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eltech
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Motor Duty Cycle

09/22/2011 2:14 AM

If an induction motor runs for 30 seconds and is off for five minutes, It does this 24/7.Is there an overload out there that we can use to protect this motor? This is a 1 Hp motor.We are currently using an electronic overload with Trip Class 5.I think it needs to run 6xFLA for two minutes before it trips.We are currently running new cables to install a thermistor.
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Tornado
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#1

Re: Motor Duty Cycle

09/22/2011 2:51 AM

Another electrical question lacking really basic data, such as motor voltage and FLA. Two minutes of 6xFLA sounds much too long; the motor should be selected to accelerate the load sooner than that. 10.9 starts per hour (60 ÷ 5.5) seems a bit on the high side, but not grossly so. Do you have motor data specifying the number of starts per hour?

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eltech
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#2
In reply to #1

Re: Motor Duty Cycle

09/22/2011 4:41 AM

415Volts, FlA is 1.8Amps.Correct, about 11 starts an hour, NEMA say 28 starts for that size motor, I could not find any info. on other standards.Currently we are using a duty cycle S1 but will change to a S4.My question was about selecting the right overload for that kind of application.The load is a screw feeder.I know that we can use a gearbox with a clutch so the motor runs continuously, but cant do at this stage.
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PWSlack
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#3

Re: Motor Duty Cycle

09/22/2011 6:26 AM

If a motor is in overload and pulling 6xFLC for two minutes, it is seriously undersized and the motor overload protection is insufficient to prevent damage to the motor and possibly a fire in the facility!

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TonyS
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#4

Re: Motor Duty Cycle

09/22/2011 7:51 AM

PWS re read the OP. The motor runs for 30S but the OL's fitted require 6X FLC for two minutes to trip.

We don't know the % of FLC when the motor is up to speed.

As usual we get information in dribs and drabs.

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PWSlack
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#5
In reply to #4

Re: Motor Duty Cycle

09/22/2011 8:33 AM

6xFLC for 2min is enough to burn....

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JRaef
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#6

Re: Motor Duty Cycle

09/22/2011 10:15 AM

Firstly, your assessment of OL Class trip curves is incorrect. The Class number is the MAXIMUM number of SECONDS it will trip in at 600% of setting. So a Class 5 OL will trip in no MORE than 5 seconds at 600% of set current a Class 10 will trip at 10 sec max. etc. etc. Second, you select the trip curve based on the motor's thermal damage curve, they are designed to match. Most IEC motors are designed for Class 10, most NEMA design motors are Class 20, some Mill Duty motors and specialty centrifuge motors are Class 30 etc. The only Class 5 motors I have seen are cryogenic and submersible pump motors, because they are designed to use the immersion liquid as the cooling medium, so they have very very low tolerance for heat, including that coming from within due to overloading. Third, one major benefit of solid state overloads is what is called retentive thermal memory, which means it is always keeping track of the heat buildup in the motor, even when off line. So it will (should) know that the repeated starting is going to affect the rotor heating and will protect you from damage. But not all SSOLs are created equal, you want to read the specs in detail. Bottom line, you should be fine, but RTFM.

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eltech
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#7
In reply to #6

Re: Motor Duty Cycle

09/22/2011 6:07 PM

GA, Thank you, I had the trip class set at 5 .I was not aware that the trip class has to match the motor's thermal damage curve.
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electricalexpert65
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#8
In reply to #6

Re: Motor Duty Cycle

09/25/2011 12:03 PM

Can there be a small correction Raef? As far as I remember, IEC specifies Trip Class as time range taken to trip when 7.2 times (720%) the set current flows thro the relay and not 600% as mentioned. Am I correct?
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electricalexpert65 (1); eltech (2); JRaef (1); PWSlack (2); TonyS (1); Tornado (1)

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