Power Factor Correction
- As studied earlier, induction motors are characterized by power factors less than unity, leading to lower overall efficiency (and higher overall operating cost) associated with a plant’s electrical system.
- Capacitors connected in parallel (shunted) with the motor are typically used to improve the power factor.
- The impacts of p.f. correction include reduced kVA demand (and hence reduced utility demand charges),I2reduced I’R losses in cable upstream of the capacitor, reduced voltage drop in the cables and an increase in the overall efficiency of the plant electrical system.
- It should be noted that p.f. capacitor improves power factor from the point of installation back to the generating side. It means that if a p.f. capacitor is installed at the starter terminals of the motor, it would not improve the operating p.f. of the motor, but the p.f. from starter terminals to the power generating side will improve that is the benefit of p.f. would be only on upstream side.
- The size of capacitor required for a particular motor depends upon the no-load reactive kVA (kVAR) drawn by the motor , which can be determined only from no-load testing of the motor. In general, the capacitor is then selected to not exceed 90% of the no-load kVAR of the motor.
- Alternatively, typical power factors of standard motors can provide the basis for conservative estimates of capacitor ratings to use for different size motors. The capacitor rating for power connection by direct connection to induction motors is shown in Table.
|
Motor |
Capacitor |
|||||
|
3000 |
1500 |
1000 |
750 |
600 |
500 |
|
|
5 |
2 |
2 |
2 |
3 |
3 |
3 |
|
7.5 |
2 |
2 |
3 |
3 |
4 |
4 |
|
10 |
3 |
3 |
4 |
5 |
5 |
6 |
|
15 |
3 |
4 |
5 |
7 |
7 |
7 |
|
20 |
5 |
6 |
7 |
8 |
9 |
10 |
|
25 |
6 |
7 |
8 |
9 |
9 |
12 |
|
30 |
7 |
8 |
9 |
10 |
10 |
15 |
|
40 |
9 |
10 |
12 |
15 |
16 |
20 |
|
50 |
10 |
12 |
15 |
18 |
20 |
22 |
|
60 |
12 |
14 |
15 |
20 |
22 |
25 |
|
75 |
15 |
16 |
20 |
22 |
25 |
30 |
|
100 |
20 |
22 |
25 |
26 |
32 |
35 |
|
125 |
25 |
26 |
30 |
32 |
35 |
40 |
|
150 |
30 |
32 |
35 |
40 |
45 |
50 |
|
200 |
40 |
45 |
45 |
50 |
55 |
60 |
|
250 |
45 |
50 |
50 |
60 |
65 |
70 |
- From the above table, it may be noted that required capacitive kVAR increases with decrease in speed of the motor. As the magnetizing current requirement of a low speed motor is more in comparison to the high speed motor for the same HP of the motor.
- Since a reduction is line current and associated energy efficiency gains, are reflected backwards from the point of application of the capacitor. The maximum improvement in overall system efficiency is achieved when the capacitor is connected across the motor terminals, as compared to somewhere further upstream in the plant’s electrical system.
- However, economies of scale associated with the cost of capacitors and the labour required to install them will place an economic limit on the lowest desirable capacitor size.