Problem regarding motor speed calculation
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A 380V, 12-pole synchronous motor (500 revolutions per minute) and a 380V, 12-pole asynchronous motor (493 revolutions per minute), operating under rated load. Question: How many revolutions will each of the two motors have made after 1 minute? What will they be after 10 minutes?
The speed formula for a three-phase asynchronous motor is: n=60f/p(1-s). As can be seen from this formula, changing the supply frequency f, the number of pole pairs p of the motor, and the slip rate s can all be used to adjust the speed. During rated operation, the speed remains constant; the motor speed stays at 500 revolutions per minute or 493 revolutions per minute. The theoretical analysis is as follows. In fact, there may be slight variations; the frequency changes a bit, and this might not be noticeable in the short term, but it becomes apparent over time.
A synchronous motor rotates at 500 revolutions per minute; after 10 minutes, it will be rotating at 5000 revolutions per minute. The rated speed of the asynchronous motor is 493 revolutions per minute, and this value changes depending on the load. When the load is light, the speed is higher; when the load is heavy, the speed is lower. After 10 minutes, the speed is not necessarily still 493 revolutions per minute.
On the premise that the load remains unchanged, there are two answers: The first: after 1 minute: 500 revolutions, 493 revolutions. 10 minutes later: 10*500=5000 rotations, 10*493=4930 rotations. The second case: 1 minute later: 500 rotations, 493 rotations. 10 minutes later: 10*500=5000 rotations, 9*500+1*493=4993 rotations. I’m not sure which answer is correct
I think it’s 4993. Right?
There is a gap between theory and practice, which is related to the motor load and power supply frequency; large fluctuations in power supply voltage also have an impact. To obtain accurate data, it is recommended to use an electronic speedometer; it will give you an exact figure.
Such questions can only be answered theoretically; I agree with the moderator’s answer – to obtain actual data, the values will certainly vary from one time period to another