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Motor Trip Class

11/02/2020 6:44 PM

I could not get any clarity regarding trip class setting, There are 2 thoughts on this , both make sense to me however both cant be correct.

1. Depending on type of Load will depend on trip class setting, example high inertia load may be trip class 5.

2.Trip class is determined by motor specification, If the motor can handle stall current up to 20s without failure, then trip class 20 can be used.

Please share your thoughts.

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#1

Re: Motor Trip Class

11/03/2020 1:35 AM

"The motor trip current does not determine the choice of trip class. Trip class specifies the length of time it will take for the relay to open in an overload condition. Classes 5, 10, 20 & 30 are the most common. Class 5, 10, 20 & 30 overload relays will trip within 5, 10, 20 & 30 seconds respectively at 600% of motor full load amps. Class 5 is usually used for motors requiring extremely fast tripping. Class 10 is commonly used to protect artificially cooled motors such as submersible pump motors of low thermal capacity.. Class 20 is usually sufficient for general purpose applications. Class 30 is usually required for high inertial loads to help prevent nuisance tripping.

Class 20 is used for all general purpose motors that have no special trip class instruction requirements provided by the manufacturer. If the motor requires a faster trip class the manufacturer's instructions should state that. If the motor has been designed to be compatible with a slower trip class, that should also be stated by the manufacturer. If you encounter a motor that has smaller dimensions that what is standard for the power and speed rating, you should check with the manufacturer to make sure that it doesn't need a faster trip class. If you encounter an application that has a high inertia and class 20 overloads trip when the motor is started, check with the manufacturer to see if class 30 overloads are permissible. If they are not, the application may require a special or oversized motor.

This information is based on NEMA Trip Classes. "

https://electronics.stackexchange.com/questions/218075/what-calculations-determine-the-choice-of-the-overload-relay-trip-class-to-prote

https://www.gt-engineering.it/en/Insights/nema-classes-vs-ansi-tripping-curves

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#7
In reply to #1

Re: Motor Trip Class

11/29/2020 12:41 AM

Motor Insulation class determines the maximum temp. the motor can develop without injurious heating which is related to load .Mostly class F insulated motors are in use to day which can go up to 145Deg C including ambient leaving about 10Deg cushion for hot spots.Motors depending upon ratings are provided with protection devices.LV lower rated motors are provided with thermistors with alarm and trip setting.LV motors above 500KW and MV motors are provided with RTD;s with Platinum resistance device.Large motors have differential protection with digital or electromagnetic relays with accuracy class of PS.

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#2

Re: Motor Trip Class

11/03/2020 6:26 PM

It is ONLY about the motor design, not the load.

Almost all motors designed to NEMA MG-1 specifications (North America), will be Class 20. Exceptions are submersible motors for pumps are usually Class 10, some are Class 5, and "Mill Duty" motors are sometimes Class 30, but that is becoming more and more rare because Mill Duty motors were notoriously inefficient and energy efficiency standards have trumped them in most applications.

Almost all motors designed to IEC (International) specifications will be Class 10. There are few exceptions.

High inertia loads, like centrifuges, are allowed to be started with the overload bypassed, but once running, are usually still Class 20 in terms of the long time running protection.

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#3
In reply to #2

Re: Motor Trip Class

11/03/2020 10:56 PM

Thank you very much for the info, I wont change the trip class from 10 to 20 until I get the correct motor specs. We have four 4kw Siemens motors on a Gantry crane for translation running of a single VSD, There has been frequent trips on one of the 4 overload protection relays, even though the motors are cool. Easy fix was to change trip class, but I will investigate some settings regarding torque at low speed.

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#4
In reply to #3

Re: Motor Trip Class

11/04/2020 2:31 AM

Torque is something that the load presents to the motor to overcome.

At stall, the current is the same irrespective of what the motor is connected to.

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#5
In reply to #3

Re: Motor Trip Class

11/06/2020 2:13 AM

Not all OL relays are suitable for use on the load side of a VFD, the harmonics in the VFD output cause them to heat faster than they should and nuisance trip. Standard bi-metal OL heaters are the worst in this regard, they are designed to operate at 50 - 60Hz only. Some mfrs offer special versions that are compensated for operation on the load side of a VFD, but you have to look for them. Eutectic meting alloy type OL relays were best, they did not react to harmonic heating up to 400Hz output. But they have been outlawed everywhere except North America due to RoHS (Reduction of Hazardous Substances) restrictions.

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#6
In reply to #3

Re: Motor Trip Class

11/06/2020 3:51 AM
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