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Alarm / Trip Settings for Class F/B Motor

Alarm / Trip Settings for Class F/B Motor

Alarm / Trip Settings for Class F/B Motor

Motor Insulation class is F with B temperature rise.

The Motor vendor recommends to set the alarm / trip value at 140 degree Celsius and 155 degree Celsius respectively.

Max Ambient temperature at Site: 45 degree Celsius

The vendor says if alarm / trip value set at 120 degree / 130 degree, the PT 100 will constantly trigger alarm and trip often.

This is not typical of what I'm accustomed to, as we typically set the Alarm / trip value at 120 degree Celsius and at 130 degree Celsius respectively for Class F/B motor insulation class.

So I asked the vendor to set at 120 degree / 130 degree, he comes back with a variation order saying it is a design change.

I think what I asked is quite typical. Any reason for vendor's argument?

RE: Alarm / Trip Settings for Class F/B Motor

These are embedded winding RTDs? Is the motor nameplate rating based on 45 C ambient? I have a hard time understanding how changing a setpoint is a design change. Are you sure you're getting a Class B rise?

RE: Alarm / Trip Settings for Class F/B Motor

Yes, the Motor has embedded winding RTDs and 45 C ambient. Vendor has clearly put 75 degree temperature rise in his datasheet.

His verbatim reply,

RE: Alarm / Trip Settings for Class F/B Motor

I think from the information provided is that you ordered a Class F/B motor.

Quote (http://www.drivesandautomation.co.uk/useful-inform...)

Modern insulation materials means Class F insulation is commonly used for motor windings. With modern designs, a 'Class B' temperature rise is readily achievable. Therefore Class F insulation with a Class B temperature rise gives us a thermal margin of 25°C, potentially increasing the life of the motor by up to 5 times.

What the vendor is trying to give you is perhaps a Class F motor with a Class F temperature rise. Expect life to be less than with a Class B temperature rise. The vendors maximum temperature for class F insulation is correct, but if you contracted for a class B temperature rise, that should control, the additional motor life should be worth something.
It seems like the question is a commercial one, did you already pay for the additional life provided by the class B temperature rise.

RE: Alarm / Trip Settings for Class F/B Motor

nick See if you can get a copy of the as-tested results. These should include a heat run under rated load (or at least current) conditions. Check for measured temperature rise above shop ambient - and see if they adjusted for the rated 45 C condition (should be just a bit more rise than when they tested at a room temp of maybe 25 C).

My guess is that the "design" rise should mean they're no more than 45+75=120 C, and probably more like a total temp of 117 to 118 C when tested in their factory. If you plan to operate at full load and high ambient, it is quite possible you will be getting consistent nuisance alarms (although NOT trips) when your settings are 120 and 130 C respectively. It is possible that things didn't quite go according to design, and the ACTUAL temperature rise might be a few degrees warmer still - perhaps enough to actually "trip" on overtemperature when set for 130 C.

Motor vendor is protecting the actual insulation life (with Class F limits); if you want to be more restrictive, that's up to you. That being said - the more normal approach is 10 C differential between alarm and trip temps (not the 15 being quoted to you).

Converting energy to motion for more than half a century

RE: Alarm / Trip Settings for Class F/B Motor

I think you should be referring to the factory acceptance test report.
Data sheet is a preliminary design document and doesn't reflect tested values for heat run test etc.
Presuming the test results are better than as in data sheet (which happens normally), the maximum temperature is expected to be 120 degC @ rated load and rated ambient. Hence, I can't understand how Alarm setting of 120 degC and Trip setting of 130 degC can cause problem in service. The factory acceptance test may hold the key (you are probably not getting the motor promised vide vendor data sheet).

RE: Alarm / Trip Settings for Class F/B Motor


An F/B rating indicates the insulation class and allowable temperature rise thus the maximum winding temperature is 155 °C but the allowable temperature rise is 80 °C Doing the math: 80 (B) + 40 (ambient) + 10 (hot spot margin) = 130 °C as a maximum temperature, rather than the 155 °C that you have for a motor designed with insulation class F. The motor F/B gives you a 155-130= 25 °C additional safety margin so operating the motor in normal conditions the insulation will last longer and the motor will have a longer life expectancy.

My suggestion is to start the settings with alarm at 135 ° and trip at 140 °C obviously the motor will not result in damage because it is already Class F, but if the goal is to increase the life expectancy of the motor the above settings will make sense- Once you start with the recommended settings you need to know the motor behavior tracking the temperature in order to know it will be necessary to change the parameters (decrease or increase).

Best Regards


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