Does electric motor reactance at given speed is constant according to rotor position (angle)?
For switched reluctance it is not. But for three phase induction motor?
Considering there is no any noise.
Electronic – Current waveform electric motor
currentelectricinduction motormotorwaveform
Related Solutions
A standard 3-phase, AC motor has the stator windings distributed in the slots to form poles that do not have much saliency. A switched reluctance motor has both salient rotor poles and salient stator poles. You would need to rewind the stator to have a motor like that.
A synchronous reluctance motor has a salient-pole rotor, but distributed windings in the stator to form non-salient poles. To convert an induction motor to a synchronous reluctance motor, you could use the stator as it is and machine the rotor to look something like this:
You can get some induction-motor starting torque from the remaining rotor bars, but the best way to operate the motor would be with a variable-frequency drive (VFD). The VFD will provide reluctance torque from stand-still. That may or may not suit your purpose.
Of course, you may have some difficulty with rotor balance.
the speed of the rotor of a BLDC motor is proportional to the frequency of the three phase current
Beware of the above phrase: there are two frequencies involved. The most important is the synchronous one, i.e. the commutation of the phases carried by electronics in order to generate a rotating magnetic field. For example, a BLDC motor rotating at 1 turn/second could have a driving frequency of 6 Hz (six-step for one turn). For this frequency, I would not say that the speed is "proportional", I would say that it has to match exactly. Try to vary the frequency while the motor is running, and bad things will happen (big vibrations, big current consumption and spikes, heating). In reality, the frequency of the phases is dictated by the motor speed, not the contrary. To vary the speed of the motor you vary the voltage to it (using the duty cycle of the PWM).
The other frequency involved is normally around 20 kHz, it's the frequency of the PWM used by the output bridge, but it has nothing (or very little) to do with the speed of the motor. The inductance of the motor has a lot to do with this frequency.
When the speed of the motor increases, it draws less current because:
- The BEMF increases and contrasts the power supply.
- The commutation times decrease, so there is less time to pump current in the coil. As soon as the current start to rise, it's time to give up to pass to the next coil. A lower inductance lets the current rise more quickly than a higher inductance.
Best Answer
Induction motors are designed to avoid reluctance variation according to rotor position. That also avoids reactance variation with rotor position. To avoid reluctance variation, it is necessary to avoid variations in the air gap that would occur if the rotor bar slots in the rotor iron and the stator winding slots in the stator iron are aligned with each other at specific rotor positions. The number or rotor and stator slots must be selected to avoid alignment as much as possible. In addition the rotor slot are configured so that they are not parallel to the shaft, but "skewed" at an angle with the shaft.
With all types of synchronous motors, the rotor is aligned with the rotating stator magnetic field. In that case, the effective reactance is constant because of the nature of than alignment.
Here is a question about skew with answers:
Skew angle in squirrel cage induction motor
The diagram below is from Phillip L. Alger The Nature of Polyphase Induction Machines Copyright, 1951 by General Electric Company. It shows a cross section view of "a typical small induction motor, with a 4-pole, 3-phase 36-slot stator winding." While not discussed in the chapter containing the diagram, the diagram shows that relatively few rotor bars are aligned with stator slots for any given angular rotor position. Also, the stator slots are shaped to maximize the iron path for magnetic flux and minimize the effect of air-gap variations. The copper, aluminum and insulating materials in the air spaces is effectively equivalent to air in the spaces in terms of magnetic permeability, reluctance and reactance.