Question

Design of DC Motor Starter In order for a dc motor to function properly, it must be protected from physical damage during the starting period. At starting conditions, the motor is not rotating, and armature generated voltage EA = 0 V. Since the internal resistance of a normal DC motor is very low, a very high current flows. It is possible for a motor to be severely damaged by such currents, even if they last for only a moment. A solution to the problem of excess current during starting condition is to insert a starting resistor in series with the armature to limit the current flow until EA can build up to its rating value. This starting resistor must not be in the circuit permanently, because it would result in excessive losses and would cause the motor's torque-speed characteristic to drop off excessively with an increase in load. In modern practice, a starting resistor is made up of a series of pieces, each of which is removed from the motor circuit in succession as the motor speeds up, in order to limit the current in the motor to a safe value. In this design problem, an automatic starter circuit is to be designed for a 4-point shunt motor rated at 15-hp, 240-V, and 45-A. The armature resistance of the motor is (0.5 + 0.01) Ohm, and the shunt field resistance is (120 + 3) Ohm. The motor is to start with no more than (150 + 3) percent of its rated armature current, and as soon as the current falls to its rated value, the starting resistor stage must be cut out. Draw a neat diagram of a 4-point dc shunt motor starter. Determine the number of steps required in the starter and the value of the resistors in each step?

Answer #1

Design of DC Motor Starter
In order for a dc motor to function properly, it must be protected
from physical
damage during the starting period. At starting conditions, the
motor is not rotating,
and armature generated voltage EA = 0 V. Since the internal
resistance of a normal DC
motor is very low, a very high current flows. It is possible for a
motor to be severely
damaged by such currents, even if they last for only a moment.
A...

In order for a dc motor to function properly, it must be
protected from physical damage during the starting period. At
starting conditions, the motor is not rotating, and armature
generated voltage EA = 0 V. Since the internal resistance of a
normal DC motor is very low, a very high current flows. It is
possible for a motor to be severely damaged by such currents, even
if they last for only a moment.
A solution to the problem of...

A 5-kW, 220-V shunt motor has an armature resistance of 0.15 ?
and a field-circuit resistance of 100 ?. The motor delivers rated
output power at rated voltage when its armature current is 20 A.
When the motor is operating at rated voltage, the speed is observed
to be 1000 r/min when the machine is loaded such that the armature
current is 15 A. In order to protect both the motor and the dc
supply under starting conditions, an external...

A 200 V DC, 1,800 RPM shunt connected motor has an armature
resistance of 0.4 ohm and a brush resistance of 0.4 ohm; brush
voltage drop of 6.0 V, independent of load. The resistance of the
shunt field is 230 ohm. The full load rated line current is 40 A.
When the motor is operated at no load the line current is 5 A and
the speed is 1750 RPM.
Calculate speed at rated load
Calculate speed at .3 rated...

1) A DC machine induces an EMF of 240 v at 1500 rpm. Find the
developed torque for an armature current of 25A.
2) Draw and explain all characteristics of d.c. Shunt motor.
3) State accessitivity of 3 point starter.
4) A four pole wave connected dc armature has 50 slots with 10
conductors per slot. The armature is rotated at 1000 rpm of the
useful flux per pole is 30mwb, calculate the amount of EMF
induced.
5) A 6...

The full-load armature current of a 5 hp, 150 V shunt dc motor
is 30 A. The field (shunt) resistance and the armature resistance
of the motor are ????F = 125 ? and ???????? = 0.15, respectively.
At a random speed of 1800 rpm magnetization curve of the motor has
measured and is shown in the table below. ????0 (V) armature
voltage 10 90 110 140 170 185 ???????? (A) field (shunt) current 0
0.82 1 1.2 1.7 2.88 Calculate...

A 230-V DC shunt motor has armature circuit resistance of 0.25 ?
and field circuit resistance of 115 ?. At
full load armature draws a current of 38 A and the speed is
measured at 1050 rpm. Find the developed
torque.
The field rheostat is now changed so that total field resistance is
144 ?. Find the speed under this condition.
Assume that the armature current does not change. Assuming the
rotational losses amount to 600 W, find the
efficiency...

Question 2
The full-load armature current of a 5 hp, 150 V shunt dc motor
is 30 A. The field (shunt) resistance and the armature resistance
of the motor are ????F=125 ? and ????????=0.15, respectively. At a
random speed of 1800 rpm magnetization curve of the motor has
measured and is shown in the table below.
EO(V) armature voltage
10
90
110
140
170
185
IF (A) field (shunt) current
0
0.82
1
1.2
1.7
2.88
Calculate the following:
a....

A separate excited DC motor is running at 1000 rpm when the
armature voltage is 600 V. The motor has a field constant Kphi = 4
V.sec and the armature resistance of the motor is 0.8 Ohms.
i. Calculate the value of the resistance of the dynamic braking at
full load. The armature current should be limited to 200 % of the
rated value.
ii. If the motor operates in regenerative braking at a speed of
1200 rpm, calculate the...

2. During starting of an automobile, a switch engages the
12-volt battery to the DC starter motor, which cranks the engine.
The starter motor’s electrical resistance R = 0.01 ohm, and the
motor’s current to torque constant K =1N mA^−1. The engine presents
friction (damping b = 5 N m^s) and inertia (J = 5 N m s^2)to the
starter motor. Obtain the system equation(s), and determine the
response for a typical start.

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