Instantaneous torque in a 3-phase motor

[pvshackguy]

This isn't a homework question because nobody assigned it to me. It is a theoretical question however.

I understand that in a balanced 3-phase system with a constant load, instantaneous power is constant. Not "pretty constant" but a flat line. This suggests to me (warning: amateur) that instantaneous torque, in a 3-phase induction motor, might also be constant throughout each revolution, assuming a fixed load. But, given that slip is necessary, and that the rotor experiences a frequency of a few hertz, it may be hopelessly naive to imagine that torque isn't somewhat "bumpy."

Again - this is instantaneous torque I'm thinking of, measured throughout a machine cycle or two, not torque vs. load or any of that usual motory spec stuff. ;-)

I've exhausted the books I have at hand, and I feel as though I've seen every torque curve on the internet by now. Perhaps it's not an important question but it's burning a hole in my curiosity.

 

[jim hardy]

Look at an armature and observe the slots are skewed, not parallel to the shaft.

That's done to relieve the magnetic discontinuity as the rotor slots pass under the armature slots, smoothing out torque and reducing audible "hum".

try a search on 'squirrel cage slots skew' . There's a 1948 patent that looks interesting ,,, but on this computer i couldn't read it for some obscure software reason..

http://ecee.colorado.edu/~ecen4517/materials/InductionMotor.pdf

old jim

 

[MrSparkle]

here is a youtube breakout of an AC motor that shows the skew.

 

[pvshackguy]

You anticipated my follow-up too. Thanks.

 

[alphy]

I find your curiosity about instantaneous torque in a 3-phase motor to be valid and interesting. While it is true that in a balanced 3-phase system with a constant load, instantaneous power is constant, this does not necessarily mean that instantaneous torque is also constant.

Firstly, let's define instantaneous torque as the force that causes rotation at a specific moment in time. In a 3-phase motor, torque is generated by the interaction between the stator's magnetic field and the rotor's induced currents. As the rotor rotates, it experiences a varying magnetic field from the stator, resulting in a varying torque.

Additionally, slip is necessary in a 3-phase motor to maintain rotation and produce torque. This means that there will always be some degree of "bumpiness" in the instantaneous torque as the rotor slips and catches up with the stator's magnetic field.

However, it is important to note that the overall trend of torque vs. time will still be relatively constant and smooth, as the motor is designed to produce a consistent output. The "bumpiness" may be imperceptible and only measurable with precise equipment.

Furthermore, the frequency of the rotor's rotation, which is typically a few hertz, is much higher than the frequency of the power supply, which is typically 50 or 60 hertz. This means that the "bumpiness" in the instantaneous torque will be at a much higher frequency and may not be noticeable.

In conclusion, while instantaneous torque in a 3-phase motor may not be perfectly constant throughout each revolution, it is still relatively constant and smooth in comparison to the overall torque output. This is due to the design and function of the motor, and any variations in torque may not be significant or easily measurable. I hope this helps satisfy your curiosity about instantaneous torque in a 3-phase motor.