Jomo Kenyatta University of Agriculture & Technology
Department of Electrical and Electronic Engineering
Assignment I{30marks} EEE 2513 Machine Drive
Semester II 2022
1. Consider a constant torque motor (200 Nm) driving a linear torque load defined by
Tm= 2ωrm;
0 < ωrm < 150 rad/s
J=2.5 Kg m2
a) Accurately sketch the motor and load torque-speed characteristics.
b) Clearly mark thereon the system steady-state operating speed and torque point.
c) Solve for the system steady-state operating speed and torque.
[4 marks]
2. A motor supplies a constant 15 hp to a mechanical load for 30-day month, for which the electrical bill is $420. The
energy rate is $0.05 per kWh.
[2 marks]
a) Find the motor efficiency.
b) If the motor is replaced with one with an efficiency of 93%, determine the monthly savings.
3. From the real mechanical system below draw an equivalent system and determine developed torque T d and referred
sum total of moment of inertia JΣ, given; moment of inertia for the motor J m=0.5
kg M2, i (gear ratio)= 2 ,
efficiency η= 80%, mechanical load torque TL= 400 NM and moment of inertia for the mechanical load JL = 5 kg
M2.
[4 marks]
4. With aid of circuit diagrams explain the operation of dc motor in four –quadrants of speed torque plane show
current flow in each quadrant and the direction of power flow.
[4 marks]
5. A dc chopper is used for regenerative braking of a separately excited dc motor. The dc supply is 400 V. The motor
has ra = 0.2Ω and Km=1.2 V-s/rad. The armature current during regenerative is kept constant at Ia= 400 A and
negligible ripple. For a duty circle of 60% for the chopper, determine
a) Power returned to the dc supply
b) Maximum and minimum speed permissible braking speeds
c) Speed during regenerative braking
[ 4 marks]
6. A three-phase full-wave converter is used to control the armature and field voltage of a 20 hp, 300 V, 1800 rpm
separately excited DC motor. The ac input is a three phase, Y-connected 208 V, 60 Hz supply. The armature
resistance is Ra = 0.25 Ω, Rf = 245 Ω and Kv=1.2V/A rad /Sec. If the field current is set to maximum:
a) draw the circuit diagram for the drive and drive equation for output voltage
b) determine the delay angle of the armature converter if the motor supplies the rated power at the
rated speed
c) speed regulation if the no load armature current is 10% of rated armature current, and delay angle
remains same as in (ii) above
[ 6 marks]
7. The speed of a 20-hp, 300 V, 900 rpm separately excited dc motor armature voltage and field voltage are controlled
by a three phase full converter. The ac input to the armature and field converters is three-phase, Y-connected, 208
V, 60 Hz. The armature resistance Ra= 0.25Ω, the field circuit resistance is Rf = 145Ω, and the motor voltage
constant Kv=1.2 V/A rad/s. The viscous friction and no load losses can be considered negligible. The armature and
field currents are continous and ripple free. Determine
i.
Delay angle of the armature converter αa, if the converter is operated at maximum field current and the
developed torque is Td= 116 N.m at 900 rpm
ii.
Speed of the motor, if the field converter is set for maximum field current, the developed torque is T d=
116 N.m and the delay angle of the armature converter αa=0
iii.
For the same load demand as in (ii) determine the delay angle of the field converter if the speed has to
increase to 1800rpm.
[ 6 marks]