Are the stator windings of a motor designed for part winding starting connected in parallel or series?
The nameplate of a part winding motor indicates a full load current rating of 72 amperes. What current rating should be used when sizing the overload heaters?
The correct answer and explanation is :
Stator Windings of a Motor Designed for Part-Winding Starting:
In a part-winding start motor, the stator windings are typically connected in parallel for the starting method. The part-winding start is commonly used in larger motors to reduce the inrush current during startup. In this method, only a portion of the stator windings is energized at first, and once the motor reaches a certain speed, the rest of the windings are energized to bring the motor to full speed.
In the starting configuration, the motor is designed with separate sets of windings, often in parallel, so that when one set of windings is energized, the motor starts with reduced current. This approach allows for a smoother, less impactful start, especially for large motors where the full winding inrush could be excessively high.
Once the motor reaches a sufficient speed, the remaining windings are connected, and the motor runs at full load with both windings in operation.
Sizing Overload Heaters for Part-Winding Motors:
For sizing the overload heaters of a motor designed for part-winding start, the current rating used should be based on the full-load current, but with consideration for the reduced starting load. When only part of the winding is energized during the initial phase, the current drawn by the motor will be lower than the full-load current.
Current Rating for Overload Heaters:
The motor’s nameplate indicates a full-load current of 72 amperes. However, for part-winding start motors, only part of the windings is energized at startup, so you typically calculate the overload heater rating based on the current draw of the part winding.
- The full current draw when both windings are energized is 72 A.
- For part-winding start, typically half of the total winding is energized at startup, so the motor will initially draw approximately half the current. This means the current during startup could be around 36 amperes (half of the 72 A full-load current).
To select the appropriate heater size, you typically account for both the full-load current (72 A) and the fact that the motor will start with a reduced load, so overload protection needs to be sized accordingly for full-load operation.
Thus, for overload heaters, the current rating typically used is 72 amperes, reflecting the full load, as the heater must protect the motor during normal operation after the windings are fully energized.
Conclusion: The correct current rating for sizing overload heaters would be 72 amperes, based on the motor’s nameplate full-load current.