Cyndi Nyberg
Former EASA Technical Support Specialist
“I have rewound a two-speed, two-winding motor. The high speed runs fine — the no-load current seems right. But when I test the low speed, the amps are far too high at rated voltage. It draws significantly above the rated current, at no-load! I know that the winding data is correct. What could be wrong?”
This is one of the most common problems we encounter at the EASA office. The solution is almost always the same. There are three questions we ask in return.
- What are the two speeds?
- What are the number of circuits in each winding?
- What jumpers are used to connect each winding?
Usually same conclusion
The responses to these three questions almost always lead to the same conclusion:
The two windings are connected in such a way that the de-energized winding has circulating currents which induce voltage into the energized winding. This is usually caused when one or both of the windings have parallel circuits. When the motor operates at one speed, it acts like a transformer and induces a voltage in the de-energized winding. If the deenergized winding is connected properly, the induced voltages in the legs of the parallel circuits balance each other to produce a net voltage of zero. If the de-energized winding is not connected properly, these voltages will create circulating currents. These currents induce a voltage in the energized winding, causing the high current.
Several options
Of course, the easiest way to avoid circulating currents is to use a 1Y connection enough strands of a certain size wire, so you double the turns, use half the wire, and double the circuits on one or both windings. You may also be redesigning a single-speed motor to a two-speed, and you may need to use more than one circuit on one or both of the windings to get the correct number of turns for the design. Also, for some higher horsepower designs, it may be necessary to use more than one circuit to get the correct number of turns.
Note: If the design has an odd coil grouping, you should not use more than one circuit in the winding. If it is not possible to wind both speeds using a single circuit, the next step will be to determine what jumper connections to use for both windings. The mathematical method to determine the proper jumper connection is rather complicated; however, Table 1 shows the correct pole-group connections for the various pole combinations. This table can also be found in the EASA Technical Manual in the “AC Machines” section.
Proper jumper connection
To use the table, you need to know the poles and the number of circuits for both the high speed and the low speed. By reading across, you will see for each speed, the proper jumper connection.
For example, say we have a 4 and 6 pole motor, the 4 pole winding has a 2Y connection, and the 6 pole winding also has a 2Y connection. First locate the row corresponding to the high speed poles (4) on the table. Then read across that row to find the line containing the low speed poles (6). Since we have 2 circuits in each winding, read across that line to find the number of circuits.
For this particular combination of poles and circuits, both windings need to be connected with 1-7 jumpers to prevent circulating currents.
There are, of course, some unusual groupings and pole combinations that cannot be covered in this simple table. These situations will need a more thorough analysis of which connections should be used to prevent circulating currents. However, for the most common speed combinations, it is relatively easy to determine the proper jumper connections to use in a two-speed, two-winding motor.
ANSI/EASA AR100
More information on this topic can be found in ANSI/EASA AR100
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