loquin
Well-Known Member
I've been thingking about using a permanent magnet (PM) DC motor as a generator for some time now. After a lot of poking around in the DC motors section of Burden Surplus Sales, I finally settled on this motor, and ordered one.
What was the reasoning behind choosing this motor?
I received the motor, and got around to testing it yesterday. For starters, I checked the unloaded voltage output from the generator; spinning it at various speeds with the drill press, ranging from 620 RPM up through 3100 RPM. Since the drill press tops out at 3100 RPM, I extrapolated the output voltage at 3600 and 4200 RPM, assuming a linear voltage to RPM output.
After this, at the highest RPM settings, I added a 10 Ohm 10 Watt power resistor as a load, then checked the voltage again. As expected, this caused the measured voltage to drop significantly, down to 16.5 volts. As a working approximation, electrically, a generator can be considered to be a perfect generator, with it's output feeding through an internal, series resistor. This internal 'resistor' is the winding resistance. By measuring the voltage drop with a known load, you can calculate the value of the internal resistance of the generator. In the case of the above generator, the internal resistance was calculated to be about 2.4 ohms.
After a minute or so of spinning the generator, the 10 watt resistor started smoking, so, I hit the power switch on the drill press. Then, after the resistor cooled down, I took a second 10 watt/10 ohm resistor, added it in parallel to the first, and powered up the drill press again. The generator purred along, the resistors got quite hot, but weren't smoking Measuring the voltage (14.2V) and recalculating the internal resistance (2.22 ohm) agreed with the first calculation within 7.5 percent. After 10-15 minutes, I turned off the drill press. The generator wasn't even warm! At 3100 RPM, it's internal resistance appears to self-limit it to a safe operating current.
At 3100 RPM, the generator was pumping out about 2.5 amps into the 5 ohm load, or about 31.25 watts. At a full 4200 rated RPM, I would estimate getting nearly 70 watts (3.7A at about 18.5V) out of this generator into 5 ohm load. Further testing, (at 4200 RPM,) will be needed to verify this, as well as to verify that the generator will not overheat at this output. In a 12 volt system, it's possible you could pull up to 6 amps at 14.4 volts (86 watts) from the generator, again, assuming no overheating. However, this also means that you would be generating over 80 watts in heat, inside the generator windings... I just can't believe that this would be good for longevity...)
Hopefully, this information will help others who are thinking about using this motor as a generator.
What was the reasoning behind choosing this motor?
- From reviewing the specs, I estimated that the maximum available power would be in the 30-50 watt range. Contrary to some beliefs, I think that, using efficient, high output LED lamps, 30-50 watts would be more than enough power to
- provide lighting that is more than sufficient for excellent visibility at speeds of 30-35 MPH, under almost any condition (hey - we're NOT traveling at freeway speeds - there's no need for auto intensity lighting at one-half the speed)
- provide excellent visibility to others, with bright LED head lights, tail lights and turn signals, and
- have some power left over for other applications.
- This motor is reversible, so there's no worry re the motor rotation
- The motor is rated for continuous duty - an important consideration, as it could be running for hours at a time.
- At 4200 RPM, it's a decent match for the engines we're using.
- While not waterproof, the motor is completely enclosed, so, I should only have to provide a shroud to keep water from directly striking it.
- It is a face-mount design, making the generator easier to mount.
- At 5/16th inch, the shaft is big enough to readily find bearings to support the end, and sprockets to transfer the torque from the engine. Since the motor was designed for use in a blower or pump application, it probably has bushings rather than bearings and therefore won't support much side-loading on the shaft. For this reason, you would undoubtedly want to add a bearing to the shaft before adding a sprocket or pulley, with their potentially high side thrust load.
I received the motor, and got around to testing it yesterday. For starters, I checked the unloaded voltage output from the generator; spinning it at various speeds with the drill press, ranging from 620 RPM up through 3100 RPM. Since the drill press tops out at 3100 RPM, I extrapolated the output voltage at 3600 and 4200 RPM, assuming a linear voltage to RPM output.
Code:
[FONT="Courier New"][B]RPM Voltage[/B]
620 4.4
1100 7.6
1720 11.7
2340 16.0
3100 20.5
3600* 24.4
4200* 27.6
* calculated voltage at RPM[/FONT]
After this, at the highest RPM settings, I added a 10 Ohm 10 Watt power resistor as a load, then checked the voltage again. As expected, this caused the measured voltage to drop significantly, down to 16.5 volts. As a working approximation, electrically, a generator can be considered to be a perfect generator, with it's output feeding through an internal, series resistor. This internal 'resistor' is the winding resistance. By measuring the voltage drop with a known load, you can calculate the value of the internal resistance of the generator. In the case of the above generator, the internal resistance was calculated to be about 2.4 ohms.
After a minute or so of spinning the generator, the 10 watt resistor started smoking, so, I hit the power switch on the drill press. Then, after the resistor cooled down, I took a second 10 watt/10 ohm resistor, added it in parallel to the first, and powered up the drill press again. The generator purred along, the resistors got quite hot, but weren't smoking Measuring the voltage (14.2V) and recalculating the internal resistance (2.22 ohm) agreed with the first calculation within 7.5 percent. After 10-15 minutes, I turned off the drill press. The generator wasn't even warm! At 3100 RPM, it's internal resistance appears to self-limit it to a safe operating current.
At 3100 RPM, the generator was pumping out about 2.5 amps into the 5 ohm load, or about 31.25 watts. At a full 4200 rated RPM, I would estimate getting nearly 70 watts (3.7A at about 18.5V) out of this generator into 5 ohm load. Further testing, (at 4200 RPM,) will be needed to verify this, as well as to verify that the generator will not overheat at this output. In a 12 volt system, it's possible you could pull up to 6 amps at 14.4 volts (86 watts) from the generator, again, assuming no overheating. However, this also means that you would be generating over 80 watts in heat, inside the generator windings... I just can't believe that this would be good for longevity...)
Hopefully, this information will help others who are thinking about using this motor as a generator.
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