# B If there were no back EMF

#### alv

Summary
What would be the effect if there were no back EMF in a DC motor
I know that as per Lenz's Law, back EMF moves in a direction so as to counter the flow of the changing magnetic flux which induced it in the first place. And it does so because if it flowed in the same direction it would contravene the law of conservation of energy as more and more energy would be created without a further input of energy. But what would be the case if, somehow, there were no back EMF. Wouldn't this mean that the supply current would just run at maximum? I've had it suggested that the motor would run faster and faster until it blew up and I can't find (or think of) any answer to this.

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#### mfb

Mentor
You would have to invent a new set of consistent laws of physics before that question has an answer.

#### Dale

Mentor
Motors are designed to work based on Maxwells equations, and the back EMF falls out of Maxwells equations. You can’t delete the back EMF without deleting Maxwells equations. And if you delete Maxwells equations then we have no way to tell you how a motor operates.

#### phinds

Gold Member
Summary: What would be the effect if there were no back EMF in a DC motor
To simplify what others have already told you, your question is equivalent to asking "if the laws of physics did not apply, what would the laws of physics say about <insert nonsense of your choice>".

#### ZapperZ

Staff Emeritus
2018 Award
Summary: What would be the effect if there were no back EMF in a DC motor

I know that as per Lenz's Law, back EMF moves in a direction so as to counter the flow of the changing magnetic flux which induced it in the first place. And it does so because if it flowed in the same direction it would contravene the law of conservation of energy as more and more energy would be created without a further input of energy. But what would be the case if, somehow, there were no back EMF. Wouldn't this mean that the supply current would just run at maximum? I've had it suggested that the motor would run faster and faster until it blew up and I can't find (or think of) any answer to this.
You may think this is a trivial thing to answer, but it isn't, and as mfb pointed out, when you decide to throw out the rules of the game, you need to tell us what rules are applicable now.

For example, how would you measure current anymore, now that you have throw out Maxwell equation. If you think about it a bit more, you'll realize that our measuring mechanism for many of these things DEPEND on the validity of Maxwell equation. Now that you've decided to throw that out, what do you expect to measure and how do you know what you are measuring? Why would the motor even run in the first place, since it depends on the mechanism described by rules that you've thrown out?

Many people like to come here and throw out this "what if" scenario without realizing that the "what if" has way deeper and wider implication and applicability than within the narrow confines of what they were thinking of. A lot of these things are interconnected and interrelated. You simply can't pick and choose what to keep and what to ignore.

Zz.

#### metastable

I advise against doing the following because it could damage all your hardware and is potentially dangerous, but you can get no back emf in a dc motor by putting the motor in a clamp so it can’t freely rotate and then apply a voltage. After a slight delay due to the inductance the current will run at maximum according to ohm’s law.

#### metastable

Another situation with no bemf is full throttle acceleration from standstill in any sort of permanent magnet electric vehicle.

#### metastable

In reality though, even if you clamp the motor, if you consider the esc - electronic speed controller - to be an integral part of the motor, modern controllers use clever software and pulse width modulation (PWM) to maintain the motor within software defined “current limits,” so the current would not necessarily run to maximum according to ohms law in a clamped situation.

#### marcusl

Gold Member
I advise against doing the following because it could damage all your hardware and is potentially dangerous, but you can get no back emf in a dc motor by putting the motor in a clamp so it can’t freely rotate and then apply a voltage. After a slight delay due to the inductance the current will run at maximum according to ohm’s law.
Your answer about no back EMF includes a back EMF. The "slight delay" is due to the back emf created across the inductor when the voltage is applied.

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#### metastable

Your answer about no back EMF includes a back EMF. The "slight delay" is due to the back emf created across the inductor when the voltage is applied.
Ah sorry I thought the form of back emf the OP was referring to was a “separate phenomenon” from the inductance back emf:

The term back electromotive force is also commonly used to refer to the voltage that occurs in electric motors where there is relative motion between the armature and the magnetic field produced by the motor's field coils, thus also acting as a generator while running as a motor. This effect is not due to the motor's inductance but a separate phenomenon.

Source:

#### marcusl

Gold Member
Technically, back emf comes from changing flux through a coil. It doesn’t matter if that arises from a changing current or motion through a magnetic field.

"If there were no back EMF"

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