3 PHASE MOTOR BEHAVIOR
Starting current of a motor can be 5 to 7 times its full-load line value (or even higher). At
starting, if an 8% voltage drop occurs during running condition, then a drop of 40% or more will
occur during start-up. In such conditions the motor will either:
Stall (i.e. remain stationary due to insufficient torque to overcome the load torque) with
consequent over-heating and eventual trip-out
Accelerate very slowly, so that the heavy current loading (with possibly undesirable low-voltage
effects on other equipment) will continue beyond the normal start-up period which can causes
over-heating and eventual trip-out.
During starting condition, torque is high and speed is low.
During running condition, Lower the phase motor voltages, lower will be RPM, torque and
current.
Start motor with star connection will only limit the starting line current.
Start motor with star connection, reduces phase voltages (winding voltages), and reduces
torque and input power by factor of 3.
Start motor with star connection, reduces the starting current 33% of FLC, peak starting current
becomes 1.3 to 2.6 time of FLC because motor phase voltages becomes 58% line voltages &
peak starting torque reduces to 33% of FLT.
When motor is delta connected, phase voltages becomes as line voltages, the phase current
becomes 58% of FLC, line current is 100% to FLC.
During phase failure or missing condition, healthy phases are over-load causes to trip the
motor.
During delta running condition, if delta connection are opened or loses then phase current
becomes equal to line current (FLC), can cause over-load relay to trip the motor.
If motor winding are cross-shorted to each other then it causes to trip the motor.
If star and delta connection are made at same time then it causes short circuit which instantly
trip the motor.
EXAMPLE:
NAMEPLATE:
IP55 160L IMB3
50Hz 400/690V Δ/Y 60Hz 460V Δ
18.5KW 32.5/18.8A 21.3KW 32A
Cosɸ 0.91 2940 RPM Cosɸ 0.92 3540 RPM
380-420/660-725V Δ/Y 440-480V Δ
34-32/19.6-18.5A 33.5-31A
During running condition:
Motor start at Y connection and run at Δ connection.
Motor runs on Y connection during starting for 2.5sec until motor attains 80-85% rated speed.
Motor runs on Y connection for 2.5 sec in order to attain 80-85% syn. (rated) speed then switch on Δ connection.
Current on main circuit breaker during starting 60-70A for 1 sec.
This is starting (inrush) current which is around 1.3-2.6 times of FLC. Above current is line current which is 2.15
times of FLC (rated) current.
Current on Y contactor during starting 60-70A for 1sec then 30-35A for 2.5 sec.
Even when motor start on Y connection, the starting current is same on main breaker and Y connector, in other
words starting (inrush) line current on main breaker is same as starting (inrush) phase current on Y connection
winding for 1 sec.
After 1 sec, phase current on Y connection winding becomes 33% of FLC. Above phase current is 100% of FLC until
motor switches on Δ connection.
Current on Δ contactor on running 20-22A
When motor is switched on Δ connection, then phase current becomes 58% of FLC. Line current on main breaker is
around FLC.
