Global site

ABB's website uses cookies. By staying here you are agreeing to our use of cookies. Learn more

Understanding the effects of variable speed drives on motors

Drives help ensure motors run according to process demands, helping to save energy and improve process output. But at the same time, the use of drives introduces additional considerations for motor protection such as:

  • Steep voltage pulses that can stress stator winding insulation, this can lead to sparking.
  • Steep voltage rises can also lead to reflected voltages, which can increase motor terminal voltage up to 2.5 times the nominal voltage.
  • Common mode voltages and current. This may cause sparking in motor bearings and finally bearing insulation breakdown.
  • Higher motor surface temperature rise due to reduced motor self-cooling when a motor with a shaft-mounted cooling fan is run at lower speeds.
  • In overload conditions, the motor surface temperature rise can be steep if it is not taken into account in sizing and load capacity curves are exceeded.
Protecting against voltage phenomena
Protecting against bearing currents
Protecting against motor overheating

Due to rapid switching and reflections in the cables, motors are subject to more voltage stress in the windings when fed by frequency converters than with sinusoidal supply voltage. The effect of these voltages can be an increase of up to 2.5 times the motor’s nominal voltage. This stresses the motor winding insulation and can cause it to break down, resulting in possible sparking. ABB recommends:

  • Between 500 V and up to 600 V, the motor needs to have reinforced winding insulation, or the drive must have a du/dt filter.
  • Above 600 V, the motor needs to have reinforced winding insulation and the drive is required to have a du/dt filter.
  • If the cable length between the drive and motor is greater than 150 meters and the voltage is between 600 and 690 V, the motor must have reinforced winding insulation.

AC drive can cause common mode voltages which induces voltages across motor bearings, leading to current flow through motor bearings. To protect against bearing currents, ABB recommends that:

  • IEC 280 frame motors and above have insulated non-drive end bearings in order to break circulating current paths.
  • IEC 355 frame motors and larger, in addition to the insulated non-drive end bearings, the drive also has a common mode filter installed.

To protect against motor overheating, it is essential to understand and keep the motor temperature under control. The connection between the motor running speed and load capacity must be known (load capacity curves). To ensure safe operation, the motor and drive combination needs to be sized correctly, so it does not exceed the load capacity curve, and rating plate information must be followed. To protect against motor heating, ABB recommends some possible solutions:

  • A separate constant speed fan to increase cooling capacity and load capacity at low speeds.
  • Directly measuring the motor’s surface temperature and using the data to control the shutdown of that motor.
  • Monitoring and controlling the power fed to the motor.
  • Limiting the load on the motor to pr event loads that cause higher motor heating.