Engineering Reverse Current PMDC Motor Braking Using Power Electronics

[dvscrobe]

Summary: How do you determine the armature current during the braking time?

For a simple permanent magnet DC motor with a known resistance and inductance in series with the armature, I would like to know how to determine the armature current if there was an electronic motor drive controlling the terminal voltage. During steady state conditions with the motor handling a particular load, terminal voltage needs to be a little higher than the back emf. If I needed the motor to stop as soon as possible, could I make my terminal voltage lower than the back emf? This would cause a reverse current. This instantaneous moment I can calculate but I can’t figure what my current does after, whether it is constant or gradually declines.

 

[Baluncore]

The quickest way to stop a motor is to generate a maximum reverse torque.
Torque is proportional to motor current.
So applying a maximum reverse voltage is the quickest way to stop.
But the current does not reverse immediately because of motor inductance.

The difference between generated emf and applied voltage will appear across the series resistance and inductance of the motor. The voltage across the inductance will decide the rate of change of current since; V = L * di/dt; so di/dt = V/L.

As the reversed torque decelerates the motor, it will stop, but the braking current will still be flowing. Again, that braking current cannot be reduced to zero instantly because of motor inductance, so you must anticipate zero, or overshoot and reverse the motor.

What parametric information is available about the motor?
Inductance, resistance.
Generated back emf in volts/rpm.
Current to torque relationship.
Inertia of motor and load.

What state variables can be measured in real time?
RPM, current, applied voltage.

 

[dvscrobe]

Baluncore,

I probably should have posted in homework section. Would you please take a look at the second problem? Is my thinking off? The supply voltage is being controlled by an electronic drive. My thinking is that if I lowered the terminal voltage, KVL dictates a reverse current. But that just seems odd. Thanks.

 

[berkeman]

I probably should have posted in homework section.

Thread moved to the Homework Help section.