct parameter settings.
  • Match voltage and connection type.

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Created by Inverter Drive Supermarket Ltd, last reviewed April 2026