Adjustable Speed Drives –
(a.k.a. Variable Speed Drives)

What They Are, How They Work

Application Information


AC and DC drives both continue to offer unique benefits and features that may make one type or other better suited for certain applications.


  • They use conventional, low cost, 3-phase AC induction motors for
    most applications.
  • AC motors require virtually no maintenance and are preferred for
    applications where the motor is mounted in an area not easily reached for servicing or replacement.
  • AC motors are smaller, lighter, more commonly available, and less expensive than DC motors.
  • AC motors are better suited for high speed operation (over 2500 rpm) since there are no brushes, and commutation is not a problem.
  • Whenever the operating environment is wet, corrosive or explosive and special motor enclosures are required. Special AC motor enclosure types are more readily available at lower prices.
  • Multiple motors in a system must operate simultaneously at a common frequency/speed.
  • It is desirable to use an existing constant speed AC motor already mounted and wired on a machine.
  • When the application load varies greatly and light loads may be encountered for prolonged periods. DC motor commutators and brushes may wear rapidly under this condition.
  • Low cost electronic motor reversing is required.
  • It is important to have a back up (constant speed) if the controller should fail.


  • DC drives are less complex with a single power conversion from AC to DC.
  • DC drives are normally less expensive for most horsepower ratings.
  • DC motors have a long tradition of use as adjustable speed machines and a wide range of options have evolved for this purpose:
  • Cooling blowers and inlet air flanges provide cooling air for a wide speed range at constant torque.
  • Accessory mounting flanges and kits for mounting feedback tachometers and encoders.
  • DC regenerative drives are available for applications requiring continuous regeneration for overhauling loads. AC drives with this capability would be more complex and expensive.
  • Properly applied brush and commutator maintenance is minimal.
  • DC motors are capable of providing starting and accelerating torques in excess of 400% of rated.
  • Some AC drives may produce audible motor noise which is undesirable in some applications.


Adjustable Speed Drive Application Information provided by: FINCOR Automation