Reverse Current PMDC Motor Braking Using Power Electronics

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Discussion Overview

The discussion revolves around the dynamics of braking a permanent magnet DC motor using power electronics, specifically focusing on determining the armature current during braking and the implications of controlling terminal voltage. The scope includes theoretical considerations and practical applications related to motor control and braking techniques.

Discussion Character

  • Exploratory
  • Technical explanation
  • Homework-related

Main Points Raised

  • One participant inquires about calculating the armature current during braking, questioning whether lowering terminal voltage below back emf would result in reverse current and how this current behaves over time.
  • Another participant suggests that generating maximum reverse torque by applying maximum reverse voltage is the quickest way to stop the motor, noting that current does not reverse immediately due to motor inductance.
  • This participant explains that the difference between generated emf and applied voltage affects the current, with the rate of change of current being determined by the inductance of the motor.
  • They also highlight the importance of knowing various motor parameters, such as inductance, resistance, back emf, and the current-to-torque relationship, as well as real-time state variables like RPM and current.
  • A later reply expresses uncertainty about the initial reasoning regarding reverse current and suggests that the inquiry may be more suited for the homework section.

Areas of Agreement / Disagreement

Participants express differing views on the behavior of current during braking, particularly regarding the implications of lowering terminal voltage and the effects of motor inductance. The discussion remains unresolved with multiple competing perspectives on the dynamics involved.

Contextual Notes

Limitations include potential missing assumptions about motor parameters and the specific conditions under which the braking occurs. The discussion does not resolve the mathematical relationships or the exact behavior of the current over time.

Who May Find This Useful

This discussion may be useful for individuals interested in motor control, power electronics, and those seeking to understand the dynamics of braking in permanent magnet DC motors.

dvscrobe
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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.
 
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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.
 
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.
 

Attachments

dvscrobe said:
I probably should have posted in homework section.
Thread moved to the Homework Help section.
 

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