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As expected, this EMF opposes the voltage of the source that created it; that is, the motor becomes a generator as soon as it is turned on, and the opposing (or counter, or back) EMF will increase with increasing frequency.However, the applied EMF overcomes this "reverse emf" and makes the current flow in the direction suggested by the applied EMF; in effect, the reverse EMF acts as a brake on incoming current and limits it.Won't it affect it affect the efficiency of the EV's.More over won't these back emf badly affect the electronic circuit system.

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  • \$\begingroup\$ No, the circuits are designed to cope with that. Even more: the energy is used to charge the batteries again. It is called "regenerative breaking." \$\endgroup\$ – Oldfart Dec 15 '19 at 12:59
  • \$\begingroup\$ Yes,But suppose if we want to push the car since the accelerator pedal is in rest we cant harvest that energy right ? because we need car to move ? \$\endgroup\$ – Abel C Dixon Dec 15 '19 at 13:05
  • \$\begingroup\$ Yes the car needs to move. I made a joke about that last month: there is a radio advert, Lexus I think, which says "the batteries charge when you push the brake pedal" So I said to friend: "Great, after I park the car I am going to put a brick on the brake pedal, then when I want to go home the batteries are full again". \$\endgroup\$ – Oldfart Dec 15 '19 at 13:08
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As expected, this EMF opposes the voltage of the source that created it; that is, the motor becomes a generator as soon as it is turned on ...

Not quite. It becomes a generator when it starts turning.

... and the opposing (or counter, or back) EMF will increase with increasing frequency.

With increasing rotational speed might be a better way of saying it.

However, the applied EMF overcomes this "reverse emf" and makes the current flow in the direction suggested by the applied EMF;

In normal driving the back EMF will always be less than the applied EMF as there is always at least a small load on the motor.

... in effect, the reverse EMF acts as a brake on incoming current and limits it.

Yes. Remember that the starting current for most motors is very high because there is no back EMF and severe heating is a result. Large motors may have a limit on how frequently they can be started (n times per hour).

Won't it affect it affect the efficiency of the EV's.

No. The motor controller applies the voltage required to generate the power required.

Moreover if we where about[?] won't these back EMF badly affect the electronic circuit system.

The biggest problem with back EMF is in regenerative braking. If the incoming energy isn't fed back into the batteries then the DC bus voltage could rise to the point of damaging the electronics. If this is going to be a problem then a bleed resistor is used to burn off some energy to maintain the DC bus voltage at the designed level.

Back EMF is good. It reduces the motor current as it approaches the no-load speed determined by the applied voltage.

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  • \$\begingroup\$ Does that mean nullifying the back current from the wheel motors is taken into consideration by regenerative braking system ? \$\endgroup\$ – Abel C Dixon Dec 15 '19 at 15:03
  • \$\begingroup\$ What to you mean by "nullifying"? verb: nullify, present participle: nullifying, make of no use or value; cancel out. If it's used to recharge the batteries or burnt off as heat then it is not nullified. \$\endgroup\$ – Transistor Dec 15 '19 at 15:16
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Back EMF is a good thing, it does not effect the efficiency of a DC motor at all.

Torque in a DC motor is proportional to the current through the armature and the magnetic flux across the armature.

Back EMF is proportional to flux and the speed of the motor.

These simple facts give rise to a series of desirable characteristics in DC motors. If the motor is not turning there is no back EMF. The armature current is determined by the armature resistance and Vt the supply voltage to the motor and is Vt/Ra. This gives the DC motor the capability of providing huge starting torque. Look on YouTube for some of the videos of high speed diesel electric trains accelerating away from stations.

Once the motor reaches equilibrium in driving its load, the armature current and hence torque is proportional to (Vt-Eb)/Ra Supply voltage minus back EMF divided by armature resistance. Here if the load increases, the speed decreases, hence Eb decreases, so current increases so torque increases balancing the increased load and the tendency to slow down. This gives DC motors the ability to run at almost constant speed over a wide range of torque.

It is rare in science and engineering for nature to give us a win-win yet we have this with back EMF in DC motors. If it was not for the weakness of the commutator-brush system DC motors would rule the electrical energy to mechanical energy conversion world.

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