EEE Job Interview Question Answer [ Part 7 ] Motor
EEE Job Interview Question Answer [ Part 7 ]
1.Question 1
Which of the things below about Star/Delta AC motor starter systems is true?
· -With the Star connection at √3 times the nominal input voltage, this is split by √3 again, for a complete of three
· -The Star and Delta contactors are electrically interlocked
· -The current through the stator windings is broken when the Star contactor opens, and it's restored when the Delta contactor closes
· -There may be a 30 - 50 milliseconds break in supply after the Star contactor opens and also the Delta contactor closes.
2.Question 2
Why are the Star and Delta contactors electrically interlocked?
This prevents short-circuiting between phases, because the Star and Delta contactors cannot close simultaneously
3.Question 3
Imagine that You bring a motor from Germany designed to run at 415Volts, 50 Hz. It has 37.5 kW output power, runs at 2978 RPM and has 120 N-m of rated torque. You ship it Canada and run it at 460 Volts, 60 Hz. what is going to be the new speed in RPM?
3600
P = (2*pi*N*tou)/60
N = (120*f)/P
4.Question 4
Does an engine or motor produce back EMF when the windings are empowered, yet the engine is still in the fixed position?
No
Back EMF = B x L x ω x r = 0 if the angular velocity ω = 0
5.Question 5
Which do you think is easier to control, speed in an AC motor or speed in a DC motor?
-DC
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