Speed control of a DC motor

S

Thread Starter

Salehi

my choises are:

1. when the pwm unit output is high the supply voltage be at the motor terminals and when the pwm unit output is low disconnect the Vs and let the motor rotate freely.

2. when the pwm unit output is high Vs be at the motor terminals and when the pwm unit output is low brake the motor by connecting both terminals to Vs or Gnd.

3.when the pwm unit output is high the supply voltage be at the motor terminals and when the pwm unit output is low revers Vs become to the motor terminal.

i do not need the motor to rotate in both directions but fast reaching to the desire speed is critical.
 
You can use single power mos, motor terminals are between the V+ and Drain pins. This is simplify the gate driver.

(Don't forget the freewheeling diode for motor terminals.)

At this operation motor speed easily controlled by PWM. You don't need H bridge if motor will turn single direction.

I think you need speed servo and you want low reaction time. This time indepented from motor power driver but your power driver must give enough current for motor.

You need good servo controller.
 
R

Robert Scott

The purpose of PWM in controlling a DC motor is to achieve a certain constant current level. The inductance of the motor windings will help to keep the current constant even if the applied voltage is pulsing on and off. Your "choice #1" comes the closest to this goal, but keep in mind you can't really disconnect the windings suddenly when the PWM output is low. Instead, what is usually done is the H-bridge or whatever switched drive you are using provides a path for the inductive current during the "off time". Then the motor windings see at most a single diode drop of opposing voltage (.7 v). That voltage, together with the wire resistance, acts to let the current go down a little during "off time". Then during "on time", the current goes up a little. The overall effect is that the current maintains an approximately constant level which depends on the duty cycle of the applied PWM.
 
E

Emery Premeaux

On August 3, 2003, Salehi wrote:
> my choises are:
>
> 1. when the pwm unit output is high the supply voltage be at the motor terminals and when the pwm unit output is low disconnect the Vs and let the motor rotate freely.
>
> 2. when the pwm unit output is high Vs be at the motor terminals and when the pwm unit output is low brake the motor by connecting both terminals to Vs or Gnd. <

I dont think this will create a breaking situation.
What you need here is a reversable "H" bridge. You will apply a controled pulse in the opposite direction to create the breaking action. You might need a rotational sensor to determine how much duty cycle and for what duration to apply the break without starting it rotating the other way. However (Just a guess), you can start with a 100 percent duty cycle (Vss full on in oposite direction) and quickly ramp it back to zero dependant on how fast the motor responds.

Make sure you have good diodes on the motor terminals as im making another guess here that this will create a lot of inductive feedback.

> 3.when the pwm unit output is high the supply voltage be at the motor terminals and when the pwm unit output is low revers Vs become to the motor terminal. <

This really wont get you anywhere... the motor will simply chatter back and forth or 'sing' but not rotate... See above. A fully functional standard design H bridge is your best bet, even if you dont need reverese, you can still use it for breaking.

> i do not need the motor to rotate in both directions but fast reaching to the desire speed is critical. <

The fastest the motor will ever reach full speed is when Vss is at full on (directly connected with no pulse width modulation) So, as I stated above, start up at full duty cycle, and taper it back as the rotation approaches where you want it. For a quick stop, apply a 100 duty cycle to the reverse pin, then taper it back.

now I have a question: What are the voltage and current requirements for this motor? With this information will make it easier to locate a suitable bridge. Are you planning on building the H bridge yourself or are you looking for an industrial manufacturer solution?

Emery
 
Hi,

I have a unipolar application, besically I am using a DC motor to drive a liquid pressure up or down. This is a 12V anad 14A DC motor. What kind of a scheme should I use?

Thanks,
Tim
 
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