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DC Motor
Introduction
There are 2 types of DC machines:- DC generator and DC motor
At present its use as a generator is limited because of widespread use ofAC power.
The machine is therefore, extensively used as motor in industry.
Construction
The machine consists of 2 parts:-
o Stator
- Have poles
- The poles are excited by DC current to produce magnetic fields
o Rotor
- Has a ring-shaped laminated iron-core with slots.
- Coils with several turns are placed in the slots.
DC Machine Construction
It has 2 types of windings:-
DC Machine Winding
N S
Stator with poles
Brush
Rotor
Field
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o Armature winding
- Voltage is induced on it
- Placed on the rotor
- Winding arrangement depends on how the coils are connected to form a closed
winding. 2 basic possibilities:-
The armature winding arrangements
(a) Wave winding (b) Lap winding
a) Wave connection
2 parallel paths regardless no of poles
Suitable for high voltage, low current DC machine
b) Lap connection
For a p pole machine, has p parallel path
Suitable for high current, low voltage DC machine
o Field winding
- Carry dc current to create magnetic field
- Mounted on the poles (stator)
- Subdivided into:-
a) Separately excited winding
No direct connection between field & armature windings
b) Self-excited winding
Has a direct connection between field & armature windings
It can further be subdivided into: shunt, series, compound excitation
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Principle of Operation
DC motor converts electrical energy into mechanical energy
Its operation can be described as follows:
o The armature winding is connected to a dc power supply
o Current flows through the armature winding
o At the same time, the field winding is also connected to a dc power supply. (Hence, the
stator becomes an electromagnet).
o Since armature is within a magnetic field, a force is exerted on the windings.
o This force causes the rotor to rotate.
o As the rotor rotates, its winding will cut the flux causing emf to be generated on the
armature winding.
o The induced voltage is called counter emf because its polarity always acts against the
source voltage.
EMF Generated
The emf generated on DC motor is given by the following equation:-
ggen kNc
ZpEMF
60
Where
Z = No of conductors in the armature circuit
p = No of poles
c = No of parallel path @ no of current path
(c = 2 for wave-winding, c = p for lap-winding)
N = Speed of armature rotation (rpm)
= Useful flux per pole (Wb)
= sradN
/60
2
Example 1
A 12-pole dc generator has a wound armature containing of 2880 conductors. Its flux per pole is0.05 Wb. The armature is turning at a speed of 200 rpm.
a) How many current paths are there in this machine?
b) What is the generated armature voltage of this machine?
c) What is the generated voltage if the machine now has lap winding?
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Types of DC Motor
The type of dc machines is classified according to the way in which their fields are excited:
separately excited and self-excited.
Separately Excited Winding
When the dc field current is supplied by an external source, the machine is said to beseparately excited.
There is no direct connection between field & armature circuits
Equivalent circuit for a separately excited dc motoris given as below:
Stator Rotor
Separately excited dc motor
AATA RIVE
Self-Excited Winding
There is a direct connection between the armature and the field winding.
The armature winding will provide the current to create the magnetic field.
It may be classified as:
a) Series excitation
- Field winding is connected in series with the armature winding
b) Shunt excitation
- Field winding is connected in parallel with the armature winding
c) Compound excitation
- Field winding is connected in series & parallel with the armature winding
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The equivalent circuits for self-excited DC motors are given as follows:
)( fAATA RRIVE AATA RIVE
Series DC motor Shunt DC motor
)( 2fAATA RRIVE
Compound DC motor
Example 2
Find the armature current and the counter emf for a shunt motor running at 1500 rpm at 51A with a
120 V source, 120 field winding and 0.1 armature resistance.
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Power Flow Diagram
Illustrates power flow from input to output
Represented as a fishbone. The branches indicate losses
In general, Pin = Pout + losses and the losses are given as follows:
Losses in DC machine
The power flow diagram
Power flow diagram of series motor
Power flow diagram of shunt motor
Power flow diagram of compound motor
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Power equation for DC motor:-
lossesPP outin Total losses = Pca + Pcf+ P
Pin = VTIL input power taken from dc supply
Pconv = EAIA developed mechanical power
Pconv = P + Pout
Motor Torque
The general equation for torque is defined as
N
PPT
2
60
Where T- Torque (Nm), N- Speed (rpm), P- Power (W), - srad
N
/60
2
For load torque (or shaft/net/load torque):N
PT outout
2
60
For loss torque:N
PTL
2
60
For mechanical torque (or induced torque):N
PT mm
2
60
Efficiency
It is given by:
LT
m
in
out
IV
PP
P
P
Speed Regulation
Speed regulation in a motor is good if the speed of the motor is relatively constant over itsnormal range.
It is usually expressed as a percentage of full-load speed given by:
%100xN
NNSR
fl
flnl
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Example 3
A 150V shunt motor has the following parameters:-
Ra = 0.50 Rf= 150 rotational loss = 250 W
On full-load, the line current is 19.5A and the motor runs at 1400 rpm. Determine:
a) The developed power
b) The output power
c) The output torque
d) The efficiency at full-load
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Starting of DC Motor
At the instant of start-up, counter emf is zero because the armature is not rotating.
So, full voltage is applied at this moment. The armature current could then be very high which
can blow fuses & disconnecting itself from the supply.
It is therefore necessary to insert some resistance in series with armature circuit to limit this
current. This practical arrangement is known as starter.
There are several types of starter, such as manual starterand automatic starter.
Manual starter for shunt motor
Automatic starter connected to a shunt motor
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DC Motor Speed Control
There are three basic methods of controlling the speed of a DC motor:
1. Armature resistance control
- Adding a resistor in series with the armature effectively increases the armature circuit
resistance.
- This results in a reduction of the steady-state speed.
- The method is relatively simple and inexpensive.
- However, there are some drawbacks. By adding a resistance means increase losses;
I2R and reduce the motors speed. Therefore, the speed of the motor that has
resistance in the armature circuit is always lower comparing to the motor that do not
have resistance in the armature circuit.
2. Field control
- This method of speed control is by changing the flux.
- To do so we connect a resistance in series with the field winding. The field current is
decreased and the speed increases with a reduction in f lux.- This method also has some disadvantages. One of that is; we can only raise the speed
at which the motor normally runs at a particular load. Another disadvantage is that the
speed is increased without a corresponding reduction in shaft load, so we will be
overloading the motor.
3. Terminal voltage control
- This method of speed control is by changing the terminal voltage of the motor.
- The most frequent method at least for shunt motors where the field winding is
separately excited.
-
The voltage control method lowers the speed in a similar fashion as the armature circuitresistance speed control method. However it does not have its drawbacks. The no-load
speed and full-load speed can be reduced all the way down to zero if desired.
- There are various ways to obtain a variable DC voltage:
a) Ward-Leonard System
- In this system, the armature voltage is controlled by varying the field current of
DC generator. By using this system, the speed of DC motor can be smoothly
varied. Other advantage of the system is it allows the DC motor to run in either
direction by reversing the field current.
b) Solid-state Speed Control
- In recent years, solid-state converters have been used as a replacement for
Ward-Leonard system.
- The converters used are controlled rectifier or choppers.
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