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Motor Speed with reduced voltage

Motor Speed with reduced voltage

Motor Speed with reduced voltage

(OP)
I understand that reduced voltage to a motor will cause the torque in the motor to drop, which will in turn cause the speed of the motor to drop as represented by the attached torque speed curve showing different torqe curves for different voltage levels.

Does the speed of the motor get reduced due to the fact that the reduced torque in the motor is only equal to the load torque and therefore does not have any additional extra torque to accelerate the load any more or any faster?

At the point circled on the attached plot, will the reduced torque equal the load torque and therefore run at the reduced speed as shown?  If we went beyoned this circled point on this same curve, would the motor then stall due to the fact that the motor torque was under the load torque curve?  

If that is the case, then what would cause the motor to go beyond the circled point and stall if the circled point was the max speed for this voltage?  The only thing that comes to mind is running the motor over base speed with the reduced voltage?   

RE: Motor Speed with reduced voltage

I hope that you are aware that this speed control technique is very inefficient and usually not used for anything but low power fans?

For all normal speed control VFDs are used. The motor voltage is varied as the frequency is varied. But it is the frequency that determines the speed, not the voltage.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Motor Speed with reduced voltage

Equating motor speed to torque assumes that the motor is 100% loaded with a linear torque load. Most motors run at less than 100% load.  

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Motor Speed with reduced voltage

I agree with others' comments.  Trying to address specific aspects of a long question:

Quote:

Does the speed of the motor get reduced due to the fact that the reduced torque in the motor is only equal to the load torque and therefore does not have any additional extra torque to accelerate the load any more or any faster?
The speed of the motor is reduced based on the fact that the [steady state] operating speed is the intersection of torque speed curves for load and motor - as motor curve shifts the operating point shifts.

Quote:

At the point circled on the attached plot, will the reduced torque equal the load torque and therefore run at the reduced speed as shown?
Yes.  Although there is no guarantee it is a safe operating point for the motor in terms of steady state thermal performance, margin to breakdown torque, starting performance.

Quote:

If we went beyoned this circled point on this same curve, would the motor then stall due to the fact that the motor torque was under the load torque curve?  
Assuming "went beyond" means further reduction in voltage, yes eventually you would stall. But again you may be in trouble before you stall.  NEMA MG1 specifies a voltage of nameplate +/- 10%.  Beyond that needs to be evaluated.
 

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RE: Motor Speed with reduced voltage

(OP)
Yes I do understand that speed control using voltage reduction is a very in-effecient way of controlling speed and that using a VFD is a much more optimal way to acheive speed control.  I simply am using this reduced voltage example as a way to understand what happens when motor torque is reduced and its effect on speed.

electricpete

What I meant "went beyond" this point was that if we moved further to the right along the motor torque curve at the same voltage level.  In other words the torque curve stayed the same but we somehow moved beyone the intersection point and were then below the load curve?  Or is this not possible due to the fact that the torque curve will intersect the load curve and set up a steady state speed which cannot be passed?

RE: Motor Speed with reduced voltage

The motor and load can both be characterized by a speed torque curve (neglecting minor transient effects).  

Whatever speed the machine is, those curves tell us the torque.  During startup the motor torque speed curve exceeds the load torque speed curve until you get to the point that they intersect.   

As long as we are to right of breakdown torque, the motor torque-speed curve is monotonically decreasing with speed... the load curve we'll assume is monotonically increasing with speed (usually the case).  The intersection is a stable operating point.  i.e. if somehow you perturb speed low, then motor torque exceeds load torque and you accelerate back to the stable operating point.  If somehow speed increases you are now in an area where load torque exceeds motor torque and you will decelerate back to the stable operating point.   

No matter what (excluding those minor transient effects) you have to stay on both curves.  And if it is not an intersection where both curves give the same torque, then you are not insteady state.

Quote:

What I meant "went beyond" this point was that if we moved further to the right along the motor torque curve at the same voltage level.
As far as I can tell that means you decreases the load torque curve and you will come to a new operating point at the new intersection.

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RE: Motor Speed with reduced voltage

I left some words out - corrected in bold:

Quote:

During startup the motor torque speed curve exceeds the load torque speed curve and the machine will accelerate   until you get to the point that they intersect.   

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RE: Motor Speed with reduced voltage

Yes, you quit accelerating when the load requires more torque than the motor produces.

In practice, you can not use variable voltage control on a 3-phase induction motor. Those curves are for a wound rotor motor where there is resistance added to the rotor circuit.
 

RE: Motor Speed with reduced voltage

(OP)
O.k. I see how the intersection of the motor torque and load torque will be the steady state operating speed of the motor and load.  If somehow the motor torque was increased or decreased by changing the speed then the motor and load would then adjust to the stable operating speed where the motor and load torques meet.  This like you said is all depending on if this intersection is to the right of the breakdown torque of the motor?

At what point will the motor stall?  Will the motor stall at a point where the motor torque is less than the load torque?  What point will this be, if the speed of the motor and load always adjust to the steady state speed?  Will this happen to the left of the breakdown torque point?

RE: Motor Speed with reduced voltage

In real life it depends a lot on the load. Not all loads are linear with speed. If you have a steady torque load independent of speed, the motor will stall to the left of the breakdown  torque point.
But you asked about torque and low voltage. Torque is related to the square of the voltage. If you cut the voltage in half, the torque curve will be scaled to 1/4 of what it was originally. What the motor does depends on the load characteristics.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Motor Speed with reduced voltage

A stall generally occurs when the load torque is higher than the mtoor breakdown torque. Once that point is reached, the motor doesn't make enough torque to keep the load running at any speed so the speed goes to 0.

Instead of trying to find curves showing the motor torque being lower try moving the load torque up to put more load on the motor.  

RE: Motor Speed with reduced voltage

(OP)
O.K so lets say I am operating a load with a motor at normal voltage.  Then ideally the motor will drive the load somwhere close to the motors rpm (with slip) and the motor/load intersection will be to the right of the motor breakdown point.  If this motor continues to get loaded the slip increases and the motor torque moves towards the breakdown point and if it goes beyoned the breakdown torque it will stall.  Is this correct.

Now lets say that the voltage to the motor was reduced for some reason and the intersection of the motor and load torque curves took place to the left of the motor breakdown torque.  Will the motor and load still operate at the new steady state speed determined by this new point?  From what was said above it will.  So if we are already to the left of the breakdown curve at this steady state point, then what point will cause the motor to stall due to lack of torque if we are already beyone the breakdown point?

I guess an example would be a reduced voltage starter.  If you tap an Autotransformer at 65% and start motor then the motor and load will operate at reduced speed until the motor goes across the line.  How do you determine from looking at the curves if the motor will not stall due to not having enough torque at this 65% setting?

RE: Motor Speed with reduced voltage

[quote]O.K so lets say I am operating a load with a motor at normal voltage.  Then ideally the motor will drive the load somwhere close to the motors rpm (with slip) and the motor/load intersection will be to the right of the motor breakdown point.  If this motor continues to get loaded the slip increases and the motor torque moves towards the breakdown point and if it goes beyoned the breakdown torque it will stall.  Is this correct.{/quote]

Yes

Quote:

Now lets say that the voltage to the motor was reduced for some reason and the intersection of the motor and load torque curves took place to the left of the motor breakdown torque.  Will the motor and load still operate at the new steady state speed determined by this new point?  From what was said above it will.  So if we are already to the left of the breakdown curve at this steady state point, then what point will cause the motor to stall due to lack of torque if we are already beyone the breakdown point?

The motor has already stalled, for lack of a better term. If you try to run almost any 3-phase induction motor like this it will quickly fail.

The breakdown torque is, in almost every case, the highest torque the motor makes. If you reduced the voltage enough that the load requires more torque than the new lower breakdown torque then everything is decelerating to a stop. There isn't another part speed where the torque is higher than the breakdown torque so the motor won't "catch" the load and start operating at part speed.

Quote:

I guess an example would be a reduced voltage starter.  If you tap an Autotransformer at 65% and start motor then the motor and load will operate at reduced speed until the motor goes across the line.  How do you determine from looking at the curves if the motor will not stall due to not having enough torque at this 65% setting?

The tap voltage is supposed to bring the motor to almost full speed. Otherwise, the autotransformer starter was a waste of money. If you transition at say 60% speed then you end up with almost the same current level as a full-voltage start and completely negate the reason for the autotransformer starter.
 

RE: Motor Speed with reduced voltage

Quote:

Now lets say that the voltage to the motor was reduced for some reason and the intersection of the motor and load torque curves took place to the left of the motor breakdown torque.  Will the motor and load still operate at the new steady state speed determined by this new point?
It depends.  Setting aside the fact that there is protection that may trip the motor, there are two kinds of intersections - one is a stable operating point where perturbation in speed pushes the motor back to the operating point.  Another is an unstable operating point where small perturbation in speed pushes the machine away from the operating point.   We already talked about a stable oeprating point above.  Let's say for simplicity your load had a constant torque of 90% breakdown torque... and after a transient you find yourself at the point where that horiziontal curve intersects the torque speed curve to the left (lower speed) of breakdown torque.  It is not a stable operating point - a small decrease in speed would cause load torque exceeds motor torque which would cause further decrease in speed... motor would decelerate all the way to 0 assuming no change in load torque (and again ignoring protective trips).

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RE: Motor Speed with reduced voltage

By the way I'm not saying that is a realistic load torque curve (you couldn't accelerate it during start), it was  just the easiest example I could think of to illustrate a non-stable operating point without being able to draw a picture.  You could also have an unstable intersection to the left of breakdown torque where both curves (motor and load) are monotonically increasing with speed, but the motor curve is steeper than the load curve.  

And a perturbation pushes you in either direction.  If the perturbation increased speed it would continue to accelerate.  If decreased would continue to decelerate.  

The unstable intersection has not much significance in practical operation because the motor won't remain there in steady state.   

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RE: Motor Speed with reduced voltage

It is totally correct. In a clearer way, if the motor continues to load up, the motor torque is still the same. The speed will decreases due to not enough torque to drive the load. If the motor still increases the with load, the speed will reduce more until it will stall. It is a matter of fact.

If the voltage to the motor is reduced for some reasons. The torque is reduced in proportion to the square of the voltage applied. But the load torque is still the same. It is also same with the above case when you increase the load. If the voltage continues to reduce, the motor torque will reduce more. As a result, the motor speed will reduce more until the motor stalls due to lack of torque.

This is another view regarding to application. When you size the motor, you must collect the load torque & motor torque information. The motor torque must be higher or same as the load torque. With Autotransformer starting, you must consider the load and the motor. Load should be applied and increased after the motor starts successfully. For the case that the load must be there at the starting period, just pause and think of another starting method or a bigger size of the motor. It may be Direct-on-line starter or something similar. For Autotransformer case starter, we must refer to its own curve.

Regards,   
 

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