service fator of motor

[sunloco]

I often see the service factor of motor when I select e-motor , but I don't know the exact meaning of service factor,I often see factor 1.1 and 1.0 , Could any one explain what is service factor and what do we need to pay attention to when we select factor ?

 

[electricpete]

The way I look at it, service factor may provide a little extra margin for temporary overloads or increased life, but shouldn't be relied upon for steady state operation.

Operation at service factor load will allow insulation temperature approx 10C above rated continuous temperature which equates to approximate doubling of insulation aging rate (in a theoretical sense).

Operation at service factor also can put increased stress on other components beside the insulation.

Different people appraoch service factor differently. Try searching the motor forum. Hopefully some folks here on the pump forum will add their perspective as well.

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[PUMPDESIGNER]

Service Factor is used by the majority of commercial pump manufacturers in their standard products, "my 20hp pump pumps more than your 20hp pump", mentality.

As usual, electricpete has the correct scoop, the pump design should not use the service factor, leaving room for field problems such as low voltage, unbalanced voltage, wire size, poor splices, higher flow rates than planned, sags in the power grid voltage, and for longer motor life.

Our solution is to trim (or have the manufacturer trim), the impeller such that no service factor is used anywhere on the pump curve, even at flow rates higher than the BEP or design flow rate. This can be hard to accomplish though, far more time and effort is required to get this done.

PUMPDESIGNER

 

[stanier]

WHere large service factors are specified this has an impact on the motor efficiency and power factor. This leads to higher costs of energy for the life of the plant and for load factor correction equipment and the building it is housed in.

You may find that it is less costly to replace the motor than to live with reduced efficiency and the Capex for increase power factor correction equipment.

Specifiers belive in their ignorance that service factors are good for the plant. Overdesign can be as bad as under desgin.

eg What is the point of designing a motor for a pump for the full HQ curve when the static head and system friction will prevent the pump ever reaching that point? Costs increase for the larger motor, coupling and baseplate on the pump. Then there is the foundation, cable size, switchgear, switchroom, transformer etc all increasing for no real benefit.

Unless the duty of a pump is to be increased or the unknowns are so great dont use a highly inflated service factor.

 

[stanier]

World Pumps Magazine has an article of oversizing pumps in their just released electronic version. This link may not work as you have to be a registered subscribner but that is free so just cut the link back and find World Pumps.

http://www.processrequest.com/apps/display.asp?id=23695439,ZcbcejiahjEC&oid=UdedcBC

 

[PUMPDESIGNER]

I agree with Stanier, my post only stated one side of the issue. We find that people usually overdesign the pump because they are not sure of their hydraulics.
But there are other problems to consider also.
We use concentric, modified concentric, and turbines.
The concentric and modified concentrics are naturally less efficient than the volute designs, but the few points of efficiency are well worth it to get rid of radial thrust causing short life on the bearings.

Again, it will come down to hiring a professional who cares and takes the time to do it the way you need it done, and who is not afraid to lose work because he charges more.

PUMPDESIGNER