# Controller Frequency and Motor Saturation



## buzzforb (Aug 16, 2008)

Could someone help me understand how increasing the frequency of the voltage being applied to a motor woud effect the saturation level at max rated voltage.


----------



## major (Apr 4, 2008)

buzzforb said:


> Could someone help me understand how increasing the frequency of the voltage being applied to a motor woud effect the saturation level at max rated voltage.


Hey buzz,

I guess you're talking about an induction motor. Increasing frequency would lower the volts per hertz ratio and lower the flux. Saturation level could be an issue if you increased the voltage too much for a given frequency.

Regards,

major


----------



## buzzforb (Aug 16, 2008)

Thanks for the rply major. I was talking about an induction motor. I guess what i am getting at is, if an iduction motor rated at 60Hz is driven at a higher frequency, say 120Hz, what is the effect on the motor. Does the rotor saturate more quickly or less quickly in relation to applied voltage.


----------



## major (Apr 4, 2008)

buzzforb said:


> Thanks for the rply major. I was talking about an induction motor. I guess what i am getting at is, if an iduction motor rated at 60Hz is driven at a higher frequency, say 120Hz, what is the effect on the motor. Does the rotor saturate more quickly or less quickly in relation to applied voltage.


Buzz,

Let's say it is a 230 Volt AC, 60 Hz motor. Volts per Hertz (V/H) is 230/60. Running that motor at 230V/120Hz is a lower V/H and it will not saturate. Flux will actually be less.

Saturation becomes a problem when the rated V/H is exceeded. So running the motor at 460V/60Hz or 230V/30Hz is a problem.

The effect on the motor running at lower V/H is less torque.

The effect on the motor running at higher V/H is saturation causing excessive losses and noise.

This is why AC drives vary the voltage in proportion to the frequency. Above base frequency they cannot raise the voltage and keep the V/H constant. So, the V/H falls and the motor loses torque capability. AC drives can run standard 60 Hz motors up to several times base frequency, but torque (and power) is derated.

Hope that helps,

major


----------



## buzzforb (Aug 16, 2008)

I am wrong in thinking that at a higher frequecy more voltage can be applied to the motor. If this is true, what is to be gained in doing so? The only thing i can think of is quicker motor response based on the higher frequency being used. If not true, what would be the reason for driving a motor at a higher frequenc? I am asking because i am thinking about how ac controllers work at higher frequecies using field oriented/ vector control. I probably should get back to reading and research but sometimes it helps to have someone explain from personal experience and understanding rather than words and formulas.


----------



## major (Apr 4, 2008)

buzzforb said:


> I am wrong in thinking that at a higher frequecy more voltage can be applied to the motor. If this is true, what is to be gained in doing so?


Hey buzz,

I assume you speak of fundamental frequency, like 60 Hz. So higher frequency means higher RPM. When you can keep the V/H, meaning higher voltage with the higher frequency, you keep the torque capability. So it means more power. More RPM at the same torque; more power.

If you can't raise the voltage in proportion to the frequency, you get higher RPM and less torque, so not more power and eventually less power.

The problem (one of them) with using industrial 60 Hz induction motors is the base voltage is 230 (or higher). So to run up at 300 Hz and get 5 times the power, where do you get 5 times the voltage, 1150 VAC? Which would take a 1600 volt battery.

To solve this problem, the motor must be wound differently. Such that it would be like a 50 VAC motor at 60 Hz. Then at 300 Hz you'd need like a 350 volt battery. Rewinding the motor is not a trivial task. But if it can be done, then you get like 5 times the power from the smaller frame size. But this brings alone other issues. Gearing to the high speed, bearing lube, thermal management, rotor balance are some. Things that can be dealt with, but takes time, effort and money.

Regards,

major


----------



## buzzforb (Aug 16, 2008)

1.Was i correct in saying that the frequency that the motor is driven at determines the response time to changes in the working envirioment. If a motor is run at 120Hz then the return infromation generated by Hall sensors will lead to quicker error sampling and correction.
2. 230v motor driven at 120Hz will allow for applied voltage of 460v. Correct?

Thank you for taking the time to walk me through this.


----------



## major (Apr 4, 2008)

buzzforb said:


> 1.Was i correct in saying that the frequency that the motor is driven at determines the response time to changes in the working envirioment. If a motor is run at 120Hz then the return infromation generated by Hall sensors will lead to quicker error sampling and correction.


Hey buzz,

Not quite sure what you're getting at. Any feedback to the control system from Hall sensors or the like should be many, many times faster than the fundamental frequency. On a medium size induction motor, you'd probably have like a 5 or 10 kHz carrier. Information should be gathered and processed like 100 times faster.



> 2. 230v motor driven at 120Hz will allow for applied voltage of 460v. Correct?


If you have a 230 VAC/60 Hz motor and run it at 120 Hz, it would need 460 VAC to get the same torque capability as it produced at 230 VAC/60 Hz. The motor would be twice as fast with twice the power. It will also have twice (or slightly more) losses. So, might not survive long.

Regards,

major


----------



## buzzforb (Aug 16, 2008)

Thanks for the help, its back to the books for me.


----------



## buzzforb (Aug 16, 2008)

Major,
There are some guys on the EVtech thread thta have been discussing rewiring an AC motor for lower voltage so that they can gain RPM by driving at a higher frequency. i know it ties into the conversation that we have had here. What efficiency loss will result and will this modification experience the same eddy? losses that you spoke of earlier in this thread?


----------



## major (Apr 4, 2008)

buzzforb said:


> Major,
> There are some guys on the EVtech thread thta have been discussing rewiring an AC motor for lower voltage so that they can gain RPM by driving at a higher frequency. i know it ties into the conversation that we have had here. What efficiency loss will result and will this modification experience the same eddy? losses that you spoke of earlier in this thread?


Hi buzz,

That type of rewind usually reduces the number of turns per coil and increases the effective wire size. So, if the slot fill factor remains equal, you end up with the same resistive losses for an equal power input to the motor. However, the goal is to increase the frequency, or RPM, which are proportional. This will increase the core losses. So, unless you take steps to mitigate this, the motor will be less efficient. That would be higher grade steel, thinner laminations, smoother rotor, better bearings, etc.

Regards,

major


----------

