My motor is a bit of a mystery. There's no real documentation to be found laying around the internet about it. So it stands to reason that I would need to test it out for myself to get some baselines.
Mainly, I wanted to know the effect of the field's current on the RPM output. This is important so that I don't over-rev the motor and kill it.
And from that I would be able to get an idea of the current's effect on the motor's torque. If the numbers make sense, anyway.
So, I ran some tests on it, measuring RPM for a given voltage on the armature with different voltages on the field. I plotted my results:
From that, I could calculate the motor's speed at any given armature voltage vs different field voltages. This will tell me if I need more field voltage to keep the RPMs down to a safe level at higher armature voltages.
I keep talking about field voltage, when it's
current that is actually important. But, in my case, my field coils have a resistance of 1.0 ohms, so whatever my field voltage is, there is the same amperage going through. 20 amps at 20 volts, 100 amps at 100 volts, etc.
Where was I?
Oh yes.
I noticed, pretty quick, a pretty much set pattern between the RPM per Volt and the field
current. Double the field voltage, and the RPM/V goes down by almost exactly 10. That's convenient.
So, if that holds true(and it does from my observed figures), I can easily calculate that if the field goes up by, say, 25%, that the RPM/V will go down by 25% of that 10. Or if it goes up by 50%, RPM/V will go down by 5 (50% of 10). Etc, etc, etc.
I wasn't expecting it to be so
linear right off the bat, after looking at field saturation curves. I expected to see a "knee" in my numbers. So I read up on why the field saturates the way it does and how current can continue to increase the field strength after said saturation, only it does so linearly. Which tells me that at ~12v, my field is already at saturation.
That makes sense, since Sep-Ex motor's are usually designed to run the field at 1/3rd the armature voltage, and this is probably out of a 36v (or at most a 48v) forklift. You would want your field
mostly maxed out while using a reasonably low amount of power. (though that's an assumption!)
Anyway, with it being nice and linear, I can plot out even higher field voltages then I would probably ever run in reality.
Thus:
The measured numbers are in the box in the upper left, and the projected are outside. Numbers along the top right half are armature voltages, numbers below each armature voltage are RPM.
I also plotted assumed/calculated torque,
based on the RPM/V change, and using 22v(24v minus load losses) on the field as a baseline. That's probably about where I'll set the baseline for said field voltage/current.
Assuming that power output stays the same and efficiency doesn't take a major hit, we can tell that torque will go up directly relative to how the rpm/v goes down. A 10% lower RPM/V will result in 10% more torque, for example.
And note that the % of difference could be referenced at any different voltage; it simply shows torque relative to said baseline. For example, if I had 48v on the field, and dropped it down to 12v, I would only have 70% of the torque I was before.
Assuming the armature current remains the same. Oh, all these variables!!
Anyway, I now have some useful information on my motor, so now I'll know how to expect it to behave.
Motors around this size are known good to be run up to 5500rpm. Technically 8" Advanced DC are good for burst up to 8000rpm, but who's to say if ADC built these as well as they built the 8" series-wound motors that are widely used in the (old) EV world?
So, back to reality...I can run the motor safely all day long at 72v with only 12v on the field. I can run it all day long at 96v with 24v on the field. If I need more power, I can run it for short periods at 120v with 48v on the field.
Whether or not the motor will like doing ~5000rpm without advanced timing, I won't know until I try. Sep-Ex motors are kept at neutral timing, so as not to mess up regenerative braking. Not that I will have regen with this controller.
How much power? How much torque? In reality, I won't know the specifics without testing/trying it. Even if I contacted ADC, they probably don't have any info on it above 48v.
I can probably assume it will be similar to the 8" series wound ADC motor, the 203-06-4001 (this is a 203-02-4007). Which can put out a continuous 14.4kw/19hp @ 96v. And, supposedly, a peak of 51kw/68hp & 100 ft-lbs of torque @ ~600 amps. I won't have the transmission-crunching torque of a series-wound motor at 0 rpm(stall), but I would assume similar torque at "operating" RPMs.
If I'm cramming 51kw in to it at 2000rpm, either I'll get a similar amount of torque out of it, or I'll generate a
lot of excess heat. Conversation of energy and all that.
I'll find out.
That's at 600 amps. My controller is rated for 1000...
Yeah, I'll put some conservative settings in to it and take it from there.
FWIW, Mechman's 7.2" SepEx was running at 3800 rpm @48v Field/72v Armature IRRC. My motor is ~10% bigger, and projects to run 10% slower at the same voltages...hopefully confirming that I'm not completely out to lunch!
On a side note regarding motors, I took a peek in at the ones in our electric indoor forklifts at work recently, curious what they're using these days, since they're new. Low and behold, it turned out to be an AC motor! 7.3 kw @24v (though it's on 36v) for the 1-hour rating. That's a lot of amperage (~300a) to be able to maintain, out of a motor about the same size as mine. Must be more efficient.
At any rate, maybe there will be a supply of affordable AC motors for EV conversion down the road, when they start parting these things out...
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