Back E.M.F. & Torque Output of a Permanent Magnet DC Motor

In summary: This back emf is proportional to the speed at which the armature is rotating. At a particular speed, it becomes equal to the applied voltage. This is the maximum speed of the DC motor. As the speed builds up, back emf increases, voltage across armature drops, current drops, and therefore torque starts to decrease. Finally, when speed is maximum, applied voltage= back emf, no current, and no torque.
  • #1
OutCell
34
0
Hi,

What is the effect of "back e.m.f." on the torque output of a permanent magnet d.c. motor? I googled it but didn't get any results.. I just need a brief description to understand the concept..

Thanks :blushing:
 
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  • #2
when armature of dc motor rotates, it also acts as a generator also n generates a counter emf(lenz law) which is called back emf. now this back emf is proportional to the speed at which the armature is rotating. at a particular speed it becomes equal to the applied voltage. this is the maximum speed of the dc motor.
at the start, when back emf is zero, voltage across armature(applied voltage-back emf) is maximum, current is maximum, therefore torque is maximum(torque is proportional to current in the armature, T = I*L*B*r, I current, L length of conductor,B magnetic field, r = radius, school physics). as the speed builds up, back emf increases, voltage across armature drops, current drops and therefore torque starts to decrease.
finally when speed is maximum, ie applied voltage= back emf, no current, no torque. therefore at no load n maximum speed, motor has no torque
 
  • #3
ank_gl said:
when armature of dc motor rotates, it also acts as a generator also n generates a counter emf(lenz law) which is called back emf. now this back emf is proportional to the speed at which the armature is rotating. at a particular speed it becomes equal to the applied voltage. this is the maximum speed of the dc motor.
at the start, when back emf is zero, voltage across armature(applied voltage-back emf) is maximum, current is maximum, therefore torque is maximum(torque is proportional to current in the armature, T = I*L*B*r, I current, L length of conductor,B magnetic field, r = radius, school physics). as the speed builds up, back emf increases, voltage across armature drops, current drops and therefore torque starts to decrease.
finally when speed is maximum, ie applied voltage= back emf, no current, no torque. therefore at no load n maximum speed, motor has no torque

Thanks mate! :rolleyes:
 
  • #4
no probs dude. pleasure to help u
 

1. What is back electromotive force (EMF) in a permanent magnet DC motor?

Back EMF is the voltage generated in the opposite direction of the applied voltage in a permanent magnet DC motor. This is caused by the motor's rotation, which creates a magnetic field that opposes the original magnetic field created by the applied voltage.

2. How does back EMF affect the operation of a permanent magnet DC motor?

Back EMF has a significant impact on the speed and torque output of a permanent magnet DC motor. As the motor speeds up, the back EMF increases, which reduces the difference between the applied voltage and the back EMF. This results in a decrease in the motor's torque output.

3. Can back EMF be eliminated in a permanent magnet DC motor?

No, back EMF cannot be completely eliminated in a permanent magnet DC motor. However, it can be reduced by using a higher voltage power supply or by adding a diode in parallel with the motor to allow the back EMF to dissipate.

4. How does back EMF affect the efficiency of a permanent magnet DC motor?

Back EMF can have both positive and negative effects on the efficiency of a permanent magnet DC motor. On one hand, it can reduce the torque output, resulting in a decrease in efficiency. On the other hand, it can also reduce the current flowing through the motor, which can improve efficiency.

5. How can back EMF be calculated in a permanent magnet DC motor?

Back EMF can be calculated using the equation E = K * N * ω, where E is the back EMF in volts, K is a constant, N is the number of turns in the motor's windings, and ω is the motor's angular velocity in radians per second. The value of K can be determined experimentally or by using the motor's specifications.

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