Capacitor charging using stepper motor

[hubble86]

Hello, I have got a small 4 wire 200 step 2-phase stepper motor which I wish to use a 10mF 63V capacitor. I am getting around 6Vrms (open circuit) at each of the two coils when rotated at around 350 rpm. How do I step up the voltage and charge the capacitor with good efficiency(low power loss as a result of I2R heat, if any)?

Thanks.

 

[jiripolivka]

Hello, I have got a small 4 wire 200 step 2-phase stepper motor which I wish to use a 10mF 63V capacitor. I am getting around 6Vrms (open circuit) at each of the two coils when rotated at around 350 rpm. How do I step up the voltage and charge the capacitor with good efficiency(low power loss as a result of I2R heat, if any)?

Thanks.

I do not see why to use a stepper motor to charge a capacitor. Using a DC power supply can do it more efficiently without it.
Otherwise, 6 V rms means almost 9 V peak which can be the voltage on the capacitor. 10 mF connected to one coil of a stepping motor may affect motor function as it delays coil current.

 

[hubble86]

Thanks jiripolivka for your informed response. I am trying to build a small portable pull-string type emf generator for which I intend to use this small stepper motor to charge the capacitor. Capacitors are fast easy to charge. Low voltage high capacity super capacitors are very costly here if at all available. So, the only way to increase the capacity is to charge the capacitor to higher voltage.

Thanks.

 

[BradtheRad]

If you are getting AC from the motor, then you can use a voltage multiplier to boost the voltage, while turning AC into DC.

 

[hubble86]

If you are getting AC from the motor, then you can use a voltage multiplier to boost the voltage, while turning AC into DC.

I really don't know the nature of output waveform from stepper motor nor am I having a CRO to find it out. Can I use boost converter in this case?

Thanks.

 

[BradtheRad]

A boost converter accepts DC, and outputs DC.

Obviously it is important that you find out whether the motor produces AC or DC.

Try hooking up two parallel led's to it, in opposite directions.

 

[hubble86]

A boost converter accepts DC, and outputs DC.

Will rectified voltage from motor work in this case?

Obviously it is important that you find out whether the motor produces AC or DC. Try hooking up two parallel led's to it, in opposite directions.

It is definitely not DC. It can be trapezoidal or something along those lines. LEDs are lighting up both ways. Can I use PC based oscilloscope with proper resistors to find out waveform?

 

[FvM]

It's quite obvious that the output voltage must be AC, simply because the average voltage across an inductor (= a motor winding) must be zero. Only a DC motor/generator with commutator can output DC voltage.

I don't believe that the exact waveform matters much for your purpose, assuming a sine voltage can't be completely wrong, even if it's somehow distorted.

Besides open circuit voltage (e.m.f.) you would want to determine the short circuit current respectively the windings impedance. This gives you an idea of expectable output power. You might find windings R and L numbers in the motor datasheet (R can be easily measured of course).

I presume that the motor inductance forms a considerable part of motor impedance at your intended operation frequency. The L/R corner frequency can be expected in a few 10 Hz range, but you are operating the motor at about 300 Hz if I calculated right.

 

[hubble86]

It's quite obvious that the output voltage must be AC, simply because the average voltage across an inductor (= a motor winding) must be zero. Only a DC motor/generator with commutator can output DC voltage.

I don't believe that the exact waveform matters much for your purpose, assuming a sine voltage can't be completely wrong, even if it's somehow distorted.

Yes, it can be. And yes, the exact waveform doesn't really matter

Besides open circuit voltage (e.m.f.) you would want to determine the short circuit current respectively the windings impedance. This gives you an idea of expectable output power. You might find windings R and L numbers in the motor datasheet (R can be easily measured of course).

I have already measured the resistance of each coil to be around 0.6ohm. The motor is quite old of astrosyn make. So, the datasheet is no longer available. In that case, how do I measure inductance?

I presume that the motor inductance forms a considerable part of motor impedance at your intended operation frequency. The L/R corner frequency can be expected in a few 10 Hz range, but you are operating the motor at about 300 Hz if I calculated right.

I tried to theoretically calculate the frequency of the wave. It is a 200 step motor rotating at 4 RPS, so 200*4=800 before rectification. After it passes through the rectifier it is 800*2=1600Hz. I am not sure whether it is a correct way to calculate.

 

[FvM]

A step sequence cycle is comprised of 4 full steps, you should get 200 Hz not 800 Hz output. But why don't you check with an oscilloscope?

 

[hubble86]

But why don't you check with an oscilloscope?

Because I am not having any. Is it okay to use pc based scope like given here

 

[BradtheRad]

To use a boost converter, you'll need to run the motor output through a diode bridge. It will cause 1.2V drop, which is a bite from a supply that is 9V peak.

This will give you DC, and then you're going to chop it into pulsed DC.

But if you feel like experimenting, here's a different technique to try.
A series LC can step up voltage of an AC sinewave.

The values must be chosen specifically for (a) frequency and (b) how high a voltage you want your capacitor to charge to.

For the frequency I used 200 Hz.



I gave the capacitor load a small value. It charges quickly so that the entire timeline can be seen in the scope trace.

This is only a theoretical simulation. It is untested. Real charging current will depend on the real output of your motor.

As you can see, volt levels can easily get out of hand (especially with no load). To limit how high the voltage gets, you can install a zener, or a resistive load.

 

[schmitt trigger]

Because I am not having any. Is it okay to use pc based scope like given here

If you are serious about electronic circuits, you have to buy, beg or borrow one.
Its like running a marathon without the proper shoes. It can be done but will limit you.

 

[hubble86]

After days of trying to choose between different ways, I chose to go with the CW multiplier. Tried to charge for more than 30 sec with 1n4007 and 10 nF cap system, the 10 mF cap got charged to only 500 mV. I think I have got wrong value capacitors. Here, I tried to simulate with 7 VA 800Hz source, and different stages of 10 uF cap and 1n4007 in TINA, got this result as shown in the attachment. Where am I going?

 

[FvM]

In both circuits, the 10 mF is almost charged to the generator peak-to-peak voltage minus rectifier voltage drops, which is the maximum voltage that can be achieved.

What are you complaining about? What do you expect?

 

[hubble86]

I was not able to charge the 10 mF cap with 10 nF and 1n4007, but could only reach to 500 mV in 30 sec. What I am asking is if I can use 10 uF cap instead of 10 nF?

Thanks.

 

[BradtheRad]

When doing these early explorations...

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

The scope trace covers 1/9 second (simulated).



Theoretically your capacitor can reach 31V or so.

Notice the 10 uF capacitors. The generator puts out 56 mA. However if your motor can produce 500 mA, then consider using 100 uF.

 

[hubble86]

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

 

[BradtheRad]

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

You can use your DC ammeter or your AC voltmeter.

Your ammeter will give you some idea although it's more complicated than measuring plain DC. Since it probably reads DC only, put a diode inline.

Attach a diode going in the other direction, in case it's needed. Watch that it doesn't overheat.

Start with the ammeter range set on maximum Amperes. This could be important if your meter contains a 200mA fuse.

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You can also try attaching resistors of various values, one at a time as you measure AC volts across it, while spinning the motor.

 

[hubble86]

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.

I think the frequency is not 200 Hz. Here is why. The motor is being rotated four times (not a 4 step sequence as assumed by you) a second. Since the stepper has 200 steps so it comes out to be 200 steps * 4 rotations in a second = 800 cycles a second. I hope I am not missing something here.