Motor temperature trip

Normally we specify Motor Insulation to Class F with Temperature rise limited to Class B. The extra margin of temperature rise allowed due to Class F, will give advantage during motor starting, specially from Hot as well as during Over Loading/Blocking condition during running. The life of Motor also is a function of insulation.

I hope any motor manufacturer can throw more light.

BNDas, Kolkata, India

Hi Das ,

Seems to be a convincing answer . The extra margin , from B Class to F Class , is to meet the overload or any other abnormal situation , I agree .

Thanks again ,

Voltamps

Check with the manufacturer for heat limits because materials used at different times in production change and some turn out not to work well while others do. Some also have recalls and high faliure rates and only tell the people who call about the motors. That is my experience.

hi,

maximum temprature for f class is 155 deg celcious this is theoritical partical you set 70%.

thanks

gopal

Class F insulation give allowable temperature rise 105oC for 1.0 SF, and 115oC for 1.15 SF. To get operation temperature tolerance for F insulation class, consider this : ambient temperature+allowable temperature+10oC.

Said that your motor has SF=1.15 and your ambient temperature is 40oC, so temperature tolerance = 40oC+115oC+10oC = 165oC. If your SF is 1.0, then you'll have 155oC temperature tolerance. Set the temperature protection to 165oC for 1.0SF or 155oC for 1.15SF. but beware of your ambient temperature!

The 10oC can be neglected as it value of tolerance, increasing 10oC from maximum temperature will decrease the lifetime of motor to about one-half, so you can set to 155oC for SF=1.15 and 145oC for 1.0.

cheers,

If want to limit to class B then set at 120Deg C.Normally 10Deg is allowed for hot spot and hence 145 is the best if u are using class F to F limits.Class F to b is old fashioned and wastes power as motor is operating at lower load and hence lower eff and PF

Dear Nesubra,

About relationship amongs loads, efficiency, and power factor, you are free to direct your self to EASA (Electrical Apparatus Serivice Association of U.S.A) article about "Understanding Motor's Efficiencies". No fix statement which said "lower load, lower efficiency", just observe the graph of characteristic.