Another sparkies question about generators

Seems we are having basic math problems again. Look at it this way (which is presumably what Jim Pentagrid meant). You have a power rating for a motor. The question was, how much of the *motor's power rating* is available if it is run as a generator?

Assume the motor is 80% efficient and that it is exactly as capable of being a generator as it was a motor; i.e., also 80%. Take the input power rating for the motor X 80%. This is the input power rating of that motor ran as a generator. Incidentally that amount of power is input as mechanical motion. Now, the motor-as-generator, is 80% efficient. The mechanical input power is converted to electrical power at 80%. That would be 80 X 80 or aprox.

64% of the original input power rating for the motor.

Bob Swinney

How are you combining the outputs of the pairs? Since each pair produces a different phase, you can't just combine them across a resistive load. You'll have phase cancellation if you try that. But if you diode rectify each phase, you can then combine them.

For maximum output, you need to use a bridge rectifier for each phase (each winding), then combine all the DC + and all the DC - leads from all the bridges to form the net output to feed to the resistive load. (If you're sure you know the winding senses, you could star connect the windings and use individual rectifiers, but the bridge is insensitive to phasing sense, and produces 1.41 times as much output voltage too.)

Alternatively, if you don't want to rectify to DC, you can use a separate resistive load for each pair. Each load only gets a fraction of the total output, but at least you won't have phase cancellation. This would require

4 bulbs for your lighting application, but that's not necessarily bad. It gives you redundancy.

Note that saturation isn't what would limit current at high RPM (you can't saturate a PM alternator). What could is the increase in reactance of each winding as the frequency (RPM) is increased. Inductive reactance increases proportionate to frequency. This reactance is effectively in series with the load, and will limit the maximum current the load can draw. But that shouldn't be an issue for any reasonable RPM.

Note that conventional auto alternators are 6 phase (2 more than you have), and they work up to 10,000 RPM. So I don't think that the number of poles is an issue.

Gary

No. You can't count the efficiency twice. The motor is 80% efficient, so it can convert 80% of the electrical power fed to it to mechanical motion. Conversely, when used as a generator, it can convert 80% of the mechanical power fed to it to electrical power.

In both cases, the missing 20% goes to heating the armature resistance (ignoring bearing and windage losses). You can't *count that twice* because it can't be used as a motor and a generator *simultaneously*. It is used as one or the other, and in each case, efficiency is 80%, *not* 64%.

Lets run some numbers. Suppose we have a motor rated at 1 hp electrical (746 watts) input. If it is 80% efficient, then it will produce 0.8 hp (596.8 watts) on its output shaft. Now if we decide to use it as a generator, we feed it 1 hp (746 watts) mechanical on its *input* shaft, and then we can extract 0.8 hp (596.8 watts) electrical on its *output* terminals. All we've done is show that, as an electrical machine, the motor is reversible. Efficiency is the same either way.

Now I'll grant that if you were silly enough to only feed the generator with 0.8 hp on its input shaft, you'd only get 477.4 watts out of it. But that doesn't mean its efficiency has been reduced to 64%. You just arbitrarily decided not to feed in enough power to get 596.8 watts out at the generator's 80% conversion efficiency.

Gary

Bob Swinney wrote: ...

no no no, this is the _output_ power rating used as a generator!!

So the input to get the rated output would be 100/80 = 125% of rating. A 20% loss in the windings gives 100% of rating as output.

Bob

Nope. The first figure arrived at was the power input rating of the motor. Other calculations are based on this.

Bob Swinneuy

Well, I wanted to use a stepper but couldn't get one to put out enough juice. But, a drill motor from HF draws 11 amps when loaded. It spins a gearbox with a 36 t0 1 ratio with 500 rpm out. So the motor spins at

18,000 rpm. Spinning this same motor at 10,000 rpm gives 9 volts @ 1.5 amps for 13.5 watts. Since I was looking for 12 watts that will be O.K.. If LEDs are used with a six volt battery,( the one from the drill), then that should be plenty of light. I need to figure out how to mount the motor with a belt drive to get the proper rpm and seal the motor so it lasts more than a week exposed to salt air. ERS

I have a string trimmer permanent magnet motor in my junk waiting for a possibel project. You can have it if you want it. I will dig it out and let you know what I can. It is several times larger than a bike generator and could be what you want.

Dan

I did not recall correctly. The string trimmer motor is a series wound motor and does not work well as a generator.

Dan

String trimmer motors make pretty decent cieling fans powered off a PV solar panel.

Every brushless motor I've worked with has had a PM armature--no excitation required. Else why would it be called brushless?

Though I've never tried it, I'd expect one would make a very good 3-phase generator. They certainly do a good job of pumping up the bus voltage on the servo amp when delivering negative torque to an overhauling load.

I have used a brushless motor as a synchronous motor (i.e., no commutation) from the output of a VFD.

Ned Simmons

Just wanted to be sure you understood the wiring of a polyphase device. If you get out of phase windings in parallel, you get low output and lots of heat.

Separate issues. No field excitation is required because it's PM. It's brushless because electronics accomplish commutation, but electronics don't do anything without voltage to work with.

I agree, but it may be necessary to substitute diodes for the internal elex.

For 12 watts a battery would be a lot simpler if perhaps not nearly as much fun.

Not as much fun and not guaranteed to have power for the lights as long as the motor is running. Besides, I don't want a big battery pack. Now, don't tell me about reliability and how more things to supply power just means more thing to go wrong :). ERS

How about a small 12V battery, say 8 to 12 Ah, and two generators or alternators, one belt driven off the auxiliary, if it doesn't already have one, and one on the mast wind driven.

Ted

I would much rather help you succeed than be a failure forecaster!

Have you thought about if you'll need voltage regulation, and if so how you'll accomplish that?

You may want a motor with rated voltage higher than 12 volts. Most outboards max out at about 5000 to 5500 RPM but you may want lights at less than wide-open throttle. Small DCPM motors typically spin pretty fast at rated voltage so they'd need to spin similarly fast to generate rated voltage. However, a higher-voltage motor will produce more volts per RPM because they have more turns on the windings. 90 volts is a common DCPM voltage, I don't think a 1-amp 90VDC motor would be very big and it would make 12 volts at fairly low RPM.

I'm going to Ax-Man today to see what they have in motors.

I thought that's what I said?

I assume you're talking about substituting diodes for the output stage of the amp here. Obviously if you're using the motor as a generator, there's no need to actively commutate the motor windings, but you do need to rectify the motor's output if you want DC.

Ned Simmons

The motor I tested that comes closest to what I want is 2.25 long and

1.5 dia.. At 10,000 rpm it was putting out 9 volts @ 1.5 amps. It was $10.00 at H.F.. It came complete with a drill chuck, batteries, charger and a drill motor case wrapped around it. For 8 bucks they have one with a keyed chuck. If two were connected in series then spun to 15,000 rpm max then with a cheap regulator the batteries could be charged and the lights lit. Now, if a similar sized motor came along with a higher voltage output so only one would be needed then that would be great. I'm curious to see what Ax-Man has. ERS

Excellent description. It convinced me.

Jim

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I only meant that not requiring field excitation is not why it's called brushless. Some PM motors do have brushes.

If the brushless motor has internal commutation electronics they may need to be circumvented with diodes. Maybe some use external amps?