Chuck Yung
EASA Technical Support Specialist 

When a service center’s test panel is designed with a limited number of incrementally stepped voltages, there are times when you need a voltage that is not available. In such cases, when you need a non-standard 3­phase voltage, one workaround is to use a 3-phase motor as an autotrans­former. This works to step down, or to step up, the available voltage. 

There are three key safety consid­erations here: 

• Never energize part of a winding with more than the voltage that portion of the winding is subject to during normal operation. 
• Do not exceed the voltage rating of the ground insulation. 
• Select a motor that can handle the no-load current of the motor to be tested. 

If a 9-lead, wye-connected motor is operated on 460 volts, the voltage measured between jumpers (4-7), (5­8) and (6-9) would be 230 volts (Figure 1). So we could apply up to 230 volts to the jumpers of the motor without exceeding the voltage rating of the part of the winding being energized. 

Produce voltages not available 
That means a 9-lead, wye­connected motor can be used as in Figure 1 to produce voltages not available from your power supply. With a 9-lead, wye-connected motor, you can produce multiples of 50% and 200% of each voltage tap you have on your test panel. 

For example, you only have 230 volts available, and need to run a 460­volt, 60 Hz motor. 

Use a 9-lead, wye motor con­nected for 460 volts. Connect the line leads of the motor you want to test to leads 1, 2 and 3 of the supplemental motor. Run the supplemental motor with 230 volts applied to the 4-7, 5-8, 6-9 jumpers. The voltage at the line leads (1-2-3) will be 2 x 230 volts, or 460 volts. 

Conversely, you might want to perform an open-stator test of a motor rated 460 volts, but the lowest 3­phase voltage available is 120 volts. Connect a 9-lead, wye motor for 460 volts, and apply 120 volts to the line leads. Between the 4-7, 5-8, 6-9 jumpers, you will have 60 volts. A typical stator, when the stator only (that is, with the rotor removed) is energized at 15% of rated voltage, draws approximately full-load current. 

Additional benefits 
A 12-lead motor can be used in the same manner when connected wye, with some additional benefits. Assuming a large 12-lead motor operates connected 2-delta for 460 volts, we can connect it 2-wye for 796 volts, 1-delta for 920 volts, or 1-wye for 1593 volts. Without exceeding the design flux, we can connect it 2-wye, and apply up to 400 volts to the 4-7, 5-8, 6-9 jumpers and obtain 2 times that voltage between leads 1, 2 and 3. Safety reminder: Do not exceed the voltage rating of the ground insulation. 

Connect a 12-lead motor (Figure 2a) in the high-voltage wye configu­ration (Figure 2b), and you now have a transformer with more voltage options than the 9-lead winding. Because the motor is rated 460 volts in delta configuration, when con­nected as a wye, we can apply up to the rated voltage for each path, and obtain a higher total output voltage. 

Table 1 illustrates the percent of applied voltage available using a 9- or 12-lead motor, while Table 2 gives an example of the additional voltages attainable for one test panel. A 12-lead motor can be connected in a wye-delta combined configuration (Figure 3) to produce 2.73 times the input voltage.

A very good test panel might offer 3-phase voltage taps of 60, 120, 230, 460 and 575 volts. Using a motor to supplement those options results in greater versatility as the options in Table 2 show. 

Caution: The motor selected for this field-expedient autotransformer “primary” must be sized large enough that the load does not exceed the rated current of the “primary” winding. While a typical 2- or 4-pole motor draws approximately 1/3 of FLA when operating uncoupled at full voltage, motors with more poles often draw half or more of the full load amperage. The autotransformer motor should be rated for at least one-third the FLA rating of the motor you plan to test. 

One last tip 
If you find yourself needing this workaround very often, consider using a surplus wound-rotor motor to build a variable-voltage autotrans­former, as described in Section 8 of the EASA Technical Manual. It gives fully adjustable voltage from zero volts to twice the voltage input, as well as natural soft-starting to reduce in-rush current.

Categories: Service center equipment

Tags: Test equipment Motor testing

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