# Is testing Series DC motor unloaded of value?



## puddleglum (Oct 22, 2008)

I have two 11" forklift motors that would both be potential candidates for an EV, but in the absense of any speed/torque data, I'm wondering if any useful information can be determined by testing the low voltage free run speed. 

One motor is a GE motor with a 49 bar comm and 9 1/4" long yoke. The other is a Hitachi motor with a 53 bar comm and 8 1/4" long yoke. The one spec I have for this motor is 1300 rpm and 55 lb/ft. torque at 44 volts and 280 amps. 
I've read here that Motor speed is slower with a higher bar count motor and torque is lower for a given voltage. I also understand that motor resistance is going to affect speed but that a free running motor has almost no resistance.

I did a short test of the free run test of both motors using a 12 volt batt. The GE motor was around 1300rpm and 35 amp draw. The Hitachi motor was 1500rpm and 48 amp draw. Testing method had a margin of error, but even so, can I learn anything more from this kind of test beyond the motors turn? Will this tell me anything useful about which motor will have more speed or torque installed in a vehicle?


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## Duncan (Dec 8, 2008)

Hi
Can you try and measure the resistance of both motors? - the zero rpm resistance

And also can you measure the actual battery voltage when you are doing your free run test

When you have those you can calculate the back EMF for each motor

This should give a point on the Back EMF v (Rpm x Current) graph

One you are above saturation then the back EMF should be proportional to Rpm x current

My Hitachi gave about 100v back EMF at 200 amps and 3500rpm, (100Kph)
which is why I am speed limited on a 130v pack


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## major (Apr 4, 2008)

puddleglum said:


> I've read here that Motor speed is slower with a higher bar count motor and torque is lower for a given voltage. I also understand that motor resistance is going to affect speed but that a free running motor has almost no resistance.


For all else equal, increase of commutator bar count will lower the RPM. But it will increase the torque per ampere. The ultimate torque from the motor depends on the rotor core size. The resistance on a particular motor does not change and is the same regardless of the voltage or load assuming the same temperature.



puddleglum said:


> I did a short test of the free run test of both motors using a 12 volt batt. The GE motor was around 1300rpm and 35 amp draw. The Hitachi motor was 1500rpm and 48 amp draw. Testing method had a margin of error, but even so, can I learn anything more from this kind of test beyond the motors turn? Will this tell me anything useful about which motor will have more speed or torque installed in a vehicle?


No load test are useful to see if the motor runs smoothly and balanced. Because the speed/torque characteristic is asymptotic approaching zero torque, you should not draw any conclusions about loaded performance from a no-load test. Simple things like seal drag or brush seating or air gap tolerance can throw the results way off.

You're talking 49 to 53 bars. If those motors are close to the same size, I'd expect very similar performance. Both GE and Hitachi make (made) very good machines.

major


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## puddleglum (Oct 22, 2008)

Thanks for the help guys. 
Duncan, my Hitachi motor is identical to yours so I've been watching how yours performs pretty close as this one should be the same. It is also in the best condition, but it's longer overall. If I end up doing a FWD car that length could be a problem. I did check the voltage, but since there is almost no back EMF at no load, I don't think I will tell me much. I have no way to load the motors at this point. Your data is probably better than anything I can derive from a free run test.
Major, as usual your answers clear things up for me. I won't read too much into this test. The GE motor has spent the last couple of years outside so it needs a good cleaning for sure. If I understand you, the GE motor's longer frame/armature core may give a torque/amp advantage, but the Hitachi should have a slight torque/amp advantage with the extra bar count that would even it out, so the performance loaded may be close? Would the longer core have any effect on rpm/volt loaded? You mentioned air gap, does that only really effect no load speed or does it effect loaded speed as well? 
The GE motor would probably be the easiest to install but the one I know least about. It looks just like a Warp11 with a shorter frame, but I know better than to assume it will match those specs. If I get to proceed with with my current plan, I will be voltage limited so the higher speed motor would be preferable. I guess gambling on the unknown is the price of cheap parts.


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## major (Apr 4, 2008)

puddleglum said:


> ....but since there is almost no back EMF at no load, I don't think I will tell me much.


Any time the motor is rotating under its own power, there is BEMF. At no-load with 12V applied you'd have like (for example) 11V of generated voltage (Eg or E_sub_g) in the armature or back EMF as you call it. The other 1V is dropped across the resistance in the windings and brushes.



puddleglum said:


> If I understand you, the GE motor's longer frame/armature core may give a torque/amp advantage,


The larger core will produce greater torque, not necessarily more torque/amp.



puddleglum said:


> but the Hitachi should have a slight torque/amp advantage with the extra bar count that would even it out, so the performance loaded may be close?


What I was saying was if they are the same size and 8% different in bar count, they'd have about 8% different Voltage and Torque constants. But things like difference in field turns and magnetic geometry can eat up 8% quickly. So just a rough approximation.



puddleglum said:


> Would the longer core have any effect on rpm/volt loaded?


Sure, longer core means more flux. Eg = Kv * RPM * Flux.



puddleglum said:


> You mentioned air gap, does that only really effect no load speed or does it effect loaded speed as well?


You'll notice air gap mostly at light loads. More gap requires more AT / Flux. Not so noticeable when saturated.

Hope that helps,

major


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## puddleglum (Oct 22, 2008)

Thanks again. Good information and explanations are always helpful. Just a little frustrating to find out I misunderstood a basic concept I thought I had figured out.


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