# What Determines the Power Capability of an ACIM?



## toddshotrods (Feb 10, 2009)

I'm talking physical characteristics, design, and manufacture - not inverter, batteries, etc. Everything else being up to snuff, what are the things that determine how much power an AC induction motor can deliver? For the sake of the argument, we'll assume it's a liquid cooled motor.

On the, old standby, series-DC motor it's normally cooling and mechanical commutation limits. Racers pump a couple hundred volts and up to 3000 amps of current, and the motors take the abuse and dish out incredible amounts of power.

What is ACIM's eventual Achille's Heel?


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## major (Apr 4, 2008)

toddshotrods said:


> I'm talking physical characteristics, design, and manufacture - not inverter, batteries, etc. Everything else being up to snuff, what are the things that determine how much power an AC induction motor can deliver? For the sake of the argument, we'll assume it's a liquid cooled motor.
> 
> On the, old standby, series-DC motor it's normally cooling and mechanical commutation limits. Racers pump a couple hundred volts and up to 3000 amps of current, and the motors take the abuse and dish out incredible amounts of power.
> 
> What is ACIM's eventual Achille's Heel?


Compared to the DC series wound motor which will provide increasing torque with increasing current, the AC induction motor encounters breakdown torque (BDT) which is the maximum torque output even as current continues to increase. Page 1 of this reference shows this graphically and explains the motor performance characteristics. http://www.yaskawa.com/site/dmdrive.nsf/%28DocID%29/MNEN-5JFQNV/$File/AR.MOTOR.01.pdf 

With the torque limits for the ACIM, power can be increased by raising the speed (requiring increased frequency and the appropriate voltage for the design). Generally mechanical factors limit the RPM, or control issues become unmanageable. 

Those two items together limit the peak power density for ACIM. The running, average, continuous, rated (or whatever you want to call it) power is limited by the thermal management system.


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## dougingraham (Jul 26, 2011)

I am not a motor expert but I do know a little. An ACIM must induce the magnetic field in the rotor and there are limits to the amount of flux that can be induced. That limit is imposed by both of the losses I mention later. Other than that the same issues occur in all motors. You have copper losses and iron losses. The copper losses are from the resistance of the wire which can be calculated with the simple formula (I^2)*R where I is the current and R is the winding resistance. An example of this would be assume the winding resistance is something like 0.005 ohm and you are seeing currents of 200 amps. The copper losses are going to be (200^2)*0.005 = 200 watts. If that current goes to 1000 amps the copper losses will be 5000 watts. If you push it to 2000 amps then you will have 20000 watts of loss in the copper. Iron losses are harder to understand. When you move a conductor through a magnetic field you generate a voltage. This same thing happens when the iron laminations move through the magnetic field. Only there is no place for the voltage to build up because the laminations look like a shorted wire and so the voltage that is induced into the laminations is turned into heat and they resist the motion through the magnetic field. The current that flows in the laminations is called eddy current. It is to minimize these currents and the losses that the laminations are made of thin soft iron that is insulated from its neighbor.

To lower the copper losses you want to fill the slots as full as possible with the conductor. In other words to maximize the amount of copper to minimize the resistance. If cost was no object then you would use silver as it is around 10% lower resistance for the same cross section. But this small increase would come at a huge cost. I can't imagine anyone but a racer actually winding a motor with silver.

To lower the iron losses you use thinner laminations which means more of them. There are also materials that display lower losses. I remember seeing some nickle cobalt laminations at the AstroFlight factory. These were for motors about an inch in diameter that were capable of turning at RPM's exceeding 60000 without severe iron losses. The normal laminations would have turned red hot at those rpm's. The cost of these really thin laminations of a somewhat exotic material was about $1000 for about a foot tall stack.

The good news is that since the (I^2)*R losses are an exponential you can keep them under control simply by reducing the current. The Iron losses come with high RPM so restricting the RPM will keep those losses under control. In an automobile or motorcycle we generally only have to worry about these special cases when we are accelerating and they will generally not be excessive during steady state operation unless the motor is undersized for the application. Airplanes and to a lesser extent boats would be more of an issue since they tend to operate at higher power levels more of the time. Racing would be another situation where the copper and iron losses cant be ignored.

I hope that helps.


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## toddshotrods (Feb 10, 2009)

major said:


> Compared to the DC series wound motor which will provide increasing torque with increasing current, the AC induction motor encounters breakdown torque (BDT) which is the maximum torque output even as current continues to increase. Page 1 of this reference shows this graphically and explains the motor performance characteristics. http://www.yaskawa.com/site/dmdrive.nsf/%28DocID%29/MNEN-5JFQNV/$File/AR.MOTOR.01.pdf
> 
> With the torque limits for the ACIM, power can be increased by raising the speed (requiring increased frequency and the appropriate voltage for the design). Generally mechanical factors limit the RPM, or control issues become unmanageable.
> 
> Those two items together limit the peak power density for ACIM. The running, average, continuous, rated (or whatever you want to call it) power is limited by the thermal management system.


Thanks Major, that is exactly what I was looking for.  So precise and to the point that I don't have any follow up questions at the moment. I probably will later but that will keep me chewing for quite a while...






dougingraham said:


> I am not a motor expert but I do know a little. An ACIM must induce the magnetic field in the rotor and there are limits to the amount of flux that can be induced. That limit is imposed by both of the losses I mention later. Other than that the same issues occur in all motors. You have copper losses and iron losses. The copper losses are from the resistance of the wire which can be calculated with the simple formula (I^2)*R where I is the current and R is the winding resistance. An example of this would be assume the winding resistance is something like 0.005 ohm and you are seeing currents of 200 amps. The copper losses are going to be (200^2)*0.005 = 200 watts. If that current goes to 1000 amps the copper losses will be 5000 watts. If you push it to 2000 amps then you will have 20000 watts of loss in the copper. Iron losses are harder to understand. When you move a conductor through a magnetic field you generate a voltage. This same thing happens when the iron laminations move through the magnetic field. Only there is no place for the voltage to build up because the laminations look like a shorted wire and so the voltage that is induced into the laminations is turned into heat and they resist the motion through the magnetic field. The current that flows in the laminations is called eddy current. It is to minimize these currents and the losses that the laminations are made of thin soft iron that is insulated from its neighbor.
> 
> To lower the copper losses you want to fill the slots as full as possible with the conductor. In other words to maximize the amount of copper to minimize the resistance. If cost was no object then you would use silver as it is around 10% lower resistance for the same cross section. But this small increase would come at a huge cost. I can't imagine anyone but a racer actually winding a motor with silver.
> 
> ...


I don't fully understand all that, but I kinda _get it_. In my typical intuitive/interpretive manner, it makes sense in my head, without knowing exactly why it does... I also understood that part, I just didn't get how to apply it specifically to ACIM because I am used to thinking in terms of series-DC torque monsters.

Now that I get BDT, mixed with the practical limitations of spinning faster for more power, I see what I was looking for.

Thanks guys!


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