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dc motor speed regulator

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wjyates

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i have been serching the web for something that would keep a dc motor at a constant speed
when different loads are applyed.
i keep finding pwm motor speed controlers,buts thats not what i want.
is there a term or name for what i am looking for
 
You need a shaft encoder or some kind of built in tach to control the speed, and unless the load is applied very gently there is still going to be some slow down or speed up when the load changes, one way to help mitigate that is to use a flywheel, the mass of the flywill will provide and absorb extra energy beyond what the motor can normally manage. You would feed the tach or encoder output into a circuit that's designed to adjust the effective power the motor gets. For lower RPM stepper motors are also an option.
 
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is there a term or name for what i am looking for

Yes, what you are looking for is "closed loop motor speed control". This link shows an example of closed loop motor speed control using PWM.

If we just PWM speed control a motor the motor will turn at a given speed. However, as a load on the motor increases the motor will slow down. The PWM driving the motor has no way of knowing this. Therefore we do as Sceadwian mentions. We get some form of feedback as to what the actual speed of the motor is. The feedback can be derived from an optical encoder, a tachometer, or other signal types.

The link is an example of feedback using a tach with a PWM circuit. Keep in mind that PWM is just one of several motor speed control methods. A Google of "Closed loop motor speed control" should yield other methods and circuits.

Ron
 
The answer will also depend on how precisely you need to maintain the speed, and exactly what kind of DC motor you are using. The reason I say this is that if you apply a constant voltage to a good quality DC motor (low impedance windings), then the speed will be surprisingly constant regardless of load.

When you apply a certain voltage to the motor, it will run at whatever speed is necessary for the back emf to equal the power supply voltage minus a small difference. Adjusting the supply voltage will shift the operating speed higher since the motor must then spin faster to generate the cancelling back emf. So, this gives you speed control. As the load increases, the motor will simply draw more current in order to maintain the speed.

So, it may be possible to solve the problem with nothing more than a regulated power supply.
 
Hello there,


Controlling the speed of a motor is different than regulating the speed of a motor. Turning the speed up or down is different than setting a certain RPM where the motor should run all the time, regardless of load.
There are questions that come up however, like just how perfect do you have to regulate the speed.

Analog controllers work on the back EMF of the motor, because the back EMF can tell you what speed the motor is running at. Measuring the voltage and current and developing a motor drive leads to an equation that is void of friction (added friction is the main cause of slow down) so the motor speed no longer varies with friction.
This kind of circuit works without a tachometer. It might require one op amp ic chip and a transistor or two.

Perhaps you can tell us what kind of technology you want to work with, micro controller or purely analog.
 
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