Electric Motor question

[jojo13]

If a motor slowed to a halt, will the current go up or down

 

[Charles Link]

This looks a little like a homework question, but I'll assume it isn't.$\\$ I believe when an electric motor is rotating, it causes a reverse Faraday EMF in the primary current input line, thereby making for a lower current than what occurs if the motor is not rotating. When not rotating, it has a considerably higher current. $\\$ I think @jim hardy is our expert in this area. Perhaps he can also give an input.

 

[jojo13]

This looks a little like a homework question, but I'll assume it isn't.$\\$ I believe when an electric motor is rotating, it causes a reverse Faraday EMF in the primary current input line, thereby making for a lower current than what occurs if the motor is not rotating. When not rotating, it has a considerably higher current. $\\$ I think @jim hardy is our expert in this area. Perhaps he can also give an input.

Yeah, I was thinking it would then go up

 

[jim hardy]

@Charles Link is correct.

in AC motors it's called ":Locked Rotor Amps" and is designated by a letter KVA code.
see https://www.industry.usa.siemens.co...pp-man-section5-part2-speed-torque-curves.pdf

in DC motors it's called "Stall Current" and can be ten or twenty times running current,

 

[berkeman]

Yeah, I was thinking the current would then increase

You didn't answer his question about whether this is homework.

 

[Charles Link]

Perhaps with this one, the most frequently occurring case is the vacuum cleaner when it gets tangled with some string or other material that keeps the motor from turning. Other examples are electric drills and electric saws getting stuck.$\\$ Please correct me @jim hardy if this is not the case: When this condition occurs, currents are going through stationary windings, that have a voltage applied to them, that are designed to turn. With no reverse EMF (from the motion of the windings in the magnetic field of the motor) to make a lower net voltage, the currents are much higher, and in some cases could be high enough to burn out the windings, including the insulation on the windings.

 

[jim hardy]

Please correct me @jim hardy if this is not the case:

that's exactly the case.
the DC motor is easiest to visualize - when it's not moving the only opposition to current flow is resistance of the windings.
So current becomes quite high.

of course one can build a DC motor with resistance high enough to tolerate being stalled,
but at the expense of its having not a lot of torque.
You'd want that in an automobile electric window motor .
for It has to survive kids playing with the buttons
and you'd want it not powerful enough to break the kid's arm.



.

 

[Babadag]

I agree with Jim.
A universal motor is typically used as suction motor across vacuum cleaners and not an induction motor. The universal motor is a series DC-motor that is specially designed to operate on alternating current (AC) as well as on direct current (DC).
Universal motors have high starting torque, operate at high speed, and are lightweight.
The maximum armature [and field] current is limited by circuit impedance [resistance].
If the windings are not damaged then the maximum current has to be less than the admissible.

 

[jim hardy]

Thanks @Babadag
Vacuum cleaners need more RPM than an induction motor can provide.. That's because they use a small diameter impeller in the centrifugal fan that develops differential pressure .

So to you DIYers - beware of vacuum cleaner motors for hobby projects.
They produce extreme RPM .
When i was about twelve i fitted a brass blade from a floor fan to a vacuum cleaner motor.
The blades flew apart embedding metal shards in the floor, ceiling and furniture. Luckily none hit us kids.

So keep a watchful eye on your " budding mad scientist's " experiments. Stick to non-brushed motors.

old jim

 

[darkwood]

In response to the initial question we would need more detail, there are many reasons for a motor to slow to a halt but I assume we are talking about motor failing to carry on with the work it is trying to do possibly for mechanical failure while still been fully energised to do the work, several other scenarios exist that may cause a motor to slow to a halt with differing results, some by design and some by some sort of electrical or mechanical failure that can effect motor behaviour. I also have to assume we are negating protection here to expand on the consequences of say a locked rotor.