Why and how does motor current go up?

[rsdoran]

There definitely has not been any "profit" associated with my website. Yes I was hoping to use it in the Motor/Drive section. I have a few things from others already listed, always looking for more.

 

[DonsDaMan]

OK by me, then.

If you have other motor- or drive-related questions, I can try to help. I work with both AC and DC motors and drives quite extensively.

Since it's not for profit, does that mean I should cancel that Mercedes I just ordered thinking of all the residuals I was going to get from you?

Don

 

[elevmike]

Thanks for the "clarification", Don.

BTW where were you when this was a hot topic??? You should visit the forum more often....

Best regards, Mike.

 

[kamenges]

Originally posted by DonsDaMan (aka Level):
Now the stator has a current-produced magnetic field and the rotor has a current-produced magnetic field. At this instant, we have what is essentially a shorted transformer with its associated infinity-seeking current. The current is limited by the inherent resistance of the stator windings, the saturation point of the laminations, and the resistance of the rotor, and of course the impedances of the source. This high current is what we term "inrush current." Also at this instant, since we have not yet gotten rotation, all we're doing is making magnets in both the stator and rotor.

Good point. I believe this to be correct. The major current limiting factor at locked rotor has to do with resistance/impedance. Since there is effectively no rotor motion there can be very little generated EMF.

Originally posted by DonsDaMan (aka Level):
At this point, the fields are pretty much saturating the cores due to the high currents in both the stator and rotor, so maximum torque is produced; no more flux coupling is happening because we've run out of iron.

This isn't born out by the NEMA design B torque/speed curve. The locked rotor torque is less than the breakdown torque. I think this is because of the rotor's magnetic time constant. It's significantly longer than the rate at which the inducing field from the stator is moving around the rotor at locked core. If I remember right the rotor magnetic time constant of a 5HP NEMA design B motor is about 250 milliseconds. You jut can't develop a full strength, coherent magnetic field in the rotor at locked rotor if the magentizing field is changing once every 18 milliseconds. This is why field oriented control drives work as well as they do. They control the stator currents in such a way that you can develop a full strength, coherent rotor field at all times (OK, that whole Iq/Id separation thing is pretty important, too).

That being said, I think that DonsDaMan's (aka Level's) explanation captures all the major points in what causes the various current levels as a motor accelerates.

Keith

 

[DonsDaMan]

I agree that the torque goes up on the curve as rotor speed increases (after the pull up torque saddle), so my phrasing about "torque being at a maximum" was definitely wrongly worded.

And although it may have to do with the magnetic time constants, I don't believe it is. The curve is not time sensitive, it is speed sensitive... the horizontal axis is speed, that is. More accurately, I think it has MORE to do with the magnetic phase angles than with the time constants. Both must contribute to the curve.

The speed component rather than time leads one to believe that the torque is constant at each point on the curve, regardless of the reason the motor is running at that speed, or for how long the motor has been at that speed.

So, why does the torque first go down with speed, then increase with speed, before plummeting to its zero at full speed?

I think it has to do with the phase angles of the magnetic fields within the rotor and stator. It is easy to see why the torque goes to zero at synchronous speed; the rotor is not being excited because of the synchronicity between the rotor speed/windings and the flux variations in the stator; there is no induction to the rotor without relative motion. No flux means no torque.

Therefore, as the rotor declines in speed (from synchronous) relative to the excitation current, the induction increases. This should be constant, but obviously, by the curves, it is not (at least the resultant torque is not constant). The rotor's magnetic strength MUST be increasing, though, due to the increase in the relative frequency between the rotational speed and the applied frequency/speed of the stator's magnetic field. Likewise, with less CEMF and therefore greater current, the stator's flux must be increasing as well.

If the magnetics are in direct opposition, then there is no torque, as the flux opposition is directly in toward the axle/axis causing no angular force and therefore no rotation. A slight variation in this angle causes a greater torque, all else being constant. This is what I think is happening at the lower speeds below breakdown torque. The angles of the magnetics (where the torque is really coming from) are becoming less and less tangential to the surface of the rotor. In fact, I think that the torque is sometimes pushing AGAINST the rotation of the motor, causing a resultant decrease in the effective torque.

So, to say this shortly, I think that the strength of the magnets of the rotor and the stator are at a maximum at locked rotor, but the angles between the magnets are causing less of that torque to be transmitted to the rotor.

This is similar to a bicycle pedal transmitting more of its energy on the down stroke than on the horizontal stroke. The vector applied by the bicyclist's foot is tangential to the gear on the down stroke, but orthogonal on the horizontal portion of the stroke. I think similar is happening inside the motor during the "saddle" portion of the curve.

Like I said at the beginning of this post, I think that while the time constant of the magnetics are playing a part in the torque to speed curves' shape, I think its shape has more to do with the phase angles between the rotor and stator magnets.

And, yes, I've been wrong before so it wouldn't surprise me if I was wrong again, here.

 

[kamenges]

I don't think I stated my response exacly right either.
The torque part of my response is mostly based on a motor simulation I saw several yaers ago. I think DonDaMan is correct in that the peak torque at locked rotor may be the highest peak torque at any part of the start cycle. However, the torque magnatude, and just as importantly the direction, do not stay constant at locked rotor. The magnitude and direction change asa the stator field moves out in front of trhe rotor field and then comes back in from the back side.
I think DonDaMan's locked rotor explanation assumes that the rotor magnetic field can follow the stator magnetic field with some constant phase relationship at locked rotor. I don't believe it can. I think the magentic time constant of the rotor will keep sections of the rotor magnetized after the magnetizing effect of the stator passes the rotor. In fact, any given section of the rotor will maintain some level of residual magnetic field until the stator magnetic field comes back around for another pass. At this point your stator and rotor magnetic fields would have the opposite relationship than they had when the rotor magnetic field was initially established. This would create a torque in opposition to the normal motoring torque. Granted, the integral of the instantaneous torque is positive so the motor will accelerate.
So when I said ytou can't generate a full strength coherent field on the rotor at locked core, I stated that wrong. It's more that you can't keep the correct phase relationship with the magentic field that is there.

Keith