Another sparkies question about generators

They had several different sizes, including a couple of 24-volt motors. Prices ranged between $5 and $10 for the smaller ones -- sizes like you describe and a bit larger. In general, the larger the diameter you can use the more volts per rev you'll probably get because there's more room for magnets and peripheral velocity is greater for given RPM. I didn't see any 90-volt PM motors this time.

If you could find a power seat motor at a junkyard, that might work very well. I have one here that's 2.56" dia and would be a little under 4" long if one cut the wormgear drive off or made a new endcap. It turns about 113 RPM thru 44:1 worm reduction, which makes me think it would generate pretty close to 12 volts at about 5000 RPM. The armature resistance is only 0.175 ohms so IR drop would be very low. I think there's be no problem getting several amps out of it. It draws about 15 amps before it starts slowing down noticably.

AxMan had some windshieldwiper motors with gearbox attached, but they wanted way too much for them -- $19.95, I think, and you don't need gear reduction.

Reply to
Don Foreman
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Jim Pentagrid is correct that the power will be much less if the speed and field are the same. But you can either increase the speed or field to make the voltage the same.

Dan

Reply to
Dan Caster

Sounds like we're imagining different classes of motor here. When I hear "brushless motor" I think servo, where the motor typically includes only the field windings, Hall sensors to provide rotor position feedback for commutation, and an encoder for position and velocity feedback to the servo control loop. The power and control circuitry is external to the motor.

I forget there are other motors for less demanding apps that integrate the drive electronics in the motor package with only the power leads, and perhaps speed control lines, brought out. I suspect this is the sort of motor you're talking about when you refer the "internal elex"?

Rectifying the output of the servo type would be trivial, though the issue of voltage regulation you mentioned elsewhere remains. As you say, not so simple for the integrated motor.

Ned Simmons

Reply to
Ned Simmons

Right, but not 60% of its input power as a generator. Efficiency is still

80%, you've just arbitrarily limited the amount of input power you're allowing to be applied to the generator input shaft. Efficiency is still always the ratio of output power to input power, and now the input power is at the shaft instead of at the winding terminals.

Gary

Reply to
Gary Coffman

Yes and for his conditions the input shaft speed is indeed specified as being the "SAME SPEED" as the motor speed on the nameplate - he made that even clear enough that it sunk in for me. So for his given boundary conditions the explaination was quite enlightnening for me.

Jim

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Reply to
jim rozen

Yeah, I'd picked up on his same speed and same current (torque) parameters. What I objected to was the statement that *efficiency* as a generator was only 64%. That was using inputs and outputs incorrectly, ie input as a motor and output as a generator. You can't do that when calculating efficiency. It is always power out divided by power in *at the same instant*. To use figures from two different modes of operation doesn't give efficiency.

Note also that it isn't mandatory to follow his limits of same RPM or same current either. Those were just his arbitrary choices. Increasing either speed (voltage) or torque (current) will allow greater input power, and greater output power, up to the thermal limits of the motor/generator.

Gary

Reply to
Gary Coffman

Gary sez:

" Note also that it isn't mandatory to follow his limits of same RPM

I thought Jim Pentagrid covered that very well myself. His choices were made in accordance with preserving the parameters addressed in the original question. As Jim Rozen stated, boundary conditions were specified at the outset.

Bob Swinney

Reply to
Bob Swinney

Reading carefully is definitely helpful! Confusion arises from the "same speed" assumption. I agree to the same-current constraint but the speedlimit should be no-load speed which the motor presumably can handle without damage. If the motor operated as a generator at no-load speed then generated EMF is rated terminal voltage, terminal voltage is that minus IR drop or 80%. We now have same current hence same losses and same torque, same power in and power out hence same efficiency, terminal voltage is 80% of nameplate rating when run as a generator.

Both Gary and Jim allude to thermal limit hence max current. I concur. They both also noted that spinning the motor faster as a genny makes more volts (more power in, more power out) while loss is proportional only to current. In this case it would actually be

*more* efficient, the situation being analogous to operating a motor at 20% above rated voltage with same torque and current. In Jim's example, if it were spun 20% faster than no-load speed then it would produce rated terminal voltage at rated current. I doubt if 20% overspeed would present a problem other than reducing brush and bearing life a bit.
Reply to
Don Foreman

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