POWER STAGES of a MOTOR

POWER STAGES OF MOTOR

Electric Motor Absorbed Power Estimation

Very often one needs to know the power absorbed by say, a pump or a carrier or fan or some other equipment driven by an electric motor. If one has a three-phase electric power meter and one measures the three phase currents and voltages then the power can be calculated directly.

Most often however all one has is the current reading from the MCC panel ammeter, the method outlined below can be used to estimate the power absorbed by the driven machine.

P = η·√3·V·I·cosφ

where
P is absorbed power in watts
η is motor efficiency
V is applied voltage
I is absorbed current in amps
cosφ is the power factor

The applied voltage is usually known or can easily be measured, the current we have, our problems are the efficiency and the power factor. Electric motor catalogs usually state the efficiency and power factor at full load and at various part loads(for example at no load, 25%, 50%, and 75%).

Curves can be fitted to these points (as shown), but our problem is that we don't know the absorbed power and in fact that is precisely what we want to calculate: an iterative solution is required; ie.

• guess the absorbed power
• calculate efficiency and power factor from fitted curves
• calculate power from
  P = η·√3·V·I·cosφ
• use this as new estimate of power to recalculate efficiency and power factor
• repeat until solutions converge

The various power stages in a d.c. generator are represented diagrammatically in Fig. (1.39).
A – B = Iron and friction losses
B – C = Copper losses

(i) Mechanical efficiency

(ii) Electrical efficiency

(iii) Commercial or overall efficiency

Unless otherwise stated, commercial efficiency is always understood.
Now, commercial efficiency

,

Condition for Maximum Efficiency

The efficiency of a d.c. generator is not constant but varies with load. Consider a shunt generator delivering a load current IL at a terminal voltage V.

The shunt field current Ish is generally small as compared to IL and, therefore, can be neglected.

The efficiency will be maximum when the denominator of Eq.(i) is minimum i.e.

The load current corresponding to maximum efficiency is given by;

Hence, the efficiency of a d.c. generator will be maximum when the load current is such that variable loss is equal to the constant loss. Fig (1.40) shows the variation of η with load current.