ERT105/EET 103
Tutorial 6 – DC and AC machine
DC motor and generator
1.
Compare the equivalent circuit for shunt DC motor and DC generator. Show the equation
of voltage terminal, VT, field current, IF and armature current, IA for both circuits.
2.
A 4-pole DC shunt motor working on 220 V DC supply takes a line current of 3 A at no
load while running at 1500 r/min. Determine the speed when the motor takes a line
current of 50 A. Assume armature and field resistances as 0.2 Ω and 400 Ω respectively.
[Ans: n2 = 1435.766 r/min]
3.
A 250 V DC shunt motor takes 6 A line current on no load and runs at 1000 rpm. The
field resistance is 250 Ω and armature resistance is 0.2 Ω. If the full load line current is
26 A, calculate the full load speed assuming constant flux.
[Ans: n2 = 984min]
4.
A DC series motor is running with a speed of 850 rpm taking a current of 25 A from the
supply. If the load is changed, the current drawn by the motor is increased to 50 A.
Calculate the speed of the motor on new load. The armature and series field winding
resistances are 0.2 Ω and 0.3 Ω, respectively. Assume the flux produced is proportional to
the current and the supply voltage is 250 V.
[Ans: n2 = 416.33 rpm]
5.
A 10 kW shunt generator having an armature circuit resistance of 0.75 Ω and a field
resistance of 125 Ω, generates a terminal voltage of 250 V at full load. Determine the
efficiency of the generator at full load, assuming the iron, friction and windage losses
amount to 600 W. [n=80.5%]
AC machine
1.
A 415 V, three phase and 50 Hz AC motor running at 1480 rpm at no load and 1300 rpm
at full load.
i.
ii.
iii.
2.
A 180 V, three phase, 8 pole and 50 Hz induction motor running at a slip of 5 %.
i.
ii.
iii.
iv.
3.
Calculate the poles of this motor. [p = 4]
Calculate the slip at full load. [s = 13.3%]
Calculate the speed and rotor frequency at half of the rated load. [3.325 Hz]
Calculate the syncronous speed of this motor. [nsync = 750 rpm]
Calculate the rotor speed of this motor. [nm =712.5 rpm]
Calulate the slip speed of the rotor. [nslip = 37.5 rpm]
Calculate the rotor frequency. [fr = 2.5 Hz]
A 415V, 15 kW, 4 pole, 50Hz induction motor operating at full load with conditions as
shown in the table Determine:
Slip
Mechanical losses
Core losses
i.
ii.
iii.
iv.
v.
the shaft speed. [ nm=1380 rpm]
the load torque. [tload=103.8 Nm]
the induced torque. [tconv=107.95 Nm]
the rotor frequency. [fr=4 Hz]
the efficiency of the motor. [n=96.15%]
8.0 %
200 W
400 W