Question { Sasken, 7213 }

why generaters need four earthings

Answer

TWO NO.S OF EARTHING ARE FOR BODY & OTHER TWO NO.S ARE FOR
NEUTRAL REQUIRED FOR ALTERNATOR.

Question { Shakthi Power Systems, 54203 }

How to select a D.G set for calculated load of 110kW..?

Answer

CONSIDERING 110kW AS INDUCTIVE OR MOTOR LOAD
Ir (RUNNING CURRENT) WILL BE 110*1000/1.732*415*0.8
Ir = 191.3 AMPS

CONSIDERING MOTOR STARTER AS STAR-DELTA,
Is (STARTING CURRENT) = 3 TIMES OF Ir (RUNNING CURRENT)
Is = 3 * 191.3 = 574 AMPS

IF MOTOR WILL START ON NO-LOAD CONDITION, THEN STARTING
CURRENT WILL ALWAYS BE LESS THAN 574 AMPS

DG SET RATING SUITABLE TO START 110kW MOTOR LOAD IS 365KVA

365KVA DG SET AT 0.8 PF WILL DELIVER 507 AMPS. SINCE DG SET
ENGINES ARE PRIME RATED I.E. AFTER EVERY 12 HRS, DG SET CAN
BE RUN ON 110% LOADING FOR 1 HOUR.

SO, 110% OF 507 = 557.7 AMPS,

HENCE, 365KVA DG SET IS SUITABLE TO START A MOTOR
(INDUCTIVE) LOAD OF 110kW


WILL BE IF MOTOR WILL BE STARTED ON NO-LOADSINCE MOTOR
SHALL START ON NO-LOAD