Cap kVAR = kW ( TanØ1- TanØ2)
In the end it all boils down to the following.
Practically this is the most useful, because;
1. Nameplate gives kW, or kW = 746 x HP/1000
2. Nameplate gives PF, usually around 0.8.
. . So 0.8 is CosØ1, so calculate Tan Ø1.
3. Correct PF only to Min 0.9, Max 0.95.
. . Motor unlikely to be fully loaded, so 0.9 OK.
. . So, CosØ2 = 0.9 gives TanØ2 = 0.48
4. PF capacitors are not specified Farads or µFds,
as expected, but in KVARs.
5. Just order next value up from calculated kVAR
value, given by above formula.
. . Also provide supply voltage, single or three phase,
frequency 50Hz or 60 Hz.
Example: Calc kVARs for 75HP, 400v 3Ph, PF=0.8 motor,
. Correct to 0.9 PF.
Ans. . Cap kVAR
= kW ( TanØ1- TanØ2)
= (746 x 75/1000) [Tan ( Cosˉ ¹ 0.8) - Tan( Cosˉ¹ 0.9)]
= 55.95[ 0.75 - 0.48]
= 15.1 kVAR
This will usually be supplied in a single unit,
the manufacturer internally connects on star or delta.
Supplier should have data on his line of equipments.
Ensure it has discharge resistors for safety.
You provide HRC fusing for short circuit protection.