# How many volts to spin an unloaded DC motor to 3k?



## elevatorguy (Jul 26, 2007)

Not sure if this helps, but 12 volts on my warp 9 was about 2200 unloaded. I used an inexpensive non contact tachometer to check mine. Why does it need to be 3000 RPM?


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## major (Apr 4, 2008)

Batterypoweredtoad said:


> I need to spin my citicar motor to a constant 3k rpm and hold it around there to seat my Hellwig brushes. I have 2 A123 7s3p modules available. That gives me roughly 22V and 60AH. I have no idea what 3k rpm sounds like. Should it be ok or should I find a lower voltage source or a load?


Hi toad,

I assume you have a series motor. The speed curve is so steep at or near no-load that it is practically impossible to tell. And then the RPM will change quite a bit as the motor heats up and also as the brushes seat and comm films. It is not necessary to be exactly at 3000 RPM to seat the brushes. That is a decent number to shoot for. 

If the motor is free running (nothing coupled to the shaft), I'd stick with 12V or thereabouts. 24V may get you well above 3000 RPM. If the motor is coupled to the drive train spinning the wheels, it will put enough load on the motor to keep the RPM lower and 24V may be o.k. You might be able to use the speedometer to check. If you have no way to check the motor RPM, I recommend you do not use the 22-24V battery. No sense risking overspeed damage. 

Having some current flowing helps seat brushes. Higher current seats them faster. So spinning the wheels is better than the empty shaft. I use an old 12V battery with a 50 Amp charger across it. With wheels spinning, once things warm up, I think I'm at about 3000 RPM on a WarP9 I've been breaking in. Sounds like it; I'll see if I can check it on Monday.

major


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## Batterypoweredtoad (Feb 5, 2008)

I hooked up the 24V battery bank to the motor with it attached to the axle. I ran it for an hour that way and then decided to attach the speedo  









I just ran the calcs-60 mph is approximately 7200 RPM. Tough little bugger! Now I have it spinning on my power supply with the volts set at 12. It still sounds like it is spinning fairly fast. The amps sat slightly above 8 for an hour or so and now they have fallen to 7.5-7.8. I'm thinking of dropping the voltage to 10.


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## Batterypoweredtoad (Feb 5, 2008)

Dropped the V to 10 and it still sounded fast. Major-any preference on amps? I went to constant current instead of Volts and it sounds nice at 7A. That dropped my Voltage to ~9.5.


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