Download tutorial2(que).

TUTORIAL 2
DC MACHINES
EMT 113 FUNDAMENTAL ELECTRICAL ENGINEERING
UNIVERSITI MALAYSIA PERLIS
Tutorial 2
- DC Motor Question 1
a)
Sketch the equivalent circuit of Series and Cumulatively Compounded DC Motor.
Compare torque-speed characteristic for both motors.
b)
A Shunt motor rotating at 1500 rpm is fed by a 120 V source. The line current is
51 A and the shunt-field resistance is 120 Ω. If the armature resistance is 0.1 Ω,
calculate the following:
i.
ii.
c)
The armature current, IA
The mechanical power developed by motor
A 220 V, 7 hp series motor is mechanically coupled to a fan. The motor draws the
current of 25A and runs at 300 rpm. The torque required by the fan is
proportional to the square of the speed. Given RA = 0.6 Ω and RS = 0.4 Ω.
Neglect armature reaction and rotational loss.
i.
ii.
Determine the power delivered to the fan and the torque developed
by the machine without external resistance connected to the
armature circuit (Rext = 0)
The speed is reduced to 200 rpm by inserting an external resistance
(Rext) in the armature circuit. Determine the value of this resistance
and the power delivered to the fan.
Question 2
A Shunt generator has a field resistance of 100 Ω. When the generator delivers 100kW,
the terminal voltage is 130V, while the induced voltage, EA is 143V. Determine:
i.
ii.
SMI/EMT113/TUTORIAL2
The armature resistance
Induced voltage when output is 40kW and terminal voltage is
150V.
TUTORIAL 2
DC MACHINES
Question 3
A six pole machine has a flux per pole of 30 mWb. The armature has 536 conductors
connected as a lap winding in which the number of parallel paths (a) is equal to the
number of poles (p). The DC machine runs at 1050 revolution per minute (rpm), and
delivers a rated armature current of 225 A to a load connected to its terminals.
a) Describe field winding and armature winding
Calculate:
b) Machines constant, K
c) Generated voltage, EA
d) Electromagnetic torque, τe
e) The power developed by armature, Pdev
Question 4 (Chapman Example 9-1, pg. 541)
A 50hp, 250V, 1200 r/min DC Shunt motor with compensating windings has an armature
resistance (including brushes, compensating windings and interpoles) of 0.06Ω. Its field
circuit has a total resistance Radj + RF of 50Ω, which produces a no-load speed of 1200
r/min. There are 1200 turns per pole on the shunt field winding.
a)
b)
c)
d)
Find the speed of this motor when its input current is 100A
Find the speed of this motor when its input current is 200A
Find the speed of this motor when its input current is 300A
Plot the torque-speed characteristic of this motor
Question 5
A 240 V shunt motor runs at 850 r/min when the armature current is 70 A. The armature
circuit resistance is 0.10 Ω. Calculate the required resistance to be placed in series with
armature to reduce the speed to 650 r/min when the armature current is then 50 A. The
field current is constant.
SMI/EMT113/TUTORIAL2
TUTORIAL 2
DC MACHINES
Question 6
A 50 hp shunt dc motor has a 280V of armature current when 10 A of field current is
supplied and have the followings values:
VT =240 V
RA = 0.065 Ω
RF = 10 Ω
Radj = 14 Ω
At a speed, ωm of 125.67 rad/s of , the rotational loss is Ploss = 1450 W. If this motor
drive a load that demand a torque τout = 250 Nm-1independent of the speed, calculate
the motor speed and efficiency.
- DC Generator Question 7
A 100 kW, 230-V, shunt generator has RA = 0.05Ω and RF=57.5Ω. If the generator
operates at rated voltage, calculate the induced voltage at:
a) Full load
b) Half load
-Neglect brush contact drop
Question 8
A 10kW, 125 V- compounded generators has rotational losses amounting to 580 W. The
shunt field resistance is 62.5 Ω, the armature resistance 0.12 Ω and the series field
resistance 0.022 Ω. Calculate:
a) Full load efficiency for long shunt connection
b) Full load efficiency for short shunt connection
c) Compare your result between (a) and (b)
Question 9
SMI/EMT113/TUTORIAL2
TUTORIAL 2
DC MACHINES
A generator shown in figure 1 supplies a 20 kW load at 100 V through cables of
resistance,R = 200 mΩ. If the field winding resistance, Rf = 40Ω and the armature
resistance, RA = 50 mΩ, calculate the terminal voltage and the electromagnetic
force generated in armature, EA.
SMI/EMT113/TUTORIAL2