Efficiency of DC Motors with Example

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Efficiency of DC Motors with Example: EFFICIENCY OF dc MOTORS

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Power flow of DC Motor


Figure shows the flow of power in a dc motor. The various losses are as follows:

power-flow-of-dc-motor

Constant Loss


Pi = core loss (including stray load core loss)
Pwf = windage and friction loss (at specified speed)
Psh = shunt fi eld copper loss (in a shunt machine)
Pk = Pi + Pwf + Psh = total constant loss

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Variable Loss


PC = I2aRa
= Copper loss ( inclusive of copper loss in series winding in a series motor and also
inclusive of stray-load copper loss)


Total motor loss,
PL = Pk + Pc
Pm = Pe = EaIa


DC Motor Efficiency Equation

The motor efficiency is given by

efficiency-of-dc-motor

Under load, IL = Ia; If is small in a shunt motor.


Then

efficiency-of-dc-motor-copper loss iron loss

or Ia2Ra = Pk


or Variable loss = constant loss


which is the same condition as was shown in a transformer.

Example Problem

A 50 kW, 230 V dc slums motor takes a current of 14.5 A when running light at 1640 rpm. The armature
and fi eld resistance are 0.15 W, and 120 W respectively. Estimate the motor effi ciency when the motor
is drawing 215 A. What would be the maximum effi ciency of the motor and the load current at which
it would occur?

shunt-generator-motor

dc motor efficiency calculation load and no load

For maximum effi ciency,
0.15Ia2 = 3311
or Ia = 148.6 A
Il = 148.6 + 1.92 ª 150 A
PL = 2 × 3311 = 6622 W=6.622 kW
Pin = 230 × 150 = 34.5 kW

η= (34.5 -6.62)/34.5= 80.8%

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