Motor Control Circuit: Minimizing Power Draw

[rako77]

Hi I'm new to this forum and currently a first year engineering student. I'm trying to build a motor control circuit so I can minimize the power draw. And I feel I need a bit of direction. I started with a 6v battery source a 6v motor: http://www.jaycar.com.au/productView.asp?ID=MD7000&CATID=9&form=CAT&SUBCATID=726 and a DPDT switch. but this was too much power under load since I was aiming to get the motor under 1 watt so I looked around and read about PWM (Pulse-width modulation) and that it is commonly used to control motors and that it is possible to make a simple PWM control with a 555 timer. I tried following this http://www.dprg.org/tutorials/2005-11a/index.html but it didn't seem to output enough voltage although I substituted an led in place of the motor and it worked alright I believe the voltage was too low.

So I'm looking for a bit of direction on pwm and motor control in general there are a few other pwm circuits which I have looked at although I'm not sure what is best:
http://www.electronics-lab.com/projects/motor_light/005/index.html
http://www.electronics-circuit.com/advanced_DC_Motor_Control.html
http://www.discovercircuits.com/DJ-Circuits/simplepwm2.htm
Maybe some pointers on 555 timers may help as well.

also I know some basic circuit analysis for dc and ac and I was wondering if anyone can point me in the right direction of how I would be able to analyse a motor in a circuit?

 

[Mike_In_Plano]

In fairly simple terms, you can get away with modeling your motor as an emf source, such as v=k*RPM, where k is a motor constant of volts per RPM. In series with this, you place a resistor, R, which you can measure. It represents the resistance in your windings. Then, very important, you add an L in series. This is the leakage inductance in your motor windings. This is very important, since it makes PWM control possible.

In actual operation, the motor will have loses due to the PWM based current fluctuations causing magnetic losses in the armature, camutator based losses, friction and and windage (air speed) losses.

Generally, most people use a low on resistance MOSFET to chop (PWM) power to their motor. You also need a good rectifier across the motor, since the PWM currents continue to flow through the rectifier even when the MOSFET is off.

I noticed in the notes you found that the rectifiers were common 1n4007 and that variety - you don't want to use these as they have very poor performance when switching quickly. For most low voltage motors, schottky type rectifiers are great.

Typically, switching at anywhere from 6 kHz through 20kHz works good. If you go too low or too high, your effeciency drops off. If you have problems with noise from the motor, don't put a capacitor across it. Instead, put a capacitor in series with a resistor across. This is called an RC snubber, and essentially serves to eat up your high frequency noise, while leaving the rest of the PWM circuit to work. A good starting place for a snubber is about 22 ohms in series with about 1nF.

Best Wishes,

Mike

 

[pantaz]

Take a look at the http://tech.groups.yahoo.com/group/osmc/" Yahoo group.

 

[skeptic2]

I'm trying to build a motor control circuit so I can minimize the power draw.

I don't understand this statement. For DC motors, generally the voltage across the motor is proportional to the speed of the motor (minus the IR drop of the motor) and the current through the motor is proportional to the torque. The only way I can think of to reduce the power draw is to reduce the speed or to reduce the load.

If you are using a power source with low source impedance, PWM should increase your power draw because of losses in the PWM circuit and from increased eddy currents in the motor. On the other hand if you are using a rheostat to control the speed, PWM would be an improvement. If this is what you are trying to do, the higher the PWM frequency the higher the eddy current losses in the motor will be and the hotter the motor will get.

I once built a motor controller for a model train engine so it would start and stop slowly and smoothly. My approach was to find the resonant frequency of the motor by finding the frequency at which the minimum voltage would move the motor. That frequency was about 70 Hz so that was the frequency of my PWM source. Instead of using a 555, I used a triangle wave generator and a comparator that compared the triangle wave voltage with a reference voltage supplied by a pot.

 

[rako77]

thanks guys. Mike_In_Plano you have helped . and pantaz I'm looking into that group and so far it seems good.

skeptic2 thanks for the information. I'm a bit new to all this but my theory was that i would slow the engine down using pwm and I think pwm turns the power on and off quickly to the engine. so if the power to the engine is only on for 75% of the time it would only use 75% of the power I know there are inefficiency due to the pwm circuitry. and the motors reaction to the frequency. That is why I though PWM would work in reducing power. The speed control of pwm does help but is not essential.

There are other option like gearing the system differently and buying a less powerful engine but if I can get pwm to work in the way I think it does then it seems like the best option as I don't need the engine to run at full speed.