motors short circuit withstanding

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kwired

Electron manager
Location
NE Nebraska
First of all motors do not pass current through to other equipment like switches, fuses, or breakers. Because of this, I don't know why they would ever need a short circuit current withstand rating.

A motor that is started across the line essentially has a short circuit initially.

Resistance of the wire that makes up the windings is the only impedance and it is not very high, but incuctance rapidly increases as the magnetic field increases resulting in a rapid drop in the current.

If you have a ground fault or winding short within motor windings you have a motor that needs rewind or replaced and any short circuit withstand ratings will not save the motor anyhow. Proper branch circuit protective device would limit amount of current and/or duration during a fault just like on any other equipment.
 

electrics

Senior Member
does it mean we dont need any other short circuit protection measure for motors other than circuit protection ??
 

raider1

Senior Member
Staff member
Location
Logan, Utah
does it mean we dont need any other short circuit protection measure for motors other than circuit protection ??

Correct, the branch circuit short circuit and ground fault protective device will provide the short circuit protection for the motor.

The separate overload device required in Part III of Article 430 will provide the overload protection for the motor and the branch circuit conductors.

Chris
 

Hameedulla-Ekhlas

Senior Member
Location
AFG
Hi, I wonder how motors are protected against short circuits? is there any rating for scc??

U.L #508 Motor Controller Short-circuit Test Rating

1 HP or less and 300 V or les 1,000A
50HP or less 5,000A
Greater than 50 HP to 200 HP 10,000A
201 HP to 400 HP 18,000A
401 HP to 600 HP 30,000A
601 to 900 HP 42,000A
901 HP to 1600HP 85,000A
 

Hameedulla-Ekhlas

Senior Member
Location
AFG
does it mean we dont need any other short circuit protection measure for motors other than circuit protection ??




Other Types of Motor Protection:

Open-Field Protection:
DC shunt and compound wound motors can be protected against the loss of field excitation by installing field loss relays in the shunt field circuit. Larger Dc mtors may race dangerously with the loss of field excitation, while other motors may not race because of friction and the fact that they are small.

Overspeed Protection:
To prevent damage to driven machine materials in the industrial process or the motor, overspeed protection is provided by controlling the power supply frequency in ac motors and by limiting the maximum shunt field resistance or armature voltage in dc motors. Typical applications included paper and printing plants steel mills processing plants and the textile industry.

Overtravel Protection:
In applications where precise postioning required control devices are used to govrn the starting stpping and reversal of electic motors. these devices can be used to control regular operation or as emergency switches to prevent the improper functioning of machinery.

Overvoltage Proctection:
Devices such as overvoltage relays provide a reduction in the level of applied voltage or maintained interruption of power to the motor circuit when excessive voltage levels are experienced. High induced voltages may occur as a result of the interruption of inductive motor control cicuits.

Reversed Current Protection:
rectifiers are used to protect dc controller in dc or three phase ac systems that can be subject to damage when experiencing phase failures and phase reversals. It is also important to provide reversed currnt protection for battery charging equipment.

Reversed Phase Protection:
Phase failure and phase reversal relays are used to prevent the operation of elevators and industrial machinery to protect motors machines and personnel from ptential hazards when phase revesal or loss occur. Interchanging two phase of the supply of a three phase induction motor will reverse its direction of rotation.

Undervoltage Release:
Undervoltage relays and two wire control are employed to initiate the temporary opening of a motor circuit on the reduction or failure of voltage. On restoration of full voltage the motor is automatically restarted.

For motors with greater than fractional horsepower ratings devices such as fuses circut breakers and selfprotected combination controllers must be installed ahead of the motor control apparatus to protect branch circuit conductors, motor control apparatus personneel and the motor itself against fault conditions that may be the result of shortcircuits or grounds.
 

kwired

Electron manager
Location
NE Nebraska
Correct, the branch circuit short circuit and ground fault protective device will provide the short circuit protection for the motor.

The separate overload device required in Part III of Article 430 will provide the overload protection for the motor and the branch circuit conductors.

Chris

I will add that the short circuit and ground fault protective device is more for protection of the conductors and other devices than the motor. Most times they are needed to operate the motor is already compromised to the point it needs replaced or rewound.

A more accurate statement would perhaps be that the SC/GFP device provides protection for the branch circuit, plus the protection does not necessarily protect the branch circuit equipment so much as it protects the property from possible overheating of any part the branch circuit in the incident.
 

electrics

Senior Member
I will add that the short circuit and ground fault protective device is more for protection of the conductors and other devices than the motor.

so what does protect a motor from short circuit?? what if a very high cross-section wire is used as feeder and its circuit breaker sc protection setting is very high and till a normal sc on the line breaks the circuit breaker a long time short circuit current burns the motor?? Why motors have no any separate protection?
 

iwire

Moderator
Staff member
Location
Massachusetts
so what does protect a motor from short circuit?? what if a very high cross-section wire is used as feeder and its circuit breaker sc protection setting is very high and till a normal sc on the line breaks the circuit breaker a long time short circuit current burns the motor?? Why motors have no any separate protection?

Motors have very specifically sized overloads. If for some reason the breaker or fuse did not open quickly the overloads would open very shortly.
 

kwired

Electron manager
Location
NE Nebraska
so what does protect a motor from short circuit??

Most times they are needed to operate the motor is already compromised to the point it needs replaced or rewound.

plus the protection does not necessarily protect the branch circuit equipment so much as it protects the property from possible overheating of any part the branch circuit in the incident.

If you have a short circuit or ground fault within the motor windings the motor is most likely toast, there is no point in protecting the motor itself, the important thing is to open the circuit before the heat that is rapidly increasing in the current path begins to damage other property.

If you have a short circuit or ground fault in the branch circuit someplace ahead of the motor the component with the fault likely may be damaged, but again the important thing is to open the circuit before other property suffers any consequences. The motor needs no special protection from this because it is not a part of the fault path and will not be affected.
 
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