4.
10 Power factor
A motor consumes both active power, which it converts into mechanical work, and
reactive power, which is needed for magnetization and which is not converted to
work.
The active and reactive power, represented in the diagram (below) by P and Q,
together give the apparent power S. The ratio between active power, measured
in kW, and apparent power, measured in kVA, is known as the power factor. The
angle between P and S is usually designated as ϕ, and the power factor itself is
designated as cos ϕ.
Power factor is usually between 0.7 and 0.9. It is lower for small motors and higher
for large motors.
Power factor is determined by
measuring the input power, voltage
and current at rated output power.
The power factor stated is subject to a
tolerance of (1-cos ϕ)/6 .
If there are many motors in an installation, a lot of reactive power will be consumed
and therefore the power factor will be lower. For this reason, power suppliers
sometimes require the power factor of an installation to be increased. This is done
by connecting capacitors to the supply which absorb reactive power and thus raise
the power factor.
Phase compensation
With phase compensation, the capacitors are usually connected in parallel with the
motor, or with a group of motors. However, in some cases, over-compensation can
cause an induction motor to self-excite and run as a generator. Therefore, to avoid
complications, it is a normal practice not to compensate for more than the no-load
current of the motor.
The capacitors must not be connected in parallel with single phases of the winding;
such an arrangement may make the motor difficult or impossible to start with star-
delta starting.
If a two-speed motor with separate windings has phase compensation on both
windings, the capacitors should not remain in circuit on the unused winding.
62 9AKK105285 EN 02-2014 | ABB Motors and Generators
Under certain circumstances, such cos ϕ Constant K
capacitors can cause increased heating without Compensation to cos ϕ =
of the winding and possibly also compen-
sation 0.95 0.90 0.85 0.80
vibration.
0.50 1.403 1.248 1.112 0.982
0.51 1.358 1.202 1.067 0.936
The following formula is used to 0.52 1.314 1.158 1.023 0.892
calculate the size (per phase) of 0.53 1.271 1.116 0.980 0.850
a capacitor for a mains frequency 0.54 1.230 1.074 0.939 0.808
of 50 Hz: 0.55 1.190 1.034 0.898 0.768
0.56 1.150 0.995 0.859 0.729
Q
C = 3.2 · 10 6· ____ 0.57 1.113 0.957 0.822 0.691
U2 0.58 1.076 0.920 0.785 0.654
0.59 1.040 0.884 0.748 0.618
Where C = capacitance, μF 0.60 1.005 0.849 0.713 0.583
U = capacitor voltage, V 0.61 0.970 0.815 0.679 0.548
Q = reactive power, kvar. 0.62 0.937 0.781 0.646 0.515
0.63 0.904 0.748 0.613 0.482
0.64 0.872 0.716 0.581 0.450
Reactive power is obtained from: 0.65 0.841 0.685 0.549 0.419
0.66 0.810 0.654 0.518 0.388
P
___ 0.67 0.779 0.624 0.488 0.358
Q=K·P
η 0.68 0.750 0.594 0.458 0.328
0.69 0.720 0.565 0.429 0.298
Where K = constant from table on right 0.70 0.692 0.536 0.400 0.270
P = rated power of motor, kW 0.71 0.663 0.507 0.372 0.241
η = efficiency of motor 0.72 0.635 0.480 0.344 0.214
0.73 0.608 0.452 0.316 0.186
0.74 0.580 0.425 0.289 0.158
0.75 0.553 0.398 0.262 0.132
0.76 0.527 0.371 0.235 0.105
0.77 0.500 0.344 0.209 0.078
0.78 0.474 0.318 0.182 0.052
0.79 0.447 0.292 0.156 0.026
0.80 0.421 0.266 0.130
0.81 0.395 0.240 0.104
0.82 0.369 0.214 0.078
0.83 0.343 0.188 0.052
0.84 0.317 0.162 0.026
0.85 0.291 0.135
0.86 0.265 0.109
0.87 0.238 0.082
0.88 0.211 0.055
0.89 0.184 0.027
0.90 0.156
Table 4.7 Phase compensation
ABB Motors and Generators | 9AKK105285 EN 02-2014 63
