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Old 05-15-2015, 01:10 PM
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citfta citfta is offline
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Join Date: Jun 2008
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As is normal on this forum there are a couple of threads that are about improving electric motors. As is also normal for this forum there seems to be a lot of confusion about what back electromotive force (BEMF) really is. There is at least one thread where the theme seems to be that BEMF is some kind of evil that must be gotten rid of. Very, Very wrong conclusion!

To help you understand what BEMF really is I need to lay some background information. Let's first look at one of the most versatile of all motors. I am talking about the shunt wound DC motor. This is a motor that has field windings that are separate from the armature circuit.

When full voltage is applied to the field windings this motor can develop high torque at very low rpm. I have worked on a 10 HP motor of this type that could move a 20 ft milling table with only 2 volts applied to the armature. And it was turning so slowly you could count the commutator segments as they moved under the brushes! Very high torque at low rpm.

How can it do that? The answer has to do with BEMF. At that low of rpm almost no BEMF is generated. However the voltage applied is also very low. So when the motor starts to turn the BEMF generated is still enough to limit the current to maintain the speed of the armature. This motor requires a cooling fan to keep it from overheating when run like this.

Where does the BEMF come from? As the armature starts moving the magnetic field of the field windings causes a current to be generated that opposes the applied current going to the armature.

As we increase the speed of the motor by applying more voltage to the armature the BEMF also increases to limit the speed of the motor by opposing again the applied voltage. This is not a BAD thing. What would happen if there was no BEMF? I can tell you from personal experience exactly what would happen.

My partner and I were troubleshooting a 25 HP shunt wound DC motor circuit. The motor was running. We accidentally bumped the relay that feed the field current to the field windings. Of course there was an immediate loss of magnetic field in the field windings. Without a magnetic field in the field windings no BEMF was produced. The armature current immediately went up to the max current the armature windings could carry limited only be the resistance of those windings. This sudden surge of current created a very load boom that shook the whole building! My boss came running out of the office to see what had blown up.

BEMF is NOT our enemy. It is the only thing that limits the current through the armature. BEMF is the natural and very efficient governor that helps us control the motor.

As I said earlier the shunt wound DC motor is a very versatile motor. One of the neat things you can do with it is to get it to turn faster than it would normally go at full armature voltage. How do we do that? By reducing the field current. Once the motor is up to full speed we can then reduce the field current by a small amount and get the motor to turn faster. This works because when we reduce the field current we also reduce the BEMF. This allows more current to flow through the armature which makes the motor speed up until the BEMF again balances out the applied armature voltage. The only drawback to this is this also reduces the torque of the motor. This is usually not a problem because we only do this after the motor is at normal full speed and therefore has whatever load it has up to speed also.

To be continued.
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