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  #1  
Old 04-22-2010, 02:36 PM
shespuzzling shespuzzling is offline
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Default Induction Motor - Torque Speed Characteristics

Can someone please explain the torque/speed characteristics of an induction motor? In particular, I am confused about the following graph:

http://www.ecmweb.com/mag/405ecm08fig2.jpg

I think maybe my understanding of torque is not accurate...I thought it was just a force that was exerted on an object causing it to spin (because there is a fixed axis of rotation). In the case of an induction motor, the torque should be dependent on the current through the rotor, which is in turn dependent on the speed of the rotor...so torque and speed should be dependent on one another. The book (Wildi - Electrical Machines) I'm reading doesn't seem to agree with me there. Also, I'm not sure why the torque starts out high (assuming it has something to do with momentum & starting from rest?), and then gets lower, and reaches the breakdown torque at a speed lower than the full load speed....if it breaks down before it gets to full load speed, how can it ever get there?

As the rotor starts to turn, the torque would be at its highest (I think) because the most amount of current is flowing through the rotor bars and thus the force on it is strongest. But then as the rotor speeds up, the current in the rotor decreases because the changing flux decreases so shouldn't the torque decrease as well?

Thank you for your help.
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Old 04-22-2010, 03:17 PM
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Looks like this motor is being driven with a fixed frequency so it has a peak in torque there (right side). Pretty much all motors have good torque at low speeds so that's why the peak at zero speed.
Imagine a normal DC motor curve which starts high and then drops off to zero added to this peak at the frequency the motor is driven at.
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Old 04-22-2010, 05:00 PM
shespuzzling shespuzzling is offline
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Hm, I just started learning about motors so I'm not sure I understand...my biggest problems have to do with breakdown torque (why does it happen at a lower speed than full load speed? how is that possible?) and the fact that a rotor can keep picking up speed even though the torque that corresponds to a faster speed should be smaller because the current in teh rotor is less.
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Old 04-22-2010, 05:21 PM
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This is the curve for a normal DC motor. The induction motor is being driven at the frequency at the far right side of your graph. The Full load speed is for maximum power output and maximum power does not occur at maximum torque because power is proportional to speed X torque.
For the DC motor maximum torque is at zero speed and although the motor is using a lot of power, the mechanical power is zero, because it is not doing any work.

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Old 04-23-2010, 12:34 AM
shespuzzling shespuzzling is offline
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I'm sorry, but I'm not following. Where does frequency come into play in the graph I posted? I am assuming that the induction motor is being fed by some constant frequency generator. My understanding was that the graph I posted was a generic example of torque/speed characteristics for all induction motors (depending on resistance of the rotor, etc...I've actually only learned about squirrel cage & wound rotors so maybe not ALL IM's, but at least those).

So, according to that graph, as a motor picks up speed, the torque starts out higher than full load torque. That I understand...when the rotor is initially at rest the flux changes fastest w.r.t. the rotor so maximum current is induced and there is a large torque. So why doesn't the torque (and speed) decrease as the rotor starts to rotate? Is there momentum that is keeping the rotor moving and less torque is required to turn it?

Then, on the graph, the torque goes down to a minimum point (pull-up point I think), and picks up again. But this all happens as the rotor is picking up speed, so that would mean that the current is getting smaller, so how is the torque getting bigger? And, in order for the rotor to be at full-load speed, the graph implies that it would have to eventually pass the speed associated with break-down torque, so why doesn't it stop before it gets to full-load speed?

Thanks.
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Old 04-23-2010, 02:12 AM
t_n_k t_n_k is offline
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An induction motor is normally sized for the anticipated full load torque. Load torque will depend on the type of load - be it a pump or fan etc. Different loads will have different torque-speed characteristics. As long as the load torque-speed characteristic lies inside the motor torque-speed envelope then normal operation is possible. The designer must be careful to ensure that the mechanical load never approaches the pull-out torque value. That result is highly undesirable.

The torque-speed graph simply shows the operating envelope for the motor over a full range of torque-speed conditions. Normally, the motor would not traverse the curve when starting from zero speed and accelerating to the particular torque-speed demand from the load. If you overlay a load torque-speed characteristic on the motor torque speed characteristic you will come up with a resultant operating point.

Suppose a load has a linear torque-speed characteristic rising from zero torque at zero speed to 80% of the motor rated full load torque. The load torque-speed line will intersect the motor torque speed line at a speed slightly higher than the full load torque speed. That is, the motor would run slightly closer to synchronous speed at 80% of full load torque than it would at its rated full load torque. During startup the motor torque-speed values would simply follow the load torque-speed line. It would [by intentional design] not approach anywhere near the pull out torque.

Suppose while running at 80% of rated load an abnormality develops in the load - let's say it's a pump in which a bearing starts to seize up. Suppose the load torque now starts to rise dramatically above the rated value. It's now possible in extreme circumstance that the motor will start to traverse the torque-speed line up to the pull out torque. Once that occurs the motor supply protection provisions will come into play - probably by isolating the motor from the supply.

Last edited by t_n_k; 04-23-2010 at 02:22 AM. Reason: clarification
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Old 04-25-2010, 10:33 PM
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Thanks, that makes things much clearer. So, are load torque-speed characteristics typically linear? Also, is the full-load torque and full-load speed exclusive to a particular load, but the general characteristics and the shape of the curve always pretty similar for a particular motor across all loads?

Does the fact that the pull-out torque occur at around 80% of the synchronous speed mean that if a load is applied with a torque equal to 2.5T (same as the value of the breakdown torque on the graph I attached), then it will accelerate to a speed that is 80% of the synchronous speed and at that point motor will stop?

Finally, for the starting torque and pull-up torque, if the load characteristics are linear from 0 speed to 80% of the full load torque, then the load torque is never equal to the starting torque or the pull-up torque. So, what do those values actually mean in terms of the load and the motor?

Thanks again, I think (hope) I have a better idea of what these graphs mean.
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Old 04-25-2010, 10:57 PM
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Quote:
Does the fact that the pull-out torque occur at around 80% of the synchronous speed mean that if a load is applied with a torque equal to 2.5T (same as the value of the breakdown torque on the graph I attached), then it will accelerate to a speed that is 80% of the synchronous speed and at that point motor will stop?
No, at zero speed the motor cannot provide a torque of 2.5T so the motor will be stalled. 2.5T is only available at 80% synchronous speed, if it was running under those conditions and slowed down even slightly it would not be able to provide enough torque and would stop.
A normal load like a fan would have low torque at low speed and the torque would increase with speed.
A rotating drum which lifts a weight on a cable is an example of a load with constant torque.
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Old 04-25-2010, 11:34 PM
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Right. Motors stall torque depends on the speed. This is usually linear in each motor, but not linear across motors in general.
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Old 04-26-2010, 01:29 AM
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Your graph represents a typical NEMA B torque curve. There are others.

Torque, as you originally surmissed, is related to current flowing within the rotor. However, to clarify, it's directly related to inphase current.

What bearing does frequency have?, in the classic formula, inductive reactance is determinent on frequency. Where does the specific frequency come from?, slip.

At full speed and down to approx 25% slip (B design), your resistive current is higher than your reactive current. At the 25% mark, you cross the threshold and reactive wins out over resistive causing a downward cascade to stall.

The curve itself comes from the addition of inphase and out of phase currents over the speed range of the motor.
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