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Why is it like that if we increase the speed of the induction motor, its core losses decrease and stray, windage losses increase?

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2 Answers 2

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An induction motor's speed is controlled by the AC supply's frequency. A higher frequency means a higher speed. Given that the stator's inductance is fixed, a higher frequency reduces the magnetization currents flowing in the stator.

This in turn reduces the onset of core saturation (caused by those magnetization currents). And, in turn, core losses decrease.

Stray inductance is caused by those stator turns that do not magnetically couple to the rotor. Stray inductance is not a power loss as such; it just reduces the power that can be fed to the rotor from the AC supply. Because it is an inductance, as operating frequency rises, the reactance also rises and this, in turn reduces the ability to push power into the rotor.

Here's the equivalent circuit of an inductor motor (single phase shown): -

enter image description here

The inductor marked \$jX_m\$ is the magnetization inductance I refer to. The inductor marked \$jX_1\$ is the stator's leakage inductance. It should also be noted that there is also a leakage inductor in the rotor, \$jX_2\$

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It is important to consider what is meant by “we increase the speed.”

The answer given by Andy aka assumes that the speed is increased by increasing the frequency of the applied power above the normal utility power frequency for which the motor is designed. He also assumes that this is done without increasing the voltage. That is one mode of operation that is commonly provided by variable frequency drives (VFDs).

With a VFD, the motor is normally started at a low voltage and frequency with the ratio of voltage to frequency held relatively constant for operation between the starting frequency and the motor’s normal rated frequency. For that mode of operation, the core losses increase as speed increases. As the frequency increases, the voltage is adjusted to maintain constant magnetizing current, but the increase in frequency increases the losses even though the flux is not increasing. Motors are sometimes driven above the normal power frequency in this mode of operation.

There may also be a question about what happens as the speed increases when the motor is started by connecting it directly to the power lines, DOL start. In that case, the rotor current is initially high and at the same frequency as the applied power. That means high core losses in the rotor. As the motor comes up to speed, the rotor current and the rotor frequency both drop. So the rotor core losses at full speed are very low. The rotor losses don’t change very much. The total core loss would decrease.

Stray losses are defined as losses other than the conventional losses, I^2xR losses, core losses, friction losses and windage losses. There are a number of sources of stray losses. Losses may be due to harmonic currents related to the rotor slots, rotor currents flowing in the rotor iron, eddy currents induced in the conductors and structural components and other phenomenon. It is not at all clear why those losses would increase or decrease with speed.

The friction loss is pretty much directly proportional to speed because it represents a constant torque. A constant torque multiplied by an increasing speed is a directly proportional increasing power. A load torque that also increases with speed would add somewhat to the loss.

The windage loss is the power required to turn the motor’s internal and external self-cooling fans or rotor fins. Any other aerodynamic drag acting on the rotor is also included. That is similar to driving a fan, so the required torque is proportional to the square of speed and the required power is proportional to the cube of speed.

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