Page 111 - Electrical Equipment Handbook _ Troubleshooting and Maintenance

AC MACHINE FUNDAMENTALS

5.12 CHAPTER FIVE

3. Mechanical losses
4. Stray losses
The stator copper losses in ac machines are the heat losses from the conductors of the sta-
tor. They are given by

2
P 3I R
SCL A A
where I is the current flowing in each armature phase and R is the resistance of the con-
A A
ductor in each armature phase. The rotor copper losses are given by
2
P I R
RCL F F
The mechanical losses are caused by bearing friction and windage effects while the core
losses are caused by hysteresis and eddy currents. These losses are called the no-load rota-
tional losses of the machine.
All the input power at no load is used to overcome these losses. Therefore, these losses
can be obtained by measuring the power to the stator at no load.
Stray load losses are all miscellaneous losses that do not fall into one of these categories.
They are taken by convention as 1 percent of the output power of the machine. The overall
efficiency of an ac machine is defined as the useful power output to the total input power:

P out
100%
P
in

REFERENCE

1. S. J. Chapman, Electric Machinery Fundamentals, 2d ed., McGraw-Hill, New York, 1991.

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