Capacitor charging using stepper motor

[FvM]

I only know the coil resistance and nothing else about the motor How to find the max current produced by motor?

Consider my comments about windings L/R in post #8. The current at 200 Hz will be limited by motor inductance, unless it's compensated by a C load.

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The relevant question is how many "pole pairs" in terms of electrical machine theory a "200 step" motor has. I keep my opinion about 4 steps corresponding to 1 full step cycle (respectively a pole pair).

I must confess that I'm a bit tired of theoretical discussions and suggest to measure the actual output frequency.

 

[hubble86]

I think there is some misunderstanding here. The output frequency of stepper motor is 200 Hz because there are 200 steps (one revolution or one full cycle) in the motor. What I am saying is that I am rotating it 4 times a second, the output cycles 800 times a second. Does it make sense now?

Also, tried to find short circuit current: it was around 95 mA. Max open circuit voltage: with 0.01 uF cap - 11 V and with 4.7 uF - 14 V.

Hope it helps.

 

[FvM]

I'm rotating a 200 step/revolution motor at 330 rpm and it generates a sine of 275 Hz.

I think the misunderstanding is on your side...

 

[mdorian]

You should use larger capacitors, with 10 uF capacitor and 200 hz the load should pull less than 3 mA otherwise the multiplier cannot work properly. Don't use a multiplier if you don't need it is very power inefficient if you really want a higher voltage you can use a a 110 to 12 v ac power transformer , an ordinary one , using the low voltage secondary as primary and then a diode bridge. What load do you intend to use?

 

[hubble86]

Sorry for delay in getting back. Was quite busy with some other urgent work.

I have completely dropped the idea of using voltage multiplier and instead have thought of using transformer. I had a 230 VAC / 12 V 200 mA adapter. Took out the transformer and connected the secondary coil to the stepper motor and measured the output at the primary: the rectified output was around 85 - 90 V. Connected 22 kohm resistor across it, the output dropped to around 50 V. Although it seems to be very lossy one can I use this transformer to safely charge a 63 V capacitor to 50 V?

Thanks.

 

[jiripolivka]

Sorry for delay in getting back. Was quite busy with some other urgent work.

I have completely dropped the idea of using voltage multiplier and instead have thought of using transformer. I had a 230 VAC / 12 V 200 mA adapter. Took out the transformer and connected the secondary coil to the stepper motor and measured the output at the primary: the rectified output was around 85 - 90 V. Connected 22 kohm resistor across it, the output dropped to around 50 V. Although it seems to be very lossy one can I use this transformer to safely charge a 63 V capacitor to 50 V?

Thanks.

I have only see crazy ideas in this thread. As a technician you should know that a capacitor rated for 63 V must never be exposed to more than that.
So use either a higher-voltage capacitor, with 90V no-load source the peak voltage can exceed 100 V. Or, use a resistor and a 63 V Zener to protect the capacitor you have.

 

[hubble86]

Thanks again for your reply. Actually I read the following somewhere in this thread:

In order to find out what maximum voltage you can reach, make the load a high resistance instead of a large capacitor.

That's why I connected this large 22 kohm resistor instead of 10 mF capacitor to find out the max voltage the capacitor can reach.

Thanks.

 

[FvM]

If you compare the effciency of a rectifier voltage multiplier with a step-up transformer, there's a disadvantage of the cascade rectifier due to the diode voltage drops. If the generator output voltage is sufficient large relative to a diode voltage, this disadvantage is however small.

I think that the main limitation of your design is set by the generator output impedance. It can't be bypassed if you use step-up trasnformer.

 

[mdorian]

Use a larger transformer , the cheap small transformers are not designed with efficiency in mind. The theoretical output voltage for you would be 6v * 220v/12V = 110v , to much , can you find a 220 to 24 v transformer to obtain 55v ? Or better one 12 v output which has a 110 input option that you can use. The output current for the transformer must be close to the current you need to draw from the stepper. In your case the theoretical power rating is 200mA*6V = 1,2W , no wonder you have such big loses. The measured power is 50V*50V/22000 ohm = 0.12W. The internal impedance of the circuit seen from the output side is ~17 Kohm