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6.4.2 DC Motor Speed Control
When using DC motors for industrial purposes, it is necessary to control the start of its
movement and regulate its speed to suit the purposes used.
The speed changes either by a resistance connected to the armature or by voltage exerted
on the ends of the motor or by changing the magnetic flux through the circuit of the field.
Both the shunt motor and the compound are like the speed control methods.
6.4.2.1 Speed Control of the Shunt Motor
The speed control methods of the shunt motor are:
1. By using variable resistance:
Variable resistance is connected with the armature circuit, the speed changes by
changing the value of the resistance by a switch that controls variable resistance
values. One of the disadvantages of this method is to reduce the efficiency of the
motor. As illustrated in Figure 6.41
2. Speed control by using voltage control:
The speed of the shunt motor can be controlled by controlling the voltage
applied to it, as in the case of Ward Leonard. However, this method is very
expensive (Figure 6.42)
3. Speed control by field:
This method is simple and low cost, as low-power field resistance is used to control the current of the field and then the magnetic flux (Figure 6.43).
Ish
IL
Shunt wdg.
Re
Ia
+
_
V
Ea
Load
FIGURE 6.41
The speed regulation of a shunt motor using resistance with the armature.
IL
Ish
R
Ia
F
V
+
Shunt
wdg.
Ea
Load
_
FF
FIGURE 6.42
Speed control by the voltage control method.
IL
Ish
R
Ia
V
+
Ea
Shunt
wdg.
F
Load
_
FF
FIGURE 6.43
Speed control by field.
6.4.2.2 Speed Control of the Series Motor
The speed control methods of the series motor are:
1. By connecting series resistance with the motor circuit:
The motor speed can be changed by adding resistance to the armature
circuit (Figure 6.44).
2. Connect resistance in parallel with the field windings:
Controlling the field current value can only be achieved by connecting resistance
in parallel with the field windings. Thus, we can control the current of the field
and thus the speed of the motor (Figure 6.45).
3. Speed control by using armature diverter:
A diverter across the armature can be used for giving speeds lower than the normal speed as shown in Figure 6.46.
R IL
Rs
Ia
+
_
FIGURE 6.44
Connect series resistance with the motor circuits.
V
Ea
Load
R
IL
Rs
Ia
+
V
Ea
Load
_
FIGURE 6.45
Resistance parallel with the field windings control the motor speed.
Armature divider
Rs
Ia
R
+
_
Ea
IL
Load
V
FIGURE 6.46
Armature diverter.
6.4.3 Starting Methods
The purpose of using different ways to start the DC motors is to reduce the current at the start,
where this current is very high, and this is illustrated by the current equations in the below:
Ia = (Vin – Eb)/Ra
(6.31)
Ia = (Vin – Eb)/(Ra + Rse)
(6.32)
This means that the current will be very high because of the low resistance to its windings,
so the resistance to start the movement until the arrival of the motor to 75% of its actual
speed after up growth of the back-electric motive force, which reduces the resistance gradually until the value of zero.
A manual or automatic initiator is used, and the initiator of the motion is a set of conductors that are conductively connected and of which any number can be added or separated
from the motor by a variable key.
Problems
6.1
6.2
What does the stator part of DC machines contain?
What are the ways to turn the armature in the DC machine?
6.3
6.4
6.5
6.6
6.7
6.8
6.9
6.10
6.11
6.12
6.13
6.14
6.15
6.16
6.17
6.18
6.19
6.20
6.21
6.22
6.23
What are the methods of feeding magnetic pole windings in DC generators?
What is the use of the commutator in the DC machine?
What is the benefit of the auxiliary poles in the DC machine?
Draw the DC generator circuit and the motor of the shunt type indicating where
the direction of the current.
Why is it not allowed to operate a DC series motor without a load?
What types of power losses in DC generators?
Draw a diagram of the power path in the DC generators.
What are the ways to speed control of the DC motor?
How can reverse the rotation of DC motors?
What is the beneficial use of the resistance to start a movement in the DC motors?
What is the theory of running DC motors?
What is the beneficial use of carbon brushes in DC generators?
What is the effect of the feeding current in the DC motor rotation speed?
A DC generator have the number of poles four, wave winding, and the number of
conductors in the armature 1000 conductor, if the electric motive force generated
200 V, the value of the magnetic flux 0.02 Wb calculated its speed.
A DC generator feeding a load of 10 Ω, 200 V, the series resistance of 0.2 Ω.
Calculate the resistance of the armature.
A DC generator that feeds a load of 22 kW. The current in the load is 88 A.
Calculate the current passing through the shunt windings if you know that the
resistance is 125 Ω.
A short compound generator feeding a load with a current of 98 A, a shunt resistance of 100 Ω, a series resistance of 0.2 Ω, and the current passing through the
shunt resistance 2 A. Calculate the armature resistance.
A short compound DC generator with eight poles, and a lap windings, the number of conductors is 1200, the speed is 600 rpm, the armature current 50 A, magnetic flux 0.02 Wb, resistance of armature windings 0.4 Ω, series resistance 0.1 Ω,
shunt resistance 110 Ω, find efficiency if the mechanical and iron losses 1330 W.
A DC power motor with four poles, the armature has a typical coil, the number
of conductors is 1000 conductor, the resistance of the series windings 0.2 Ω, resistance of the armature windings 0.4 Ω, calculate its speed if it is known that the
magnetic flux generated in each pole 0.02 Wb, the current drawn 50 amp, and
working on the source 230 V.
A DC motor is designed to reduce its speed, adding resistance in series with
the shunt windings, calculate the value of this added resistance, if the armature
resistance is known to be 0.5 Ω, and the shunt resistance is 10 Ω, the current is
pulled by the motor 60 A, the current that passes the armature resistance is 10 A,
and operates at 220 V.
A long compound DC generator that feeds a load of 2200 W, 220 V, armature
resistance of 0.2 Ω, 44 Ω of parallel resistance, and 227.5 V generated electric
motive force. Calculate the resistance of the series windings.