November 2009 Electrical Apparatus

November 2009 Electrical Apparatus

This is a summary of the Electrical Apparatus November 2009 featured technical article,  by Richard L. Nailen, P.E.     

Early in the 20th century, the high starting current of polyphase induction motors became a power supply system problem as motor ratings increased far above a few kilowatts. Some type of reduced-voltage starting was needed. Series resistance, common for d-c machines, was inefficient.

One solution was the autotransformer starter. During acceleration, a reduced voltage (typically 50% to 80%) was applied to each phase of the motor from a transformer winding tap. Because of the transformer ratio, incoming line current was reduced by the same amount as the motor torque. When full speed was reached, the tap circuit was opened, and the motor reconnected to the line.

Continuing increase in motor rating highlighted one disadvantage. Opening the starting circuit allowed the disconnected motor to begin decelerating. Its decaying magnetic field generated a terminal voltage. Reclosing the motor onto line voltage caused voltage and torque transient disturbances both electrically and mechanically damaging.

A starter modification patented in 1914 allowed momentary reconnection of part of each autotransformer as a series reactance, keeping the motor circuit closed during the transition to the running connection. Several variations of that “Korndorfer connection” are common today.

A second widely used redesign reduced the three autotransformers to two units connected in open delta. The resulting slight unbalance in motor voltage during starting was acceptable for the relatively short acceleration period.

Magnetically saturated with low impedance, autotransformers draw high excitation current. Because that opposes motor starting current in part of each transformer winding, that portion can be wound with smaller conductors as long as the motor and transformer sizes are well-matched. A much smaller motor allows transformer exciting current to predominate, risking transformer overheating.

Autotransformers are typically rated for maximum starting time of 15 to 30 seconds, repeatable at a frequency specified by the manufacturer. Those values, and limits in applicable standards, vary considerably worldwide. Starting a heavy load, or starting too often, may damage the starter rather than the motor.

To order a back issue with the full article, “A close look at the autotransformer starter,” call 312-321-9440 or visit our online webstore.

Share Button