# Limiting RPM on DC/AC motor



## dladd (Jun 1, 2011)

All of the modern programmable DC controllers can limit RPM's as long as you have a speed sensor on the tailshaft of the motor. Zilla, Soliton, Warp Drive, Synkromotive.


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## major (Apr 4, 2008)

appleseed said:


> How do i limit the RPM of a Series wound dc motor?


Hi apple,

Here's my take on part of your big question. You limit the RPM of the series wound DC motor by the load or by the input. If you use the load, then you'd better always be shaft connected and geared such that even a downhill or tow situation won't overspeed the motor and your normal load is such that it will limit the motor RPM. Direct drive does this pretty well, with the proper ratio.

The other way is to limit the input to the motor to limit RPM. This should (needs to?) be done whenever a series motor is used where it can be disconnected from the load as with a clutch or chain breakage. The common way to do this is to sense the motor RPM and when a limit (with safety factor) is reached, turn off the supply voltage to the motor. You can get fancier and instead of turning off the supply voltage, restrain it from further increase. 

Some of the recent controllers for series motors have the capability of tacho feedback from the motor and programmable RPM limit. Most older or less functional controllers do not have this feature. You can devise an external circuit to work with a motor speed sensor to turn off these controllers and prevent overspeed. This type of overspeed protection can work well but does not prevent mechanical overspeed such as downhill or towing where the motor is forced to spin too fast without electrical input control.

Other types of DC motors can be RPM limited without shaft feedback such as SepEx and PM. Here the motor characteristics are known and the controller can be programmed to limit RPM from sensing other variables. AC motors and controllers give you automatic RPM limit capability because RPM is frequency dependent or those have shaft sensors for other reasons. But again, you can still mechanically force overspeed with the right conditions.

I'll stop here except to say that maximum motor RPM rarely equates to maximum motor power.

Regards,

major


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## frodus (Apr 12, 2008)

The AC controllers I've used have limits on RPM as well, but they don't have the overspeeding issues that Series DC motors do.


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## appleseed (Sep 12, 2010)

Hey major

First, how long have you been on this forum? You have given many good answers in other threads, and even thou we didn't talk directly, you have helped me learn alot.

I realize max rpm not =max power. I was just hoping to be able to have a low gear ratio to maximize torque without hurt the motor at extended periods. Do you know some people who use direct drive and have the dc/ac motor set to rev fairly high at highway speeds? have they had any problems

What dc controllers can limit RPM? Curtis? Net gain controller?
Im pretty sure sepex or pm are either too small or really expensive for motorcycle/car ev use. I many be wrong, if so any examples i could research?

i understand the mechanical rev, its the same as a ICE.


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## major (Apr 4, 2008)

appleseed said:


> Hey major
> 
> First, how long have you been on this forum?


If you click on my name above the post you can go to my profile and view statistics.



> I realize max rpm not =max power. I was just hoping to be able to have a low gear ratio to maximize torque without hurt the motor at extended periods. Do you know some people who use direct drive and have the dc/ac motor set to rev fairly high at highway speeds? have they had any problems


Tesla and Nissan Leaf are direct drive and work very well. Those are AC drives and for sure the Tesla is a very high speed motor (12 to 14kRPM) but then has about twice the max MPH you'd need. Almost all the drag EVs are direct drive. About all the EVmotorcycles.

Direct drive can work well if you have a big enough motor and strong enough controller to get decent accel torque, and don't gear for extreme top end mph. But most EV car conversions have a tranny from the donor and just use it with a bit smaller motor and controller. I think most leave it a single gear around town and shift for highway speed. 



> What dc controllers can limit RPM? Curtis? Net gain controller?





dladd said:


> All of the modern programmable DC controllers can limit RPM's as long as you have a speed sensor on the tailshaft of the motor. Zilla, Soliton, Warp Drive, Synkromotive.





> Im pretty sure sepex or pm are either too small or really expensive for motorcycle/car ev use. I many be wrong, if so any examples i could research?


Cars, probably not. MC, SepEx 6.6" motors (like D&D) and Curtis or Sevcon drives and some PM motors capable like Agni. 



> i understand the mechanical rev, its the same as a ICE


I was talking about the electric motor being backdriven from the mechanical driveline.

Regards,

major


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## appleseed (Sep 12, 2010)

"Tesla and Nissan Leaf are direct drive and work very well. Those are AC drives and for sure the Tesla is a very high speed motor (12 to 14kRPM) but then has about twice the max MPH you'd need. Almost all the drag EVs are direct drive. About all the EVmotorcycles."


Just using the nissan since i know about it. Its top speed is about 95mph due to gearing and max rpm. Would it be harmful if someone changed the gearing so the top speed is now 75mph but the ac motor be near but not on the rpm limit for extended time? what about a dc motor? Since electric motors have little drag, maximizing torque through gearing is a good way to improve acceleration.

THe only thing i could think that would wear would be bearings but those are cheap and im sure would still last awhile


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## EV Dawg (Oct 30, 2011)

I don't think you will have an issue. Here is why:

All motors have a maximum (electromagnetically possible) rpm that is voltage dependent for DC motors, and frequency dependent for AC motors. This is one of the factors that constitutes a motors "rating" and automatically prevents immediate physical damage from overreving. As long as that rpm is lower than the rated rpm, you will not be be able to "overrev" your motor.

The only other real way to damage a motor is heat which relates to current draw: conductive resistance*current^2. Current always goes down (to a certain point) as rpm increases, so again, nothing wrong with letting the motor rev a little higher than normal.

All the those warnings that say to never let a motor run unloaded either have to do with motor requiring a load to keep the rpm lower than than the physically safe limit or the fact that there would be no air flow over the motor when it is free revving.


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## appleseed (Sep 12, 2010)

EV Dawg said:


> I don't think you will have an issue. Here is why:
> 
> All motors have a maximum (electromagnetically possible) rpm that is voltage dependent for DC motors, and frequency dependent for AC motors. This is one of the factors that constitutes a motors "rating" and automatically prevents immediate physical damage from overreving. As long as that rpm is lower than the rated rpm, you will not be be able to "overrev" your motor.
> 
> ...



i believe its called back emf? well the voltage created by the motor at a certain rpm restricts the amps from the controller, thats why a higher voltage allows the motor to run at a higher speed.

so yes it is possible that at the motor rpm limit the Back voltage (?) is higher than the voltage you feed the motor, but if you use the highest safe voltage ex 250 volts on a warp 11 hv then the motor may be able to over rev

is there a chart for motors to show back volts at listed rpm?


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