APPLYING CAPACITOR UNITS
Power Factor Correction Capacitors can be applied at individual motors, distribution panels, or on the main service panel. Fixed Capacitors can be connected at all three locations, or Automatic Capacitor Systems such as the Steelman VAR MANAGER can be installed on the main service panel. Fixed Capacitors are permanent values of KVAR connected to the electrical system, while Automatic Capacitor Systems vary the amount of KVAR that is connected based on sensing the entire electrical system requirements. If plant loads vary widely during any 24 hour period, large fixed capacitors at the main service panel are not recommended. Overcorrection may result, causing potential problems to the capacitors and adjacent connected equipment. In this case individual motor correction or an Automatic Capacitor System would be the best installation.
DETERMINE TOTAL REQUIREMENT
In order to determine the overall KVAR requirements, the normal load KW and the original power factor must be known. This information can usually be obtained from the electric utility bill or from the local power company. To compute the total KVAR required, refer to Table 3, and multiply the value found at the intersection of “Original Power Factor” and “Desired Power Factor” by the normal load KW. As an example: To improve the power factor of a 400 KW load from .77 to .92 :
KVAR = KW x Multiplier
= 400 x .403 (Table 3)
In the above example, 161.2 KVAR would be required to correct the complete system. If individual motors are being corrected, the KVAR being connected to individual motors is subtracted from the overall KVAR required for the entire system. The balance would then be connected to the distribution system. If the total KVAR required was 161.2 as noted in the above example, and 100 KVAR had been connected to individual motors, the balance of 61.2 would be reduced to 60 KVAR and connected to the distribution system. As an alternative, a VAR MANAGER 180 KVAR unit could be used on the main service to correct the whole facility.
KVAR UNITS – INDIVIDUAL MOTORS
There are four different methods of selecting the proper size KVAR units for induction motors. Choose from one of these methods below based on what information you have available.
- Use actual load measurements of KW and Power Factor. This information can be used with Table 3 to calculate the KVAR necessary for a desired Power Factor.
- Use motor manufacturer’s recommendations. Some motors are supplied with maximum KVAR recommendations.
- Use motor data supplied by motor manufacturer, when full load power factor and full load amps are known. First, multiply full load amps times voltage times S3 and divide by 1000. This result is KVA. Next, multiply the KVA times the power factor. This result is the KW. Next, use Table 3 to determine the KVAR required to raise the full load Power Factor to the desired level.
- Use Tables 1 & 2 which list the recommended sizes of KVAR units needed for correction of most induction motors to approximately 95% power factor. These tables show the proper KVAR for a given horsepower and RPM. For motor types or sizes not listed, please consult the factory.
Certain motor applications are not suitable for connecting the capacitor to the load side of the motor starter. Applications involving reversing, plugging, or frequent starts; crane or elevator motors, or any motor where the load may drive the motor, multispeed motors, or motors using open transition reduced voltage starting, must be corrected on the distribution panel or main service panel.
Do not oversize the Steelman KVAR Capacitor. It is not recommended that the total capacitor rating connected to the load side of a motor controller exceed the rating required to raise the no-load power factor of the motor to unity. A leading power factor can be worse than a lagging power factor.