INTRODUCTION
Occasionally we get asked to size power factor correction capacitors to improve the power factor of a single motor. Usually the requested improved power factor level is 90 or 95%. The necessary calculations to get the proper capacitor KVAR (Kilovolt Ampere Reactive) value are straightforward, but since we don’t do it often it is nice to have the method in writing.
PROCEDURE
The first thing needed is the full load power factor and efficiency information for the motor. On Baldor motors this can be found in either the 502 data section or the Baldor CDROM. Next, since most power factor tables are worked in terms of Kilowatts, it is necessary to convert the motor output rating into Kilowatts. The procedure for doing this is to take the motor HP multiplied by the constant for KW per HP (0.746). This will give Output KW. Then it is necessary to divide this by the efficiency of the motor (as a decimal) to get the Input KW at full load. Next, refer to power factor correction Table I going in from the left with the existing power factor and coming down from the top with the desired power factor. Where they intersect find the multiplier needed.
Next, multiply the motor Input Kilowatts by the appropriate multiplier from Table 1 to get the required KVAR of power factor correction. This value would be rounded out to match commercially available power factor correction capacitor ratings shown in Table 2.
TABLE 1 
ORIGINAL
FACTOR % 
DESIRED POWER FACTOR % 
85 
90 
95 
60 
0.713 
0.849 
1.004 
62 
0.646 
0.782 
0.937 
64 
0.581 
0.717 
0.872 
66 
0.518 
0.654 
0.809 
68 
0.458 
0.594 
0.749 
70 
0.400 
0.536 
0.691 
72 
0.344 
0.480 
0.635 
74 
0.289 
0.425 
0.580 
76 
0.235 
0.371 
0.526 
77 
0.209 
0.345 
0.500 
78 
0.182 
0.318 
0.473 
79 
0.156 
0.292 
0.447 
80 
0.130 
0.266 
0.421 
81 
0.104 
0.240 
0.395 
82 
0.078 
0.214 
0.369 
83 
0.052 
0.188 
0.343 
84 
0.026 
0.162 
0.317 
85 
0.000 
0.136 
0.291 
86 

0.109 
0.264 
87 

0.083 
0.238 
88 

0.056 
0.211 
89 

0.028 
0.183 
90 

0.000 
0.155 
91 


0.127 
92 


0.097 
93 


0.066 
94 


0.034 
95 


0.000 

TABLE 2 
3PHASE STANDARD CAPACITOR RATINGS
KVAR (Kilovolt Amperes Reactive) 
1.0 
20.0 
70.0 
1.5 
22.5 
75.0 
2.0 
25.0 
80.0 
2.5 
27.5 
85.0 
3.0 
30.0 
90.0 
4.0 
32.5 
100.0 
5.0 
35.0 
120.0 
6.0 
37.5 
140.0 
7.5 
40.0 
150.0 
8.0 
42.5 
160.0 
9.0 
45.0 
180.0 
10.0 
50.0 
200.0 
11.0 
52.5 
225.0 
12.5 
55.0 
250.0 
15.0 
60.0 
300.0 
17.5 
65.0 
350.0 
EXAMPLE:
To illustrate the procedure an example is worked as follows:
What is the KVAR of power factor correction capacitors needed to improve the power factor of a catalog number M2555T, 100 HP motor, to 95% at full load?
Step 1: Look up the existing power factor and efficiency
Efficiency = 94.1%
Power Factor = 85%
Step 2: Convert the HP to Kilowatts output. 100 HP x 0.746 = 74.6 KW
Step 3: Convert Kilowatts output to Kilowatts input by dividing by the full load efficiency.
(74.6)/(.941) = 79.3 KW Input
Step 4: Look in Table 1 to find the multiplier to achieve the desired 95% corrected power factor.
The multiplier is 0.291.
Step 5: Multiply Input KW by this multiplier.
79.3 x 0.291 = 23.1 KVAR
This gives the required Capacitor KVAR.
Step 6: Select closest value from Table 2.
22.5 KVAR
The voltage of the capacitor would also have to be specified. In this case it would be 480 volts.
CURRENT CORRECTION
In many cases when a single motor is being corrected, the capacitors are connected between the motor starter and the motor at the motor terminals as shown in Figure 1. With this being the case, the effect of the correction is to reduce the current flowing through the starter and overload relay. Since the overload heaters are selected (or adjusted) on the basis of the motor full load current, this means that the overloads will not correctly protect the motor unless the ampacity is reduced to reflect the reduced current now flowing as a result of the power factor improvement.
The motor itself will draw the same number of amps at full load as it would without the Power Factor Correction. However, the power factor correction capacitors will be supplying a portion of the current and the balance will be coming through the starter from the power line.
The new value of current passing through the overloads is given by the following formula:
Current_{new} = Motor Full Load (Nameplate) Amps X (Power Factor Original/Power Factor Corrected)
For example, in the case of the 100HP motor in the example, the heater size, which would normally be selected from the motor nameplate current at 118 amps would have to be adjusted as follows:
Current_{new} = 118 X (.85/.95) = 118 X .895 = 105.6 or approximately 106 amps
SUMMARY
A few words of caution might be appropriate. Usually it is desirable to “under correct” rather than “over correct”. If the capacitors chosen are too large there can be a number of problems, including high transient torques and overvoltage. Thus it is usually desirable not to attempt to improve power factor beyond 95%. It also usually becomes uneconomical to attempt improvements beyond 95%.
Please note: This type of power factor improvement should not be used in any situation where the motor is being controlled by a solid state device such as a soft start control or a variable frequency drive.
For more information on power factor improvement request Bulletin PF2000F “Power Factor Correction — A Guide for the Plant Engineer” available from
Commonwealth Sprague Capacitor Inc.
Brown Street
North Adams, MA 01247
