Increasing thermal capacitity of motor for larger HP

[philly]

I know that one way to increase the HP of a motor is to incerase the thermal capacity of the motor by cooling it somehow or by using higher rated widings to handle the increased heat by drawing more current. This allows the motor to draw more current and therefore produce more torqe and hp for a given speed.

Is there a limit though to how much current a particular motor design can be increased (assuming infinite thermal capacity) before the torque and hp would no longer increase? For instance if we kept increasing the current to some infinite value would the hp keep increasing or would we reach a point where the motor design wouldn't allow any more hp incerase for an increase in current such as saturation?

 

[Jraef]

Well, you can't really "increase the current of the motor" per se, all you can do is increase the LOAD on the motor, which will cause an increase in slip, which will cause an increase in current drawn by the motor. But the motor has a finite amount of torque it can produce, once you exceed the motor's torque spec, all bets are off as to how it will react. Essentially if you look at a Design B motor's torque/speed curve, you will see a sort of "double hump", where the initial torque (starting torque) is at the beginning, there is a trough as speed increases, then a second hump that is called Breakdown Torque, and the "Full Load Torque" is on the down hill side of that second hump at full (slip) speed. So by increasing load, you move backwards into that Breakdown Torque hump and the motor pulls more current to try to get back to slip speed. But if you slow the motor down too much, you fall back behind that curve, where you are now pulling more current yet developing LESS torque, and that is the definition of a Stall condition. In other words, your motor can't handle the load any longer.

So overloading a motor is sort of a dangerous game, even if you somehow artificially cool it. By that I mean there is no formula by which you can determine how the motor will react to an increase in load beyond what it is designed for, you would have to do it by trial and error. to do that, you will need to have very good data collection capabilities, i.e. RTDs embedded in the windings and bearings, torque measurement capabilities etc. etc.

PS: by the way, I am assuming you were referring to an AC Squirrel Cage Induction Motor.

 

[philly]

Well, you can't really "increase the current of the motor" per se, all you can do is increase the LOAD on the motor, which will cause an increase in slip, which will cause an increase in current drawn by the motor. But the motor has a finite amount of torque it can produce, once you exceed the motor's torque spec, all bets are off as to how it will react. Essentially if you look at a Design B motor's torque/speed curve, you will see a sort of "double hump", where the initial torque (starting torque) is at the beginning, there is a trough as speed increases, then a second hump that is called Breakdown Torque, and the "Full Load Torque" is on the down hill side of that second hump at full (slip) speed. So by increasing load, you move backwards into that Breakdown Torque hump and the motor pulls more current to try to get back to slip speed. But if you slow the motor down too much, you fall back behind that curve, where you are now pulling more current yet developing LESS torque, and that is the definition of a Stall condition. In other words, your motor can't handle the load any longer.

So overloading a motor is sort of a dangerous game, even if you somehow artificially cool it. By that I mean there is no formula by which you can determine how the motor will react to an increase in load beyond what it is designed for, you would have to do it by trial and error. to do that, you will need to have very good data collection capabilities, i.e. RTDs embedded in the windings and bearings, torque measurement capabilities etc. etc.

PS: by the way, I am assuming you were referring to an AC Squirrel Cage Induction Motor.

Great Response.

I understand what you are saying in regards to the load being the variable that increases current. I guess it is possible for the load to increase and therefore draw more than full load current thus increaseing the torque and and HP that is required from the motor. Because now this increased HP will now be higher then the motors full load rating the motor will be limited by its thermal capacity. However if you could somehow artificially cool the motor as you mentioned then you could operate the motor over its full load value drawing more current and thus providing an increased torque and HP. However as you stated the maximum torque that the motor can produce is finite and determined by the breakdown torque of the motor? Am I understanding this correctly?


So if the max torque a given motor can produce is the motors breakdown torque then is this value governed by the torque equation:

T= (Vs/Hz)(R2/(R2/s+x^2))

And taking the derivitive of this equation and setting it to 0 this would provide the maximum torque value.

So if we wanted to change this value or increase the max torque of a given motor would this be accomplished by changing the rotor resistance as the equation would indicate? I alwayst thought this was done by changing the stator somehow?

 

[philly]

So by increasing load, you move backwards into that Breakdown Torque hump and the motor pulls more current to try to get back to slip speed.

I'm not sure i completely understand this sentence?

I know by increasing load the slip increases and you move backwards on the torque curve to a new slip speed as current increases. How does the motor try to get back to slip speed by pulling more current?

 

[Jraef]

I'm not sure i completely understand this sentence?

I know by increasing load the slip increases and you move backwards on the torque curve to a new slip speed as current increases. How does the motor try to get back to slip speed by pulling more current?

The motor is always trying to return to its base speed, I probably should have used that term instead of "slip speed" because you're right, whatever speed it is running at under any given load is technically the slip speed. I've taught some basic motor classes and sometimes people get confused when I said "base speed" because they come in thinking that is the synchronous speed. So I got into the habit of calling it "slip speed", but in this context, it didn't help. Sorry.