The Ask Control Engineering blog covers all aspects of automation, including motors, drives, sensors, motion control, machine control and embedded systems. Control Engineering answers questions from readers of Control Engineering's print and online magazines, newsletters and other publications.

Induction motors with autotransformers

Bruce Konzak writes:Dear Control Engineering: Does a single phase 240 Vac induction motor have the same performance characteristics when powered by an autotransformer with a line to neutral feeder, as it does when powered normally by a line-to-line connection 120 degrees out of phase?

February 10, 2010


Bruce Konzak writes:Dear Control Engineering: Does a single phase 240 Vac induction motor have the same performance characteristics when powered by an autotransformer with a line to neutral feeder, as it does when powered normally by a line-to-line connection 120 degrees out of phase? I’m having some trouble getting consensus on this one.

We put the question to Joe Pottebaum, senior application engineer for Yaskawa Electric America, Inc. He answers:

“To answer this question definitively requires more information than what is given. Nevertheless, if we assume that the auto-transformer is suitably rated for the motor, then the motor will suffer virtually no degradation in its performance when powered through the auto-transformer. The motor will be happy as long as it sees 240 volts. However, there are some considerations.

“Typical household 240 Vac single phase service is grounded at the center-tap of the transformer secondary feeding the building. The grounded center-tap becomes the neutral (white wire) for two 120 Vac circuits throughout the building. Therefore, when 240 Vac is used directly to power a 240 volt motor, the maximum voltage with respect to ground is 120 Vac. In the description, it appears that the auto-transformer is being used to step up or step down voltage from a feeder with one side grounded. If the motor is connected between that grounded side and the auto-transformer 240 Vac tap, then the maximum voltage with respect to ground is 240 Vac, double the 120 Vac from a conventional service. Generally, this is not a problem, but does merit checking with the motor manufacturer.”

Read Who’s using industrial drives? for links to additional motor related topics.

Posted by Ask Control Engineering on February 20, 201