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Motor over current

12/14/2021 3:14 AM

Dear all

in standard how much time can motor withstand if it is draw current exceeded the full

load

thanks

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Guru
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#1

Re: motor over current

12/14/2021 3:35 AM

Well, that rather depends upon the motor. One might call the manufacturer for advice, perhaps, though bear in mind that running a motor in <...over current...> is beneficial to motor manufacturers' sales figures, motor rewinding operations' sales figures, and nothing else.

Please explain why the motor overload protection device has been set too high or bypassed as a matter of routine? Wouldn't this point to either a mismatch between the motor and its load, or a fault within the motor, or possibly both?

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#2
In reply to #1

Re: motor over current

12/14/2021 4:21 AM

Please allow me to ask the question in another way. Why use Short-time-delays protection if we know that this protection allow the fault current to flow for several cycles, which subjects the electrical equipment to thermal stress? why not use over load and instantaneous protection only

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#3
In reply to #2

Re: motor over current

12/14/2021 5:21 AM

A <...fault current...> is cleared by the circuit protective device, which may have both a thermal and a magnetic part of its response. Its job is to protect the wiring.

An <...over current...> is cleared by the motor overload device, which is a separate piece of equipment from the above. Its job is to protect the motor.

The above two are in the domain of co-ordination, which methodology is covered in the electrical standards applicable in most jurisdictions; in the UK, for example, BS7671 applies.

Why the <...we...> does any particular thing is unknown here.

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#4
In reply to #2

Re: motor over current

12/14/2021 5:30 AM

The locked rotor current draw (starting current) is significantly higher than the running current. The thermal stress during these several cycles is minimal compared to the thermal stress of a mechanically overloaded motor. If something is overheating, you have other problems than the starting current condition.

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#6
In reply to #2

Re: motor over current

12/18/2021 2:59 AM

Overload protection is effective for currents not exceeding about 2 times (could even be 1.5 times) the rated current. Instantaneous protection is for short circuit faults ith high current.

If the motor protection relay includes locked rotor protection and prolonged start protection, there may not be much use with short time delay protection.

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#5

Re: motor over current

12/14/2021 6:05 AM

The overload feature on a motor is triggered by temperature not by current generally speaking....How much temperature rise a motor can withstand is dictated by the insulation rating of the windings...

Insulation Classes for Electric Motors

ClassMaximum Ambient Temperature (°C)Maximum Temperature Rise (°C)
A4060
B4080
F40105
H40125

https://www.electricalengineeringtoolbox.com/2016/02/insulation-classes-for-electric-motors.html

https://inspectapedia.com/electric/Electric-Motor-Overheating.php

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#7

Re: Motor over current

12/28/2021 6:12 PM

Motors have a "thermal damage curve"; an amount of time they can withstand a temperature rise, and it is generally described as an I2t relationship. That is why motor thermal overloads are designed to follow an I2t curve as well.

Different motors have different starting and ending points for those curves. For AC induction motors, they tend to be based on "Classes" as defined by various organizations. For North America, we use NEMA Classes, based on the thermal damage curve having a maximum time limit at Locked Rotor Current, defined as 600%. So a Class 10 curve is defined as needing to trip within 10 seconds at 600%, a Class 20 curve is 20 seconds at 600%, a Class 30 = 30 seconds. The other end of the curve is referred to as the "pick-up point" and for NEMA motors, it is no greater than 125% of the motor FLA rating. So the i2t curve is defined as being between those two points; 125% and 600%, with a time shift on the 600% side.

IEC motors are all based on a Class 10 curve, but have different "duty cycles" defined as S1 through S9 based on load and rest times, it's much more complex.

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