Why do machine designers sometimes use a sinusoidal distribution of conductors in the slots around a stator to ensure a sinusoidal mmf produced by the stator? I can't figure out why we care about the shape of the stator flux.

As a motor, the stator flux is only required to induce a voltage in the rotor to cause it to spin....I don't see why this induced voltage must be a perfect sinusoid.

As a generator, the rotor is what induces the voltage in the stator, so as long as the rotors mmf is sinusoidal, you should be good.

Why do we care about the stator mmf?

 

They INTERACT with each other.

So,,,non sinusoidal in one portion means...

 

Okay...so what would happen if the coils in the stator were not distributed and the mmf produced looked more like a square wave? How would that effect synchronous motor operation? In the case of a synchronous generator, what would it mean for the output voltage?

 

As Kermit says, one depends upon the other...

I'll not answer your questions directly, but ponder carefully the following:

As a motor, rotor rotation is derived from the rotating magnetic fields produced by the stator.
If the flux is sinusoidal and regular, the mechanical forces imposed on the rotor
will be:
1. Constant and regular?
or
2. Abrupt, pulsating, and rough?

As a generator, the desired output from the stator windings is:
1. A sinusoid, regular and without harmonic content?
or
2. A square or sawtooth wave, pulsating violently and laden with harmonic signals?

 

As with in any power system power is only transmitted at the fundamental frequency. In other words, only a sinusoid operating at the fundamental frequency will cause power to be converted from electrical to mechanical power and vice a versa. Any interaction between windings due to flux linkages at frequencies other than the fundamental are essentially loss OR contributors to ripple torque (which is typically undesirable).

As you know, a square waveshape is rich in harmonics (especially odd tripline harmonics which contribute to zero sequence in 3 phase systems). Therefore it is most desirable to have a sinusoidal flux linkage with minimal harmonics, especially taking care to get ride of tripline harmonics (i.e. 3,9...). There are various winding structures to ensure the tripline harmonics can not exist...which is a whole nother topic.

 

Thanks everyone for the responses! I'm beginning to understand more clearly.

Can somebody please tell me if this is an accurate statement:

In an unloaded synchronous generator with non-distributed stator windings, a sinusoidal mmf from the rotor induces a sinusoidal voltage in the stator. But, when a load is connected, current flows in the stator windings and the mmf it produces will contain many harmonics and be more like a square wave, which in turn interacts with the rotor mmf. The dc current in the rotor will react to this and end up producing a distorted mmf, and thus ultimately the stator voltage will also be distorted.

DrNick, you actually bring up another point that I've been trying to understand completely...are harmonics considered waste because the average power delievered by some harmonic current multiplied by the fundamental voltage is going to be 0? The other way I think of it is that harmonics can create voltage drops across elements which the voltage source will have to compensate for. Are these accurate statements?

What if you had a current wave rich in harmonics, and an identically distorted voltage wave at the load, would the harmonics still be wasted then? Maybe I should post this in another topic, but since you brought it up I thought I'd ask.

 

the sinusoid is a mathematical representation of a circular electrical machine. If your machine goes around and around, the sinusoid is the result. If the windings are phased together, thier output/input will add toghether. If they are phased apart, thier outputs/inputs will buck each other, creating waste power that must be dissipated.

In an unloaded synchronous generator with non-distributed stator windings, a sinusoidal mmf from the rotor induces a sinusoidal voltage in the stator. But, when a load is connected, current flows in the stator windings and the mmf it produces will contain many harmonics and be more like a square wave, which in turn interacts with the rotor mmf. The dc current in the rotor will react to this and end up producing a distorted mmf, and thus ultimately the stator voltage will also be distorted.

My guess is that the sinusoid will be distorted slightly, but output will fall considerabley while heat generation climbs substantially.