Formula for calculating power output of stepper used as generator?
Formula for calculating power output of stepper used as generator?
(OP)
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.
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.





RE: Formula for calculating power output of stepper used as generator?
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.
RE: Formula for calculating power output of stepper used as generator?
They do indeed have permanant magnets.
You must be thinking of another type of motor?
RE: Formula for calculating power output of stepper used as generator?
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!
RE: Formula for calculating power output of stepper used as generator?
RE: Formula for calculating power output of stepper used as generator?
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.
RE: Formula for calculating power output of stepper used as generator?
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.
RE: Formula for calculating power output of stepper used as generator?
Mark
RE: Formula for calculating power output of stepper used as generator?
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.
RE: Formula for calculating power output of stepper used as generator?
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?
RE: Formula for calculating power output of stepper used as generator?
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.
RE: Formula for calculating power output of stepper used as generator?
Very much appreciated.
RE: Formula for calculating power output of stepper used as generator?
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.
RE: Formula for calculating power output of stepper used as generator?
RE: Formula for calculating power output of stepper used as generator?
RE: Formula for calculating power output of stepper used as generator?
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.
RE: Formula for calculating power output of stepper used as generator?
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.
RE: Formula for calculating power output of stepper used as generator?
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.
RE: Formula for calculating power output of stepper used as generator?
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.
RE: Formula for calculating power output of stepper used as generator?
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.
RE: Formula for calculating power output of stepper used as generator?
Lots of them on ebay, sorry to get this thread off topic but...... What is a good name or two that I should look for, (any tips or hints at what type to get?) I am sure my needs will be very basic in nature, I have wanted one for awhile I'd like to get a decent one.
Thanks
RE: Formula for calculating power output of stepper used as generator?
1) Tektronix.
2) Tektronix.
3) Tektronix.
Now there ARE other good scopes out there but, generally they target specialties.
The type (technology) of scope is important too.
First a point about the number of channels.. One channel is okay but two channels let you observe a control signal and the resulting response of the controlled device/system. This is what a vast majority of what people do with scopes. In my opinion you want two channels.
Now for the technology.
The standard scope (least expensive) will be your standard sweep oscilloscope. BUT! They are useful only for studying PERIODIC events. This means a repeating signal that can continually refresh the screen so you can actually see the signal. You can, if you are good with a scope, setup see non repetitive events with this type of scope. It must have sublime triggering control so you can force a single trigger when you want it then turn up the scope brightness to damaging levels and have one image flash past and stick to your eyeballs.
A more useful scope is the digital scope. Instead of just funneling a signal into a sweeping screen painter the signal is digitized and stuck into digital memory. It is fished out of this memory and displayed onto a screen (an LCD).
The LARGE benefit of this method is that very slow or non repetitive signals can be caught and stuck on the screen for your perusal. Often stored and printed out too.
Down side they cost 2-3 times more. Personally, to me, use-wise they are 5-6 times better. They are easier to get what you need out of, faster.
The last thing to mention that a lot of people miss is the scopes front end. A standard scope of any type needs the probe and the a return signal path. Normally this signal return is the scopes chassis ground->back to the wall outlet. If you are digging around in a computer, the whole thing is likely referenced to a ground. Just hook the scope ground to the computers DC supply ground and you are good to go. However if you want to look at un referenced signals you have to be very careful.... If you want to look at any power system on the line voltage side you need to consider EXACTLY where you put the scope ground lead because it is the GROUND. If you hook it to the wrong place, your scope's ground system will likely vaporize including the ground clip that you are foolishly using in your fingers as you clip it onto the wrong spot of your circuit.
There are some ways around this. Good scopes like those mentioned in the list above recognize that this situation can come up. The classic (but dangerous) method used by the canny investigator is to isolate the ground prong of the scope. This means the entire scope becomes live as it floats on the line voltage point you hooked the scope's ground to. This is why the scope maker will specify how much isolation is provided between the inside electronics and the scope's metal face plate. There are several alternatives both are much safer. First, you can buy (relatively) expensive isolated probes. The other is to buy a scope with isolated inputs. I have owned a dozen scopes. I own three at present. I always turn to my isolated scope! It is wonderful to be able to ask, "am I below the isolation limit voltage?". If the answer is yes, then I can clip on anywhere. The second channel can then be clipped on anywhere else because both channels are totally isolated from each other and the scope chassis.
A classic example would be looking at a Solid State Relay. One side is run by a 5V computer signal the other side is controlling a 480V heater. You can look at both sides of this SSR with an isolated scope at the same time, watching cause and effect.
Down side? They cost, yet again, more.
Given this information you need to consider the type circuits you are thinking about studying. If you will be messing about with line voltage circuits then you should consider an isolated scope from the get-go.
If you have other specialized interests they should, of course, be addressed too. For instance if you are going to be doin three phase power studies there are scopes that will do ALL the power measurements with push button ease. These scopes can get really pricey too.
One scope I highly recommend is Tektronix THS700 Series.http:
If you are just working on the above product it sounds like you could probably pick a place on the output of your stepper generator, and ground the system. (No isolation needed?) If you are going to be looking at signals above 40Hz repetition rates then a digital scope isn't absolutely required. If you are only interested in the output (the input being rotation), then a single channel will probably suffice.
Just get one of the types listed above and you will do okay.
RE: Formula for calculating power output of stepper used as generator?
Just what I needed to know.