# Stepper motor Alternator

### Help Support Home Model Engine Machinist Forum:

#### Toymaker

##### Well-Known Member
HMEM Supporting Member
<snip>
#2: Voltage is proportional to rate of change of magnetic field: I.E. How fast you change from N to S on any pole.
<snip>

It's not how fast poles change from N to S, but rather how fast the lines of magnetic flux pass through the conductor (ie the wires) that determines voltage output.

#### Toymaker

##### Well-Known Member
HMEM Supporting Member
Adding in a bit - - - see the following
Why isn’t there a standard electricity frequency around the world?

Things are a bit more complicated than presented.
The big hydro-electric resource - - - Itapu - - well - - - most of the production is 50 Hz but some is 60 Hz - - - turns out that Sao Paulo runs at 60 Hz but most of the rest of Brazil runs at 50 Hz - - - how's that for 'interesting'?!?

All this goofiness because when things were being developed - - - there was no agreement and someone made more money by not going with changes suggested when moving from DC to AC. (Argh!!!!!!!!!!!!!)

At least the aircraft industry got it right: 400 Hz is the standard for all airplanes regardless what country makes them.

#### Toymaker

##### Well-Known Member
HMEM Supporting Member
I finally got some time to do some tests at different rpms and under load.

1250 rpm
85 volts
943 cycles per second

1450 rpm
92 volts
1080 cycles per second

1700 rpms
103 volts
1240 cycles per second

1850 rpms
115 volts
1330 cycles per second

All readings are approximate but what they do show is the volts and cycles follow the rpms. I don't have any readings higher than 1850 rpms but I would expect the output would increase in proportion to the rpms.
If the stepper motor was geared up I wonder how high the numbers would get. The load on the motor was not that high.

Food for thought

Ray

Green Twin is right when he noted that you'll start running into problems as you get into higher frequencies. The laminated iron cores used in most motors, including steppers, when used as alternators, will become less efficient as the frequency increases. This is mostly due to eddy currents within the iron laminations. Higher frequency typically requires the use of powdered iron core or ferrite materials.

#### Steamchick

##### Well-Known Member
HMEM Supporting Member
AC Power is a lot more complex than the simple description I gave, but many mechanical folk shy away from "AC" as being "too complex" for their needs or understanding. - in my long experience. The demonstrated experience of voltage and frequency varying linearly with speed is one simple aspect that I tried to explain. But the "Whole power" is subject to "Volts x Amps" - so you have to at least add a resistive load and measure Current as well as voltage to tell the "Whole story" of what happens, and whether it is worth pursuing as a power generator. Only when you add a load will the generator apply a proper load to the motor.
K2

#### Rdean33422

##### Well-Known Member
HMEM Supporting Member
In my experiments the load was a 7 watt led light bulb so how does that compare to work done? Is this power output capable of charging a cell phone battery using a wall wart?
How would the high frequency effect the wall wart output?

Ray

#### Toymaker

##### Well-Known Member
HMEM Supporting Member
In my experiments the load was a 7 watt led light bulb so how does that compare to work done? Is this power output capable of charging a cell phone battery using a wall wart?
How would the high frequency effect the wall wart output?

Ray
Just from observation, it appears in your video that the light was glowing at a normal brightness, so if it was, your stepper alternator is making at least 7 watts of power; it could be capable of outputting more watts, but you haven't given us enough data to know.

IF 7 watts is the max output from your stepper, it would take a very long time to charge a cell phone, and the plug-in phone charger likely will not work very well at the high frequency your stepper is providing.

If you have some old incandescent light bulbs, the type with filaments inside that glow, not LED or Fluorescent, try using those instead of your 7 watt LED bulb; something around 25 watt to 100 watt. Before you plug the light into your stepper-alternator, use your meter to measure the bulb's resistance in Ohms,...should be a few hundred Ohms.

Run the test as you did with the LED light and measure the voltage. The current (in Amps) you're stepper is making will be: Amps = Volts/Ohms. And the power you're making will be Watts = Amps x Volts.

Steppers are a fun way to make a little power when used as an alternator,....but if you want to make real usable power, replace the stepper with a PM (Permanent Magnet) motor. Most any motor from a cordless power tool will work nicely, or you can buy one from eBay.

Last edited:

#### Gary C

##### Member
HMEM Supporting Member
Just to mention that filament lamps have low resistance when cold. They have a resistance ration of between 13 -15:1 between cold and hot. All filament lamps have a resistance slope including car lamps. It's the inrush current that causes a weak cold filament to fail.

#### Steamchick

##### Well-Known Member
HMEM Supporting Member
From the number of poles, a stepper motor should be a useful demonstration generator for an odd LED or three, when run by a slow running steam or hit and miss IC engine. PM Hand tool (drill) motors usually run at 10s. of thousands rpm to develop their power: I.E. Relatively small voltages from the DC battery but relatively high current: a 240W motor at 24V is drawing 10A on load. If run as a generator at less than 1000rpm you may only get 1 or 2 volts, - and similarly only tens of milliamps as a result. - The net power output being only a couple of watts. Gearing to get the generator speed you want can sap all the engine's power.
Conversely, a stepper motor, with 10 or more times the poles of a "power" motor, will generate the voltages at low revs but with very small current (amps) - as demonstrated by Ray - thus generating a few watts but at (possibly?) more useable voltages? I suspect these devices vary so much for different applications, makes, and sizes there is no simple rule to getting a stepper motor to match your application except by trial and tribulation!
I would be keen to use a diode bridge to convert the stepper motor output to DC to eliminate the high frequency from ruining any downstream (semi-conductor) devices, other than resistance wire bulbs.
Personally, I look to bicycle generators for my small steam engines, as they are designed to generate the single figure voltages and currents for torch bulbs at speeds suitable for the low speeds and low power of my engines, with small enough belt drives to look OK. They can be hidden inside "generator shaped" housings as well!
For some I use bridge diodes to rectify the variable frequency AC, then a Buck controller to get the controlled DC output for the LEDs that I have. This configuration may suit Ray - with parts such as these but that you select to match your input voltage and output voltages?
e,g.
W04M 400V Bridge Rectifier Diode 1.5A
6-24V Input to 5V 2A Output USB Charger Module DC-DC Buck Step-Down Converter

Thanks to all contributors here as I have learned a lot about stepper motors, from a zero baseline of knowledge!
K2

#### Toymaker

##### Well-Known Member
HMEM Supporting Member
Just to mention that filament lamps have low resistance when cold. They have a resistance ration of between 13 -15:1 between cold and hot. All filament lamps have a resistance slope including car lamps. It's the inrush current that causes a weak cold filament to fail.

Yep, ideally Rdean should use a wire wound resistor rated at something above 25 watts,...100 watts would be better. I have those laying around my shop, but not many people do; it's far more likely Rdean has some old light bulbs laying around the house. As long as the bulb's filament doesn't start to glow during his testing, the bulb's resistance shouldn't change too much; I'll be quite surprised if his stepper alternator manages to cause a 25 Watt bulb to glow.

#### Rdean33422

##### Well-Known Member
HMEM Supporting Member
Thank you all for the insight and knowledge. I will give the regular light bulb a try.

Ray

#### Toymaker

##### Well-Known Member
HMEM Supporting Member
From the number of poles, a stepper motor should be a useful demonstration generator for an odd LED or three, when run by a slow running steam or hit and miss IC engine. PM Hand tool (drill) motors usually run at 10s. of thousands rpm to develop their power: I.E. Relatively small voltages from the DC battery but relatively high current: a 240W motor at 24V is drawing 10A on load. If run as a generator at less than 1000rpm you may only get 1 or 2 volts, - and similarly only tens of milliamps as a result. - The net power output being only a couple of watts. Gearing to get the generator speed you want can sap all the engine's power.
Conversely, a stepper motor, with 10 or more times the poles of a "power" motor, will generate the voltages at low revs but with very small current (amps) - as demonstrated by Ray - thus generating a few watts but at (possibly?) more useable voltages? I suspect these devices vary so much for different applications, makes, and sizes there is no simple rule to getting a stepper motor to match your application except by trial and tribulation!
I would be keen to use a diode bridge to convert the stepper motor output to DC to eliminate the high frequency from ruining any downstream (semi-conductor) devices, other than resistance wire bulbs.
Personally, I look to bicycle generators for my small steam engines, as they are designed to generate the single figure voltages and currents for torch bulbs at speeds suitable for the low speeds and low power of my engines, with small enough belt drives to look OK. They can be hidden inside "generator shaped" housings as well!
For some I use bridge diodes to rectify the variable frequency AC, then a Buck controller to get the controlled DC output for the LEDs that I have. This configuration may suit Ray - with parts such as these but that you select to match your input voltage and output voltages?
e,g.
W04M 400V Bridge Rectifier Diode 1.5A
6-24V Input to 5V 2A Output USB Charger Module DC-DC Buck Step-Down Converter

Thanks to all contributors here as I have learned a lot about stepper motors, from a zero baseline of knowledge!
K2

Here's a picture of a quick test I ran a few minutes ago: I'm spinning a small PM motor I took out of an old cordless drill; it's fairly small, about the same size as the chuck on my drill. I measured the rpm at 1,012 before I took this pic. The brown cylinder shaped object in the pic is a 15 Ohm resistor which the output of the motor is connected to via the red and white wires. The meter is showing 3.66 Volts DC. Using Ohms Law, Amps= V/R = 3.66 V/15 Ohms = 0.244 Amps or 244 milliamps. And since amps times volts = watts, this tiny motor running at just 1000 rpm is producing 0.89 watts, or very close to 1 watt. At 2000 rpm I would expect about 7 VDC at close to 1/2 amp, which should be fine for charging a cell phone with no need for wall chargers. At 4000 rpm I would expect to see a bit over 14 VDC which you could plug a car phone charger into .

Again, this is a tiny motor; measuring 1.5" diameter and 2" long.

Last edited:

#### Rdean33422

##### Well-Known Member
HMEM Supporting Member
I was able to get short a quick test done.
I measured the resistance of the light bulb cold and it was 45 ohms.
The motor was running at 2000 rpms with the load connected.
The voltage was 70.

Amps = Volts/Resistance
1.55 = 70/45

Watts = Amps * Volts
108.5 = 1.55 * 70

Did I figure this right?

The bulb was less than half bright and if I could have upped the rpms higher it would have been brighter.
You see I did not have the engine secured to the bench so I was chasing it around while I was trying to get the readings. I will be better prepared next time.

Ray

#### Toymaker

##### Well-Known Member
HMEM Supporting Member
I was able to get short a quick test done.
I measured the resistance of the light bulb cold and it was 45 ohms.
The motor was running at 2000 rpms with the load connected.
The voltage was 70.

Amps = Volts/Resistance
1.55 = 70/45

Watts = Amps * Volts
108.5 = 1.55 * 70

Did I figure this right?

The bulb was less than half bright and if I could have upped the rpms higher it would have been brighter.
You see I did not have the engine secured to the bench so I was chasing it around while I was trying to get the readings. I will be better prepared next time.

Ray

Good Job!! Your math is flawless, proving your stepper is producing far more power than I had expected, and much more than you will need to charge a cell phone. As a bonus, since the bulb had begun to glow, you know that the resistance had dropped below 45 ohms, meaning your steam powered stepper is actually making more than 108.5 watts. If you want to get a better measurement of how much power your stepper is producing, try using a larger bulb,...one that doesn't glow at all or even get warm when its powered with your stepper. You could also try a two slice toaster; they're typically rated at 800 to 1200 watts, which shouldn't even get warm using your stepper alternator.

EDIT NOTE: My above statement that resistance drops as filament temperature increases is incorrect. Increasing Temperature of wire resistors will cause the resistance to also increase. Meaning your stepper is most likely making less than 108.5 watts.

Last edited:

#### Rdean33422

##### Well-Known Member
HMEM Supporting Member
It is a gas powered twin cylinder engine but it just doesn't seem right.
I need to do some more tests maybe on a different stepper motor.

Ray

#### Toymaker

##### Well-Known Member
HMEM Supporting Member
It is a gas powered twin cylinder engine but it just doesn't seem right.
I need to do some more tests maybe on a different stepper motor.

Ray

If you happen to have one, or don't mind buying one, the motors used on drones, called "outrunners" make excellent alternators. They come in lots of different sizes, power levels, and voltage ranges, and their output is 3-phase AC. So, another option for you to consider and/or play with

Last edited:

#### tractor162003

##### Active Member
HMEM Supporting Member
Toymaker
You have this wrong. (As a bonus, since the bulb had begun to glow, you know that the resistance had dropped below 45 ohms, meaning your steam powered stepper is actually making more than 108.5 watts.)
As the bulb heats up the resistance go up.

#### Toymaker

##### Well-Known Member
HMEM Supporting Member
Toymaker
You have this wrong. (As a bonus, since the bulb had begun to glow, you know that the resistance had dropped below 45 ohms, meaning your steam powered stepper is actually making more than 108.5 watts.)
As the bulb heats up the resistance go up.

Good catch,... I was wrong. I must have been thinking of power transistors ;-)

#### Steamchick

##### Well-Known Member
HMEM Supporting Member
Toymaker, whatever goes wrong in your head is exactly the same as mine... - possibly a "mathematical dyslexion" - or whatever, that I couldn't spell without Google!
I am very grateful to all those who correct my maships!
K2

#### Steamchick

##### Well-Known Member
HMEM Supporting Member
Ray. What you are demonstrating is a generator that will probably provide 210V at 6000rpm - if the insulation is good enough! But what does the rating plate say on the stepper motor? (Maybe as you are in the USA you can work on 110V at 3000rpm instead, to suit your light bulb?)
I also suspect that when you get 210V you'll power a 220~240V filament bulb nicely... and have a clue to the wattage you can generate. Don't be surprised if your gas engine does produce a couple of hundred watts at those sorts of speeds.
If you know you jet sizes, and gas pressure at wide open throttle, I can deduce an approximate "gas power" you are introducing to the engine. All the kW of gas introduced comes out as heat (bearings, water jacket, exhaust, etc.) and electrical power. If 10% efficient I can believe you are inputting 2kW of "gas" to get 200w of electrical power...
But let's have some numbers and play with the Engineering!
Thanks,
K2

#### Rdean33422

##### Well-Known Member
HMEM Supporting Member
In the first post there is a picture of the stepper motor that has all the information printed on it.

The engine is capable of higher rpms but first I must find a way to secure it to the work bench.

Ray

Replies
33
Views
3K
Replies
9
Views
1K
Replies
23
Views
998
Replies
1
Views
342
Replies
9
Views
2K