Speed control of a DC motor Using Armature Voltage

[sunny.sunman]

Design speed control circuit for a shunt DC motor (having minimum 20W power rating). The speed of

motor should vary below its rated speed (in rpm). The design should have the following specifications:

- Armature voltage control circuit for variations in speed (Three various speeds below rated

speed)

- Current limiting protection circuitry for starting current

- Control the speed using electronic components for Pulse Width Modulation

[pebe]

So what is your question?

[sunny.sunman]

i do not know what to do. i need a start and a design

[pebe]

Can I assume from its vague nature that this is homework?

It is not possible to generalize so you need to supply a lot more information. Like:
What voltage does it work at? What is the source of the voltage? What power?('minimum 20W' could be a 100HP motor. What is the starting current when a)unlimited, and b)limited? What are the three speeds?...........

[sunny.sunman]

yea it is.

the input voltage is 12v. we are using a 20W motor. and there are no three speeds we just have to control the speed.

[pebe]

You need a variable period square wave generator to give PWM (a 555?), driving a FET in series with the motor. Arrange the PWM to give slow speed at start-up to limit the starting current.

[sunny.sunman]

ya. so would this work? if yes then is it an armature voltage control circuit cause i can not really tell if it is.

[sunny.sunman]

same question for this.

[pebe]

Your first circuit will not work because the current the 39ohms base resistor of the BD129 would draw would be on the maximum limit of the 555, and a FET makes a better switch than a bipolar transistor.

The second circuit is much better. The 555 oscillator is wired in the configuration I would use. You don't need the 470µF cap across the IRF520, - it defeats the object of using PWM. The diode across then motor may not be required because the FET has an internal diode which serves the same purpose.

[sunny.sunman]

so is the second one an armature voltage control circuit?

[pebe]

The motor symbol shows a square block at each of its two terminals. I assume they are the brushes of the armature and that the field consists of a permanent magnet or a separate wound coil.
So, yes, I think it is an armature type of DC motor.

[sunny.sunman]

thank you for the help

[sunny.sunman]

just to tell you the second circuit did not work like that. i had to swap what was on pin 7 with what was on pin 3 and it worked

[pebe]

I don't know why that should have been so.

Pin7 connects to an open collector of an NPN transistor. When Pin6 goes above 8V the transistor switches on, taking the FET's gate to ground and cutting it off. When Pin2 goes below 4V the FET the transistor switches off and the 1K from the gate turns on the FET.

So whenever Pin3 is low Pin7 is also low.

[sunny.sunman]

that should have been so, maybe there might have been a mistake in my connections.but it worked when i swapped it. anyway it worked and its done thank you for the help.