Name:
Montaha Alduis
Kholoud Alsamoumi
Amani Ahsan Al-Aini
202174282
202174
202174
Department:
Biomedical engineering
Lab:
Electrical machine
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Experiment.5.
Loud characteristics of D.C shunt Motor
Objectives:
To connect and run the D.C. shut motor and to obtain its loud characteristics.
Procedures:
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1.Make the appropriate connections of the D.C. shunt wound motor according to
2.Before operation connect the magnetic powder brake to the control unit.
3.Set the control unit as follows:
Speed range switch "min":3000.
4.Put the motor into operation, and make it run to its normal speed.
5.Load the motor between no load operation and two times the normal torque.
Measure the torque T, the speed n, the armature current I, and the exciter current If
under load.
6.Calculate the values of input power P, output power P, and efficiency. 7.Using the
measured and calculated values, draw the curves for the D.C. shunt wound motor,
i.e. n, I, 1, Pi Po- f(T).
Discussion:
1.Using the load characteristics describe the typical motor, speed, torque
behavior of the DC shunt motor.
The speed of a motor is defined as the rate at which the motor rotates. The speed
of an electric motor is measured in revolutions per minute, or RPM. Torque: The
torque output of a motor is the amount of rotational force that the motor develops.
2.Describe the armature current curve.
1. Speed-Armature Current Characteristic:
If applied voltage V is kept constant, the field current will remain constant,
hence flux will have maximum value on no load but will slightly decrease due
to armature reaction as the load increases but for most purposes the flux is
considered to be constant, neglecting armature reaction effect.
From speed equation, speed N is directly proportional to back emf E b or (V –
Ia Ra) and inversely proportional to the flux φ. Since flux is considered to be
constant, so with the increase in armature current the speed slightly falls due to
increase in voltage drop in armature and the speed-armature current curve
coincides with the back emf-armature current curve. Since voltage drop in
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armature at full load is very small as compared to applied voltage so drop in
speed from no load to full load is very small
2. Torque-Armature Current Characteristic:
From the expression for the torque of a dc motor, torque is directly proportional
to the product of flux per pole φ and armature current, I a. Since in case of a dc
shunt motor the flux per pole φ is considered to be constant, therefore, torque
increases with the increase in load current following linear law, i.e., torque-
armature current characteristic is a straight line passing through origin O (Fig.
1.15). But the weakening of field due to armature reaction causes the torque
line to droop slightly, and the iron and friction losses cause it to be slightly
lower than the line representing the electromagnetic developed torque
corresponding to T = 0.159φ ZP I a/A Nm
3.Describe the efficiency curve of the DC shunt motor.
Efficiency is generally expressed as a percent- age. The efficiency of a dc motor
depends on the power output. Initially with low power outputs, efficiency is poor
due to the constant losses. As the output increases, the efficiency increases till it
reaches a certain maximum value. If the out- put is still further increased, then the
efficiency decreases due to the rapid rise in the variable losses. The motors are
normally designed to have maximum efficiency while working at or near full load.
The efficiency of a dc motor at normal operating conditions may be as high as
95%. A typical efficiency curve showing the manner in which the efficiency varies
with the output is shown in Fig. 1.
4.Why does the rotor speed of a DC shunt motor decrease only slightly with
increasing load?
This is because the torque is supplied by the current, while faster speeds increase
the back emf which not only limits its final speed but blocks incoming current,
therefore lowering torque. This is Why Torque and Speed are Inversely
Proportional in an Electric Motor
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5.How does the power delivered by a DC shunt wound motor change with
increasing load torque?
the load on the motor is increased, the armature rotation slows and back EMF is
reduced, since back EMF is proportional to speed. With less back EMF voltage and
a constant supply voltage (E), the net voltage increases. The increase in net voltage
results in an increase in armature current
6.How can the nominal data of the motor be determined from the load
characteristics?
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Experiment.6.
Load characteristics of D.C series Motor
Objectives:
To connect the D.C. series motor to D.C. supply and put it into operation. To
record and plot the load characteristics of the motor and to mark the nominal
torque on the load characteristics and deduce the nominal values.
Procedures:
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1.Connect the D.C. series motor according to the circuit shown above.
2.Before operation connect the magnetic powder brake to the control unit.
3.Set the control unit switches on the following range:
6000 1.0
MAN 1
4.Put the circuit into operation. The d.c. supply voltage is to be kept constant at
220V. during the measurements.
5.Load the motor between no load operation and two times the nominal
torque(0.477N.m),
6. Measure the speed n, I., T at a d.c. voltage of 220V.
7.Calculate the values of Pi, P, and n.
8. Using the measured and calculated values, draw I, n, Pi P, and n against load
torque T.
Vt( V) n (r/min) Ia (A) T ( N .m) Pin Pout Efficiency
(W) (W) %
220 2000 0.5 No-load 110 0 0%
220 1400 0.4 0.1 88 14.66 16%
220 1100 0.69 0.2 151.8 23.03 15%
220 900 0.95 0.3 209 28.27 14%
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Discussion:
1.Using the load characteristics describe the motor, speed torque behavior, the
current consumption curve, the efficiency curve of d.c. series motor.
From the performance curves for a dc series wound motor it is noted that:
(i) The speed of series motor falls rapidly with the increase in load, so a series
motor is not suitable for services requiring a substantially constant speed.
(ii) The efficiency increases rapidly in the beginning, reaches its maximum
value and then decreases. This is due to the fact that at light loads the friction
and iron losses are large compared with the load and the effect of these losses
becomes less with the increase in load. The armature and field copper loss
varies as the square of the current, so these losses increase rapidly with the
increase in load. The efficiency becomes maximum when friction and iron
losses are practically equal to the copper loss.
(iii) Series motor develops a starting torque comparatively greater than that
developed by a shunt motor for a given current. Hence series motors are best
suited where huge starting torque is required i.e., for streetcars, cranes, hoists
and locomotives.
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(iv) In addition to the large starting torque, there is another unique
characteristic of series motors which makes them best suited for traction
services. For instance let the streetcar ascend a gradient. In case the shunt
wound motor is used for driving the above streetcar, it will maintain the speed
of the car at approximately the same value that it had on the level ground and,
therefore, will tend to draw an excessive current.
A series motor, on the other hand, automatically slows down on such a gradient
because of increased current demand and, therefore, it develops more torque at
reduced speed. This drop in speed allows the motor to develop a large starting
torque with but a moderate increase of power. Hence under the same load
conditions, the rating of a series motor would be less than for a shunt motor.
2. Why does the speed of a d.c. series motor decrease severely with increasing
load?
The speed of a dc motor is changed by changing the current in the field or by
changing the current in the armature.
3. How can the nominal data of the motor be determined from the load ?
