Formula for calculating power output of stepper used as generator?

[bigmark1972]

I hope someone here can help, this is an odd question but here goes.

I would like to use a stepper unipolor or bipolor whatever would be better suited to generate ac power and then rectify it to DC.

My question is this, just for a number assume I am looking for 24vdc@ 2 amps how would I go about determining what stepper to use and at what RPM it would have to turn?

There are so many factors, steps per revolution, motor voltage, and I know I am missing a few other factors.

If anyone here knows of a formula or method to get the solution could someone please share it?

For anyone who can help thanks.

 

[Warpspeed]

Stepper motors have no permanent magnets or field excitation, they therefore cannot be used in reverse as an alternator to generate electrical power.

In order to generate useful electrical power from rotation, there first must be a fairly strong magnetic field inside that has to come from somewhere.

 

[bigmark1972]

I'm going to have to disagree with you. I have a stepper at work that will generate 600vac @ 2000 RPM 24v 1.8deg motor.
They do indeed have permanant magnets.
You must be thinking of another type of motor?

 

[itsmoked]

I must also disagree as most of the stepppers I have ever picked up turn nicely until you short two leads together then they get HARD to turn.

Personally bigmark, why don't you just hook one to a motor put it to a full wave rectifier, load it with a reasonable load resistor and then run it. Run it for a long while so it reaches steady state temp. Monitor its temp with a noncontact temp gun. Tell us what happens!

 

[bigmark1972]

Good idea smoked, I might tyt that. Mathematically, however, there has to be a formula to figure this out agree?

 

[Skogsgurra]

Yes. There is. Or rather, the basic physical laws apply and a formula can surely be applied. You have to have at least these numbers ready to input:

Magnetic flux vs angle.
Magnetic reluctance in the motor.
Winding number of turns, resistance, stray inductance.
And probably a lot more.

I do not think that this is a practical way to calculate possible output power. I would instead reason like this: The motor can output a certain torque at a certain speed. Torque times speed = power (in SI units). The efficiency is something between 50 and 80 percent (probably) and since efficieny works both ways, it is very likely that the available electric output power is 50 - 80 percent of shaft input power.

This is just to see if the motor is big enough to be used. I would do as itsmoked says before feeling sure about it.

 

[UKpete]

bigmark, I think skogs has supplied your answer.

To clear up any confusion between PM and reluctance types see:

http://www.cs.uiowa.edu/~jones/step/types.html

- to generate, the permanent magnet types (e.g. the hybrid stepper) will be needed.

 

[bigmark1972]

Yes that was an excellent idea, I will use both methods and see how it goes. Thanks for the help everyone.
Mark

 

[itsmoked]

Nice solution angle skogs.

UKpete that was an excellant link! I added it to my favorites. Splains Warpspeed's experience, he must hang around variable reluctance steppers!

And thinking about it bigmark, you'll get a better true efficiency number if you leave out the rectifiers from my suggestion. You will need to use a true RMS meter and take into account the phases.

 

[bigmark1972]

Torque times speed = power (in SI units)

Would speed be as in RPM or as pulses per second? I can calculate this after finding the step value of the stepper.
I would assume RPM but I'd like to know for sure. I'm not to familiar with SI units how can I convert that to watts or something more recognizable?

 

[Skogsgurra]

Rotational speed is counted in rad/second. There are 2*PI rads to one revolution. 1 RPM = 1/60 RPS and hence 2*PI/60 rad/sec. Example: 1800 RPM -> 6.28*1800/60 = 188 rad/second. A convenient rule-of-thump is that you get rad/sec if you divide RPM by ten.

Torque is always in Nm (newton-metre). One kgf-metre = 9.81 Nm.

Multiply rad/sec with Nm and get joules/second, which is identical to watts.

If you work in SI all the time, you do not need to think about a lot of conversion factors.

 

[bigmark1972]

Great info thanks!

Very much appreciated.

 

[bigmark1972]

skogs,
you seem to be a very knowledgable person and I appreciate your input. I have figured out a few generator combinations. I have one more question if you have the time.
How can I figure out the volts that will be supplied?
Here is a sample table from a motor I am considering.

A little more info:

PartNumber 42M048C2B-N
DC Operating Voltage 12
Res Per Winding (Ohms) 52.4
Ind per Winding mH 85.7
Holding Torque mN*m/oz-in 84.0/11.9
Rotor Moment of Inertia (g*m2) 12.5E-4
Detent Torque mN*m/oz-in 12.7/1.80
Step Angle 7.5°
Step Angle Tolerance ± .5°
Steps per Rev 48
Max Operating Temp 100°C
Ambient Temp Range Operating -20°C to 70°C
Ambient Temp Range Storage -40°C to 85°C
Bearing Type Sintered Bronze sleeve
Insulation Res at 500Vdc 100 megohms
Dielectric Withstanding Voltage 650 ± 50 VRMS 60 Hz for 1 to 2 seconds
Weight g/oz 145 g/5.1 oz
Lead Wires 26 AWG, UL Style 1430

I have a small sanyo stepper that's 1.8deg 24v stepper and I can generate 600vac at 1000RPM.

 

[Skogsgurra]

Wow! How did you get that graph into the posting? I have wanted to do that for as long as I can remember. I will get back with some answers (hopefully) later.

 

[itsmoked]

I was wondering that too!

 

[itsmoked]

That voltage question.. I really don't see how that can be answered without detailed flux information. Especially in the realm of strange rotor and stranger yet stator configurations.

I think yer still going to be faced with empirical determination methods.

Also just because a scope shows whopping large inductive spikes the actual loaded doing work numbers are going to be much lower. <50V I would imagine/guess.

 

[cswilson]

One tangential point here: If you can use it as a motor, you can use it as a generator. This includes reluctance motors. Anytime you decelerate a reluctance motor faster than losses would decelerate it, you are (re)generating. You can also generate on an ongoing basis.

Now, the waveforms you get from generating a reluctance motor are unipolar and not remotely sinusoidal, so they are seldom used for standard generator actions. And unlike a PM motor, you can't just spin them up without any control But with a controller commanding torque in the opposite direction of motion, you will cause the power stage to generate, and get waveforms that would be quite easy to convert to reasonable DC.

 

[bigmark1972]

As far as the pic, it's pretty easy to do, pretty simple HTML just grab a cheat sheet somewhere on the web.

I'm going to rectify the voltage into DC so spikes won't be a problem. I have not load tested it yet for actual amps but I will get to that soon.

I wonder if I could use the resistance in the coils into the formula to derive the voltage with ohm's law, if so what stepping mode would turning a stepper result in?

Actually now that I think of it I should get the same (or close to) result with a much smaller 24vdc 1.8 deg stepper just with less current. I was hoping to charge a cap to 300VDC and need less than .7amps at that voltage.

I need to load test that thing and see if I am just seeing spikes or what's going on. I wish I had a scope, but I'm too much of a newbie at this stuff to justify one just yet.

 

[itsmoked]

Yer probably not going to get anywhere near 300V Probably more like 24V at any current. Transforming that to 300V with umpteen phases would probably be impractacle too. But perhaps you can run a DC-DC for step up.

You understand that .7A x 300V = 210 watts..... This is non-trivial. Car alternators with maybe 2 HP jammed into them only output about 900 watts.

Scopes... Find one at an auction. ebay. Or a DOVEBID measurement auction(every few weeks) Or get a PC scope that runs USB to any computer. They're only 200 bucks or so and are smalllll.

 

[bigmark1972]

You understand that .7A x 300V = 210 watts..... This is non-trivial. Car alternators with maybe 2 HP jammed into them only output about 900 watts.

Gotcha, I wanted the higher voltage so I did not have to use a cap the size of a lunch box. This is part of a prototype product I hope to market one day

Thanks for the scoop on the scope.