Page 36 - Industrial Power Engineering and Applications Handbook

Theory, performance and constructional features of induction motors 111 7
Table 1.8 Derating for higher altitudes
Class E Class B
Altitude Permissible temp. rise Permissible output Permissible temp. rise Permissible output
tm) above 40°C from the curves above 40°C from the curves
(%) m)
( "C) (%) ( "C) (%)
up to 1000 1 75a 100 100 80" IO0 100
Reduction in temperature ~ 0.75 I .o - 0.8 1 .o -
rise for every 100 m above
1000 m
2000 I 67.5 90 94 72 90 94
3000 , 60.0 80 87 64 80 87
4000 I 52.5 70 79 56 70 79
aIEC 60034-1 has now substituted this by reduction in the ambient temperature.

100 lower will be this current and it will depend upon the
magnitude of magnetic induction (type and depth of
slots) and air gap, etc.
2 No-load losses
Windage losses: A result of friction between the
moving parts of the motor and the movement of
air, caused by the cooling fan, rotation of the rotor,
t 8o etc.
c Core losses (Section 1.6.2A(iv))
,P - Eddy current losses, caused by leakage flux
0
'b - Hysteresis losses, caused by cyclic magnetization
s!
e 60 of steel.
c
These may be considered up to 3-8% of the rated
8 output.
3 No-load power factor
An induction motor is a highly inductive circuit and
Altitude in metres - the no-load p.f. is therefore quite low. It may be of
40 the order of 0.06-0.10 and sometimes up to 0.15.
1000 2000 3000 4000 The above values give only a preliminary idea of the
no-load data of a motor. The exact values for a particular
Figure 1.14 Derating cutve for higher altitudes
motor may be obtained from the manufacturer.

Example 1.5
In the previous example, if altitude be taken as 2000 m, then 1.8 Effect of loading on motor
a further derating by 94% will be essential. performance

=
:. h.p. required = loo 126.8 h.p. The declared efficiency and power factor of a motor are
0.84 x 0.94
affected by its loading. Irrespective of the load, no-load
A 125 h.p. motor may still suffice, which the manufacturer losses as well as the reactive component of the motor
alone can confirm, knowing the maximum capacity of a 125 remain constant. The useful stator current, Le. the phase
h.p. frame.
current minus the no-load current of a normal induction
motor, has a power factor as high as 0.9-0.95. But because
1.7 No-load performance of the magnetizing current, the p.f. of the motor does not
generally exceed 0.8-0.85 at full load. Thus, at loads
lower than rated, the magnetizing current remaining the
A study of no-load performance suggests that no-load same, the power factor of the motor decreases sharply.
current, power factor and losses may vary in the following The efficiency, however, remains practically constant for
proportions, depending upon the type, size and design of up to nearly 70% of load in view of the fact that maximum
the motor: efficiency occurs.at a load when copper losses (Z'R) are
equal to the no-load losses. Table 1.9 shows an approxi-
1 No-load current mate variation in the power factor and efficiency with
This may be 25-50% of the full-load current and the load. From the various tests conducted on different
sometimes even up to 60%. The higher the rating, the types and sizes of motors, it has been established that the