Variable DC motor speed control

[fabbie]

can anyone please shed some light on how to variably control a DC motor speed? Is there a circuit capable of performing this task?
Furthermore, an RF signal is suppossedly to be interfaced with this circuit to control the speed the DC motor speed. Is the use of a microcontroller necessary?
Thanks in advance for any advise

 

[Nigel Goodwin]

can anyone please shed some light on how to variably control a DC motor speed? Is there a circuit capable of performing this task?

It's called PWM (Pulse Width Modulation), there are other methods, but PWM is efficient, cheap, easy, and works well.

Furthermore, an RF signal is suppossedly to be interfaced with this circuit to control the speed the DC motor speed. Is the use of a microcontroller necessary?

It would make life a LOT easier to use a microcontroller, a PIC with hardware PWM would be ideal - you can get 8 pin FLASH versions that would do this nicely.

 

[john1]

Hi,

This is a new one on me.
Please tell me more about this,

Cheers, John

 

[Styx]

Basically for all machines

Voltage relates to speed
Current relate to torque.

A brushed DC machine take this relationship to its simplest form

Speed = Ke * V
Torque = Tt * I

So by varying the voltage that is supplied to a brushed DC machines terminals, the speed can be varied.

The easiest (and most efficient) is to use PWM. What this means is to take a high voltage supply. Bu turning it ON and OFF at the DC machines terminals the average voltage as seem by the machine is reduced.

By varying the ON time w.r.t. the OFF time the voltage can be controled.

Vout = Vin*Duty

So for a PWM signal that is 50% on and 50% off (50% duty) the voltage seen by the machine is 50% that of the main DC-link.

if hte duty is 25% (25% ON and 75% OFF) then the voltage at the machine terminals is 25% of hte DC-link.

 

[john1]

Hi,

i've never heard of interfacing RF to affect the speed
of a DC motor.

Maybe he meant PWM ... ?

and interfacing ? ... that must mean including or adding

Cheers, John

 

[Nigel Goodwin]

Hi,

i've never heard of interfacing RF to affect the speed
of a DC motor.

Maybe he meant PWM ... ?

and interfacing ? ... that must mean including or adding

Radio controlled model cars - you have a radio link interfaced to a speedcontroller feeding the motor, some are extremely crude (a PCB switch moved by a servo), better ones use PWM and highpower FET's (usually a big row of them!).

 

[grandestlama]

A useful non-microcontroller option is the SG3524 IC.
**broken link removed**

You can get more details on PWM technique from Google.