AC induction motor,i use a strong clamper to lock the motor shaft

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Discussion Overview

The discussion centers around the behavior of an AC induction motor when its rotor is locked using a clamp while the power supply is switched on. Participants explore the implications of this setup, including potential overheating, the role of thermal overload switches, and comparisons to transformers.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant inquires whether locking the motor shaft will cause a short circuit, while others clarify that it will not, but rather the motor will become an electric heater.
  • There is a discussion about the purpose of the test, with some participants suggesting it is to determine if the motor can provide torque at zero radial velocity.
  • Participants mention that locking the rotor is not considered a malfunction, but it will lead to overheating until the thermal overload switch activates.
  • Questions arise regarding the operation and necessity of thermal overload switches in motors, with some participants stating that all motors manufactured by their company included such switches for safety certification.
  • Concerns are raised about the consequences of operating a locked rotor motor without an overload switch, including the risk of burning the enamel on the windings and potential fire hazards.
  • A comparison is made between the operation of a locked rotor motor and a transformer under load conditions, noting the differences in design and potential outcomes.

Areas of Agreement / Disagreement

Participants generally agree that a locked rotor condition will lead to overheating and that thermal overload switches are important for preventing damage. However, there is no consensus on the exact outcomes of operating a motor under these conditions without an overload switch, as some participants express varying levels of concern regarding potential damage.

Contextual Notes

The discussion includes assumptions about the design and safety features of motors, as well as the operational limits of locked rotor conditions, which may vary by motor type.

Who May Find This Useful

This discussion may be of interest to individuals involved in motor testing, electrical engineering, and those seeking to understand the safety mechanisms in AC induction motors.

jxlee
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I have a AC induction motor,i use a strong clamper to lock the motor shaft.Then i switch on the power supply.?
So,i want to ask is there the motor will become short circuit?
Can i know what is the cause for this malfunction?Is that the motor stator burn out or they still got other reason causing this?
 
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Quick question on what you are trying to do... Are you trying to test the characteristics of a motor, debug a motor, or are you trying to figure out why the motor cannot provide an adequate amount of torque at a radial velocity of zero?

As for whether or not it will look like a short circuit, and the answer is no. There's the stator and rotor impedances to take into account.
 


So,i want to ask is there the motor will become short circuit?
No. It will become an electric heater.

Can i know what is the cause for this malfunction?
I don't understand this question. Locking the rotor with a clamp is not a malfunction.

Is that the motor stator burn out or they still got other reason causing this?
The motor will continue to heat until the overload switches it off. When the motor cools down the overload will switch back on and the cycle will repeat.
 


TurtleMeister said:
The motor will continue to heat until the overload switches it off.

Do you happen to know why?
 


Ya.I want to test the characteristics of the motor.I want too find out if accidently the motor rotor is stuck but the motor power supply are still switch on, so will the motor burn? And what is the causes to this?
By the way,may i know what is the motor overload switch?
 


Phrak said:
Do you happen to know why?
It's a thermal overload which is located in close proximity to the windings and field core. It simply switches the main winding on and off with rise and fall of temperature.
 


This mean every motor have this thermal overload switch,and how this thermal overload switch operate?
 


I want to test the characteristics of the motor.I want too find out if accidently the motor rotor is stuck but the motor power supply are still switch on, so will the motor burn?
It's called a lock rotor test. If the thermal overload is functioning correctly then the motor will become very hot, but not hot enough to cause damage. However, a locked rotor is not a normal condition and the motor should not be purposely operated that way.
 


This mean every motor have this thermal overload switch,and how this thermal overload switch operate?
Well, all I can say is that every motor my company manufactured had an overload. We manufactured fractional hp motors between about 1/5 hp to 1 hp.

Our motors were required to have an overload for UL certification.
 
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  • #10


Thank,Sir...But if we despite the overload switch.We just supply the normal power to the motor and with the rotor locked.So ijust want to know will the motor burn out or it still can operate like before after we remove the locked?
 
  • #11


jxlee said:
Thank,Sir...But if we despite the overload switch.We just supply the normal power to the motor and with the rotor locked.So ijust want to know will the motor burn out or it still can operate like before after we remove the locked?
If you operate a locked rotor motor without an overload then the enamel on the motor windings will start to burn. It smells bad. :) This would be a fire hazard. The windings would eventually short out and trip the breaker, hopefully before it starts a fire.

The amount of time you can operate a motor with locked rotor varies depending on the motor design.
 
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  • #12


Thanks Sir...But a motor is build up with coil same as transformer.So if we let the transformer switch on with no load connect to it,is it will burn also?
 
  • #13


No. A motor is similar but not the same as a transformer. There is an air gap between the stator and rotor. A locked rotor motor is more comparable to a transformer with its output shorted. If not fused then the transformer would also burn.
 
  • #14


Thanks Sir..Thanks for the info...=)
 
  • #15


You're welcome. Glad I could help.
 

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