How does a single phase induction motor work with a non-constant power supply?

[dhruv.tara]

NOTE: This is not a homework problem.

This semester we dealt with AC motors both three phase and single phase.

Now I know that 3 phase system is associated with constant power and therefore a constant power is being fed into that motor (and hence converted into constant mechanical power)

However single phase system do not have a constant power. For unity pf load they'll get sin^2 term which is not constant. Then in those kind of motors is the mechanical power is also not constant? If that is the case shouldn't we get time dependent torque or speed or both?

 

[Averagesupernova]

Where did you get the idea that there is constant electrical power going into an induction motor? Am I misunderstanding something?

 

[dhruv.tara]

My course text mentions that 3 phase induction motor runs on constant electric power. It proofs it in sense that the power delivered to the motor in a 3 phase balanced system is constant..

 

[Averagesupernova]

That sounds like mumbo-jumbo to me.

 

[david90]

My course text mentions that 3 phase induction motor runs on constant electric power. It proofs it in sense that the power delivered to the motor in a 3 phase balanced system is constant..

3 phase power does look more constant than 1 phase but it's not constant like DC power. Maybe your text is talking about the torque @ the shaft of your motor. In this case, the power @ the motor shaft could be constant due to the inductance of the motor windings acting like capacitors. Basically the windings smooth out the ripple of 3 phase power.

 

[256bits]

3-phase does deliver constant power to a resistive load.
I am not quite sure now for an inductive load such as a motor, due to the above comments. I now stand corrected.