Part 1 A dc motor is connected to a constant emf of 15.6 V. The resistance of it
ID: 1313577 • Letter: P
Question
Part 1
A dc motor is connected to a constant emf of 15.6 V. The resistance of its windings is 2.0 ohms. At normal operating speed, the motor delivers 6.8 W of mechanical power.
(a) What is the initial current drawn by the motor when it is first started up?
A
(b) What current does it draw at normal operating speed?
A
(c) What is the back emf induced in the windings at normal speed?
V
Part 2
A transformer with 1817 turns on the primary and 386 turns on the secondary is used in an electric slot car racing set to reduce the input voltage amplitude of 170 V from the wall output. The current in the secondary coil is of amplitude 3.2 A.
What is the voltage amplitude across the secondary coil?
V
What is the current amplitude in the primary coil?
A
Explanation / Answer
1) a) 15.6/2 = 7.8 A
b)if motor is 100% efficient
mechanical power out = electrical power in
6.8W = 15.6V x current
so current = 6.8/15.6 = 0.436A
c)
supply voltage - back emf = current x resistance of winding
16.8V - backemf = 0.436A x 2 Ohms
backemf = 16.8- (0.872) = 15.928V
2)1817/386= 4.707:1.
170/ 4.707= 36.11V on the secondary.
b)There is no information there except turns ratio.
The primary current will be (3.2/ 4.707) = .0.6798A