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Two different motors, one is rated 1.49A on slow speed configuration and 1.43A on high speed. The other motor is rated 6.09A on slow speed and 6.99A on high speed. These are constant torque pole changing motors. I don't understand why one motor has an inverse speed - current relationship and the other one has a proportional current - speed relationship.

What I know is that if you keep a relatively constant voltage/speed ratio your air gap Flux will be constant, meaning your torque will be constant. I just don't understand why the current ratings aren't inverse on both motors

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  • \$\begingroup\$ I assume you are talking about induction motors? If so are they both Dahlander or separate winding pole switching. \$\endgroup\$
    – RoyC
    Nov 2 '19 at 12:30
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With constant torque, the output power will be directly proportional to speed. Therefore you would expect the input power to be proportional to speed. Since the input voltage is constant, you might expect the current to be less with reduced power, but actually the current is relatively constant and the power factor is lower. That is essentially due to the need for the magnetizing flux to remain constant.

There are a lot of variables in motor designs. Two induction motors can have the same voltage, frequency, speed and torque specifications but have significantly different rated current and power factor. There are a number of performance goals in addition to the basic requirement to design a motor with a specific speed and torque rating. Typical goals would include high efficiency, high power factor, low locked-rotor current and high safety margins for operating temperature.

For for economic reasons, manufacturers try to use the same laminations for several motor configurations. To allow interchangeability of motor brands, a limited number of standard motor diameters have been established.

The above factors lead to a lot of anomalies in the detailed performance parameters of motors.

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  • \$\begingroup\$ Good points, the question is leaving out several levels of detail and the differences in rating are not that significant. I am voting to close this as unclear. \$\endgroup\$
    – RoyC
    Nov 2 '19 at 14:36

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