[SOLVED] speed control of brushed dc electric motor

[dxtr]

Hey everyone
i'm working on a small project , its about controlling speed of a brushed dc motor using 4 quadrant chopper ,
what i don't understand is how to control the speed of the motor when its acting like a generator (the shaft of a DC motor is turned by an external force).

 

[rohitkhanna]

This is an unusual requirement - what exactly are you trying to do ? A little more detail about the objectives might help in getting a correct solution.

 

[tpetar]

Hey everyone
i'm working on a small project , its about controlling speed of a brushed dc motor using 4 quadrant chopper ,
what i don't understand is how to control the speed of the motor when its acting like a generator (the shaft of a DC motor is turned by an external force).

When you use this device by supplying electricity to + and - it act as motor, you can use H-bridge to control motor.

As generator you will regulate speed over pulley ratio and input rotating speed. See principe of dynamo.

 

[dxtr]

As generator you will regulate speed over pulley ratio and input rotating speed. See principe of dynamo.

you mean i'm going to regulate the speed of the external force ?

 

[FvM]

If you actually apply an external torque rather than drive the motor with fixed speed, there's no principle difference between motoric and generatoric mode. You control the speed by setting the motor voltage. That's what every motor inverter or DC chopper involving recuperation does, in so far I don't think it's unusual.

 

[bigdogguru]

I too am a little confused as to how these topics relate.

Are you referring to the monitoring of Back EMF (BEMF) for speed control?

If so the following appnotes maybe of interest:

AN905 - Brushed DC Motor Fundamentals (Page 6)

**broken link removed**

AN893 Source Code

BigDog

 

[FvM]

The statement in AN905 is valid for all four quadrants:

The speed of a BDC motor is proportional to the voltage applied to the motor.

In a H-bridge the voltage is set by the PWM duty cycle. But this does only work with a correct PWM modulation scheme involving synchronous switching. Some "poor mans" PWM schemes found in DIY projects don't work in generatoric mode.