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Reduced-Voltage Autotransformer Starter Performance Misunderstood

Dec 1, 1999 12:00 PM, By EC&M Staff

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The author of the article “Reduced-Voltage Starters: Choosing The Best Type For Your Application” (June 1998 issue) made an error with the following excerpt, which begins on page 52: “For example, we were pretty certain the 2000-hp motors would not start on 150% of full-load current, which would be the 50% tap on an autotransformer starter. We did expect them to start on 384% of full-load current, which would be the 80% tap on an autotransformer starter. However, we didn’t know if they would start at 254% of full-load current, which would be the 65% tap.”

I wish to present a training approach that usually prevents the confusion associated with the performance of autotransformer starters.

While starting an AC induction motor at reduced voltage, the line current and motor current may or may not be the same, depending on the type of reduced voltage starter used. In a primary-impedance starter and solid-state starter, the line current and the motor current are the same because the resistors, reactors, or SCRs are inserted in series with the line and motor. The current in a series circuit is the same at any point in the circuit.

However, in an autotransformer starter, the line current and motor current are not the same. Using a motor with a 600% inrush current and connecting to the 50% voltage tap of the autotransformer, the motor current will be 300% (50% of 600%), but the line current will be only 150% (25% of 600%). No other type of reduced-voltage starter can supply, for example, the torque of a 300% current start while drawing only 150% current from the line.

Perhaps the excerpt should read: “For example, we were pretty certain the 2000-hp motors would not start on 300% of full-load motor current (150% line current), which would be the 50% tap on an autotransformer starter. We did expect them to start on 480% of full-load motor current (384% line current), which would be the 80% tap on an autotransformer starter. However, we didn’t know if they would start at 390% of full-load motor current (254% line current), which would be the 65% tap.”

The article also states a 300% motor current start was satisfactory with the solid-state starter (and 310% or 290%). I agree. If a 300% motor current start was satisfactory with the solid-state starter, then a solid-state starter is not necessary. In this case, an autotransformer starter connector on the 50% voltage tap provides a 300% motor current start while drawing only 150% line current—outperforming the solid-state starter.

All these percentages mentioned assume an ideal transformer. In the field, the values are slightly different, due to losses (including the required magnetizing current in the autotransformer). For proper selection and application of reduced-voltage starters, you should consider both motor current and line current starting.

Mac McMillin, P.E.
Senior Application Engineer
Square D Co.
Seneca, S.C.

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