Whats a DC motor Efficiency factors & How can we have multy speed without a resistor

1. Jan 6, 2009

ahrouhi

I am working in an Auto Parts manufacturer company which one of its production is DC motors used in different wheicles. I have two type of question about a DC motor :

1: we have a new model DC motors which get too hot. whats the reason? what does the effect of the thickness of cooper wire of armature ? if we want to reduce the Current can we use more thicker wire in armature? whats the effect of thicker wire on RPM?whats the effect in number of wounded wire? does they related to RPM? would you guide me by equations and formulas too?

2: our DC motor have 2 speed. high and low. we use a resistor to provide multiple speed. is there any other way to produce multiple speed beside resistor?

2. Jan 6, 2009

Staff: Mentor

Re: Whats a DC motor Efficiency factors & How can we have multy speed without a resis

Don't know about the hot motor, but you don't change speeds by dropping the voltage. You slow the motor speed with PWM of full voltage.

3. Jan 6, 2009

ahrouhi

Re: Whats a DC motor Efficiency factors & How can we have multy speed without a resis

would you explain more about PWM? i am nat familiar with that.

4. Jan 6, 2009

Staff: Mentor

Re: Whats a DC motor Efficiency factors & How can we have multy speed without a resis

Pulse width modulation. In the context of motor drive, it means turning the power on and off at some rate, with a duty cycle that controls the speed of the motor. For a 12V DC motor for example, you could drive it with a 1kHz rectangular wave, with the ON time versus OFF time adjusted for whatever speed you are trying to achieve. Full ON all the time is max speed, and say, 70% on time is half speed, or whatever the motor does with 70% ON time.

5. Jan 6, 2009

ahrouhi

Re: Whats a DC motor Efficiency factors & How can we have multy speed without a resis

Dear friend. in industry we should consider many things such as price, durability... i am very eager to know that using an electronic board for PWM ,on our project , in compare with the traditional method, using resistor,is economical or no? presently we should consider about 1.5 \$ for resistor.
meanwhile using On & Off for reducing the speed does have any side effect such as reduction the life cycle of DC motor? because any On & Off seems to be like an impact on system.

6. Jan 6, 2009

Redbelly98

Staff Emeritus
Re: Whats a DC motor Efficiency factors & How can we have multy speed without a resis

PWM will be more efficient, since you will not be wasting power heating up a resistor.

The on/off is done very quickly, so things like motor speed and also temperature do not change appreciably during an on/off cycle. So the types of things that cause problems--current surges, thermal cycling--do not occur.

It sounds like you need to consult in person with an electrical engineer familiar with DC motors. They could look at your motor in operation and see better what might cause the overheating. Or perhaps your company should hire such a person full time.

7. Jan 7, 2009

ahrouhi

Re: Whats a DC motor Efficiency factors & How can we have multy speed without a resis

many thanks for your suggestion of using PWM. would you please guide me how to calculate the specification of PWM? our DC motor normal speed (normally is high) is 2700 RPM. for reduction to 2200 RPM what should be the parameters or specifications of PWM? (1 khz rectangular wave would be adequate?)

secondly , i am still in need of knowing the effect of thickness of cooper wire of armature in current and speed and torque of a DC motor. please help me in this matter too.

8. Jan 7, 2009

BrockLee

Re: Whats a DC motor Efficiency factors & How can we have multy speed without a resis

Qoute -secondly , i am still in need of knowing the effect of thickness of cooper wire of armature in current and speed and torque of a DC motor.- EndQoute

According to wikipedia's Definition and Examples of Inductance Measuring, which you can find by looking up Henry's on there website. There is no need to calculate the wire's gauge(outerRadius). It Is merely a function of total windings, diameter of windings, and Current. But this principle can help you make a simple potentiometer or more precisely two coils one stuck in the other and the closer and the larger the surface area within the lines of flux will speed up the motor. but unless you have a well put together power system. no, not just a car battery and a coil(dangerous), it will work for producing larger currents and lower currents. unless the motor is brushless and in that case(which is what im thinking) a switch with multiplying oscillators is a better idea. As with torque and rpm's a Transmission is needed. but once you have got a cheepo one(Cough.. Grainger) you can assort Sprockets and Gears until you have a proper Torque. Remember Its only a Ratio ;)

9. Jan 8, 2009

ahrouhi

Re: Whats a DC motor Efficiency factors & How can we have multy speed without a resis

thanks, i found many good information in wikipedia. but still waiting for your guidances. experience is precious and i respect to your experience and eager to hear that.

10. Jan 11, 2009

ahrouhi

Re: Whats a DC motor Efficiency factors & How can we have multy speed without a resis

please send me some more circutes for designeing PWM . and let me know haw to calculate the frequency to adjust my motor speed from 2700 to 2100?