# Calculating DC series motor torque???



## major (Apr 4, 2008)

blwncrewchief said:


> Can anyone show me how to calculate the back EMF volt per rpm?


Hi blwn,

V/rpm does not work for you with series motors because the flux changes with load. That constant is used a lot for PM motors because the flux is constant.

For the series motor, you have to use the RPM vs torque (or current) from the motor curve. Or get a whole bunch of design parameters and spend a load of time calculating it.

Now, at a given load on a motor curve like that, you can proportion the RPM up and down by the ratio of a given voltage to the voltage at which the curve is drawn. This is a pretty good approximation.

Regards,

major


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## blwncrewchief (May 29, 2008)

OK, let me back myself up a little here since I have taken myself off track. I'll back up to 72v to start to match the graph given.

1) So would maximum ft pounds of torque be flat from 0 rpm to approximately 1500 rpm with 72v and 400a at about 110ft pounds?

2) So if I doubled the available voltage to 144v this would approximately double the rpm that maximum torque would carry out to approximately 3000 rpm?

3) After that rpm, torque(current) would fall off by rpm according to the curve of the graph?

Am I still all wet 

Thank for your help and patience major.


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## major (Apr 4, 2008)

blwncrewchief said:


> OK, let me back myself up a little here since I have taken myself off track. I'll back up to 72v to start to match the graph given.
> 
> 1) So would maximum ft pounds of torque be flat from 0 rpm to approximately 1500 rpm with 72v and 400a at about 110ft pounds?


Yes. The torque is 110 lb.ft. if the current is 400 amps, regardless. At 72 volts applied to the motor, 400 amps produces 110 lb.ft. and the RPM is about 1500. At lower RPM and 400 amps, the torque is 110 lb.ft., but the motor voltage is lower. This is done with the motor controller.



> 2) So if I doubled the available voltage to 144v this would approximately double the rpm that maximum torque would carry out to approximately 3000 rpm?


Yes, that is a reasonable approximation at 110 lb.ft. (400 amps).



> 3) After that rpm, torque(current) would fall off by rpm according to the curve of the graph?


Yes.



> Am I still all wet


You look dry to me



> Thank for your help and patience major


You're welcome,

major


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## blwncrewchief (May 29, 2008)

I think I got it now. Thanks again major


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## tylerwatts (Feb 9, 2012)

Gents, can I ask how the initial torque rating for a motor is determined please? must it be dyno tested, or calculated? I ask in relation to determining the torque ratings of second hand motors (eg forklift motors) where specs are not available. Would I need a rated torque, rpm and voltage value to determine the capabilities of the motor?
Thanks
Tyler


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## major (Apr 4, 2008)

Hi tyl,



tylerwatts said:


> Gents, can I ask how the initial torque rating for a motor is determined please?


Yes.



> must it be dyno tested, or calculated?


Yes.



> I ask in relation to determining the torque ratings of second hand motors (eg forklift motors) where specs are not available. Would I need a rated torque, rpm and voltage value to determine the capabilities of the motor?


That's the tough one. Typically the best you can do is take a wild a$$ guess, get some photos and post up on this thread http://www.diyelectriccar.com/forums/showthread.php/using-forklift-motor-and-choosing-good-7598.html You may be able to find the motor in question or similar motors used by others in that thread, in the garage or on the EValbum.

If the motor has a nameplate with the rating in hp and RPM, rated torque is a simple calculation (hp = RPM * lb.ft. / 5252). If not, then the size of the motor is generally a decent indication of torque capability. Take a look at the torque curves for the Warp line of motors. These are similar characteristics to fork motors. 

With EVcar conversions, when you keep the multiratio transmission, and you're off by 10 or 20% on the motor torque estimation, just use a different gear 

Regards,

major


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## tylerwatts (Feb 9, 2012)

Thanks Major
Actually looking for a direct drive DC bike conversion, wanting to up the volts (for rpm) and have a deent top end power output to maintain a decent cruise speed.
Motor in question:
http://www.ebay.co.uk/itm/250969120043?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649
Decent continuous power and good voltage rating. I think...
Thanks
Tyler


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## major (Apr 4, 2008)

tylerwatts said:


> Thanks Major
> Actually looking for a direct drive DC bike conversion, wanting to up the volts (for rpm) and have a deent top end power output to maintain a decent cruise speed.
> Motor in question:
> http://www.ebay.co.uk/itm/250969120043?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649
> ...


Bike hey  Might be a biggy for a 2-wheeler. But they make big cruisers. Get a weight on it. Schabmuller makes a good motor. You have the RPM and power so can calculate the rated torque (at 160A). That will be on a 20% duty cycle (says 53 but probably is S3) which I think is 10 minute base, so 2 minutes on 8 minutes off. Likely would take much higher current for short bursts, so more torque for launch. Maybe o.k. for higher voltage, probably in fact.

It is a pump motor, so.......It may have internal spline (no shaft extension). Unidirectional (correct for your bike?). Might be compound wound (series needed for standard controller).

Price is fair to high, IMO. Condition unknown.

And on a bike, you can adjust the sprockets for drive ratio if you need. Most bikers do to get it optimized for their taste.

Cheers,

major


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