Norman Martinez 200327562
Unit 3: Homework
From the Electric Motor Control (Tenth Edition) by Stephen L. Herman Textbook, answer all the problems selected below.
Section 1- unit 3
Problems 2, 3, 6, 7, 32 and 33
2. How many poles are required on motor starters for the following motors:
(a) 240-volt, single-phase induction motor: 2 poles
(b) 440-volt, three-phase induction motor: 3 poles
3. If a motor starter is installed according to directions but will not start, what is a common
cause for the failure to start?
The common cause is that there are no overload heaters.
6. What is the phase relationship between the flux in the main pole of a magnet and
the
flux in the shaded portion of the pole?
The relation is 90 degrees apart.
7. In what AC and DC devices is the principle of the shaded pole used?
Magnetic starters on contactor section Relays
In DC coils to provide a time delay in the decay of flux in the coils. In AC magnets to minimize the noise and wear.
32. What is the difference between a grounded control system and floating control system?
A grounded control system is electrically tied down and referenced to the Earth, which is abbreviated GND. Floating control systems are not electrically connected to the Earth. In practical matters, the grounded systems aid in troubleshooting a problem because it permits one lead of the voltmeter to be connected to any grounded point and the other lead can be used to test voltage at various locations throughout the circuit. 33. A control transformer has a primary voltage of 480 Volts and a secondary voltage of 120 Volts. The transformer has a power rating of 0.5 kVA. What is the maximum output current of the transformer?
I
=
VA
E
=
500
VA
120
V
=
4.16
A
