Handbook of Electrical Engineering

100 HANDBOOK OF ELECTRICAL ENGINEERING

a) The rotor impedance increases.

b) The rotor emf reduces.

These effects result in the supply current is being nearly constant during most of the run-
up period.

The rotor speed cannot reach the same speed as that of the stator field, otherwise there would
be no induced emfs and currents in the rotor, and no torque would be developed. Consequently when
the rotor speed is near to the synchronous speed the torque begins to decrease rapidly until it matches
that of the load and rotational friction and windage losses. When this balance is achieved the speed
will remain constant.

5.2 Essential Characteristics

The most significant design characteristics of interest to power system engineers in the oil industry
are:-

• Torque versus speed.


• Stator current versus speed.

Characteristics such as efficiency and power factor at running conditions have traditionally been
of secondary importance, but nowadays with an emphasis on energy conservation more attention is
being paid to efficiency in particular. The main objectives in the choice of a motor are that:-

• It creates plenty of torque during the whole run-up period.


• It can be started easily using simple switching methods.


• It is a ‘standard’ design from a manufacturer.

5.2.1 Motor torque versus speed characteristic

Many of the electrical engineering textbooks that include the subject of motors in their contents
describe the equivalent circuit of an induction motor as a series and parallel combination of resistances
and reactances, see References 1 to 8. The equivalent circuit usually defines the situation for one of
the three phases and so care needs to be taken to ensure that the final results obtained apply to the
complete motor. Care is also necessary in using the ohmic data from manufacturers, they may have
either star winding values or delta winding values and the choice may not be obvious. The equivalent
circuit of most practical use is shown in Figure 5.1 for one star connected winding, where:-

s = slip=

stator frequency−rotor frequency

stator frequency

per unit

=

(f orω)−(frorωr)

(f orω)

per unit

Vs = supply voltage per phase.
I 1 = supply and stator current per phase.
I 2 = rotor current per phase.
Rc = resistance representing the iron core eddy current loss. In some situations the
manufacturer may add to this a component to represent friction and windage