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Old 10-12-2012, 12:11 AM   #1 (permalink)
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Anyone know how to figure out DC motor specs by measurements?

I bought a conversion - 1990 chev sprint, electrically less done that I thought - from someone and I'm trying to determine the ratings of the DC motor. The wiring was a bit odd, so I am checking out everything that I can. I'm looking for some rules of thumb to estimate the rated current of the field and of the armature. Estimating the rated voltage would be nice, too.

The motor is claimed to be a surplus brushed DC aircraft motor, 21 HP, rated 120 VDC, separately excited.

Field coil - posts are sized for about #12 wire.
Armature has 3/8 posts, looks like they are big enough for ... 150 - 200A, at a guess.
There are 6 brushes on the armature. The angle is hard to measure without complete dis-assembly. I'd guess maybe 12 degrees off vertical. The brushes are angled for clockwise rotation from the drive end.
The motor turns clockwise from the drive end, which is not the correct rotation to move the car forward. I'll post a separate question about how to reverse rotation (brush holders, etc).

Measurement with one 12V battery for both field and armature:

Unloaded - 200A surge draw when both Field coil and Armature are energized at the same time (120A surge on the field, 80A surge on the armature?) 770 RPM unloaded. It settles down to 7A for the field, 2.9A for the armature.

Measurement with one 12V battery for accessory (lights, contactors, field) and one for the armature so field is energized with the ignition, armature is energized with the accelerator pedal microswitch:

Motor partially loaded, coupled to the standard transmission (in neutral)
Field coil draws about 10A at 12VDC after it is warmed up (after about 60 seconds).
The Armature draws about 20A after it is warmed up.

Motor loaded, first gear or reverse, moving the car (and me). The motor and armature draw about 140 amps, after acceleration, moving steady state.

Motor loaded, first gear or reverse, brake on, motor stalled. The motor and armature draw about 200 amps (very short test).

No measurements of voltage sag during tests. I had one more meter but it didn't occur to me.

Any suggestions would be appreciated.

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Old 10-18-2012, 07:09 AM   #2 (permalink)
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Does it say if the 21 Hp is a continuous rating or for just a limited duration?

let's assume it is continuous. 21 Hp is about 15,700 watts, so the 120 VDC rating let's us calculate the current as 130 Amps continuous for the armature.

From your measurements at 12 VDC the field resistance is about 1.7 ohms, so at 120 VDC it will draw about 70 amps. i would not want to energize the field all the time, only when the pedal is pressed.

A brush-type DC motor has a fairly linear response, so scaling up the voltage to 120 from 12 will increase the no-load speed x10, so the motor voltage constant is 7700/120 ~ 64 rpm/volt. The motor torque constant is 1/7th of the voltage constant, or 9.2 amps per ft-lb. Dividing the 130 amps by the torque constant indicates that the motor would make about 14 ft-lbs of torque at 7700 rpm on a 120VDC power supply.

As the motor is loaded the current will increase and the speed will decrease. When the motor is stalled at 120 VDC the armature currents could go up to 2000 amps and generate 200 ft-lbs of torque. My guess is that it is an aircraft starter motor, not intended for continuous operation. As such it is probably not reversible, and probably not really suitable for an electric vehicle. Does it have any sort of cooling fans/vents/ducts--that would be another clue?

It may still be possible to run this motor in an EV by using lower voltages with high capacity batteries, e.g. Lithium cells, depending upon the transmission. Good luck to you.

Last edited by kennybobby; 10-18-2012 at 07:35 AM..
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Old 10-19-2012, 12:50 AM   #3 (permalink)
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Quote:
Originally Posted by kennybobby View Post
Does it say if the 21 Hp is a continuous rating or for just a limited duration?

let's assume it is continuous. 21 Hp is about 15,700 watts, so the 120 VDC rating let's us calculate the current as 130 Amps continuous for the armature.

From your measurements at 12 VDC the field resistance is about 1.7 ohms, so at 120 VDC it will draw about 70 amps. i would not want to energize the field all the time, only when the pedal is pressed.

A brush-type DC motor has a fairly linear response, so scaling up the voltage to 120 from 12 will increase the no-load speed x10, so the motor voltage constant is 7700/120 ~ 64 rpm/volt. The motor torque constant is 1/7th of the voltage constant, or 9.2 amps per ft-lb. Dividing the 130 amps by the torque constant indicates that the motor would make about 14 ft-lbs of torque at 7700 rpm on a 120VDC power supply.

As the motor is loaded the current will increase and the speed will decrease. When the motor is stalled at 120 VDC the armature currents could go up to 2000 amps and generate 200 ft-lbs of torque. My guess is that it is an aircraft starter motor, not intended for continuous operation. As such it is probably not reversible, and probably not really suitable for an electric vehicle. Does it have any sort of cooling fans/vents/ducts--that would be another clue?

It may still be possible to run this motor in an EV by using lower voltages with high capacity batteries, e.g. Lithium cells, depending upon the transmission. Good luck to you.
Thanks for the response! Your posting uses some assumptions, but I think it will help me (and I'm sure others) to estimate some parameters for used DC motors.

The motor has no markings. I'm told (on the DIYelectricCar forum) that it is a 'jack & heinz #g23 aircraft generator'. I'm having trouble finding out the specs for that model online.

So I don't know if it is actually 120VDC. But given that assumption, I follow some of your calculations.

Where did you get the 1/7 of the amps per foot-lb of torque come from? Is that a rule of thumb? Does it use the motor diameter? Did I miss the link to some of the numbers in your post?

2000 amps at stall with 200 foot-lbs is great, but again I don't understand where those numbers came from. I'd like to, though.

There is no cooling fan. No ducts that are visible. I'm told that the jet engine (in the plane that used this motor/generator) produces a BUNCH of compressed air and that the air can be 'bled' to cool the motor. If it is run hard, a *LOT* of air would be required. At EVCON 2012 I was told that some of the used aircraft starter/generators can require so much air for cooling that the compressor or blower required to keep it from melting has more horsepower than the electric motor. I have not been able to verify this information so far.

I'll be monitoring the motor temperature as well as the armature temperature quite closely.

Yes, I have been told that this motor is not likely a great candidate for an electric car. It's likely good enough for me to use for a bit of comparative testing against a surplus AC motor.

Last edited by thingstodo; 10-19-2012 at 09:32 PM.. Reason: typos
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Old 10-20-2012, 12:14 AM   #4 (permalink)
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Oh yeah i see that i did make a big assumption---that it actually was a motor! lol

A generator will make a lousy motor--it may seem to run like a motor when you connect the power, but just because you can doesn't mean you should...

Nevertheless, for dc motors the speed is determined by the voltage and the torque by the current.
We use motor constants that are linear characteristics determined from dynamometer test data. Kt is the motor torque constant: how many ft-lbs of torque are generated per amp. Kb is the motor voltage constant (back-emf constant): how many volts per rpm. When using these units of measurement the Kt is equal to 7.040 times the Kb, so the rule of thumb is 7x .

You made some test measurements using 12 volts such as the no-load speed and current, and the zero-rpm stall current, which is the same sort of data that we measure on the dyno. From your data i made calculations of those motor constants and winding resistances, etc.

At the 21 hp (15kW) with 120 volts we got the 130 amps. From the 12 volts 770 rpm no-load speed we got the Kb, from there got the Kt. You did a stall test and got 200 amps at 12 volts. Scale it up to 120 volts and the current goes up x10 = 2000 amps, then use the Kt to calculate the corresponding torque, ~200 ft-lbs. Of course it may melt the wires at 2000 amps, but that is how much current it will draw at 120 volts.

A motor would be able to handle that current but i doubt the generator would. If you have no nameplate spec data then all bets are off--best to start at low voltage and work your way up. You need some sort of motor controller to adjust, regulate and limit the current into the motor, otherwise it may melt down, even with 12 volts, 200 amps is serious current and can do some damage.
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Old 10-20-2012, 02:39 AM   #5 (permalink)
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How big is this motor? a lot of people go by diameter alone, but if it has the kind of forced cooling you talk about then it might be a higher output... but then again that is with the forced cooling! over heating any motor will kill it.
Bottom line of course is that under sizing your controller will be a bad idea and that higher voltage with a controller/battery pack is better then lower, a 72v 400amp speed controller is not enough for a car, golf cart sure, but not a car, a 144v 500amp speed controller will work for a lot of cars but people who ask a lot of their car will find it to be lacking, you are not going to find many lead acid batteries that can discharge past 600 amps for more then a few seconds but if your controller can't handle that much you will feel it, lithium on the other hand is more likely to be able to handle high discharge rates like that but still going with higher voltage is always nice, it keeps the wire size small.
Most motors can handle higher voltage... until they burn up and that is where cooling comes in, finding someone who knows what your motor can handle is going to be really hard, so get some thermocouples and find out for your self how hot it gets and unless you can find something saying it can handle over 350F then keep it under that.
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Old 10-20-2012, 10:11 PM   #6 (permalink)
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Quote:
Originally Posted by Ryland View Post
How big is this motor?
It's a bit over 7 inches, but under 7.5. My caliper does not go that wide

Quote:
... but if it has the kind of forced cooling you talk about then it might be a higher output...
As I mentioned, I have not confirmed that one as yet. The information on the motor appears to be paper-based and so a bit more challenging to locate. I may have to look at old catalogs in the library!

Quote:
Bottom line of course is that under sizing your controller will be a bad idea and that higher voltage with a controller/battery pack is better then lower
I'd like to throttle the controller to avoid melting the motor. Do any of the controllers have RTD or thermocouple inputs?

Lead-acid will be used for testing. Likely LiFePO4 (CALBs) if it appears to be workable after testing.
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Old 10-21-2012, 02:06 AM   #7 (permalink)
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most controllers have a half speed input, mostly used for reverse but also used to have kick on when the batteries are getting low, a simple snap switch could also be used to click the controller in to half speed to keep it from over heating as much, two snap switches, one that clicks on 10F cooler with a dash light would be a nice idea too, placing it next to the air output or almost touching the rear end of the motor shaft would be the two places I would place them, in an ideal world you'd have a thermal couple measuring brush temp but that gets more complected but it's done sometimes.
Snap switches are cheap enough that I would get some that are set for around 200F to 250F because the areas you can place them are going to be cooler then the parts that you want to protect.
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Old 10-21-2012, 10:59 PM   #8 (permalink)
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Temperature switches

200 - 250F. That sounds reasonable.

Any idea what the 'half speed' input to the controller is called on a typical controller?
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Old 10-23-2012, 12:29 AM   #9 (permalink)
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Quote:
Originally Posted by thingstodo View Post
200 - 250F. That sounds reasonable.

Any idea what the 'half speed' input to the controller is called on a typical controller?
It's called "Half speed/reverse", you can also wire in a resister in to the throttle on a relay to cut the throttle in half if your speed controller doesn't have a half speed/reverse input.
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Old 10-23-2012, 03:09 PM   #10 (permalink)
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See if you can contact this guy--he's using one like yours and may know all about it...


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