Comparing slip ring motors with standard ac motors

Standard squirrel cage motors have a fixed value of starting and breakdown torque, whereas slip ring motor characteristics can be changed by adding or removing rotor resistance. Standard squirrel cage motors also have a fixed nominal rating that is almost always based on S1 duty. Slip ring motors can have many different ratings depending on the duty cycle for which they are specified. The table below illustrates this:


Take note

  • Slip ring motors allow different values of external resistance to be added to the rotor, thus controlling the torque over a wide speed range.
  • Slip ring motors can be replaced with speed controlled squirrel cage motors.
  • Selecting the appropriate squirrel cage motor and VSD requires careful engineering, but is both practical and cost effective.


Published as

Slip ring motor conversion to VSD driven standard squirrel age ac motor

By J van Niekerk, Zest WEG Group

Electricity+Control, January 2013 (pages 12 – 15)