SIZING A CAPACITOR TO CORRECT THE POWER FACTOR OF MULTIPLE MOTORS (kVAR vs. HP)
Question: Can you come up with a ruleofthumb for sizing a single capacitor to correct the power factor of multiple motors?
Solution:
If we look at ranges in the Myron Zucker, Inc. CALmanual (Table 3, in the 1800rpm column), we see that the kVAr as a percentage of HP goes from 50% for a 2HP motor to 24% for a 500HP motor. This can also be described as a multiplier times HP to determine kVAr. For example, 0.50 would be the multiplier for a 2HP motor, resulting in a 1kVAr capacitor.
This inconsistency is due to the following:
 Inductances vary by motor manufacturers, affecting the resultant magnetic currents. Magnetic current being motor noload current.
 These tables are based on the average magnetic current of all motor manufacturers.
 The capacitor manufacturers have selected the nearest standard kVAr rating for these motors.
Using the table above, if you consider all the multipliers used, you will find that the average is 0.317. But if we use a 0.33 multiplier as a rule of thumb, we get the following results:
HP

Results

210

Low kVAr Values

1525

O.K.

30

High kVAr Values

4050

Low kVAr Values

60100

O.K.

125300

High kVAr Values

So let's use this ruleofthumb on some multiple motors and see what happens.
Example: For 1800rpm "Tframe" nema "Design B" motors:


kVAr Values


HP

by Table 3

RuleofThumb


25

7.5 kVAr

8.25 kVAr


50

17.5 kVAr

16.5 kVAr


75

25 kVAr

25 kVAr


100

30 kVAr

32.5 kVAr



Totals:

250 HP

80 kVAr

82.5 kVAr

Results:
(1) 250 HP by Table 3 is 60 kVAr.
Total 250 HP using individual capacitors is 80 kVAr.
Total 250 HP using ruleofthumb is 82.25 kVAr.
Now we have (3) different kVAr values: 60, 80, and 82.25 kVAr.
Using Total HP (250) by Table 3 results in 60 kVAr. Larger motors have lower magnet currents. So if we add up the HP and take the total (250 HP) and use Table 3, we end up with a lower power factor on your system.
Using the ruleofthumb, we end up with a higher kVAr sometimes, but only by an insignificant amount. So if only the total HP is known, using the ruleofthumb 0.33 multiplier will get us in the ballpark.
But if individual motor HP is known, Myron Zucker, Inc. recommends using the individual capacitor amount of 80 kVAr, for the following reasons:
 These Tables have been used for more than 40 years with no known problems.
 The higher kVAr value 80 kVAr will not cause a leading power factor.
 On the electrical system or main bus, there are other loads that also need to be corrected: small motors and inductive loads
 The only disadvantage with the above is that, if some motors are not on the system  let's say half (50%) or more of the motors are offline  the result could be a leading power factor. If the motor program (time on/off) is known, then the average HP total should be used from Table 3