When a motor is stuck, why does current (amps) go up?

[madison069]

To start out with, it's not just one equation to describe a electric motor.

Here is a link that can explain the theory of a electric motor better then most of us can tell you in a simple post.

http://www.reliance.com/mtr/mtrthrmn.htm

Maybe you will get a better understanding of why the motor current draw increase when the motor stalls out.

 

[Jacobson]

This sums it up very well: From http://forums.mikeholt.com/showthread.php?t=52458

Quote:
The current drawn by an induction motor is a function of the rotor speed. The applied voltage creates a rotating magnetic field. When the rotor is rotating at the same speed as this magnetic field, the input current is minimum.

When the rotor is stopped, the input current is a maximum. This is because the turning rotor produces a back EMF that reduces the input current. If the rotor is not turning, there is no back EMF, and the input current is the highest. This is refered to as the "locked rotor current" because it is the same current that would be drawn if the rotor were locked in place so it can't turn.

When a motor is first started, the rotor is stopped, so it draws the locked rotor current. The motor quickly starts turning, and the rotor current decreases. The current keeps droping until the rotor gets up to full speed. At that point the current is at minimum. (Full speed is slightly less than the speed of the rotating magnetic field, and depends on how much torque the motor must produce to turn the load).

In most cases, I think sizing the breaker at 250% of rated current is enough to prevent tripping on startup. It might not be enough if the motor load has a large amount of inertia, or if the motor has a particularly large locked rotor current.

Here is how someone explained it to me:

As current flows through the input motor coil of wire, the electromotive force causes the secondary coil to rotate, this relative motion between the two coils on the “shaft” portion of the motor induces a “Back Electromotive Force” in the input motor coil. This Back EMF limits the current flow in the motor and dictates the maximum spin rate for the voltage applied to the motor. So, If the motor is Stuck, i.e. not spinning no Back EMF is produced by the “shaft” coil of wire and so in the primary coil/ input has no “resistance” Thus current surges and burns out the fuse.


So, in conclusion, it seems the bottom line is that the operation of the motor itself creates resistance. Then, due to Ohm’s Law (Voltage = Current * Resistance), the current varies indirectly with resistance? So, when the motor stops, the back resistance goes down, and current goes up (with voltage staying constant)

 

[joel63]

Here is how someone explained it to me:

As current flows through the input motor coil of wire, the electromotive force causes the secondary coil to rotate, this relative motion between the two coils on the “shaft” portion of the motor induces a “Back Electromotive Force” in the input motor coil. This Back EMF limits the current flow in the motor and dictates the maximum spin rate for the voltage applied to the motor. So, If the motor is Stuck, i.e. not spinning no Back EMF is produced by the “shaft” coil of wire and so in the primary coil/ input has no “resistance” Thus current surges and burns out the fuse.


So, in conclusion, it seems the bottom line is that the operation of the motor itself creates resistance. Then, due to Ohm’s Law (Voltage = Current * Resistance), the current varies indirectly with resistance? So, when the motor stops, the back resistance goes down, and current goes up (with voltage staying constant)

You're getting warm with your understanding on this subject.

Keep studying and soon you will be explaining it to others.

Remember, Inductance.

 

[Wakefield]

"Inductive impedance" vs.(or plus)resistive impedance?

Study of different kinds of AC motors-
there was one kind demonstrated,they were a pair wired together and with power on they sat still but turning the armature of one of them caused the armature of the other one to turn the same amount!

Capacitor start and permanent capacitor motors?
Air conditioner motor that doesn't have enough torque to start a load (unless Freon pressure against its pump has bled off) but when spinning has more than enough torque?
Centrifugal switch on an old washing machine motor-it runs but won't spin up from a stop unless the centrifugal switch works and closes its contact-which makes another winding hot in the motor but that winding gets kicked open (not hot) by the centrifugal switch once the motor gets up some speed-that motor doesn't have a capacitor but someone said that putting a big capacitor in series with the centrifugal switch would make the locked up torque of the motor even stronger

 

[HubbaBubba]

If you have followed the post this far...(like R. Simmons would say)..STOP the insanity.

 

[Wakefield]

Maxwell's Equations-heady stuff?
Is that what Einstein was brought up on? (In other words some advanced physics)

I see there is a Wikipedia article about James Clerk Maxwell

 

[Steinmetz]

When a motor is stuck, it can cause a spike in current.
This can melt a wire, or blow a fuse.

But why? What exactly is happening here?
Which values are going up, and which are going down?

Volts = Amps * Resistance
Watts = Amps * Volts

The "locked rotor" current value for an electric motor is always the highest.

 

[Steinmetz]

Am no electrician but a motor is a resistance device.

That's why a motor draws allot more AMPS on start up. You need to increase the Amps to overcome the resistance so current will flow. Once the motor is up to speed less amps are needed to maintain current flow.

I see it all the time at work. A starter motor will jam and glow white with heat. The energy has to go somewhere so its converted into heat.

Edit: Iam talking about DC here. Mainly 12-24v

Actually, a motor is an inductive load.

 

[joel63]

In addition to your learning about electric motors learn about Eli the Ice Man.

http://physics.bu.edu/~duffy/sc545_notes06/ELI.html

 

[Pantsfall_McFixit]

When dealing with AC motors the there is more complication going on since the current is constantly switching directions. AC adds layers of electrical and magnetic equations on top of the old Ohms Law principles. It's not magic, but since so many things are going on at the same time, it can be hard to understand. I've been trying to grasp the whole picture for years; it's easier to look at one of the many concepts going on at the same time instead of all of them at once.