Class F motor Vs Class F/B motor - constructional difference 4

[NickParker]

What is the constructional difference between a Class F insulated motor and a Class F insulated motor with a Class B temperature rise? I understand that the cooling circuit is the differentiating factor, but how is the cooling circuit enhanced in a Class F motor with a Class B temperature rise?

 

[jraef]

Enhanced? I would think it’s the other way around!

Class F insulation is supposed to have a HIGHER allowable temperature rise than Class B, 25deg C higher in fact. So the real question would be, what’s WRONG with their cooling design that they cannot attain the proper rise value from the Class F insulation?


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[RRaghunath]

There is no difference in construction of the motor itself. Of course the Class-F/B motor needs to have bigger coolers.
It is the customer choice that he wants to buy a Class-F insulated motor but insists the temperature rise of the motor at full load should not exceed the limits set for Class-B motor.
In order to have lower temperature rise as above, the motor needs to have more cooling.
If Class-F motor has CACA (Closed air Circuit Air) cooling, Class-F/B motor may have CACW (Closed Air Circuit Water) cooling.

R Raghunath

 

[Gr8blu]

The first temperature reference (Class F, in this case) refers to the thermal capability of the insulation. Above this temperature (F = 155 C), the insulating properties and/or mechanical integrity of the insulation system change, possibly resulting in injury to personnel or equipment.

Operating an insulation at LESS than its allowable max temperature (i.e. at a "B" rise instead of the full "F") effectively extends the life of the insulation by a measurable amount, barring other unforeseen mechanical issues. The max temperature for a "B" insulation is 130 C ... which is a 25 C differential compared to the "F". For each 10 C below, life is extended by a factor of 2. This means that an "F" insulation operating at "B" temperatures will last roughly 5X longer than the same system operating at "F" temperatures.

Similarly, when operating higher than the design temperature, insulation life is halved for every 10 C.

Note that for most industrial machines, operating the windings at a Class B temperature rise above a 40 C ambient will result in a frame surface temperature that is (usually) hot enough to boil water - and cause a 3rd-degree burn after 2 seconds of contact with bare skin.

Converting energy to motion for more than half a century

 

[electricpete]

I agree with the previous two posters.

Many users specify Class F insulation with Class B rise as a means to adding thermal margin (for a perceived gain in reliability).

You could arguably achieve a similar result by ordering Class F insulation rating and a higher horsepower rating than you otherwise would, but that might impose constraints/requirements that you will never take advantage of (higher torque capability, efficiency at a far higher load then you ever intend to operate at). It may also result in increased starting current.

Service factor might be somewhere in the middle between those options depending on how you look at it. It requires the motor to operate at a given overload with specified temperature rise.

 

[edison123]

Class F/B machines will have more copper and hence will be bigger with a longer life of 25 years or more than class F machines.

Muthu

http://www.edison.co.in

 

[RRaghunath]

Thanks Muthu Sir for correcting me.

R Raghunath

 

[wcaseyharman]

For what it’s worth, Large utility generators almost always are insulated for class F and operated at class B temperatures.

My recollection is class F per standard is expected to last 20,000 operating hours if continuously at 155 C, which is less than 3 years (others on this forum know these details much better than me I’m sure). Lowering the temperature by 25 degrees obviously extends the life a lot assuming an 100% increase for every 10C.

 

[edison123]

Class F/F machines give me more business than F/B. I have seen even reputed OEM's cutting the factor of safety seen in older machines.

Muthu

http://www.edison.co.in