Speed control of dc motors

ELECTRICAL MACHINES
D. C. MOTORS SPEED CONTROL

2
INTRODUCTION
• The principal advantage of a d.c. motor is that its speed can be changed
over a wide range by simple methods
• Such a fine speed control is generally not possible with a.c. motors. In a.c
motor speed control is very difficult compare to d.c machines.
• Fine speed control is one of the reasons for the strong competitive position
of d.c. motors in the modern industrial applications. Specially in Textile and
Rolling mills.
Lecture Notes by Dr.R.M.Larik

3
SPEED CONTROL OF D.C. MOTOR
Speed of a d.c. motor is given by:
N 
Eb
Φ
or N = K
V− Ia R
Φ
rpm (as Eb = V − IaRa)
where R = Ra for shunt motor
R = Ra + Rse for series motor
From above, it is clear that there are three main methods of controlling the
speed of a d.c. motor, namely:
i) By varying the flux per pole (ϕ), known as flux control method
ii) By varying the resistance in the armature circuit, known as armature
control method.
iii) By varying the applied voltage V, known as voltage control method

4
SPEED CONTROL OF D.C. SHUNT MOTORS
➢ Flux control method
• Based on the fact that by varying the flux Φ, the motor speed (N 
1
Φ
)
can be changed and hence the name flux control method
• In this method, a variable resistance is placed in series with shunt field
winding
• We can only raise the speed of the motor above the normal speed
• Generally, this method permits to increase the speed in the ratio 3:1.

5
SPEED CONTROL OF D.C. SHUNT MOTORS (Contd.)
i) This is an easy and convenient method.
ii) It is an inexpensive method since very little power is wasted in the shunt
field rheostat due to small value of Ish.
iii) The speed control exercised by this method is independent of load on
the machine.
➢Advantages
➢Disadvantages
i) Only speeds higher than the normal speed can be obtained since the
total field circuit resistance cannot be reduced below Rsh (the shunt
field winding resistance)
ii) There is a limit to the maximum speed obtainable by this method
because if the flux is too much weakened, commutation becomes
poorer.

6
• Based on the fact that by varying
the voltage available across the
armature, the back e.m.f and hence
the speed of the motor can be
changed.
• Done by inserting a variable
resistance RC (known as controller
resistance) in series with the
armature
N  V − Ia (Ra + RC)
where RC = controller resistance
➢Armature Control Method

7
• Due to voltage drop in the RC, the back e.m.f. (Eb) is decreased
• The speed of the motor is reduced as N  Eb
• The highest (normal) speed obtainable is that corresponding to Rc = 0
• This method can only provide speeds below the normal speed
➢Armature Control Method (Contd.)
Disadvantages
• Power loss in the Rc is high since it carries full armature current
• The speed varies with load as the speed depends upon Ia Rc
• The efficiency of the motor are reduced.
• This method results in poor speed regulation.
Due to above disadvantages, this method is seldom used to control the
speed of shunt motors.

8
o In this method, the field current is supplied from a separate source.
o This method avoids the disadvantages of poor speed regulation and low
efficiency as in armature control method
➢Voltage Control Method
✓ Multiple Voltage Control
• In this method, the shunt field of the motor is connected permanently
across a-fixed voltage source.
• The armature can be connected across several different voltages
through a suitable switchgear so that voltage applied to the armature
can be changed

9
➢Voltage Control Method (Contd.)
✓ Ward-Leonard System
The Ward Leonard system is very important and useful method of
controlling the speed of a d.c motor.
The adjustable voltage for the armature is obtained from an adjustable-
voltage generator while the field circuit is supplied from a separate
source.
G: Generator; E: Exciter FC: Field ControllerLecture Notes by Dr.R.M.Larik

10
✓ Ward-Leonard System (Contd.)
Advantages
a) Motor speed can be adjusted through a wide range without resistance loss
b) The motor can be brought to a standstill quickly, simply by rapidly reducing
the generator voltage resulting in reduction of the back e.m.f. of the motor,
and establishing dynamic braking.
c) This method is used for the speed control of large motors when a d.c.
supply is not available.
The disadvantage of the method is that a special motor-generator set is
required for each motor and the losses in this set are high if the motor is
operating under light loads for long periods.

11
NUMERICAL PROBLEMS (Contd.)
Problem 4(a)
A 220 V, 15 kW, 850 r.p.m. shunt motor draws 72.2 A when operating at
rated condition. The resistances of the armature and shunt field are 0.25 
and 100  respectively. Determine the percentage reduction in field flux in
order to obtain a speed of 1650 r.p.m. when armature current drawn is 40 A.
Solution:
Voltage supply = 220 V DC, Motor rating = 15 kW, N1 = 850 rpm, IL = 72.2 A,
Ra = 0.25 , Rsh = 100 , N2 = 1650 rpm, Ia2 = 40 A
% reduction in field flux ? (at 1650 rpm)
N2
N1
=
Eb2
Eb1
x
Φ1
Φ2
Ish =
V
Rsh
=
220
100
= 2.2 A
Ia1 = IL – Ish = 72.2 – 2.2 = 70 A

12
Problem 6(a)
A 240 V d.c. shunt motor has an armature-resistance of 0.25 ohm, and
runs at 1000 r.p.m., taking an armature current 40 A. It is desired to
reduce the speed to 800 r.p.m.
(i) If the armature current remains the same, find the additional
resistance to be connected in series with the armature-circuit.
(ii) If, with the above additional resistance in the circuit, armature current
decreases to 20 A, find the speed of the motor.

13
Problem 6(a)
Solution:
Supply = 240 V DC, Ra = 0.25 , N1 = 1000 rpm, Ia1= 40 A, N2 = 800 rpm,
Ia3 = 20 A
Additional resistance ? (for Ia1 = Ia2), Additional resistance ? (for Ia3 = 20 A)
Back e.m.f. Eb1 = V – Ia1 Ra
= 240 – 40 x 0.25 = 230 V
Eb1  1000 r.p.m., Eb2  800 r.p.m.
Eb2
Eb1
=
800
1000
Eb2
230
=
800
1000
Eb2 = 184 V

14
Problem 6(a)
Solution (Contd.):
Also Back e.m.f. Eb2 = V – Ia2 (R + Ra)
184 = 240 – 40 x (R + 0.25)
R = 1.15 
Back e.m.f. Eb3 = V – Ia3 (R + Ra) = 240 – 20(1.15 + 0.25) = 212 V
Eb3
Eb1
=
N3
N1
212
230
=
N3
1000
, and N3 = 922 r.p.m.
Problem 6(b): A 250 V shunt motor has an armature current of 20 A when
running at 1000 r.p.m. against full-load torque. The armature resistance is
0.5 . What resistance must be inserted in series with the armature to
reduce the speed to 500 r.p.m. at the same torque? What will be the speed
if load torque is halved with this resistance in the circuit? Assume the flux
to remain constant throughout and neglect brush contact drop.Lecture Notes by Dr.R.M.Larik

15
SPEED CONTROL OF SERIES MOTORS
➢Flux Control Method
In this method, the flux produced by the series motor is varied and hence
the speed.
✓Field diverters
• A variable resistance (field diverter)
is connected in parallel with series
field winding
• Some portion of the line current is
shunted from the series field
winding, thus weakening the field
and increasing the speed

16
SPEED CONTROL OF SERIES MOTORS (Contd.)
➢Flux Control Method (Contd.)
✓ Armature Diverter
• To obtain speeds below the normal
speed, a variable resistance
(armature diverter) is connected in
parallel with the armature
• The diverter shunts some of the line
current, thus reducing the armature
current.
• For a given load, if Ia is decreased, the flux Φ must increase (T  ϕ Ia).
and the motor speed is decreased (as N  Τ1 ϕ).
• By using the armature diverter, any speed lower than the normal speed
can be obtained.

17
✓ Tapped Field Control
• The flux is reduced by decreasing the
number of turns of the series field
winding, hence, speed is increased.
• The switch S can short circuit any part
of the field winding, thus decreasing
the flux and raising the speed.
• With full turns of the field winding, the
motor runs at normal speed and as the
field turns are cut out, speeds higher
than normal speed are achieved.

18
✓ Paralleling Field Coils
• This method is usually employed in the case of fan motors.
• By regrouping the field coils several fixed speeds can be obtained.

19
✓Armature-resistance Control
• A variable resistance is directly
connected in series with the supply to
the complete motor to reduce the the
armature voltage and hence the speed
falls.
• By changing the value of variable
resistance, any speed below the
normal speed can be obtained.
• This is the most common method employed to control the speed of d.c.
series motors.
• Although this method has poor speed regulation, this has no significance
for series motors because they are used in varying speed applications.

20
Problem 8
A d.c. series motor runs at 1000 r.p.m. when taking 20 A at 200 V. Armature
resistance is 0.5 . Series field resistance is 0.2 . Find the speed for a
total current of 20 A when a diverter of 0.2  resistance is used across
the series field. Flux for a field current of 10 A is 70 per cent of that for 20 A.
Solution:
Supply = 200 V DC, N1 = 1000 rpm, Ia = 20 A, Ra = 0.5 Ω , Rse = 0.2 Ω,
Φ2 = 0.7 Φ1
N2 ? (at total current of 20 A with diverter of 0.2 Ω)
N2
N1
=
Eb2
Eb1
x
Φ1
Φ2
Back e.m.f. without diverter resistance
Eb1 = V – Ia (Ra + Rse)
= 200 – 20 (0.5 + 0.2) = 186 V

21
Problem 8
Solution (Contd.):
Diverter resistance = Series field resistance
As the motor current 20 A is divided between the two resistances 10 A flows
through series field and produces 70% flux as compared to 20 A.
Combined resistance =
0.2
2
= 0.1 Ω
Motor resistance = 0.5 + 0.1 = 0.6 Ω
Back e.m.f. Eb2 = V – Ia2 (Ra + Rse/div)
= 200 – 20 x (0.5 + 0.1) = 188 V
N2
1000
=
188
186
x
1
0.7
N2 = 1444 r.p.m.

22
SERIES - PARALLEL CONTROL
• In the system, used in traction, two (or more) similar d.c. series motors are
mechanically coupled to the same load.
• When connected in series, each motor armature will receive one-half the
normal voltage, hence, the speed will be low.
• When connected in parallel, each motor armature receives the normal voltage
and the speed is high Lecture Notes by Dr.R.M.Larik

23
SERIES-PARALLEL CONTROL (Contd.)
➢Series-Parallel and Resistance Control
• At standstill, the motors are connected in series via a starting rheostat.
• The motors are started up in series and starting resistance is cut out step by
step to increase the speed.

24
SERIES-PARALLEL CONTROL (Contd.)
➢Series-parallel and resistance control (Contd.)
• To increase the speed further, the two motors are connected in parallel and at
the same time the starting resistance is connected in series.
• The starting resistance is again cut out step by step until full speed is attained.
• Then field control is introduced.Lecture Notes by Dr.R.M.Larik

25
SPEED CONTROL OF COMPOUND MOTORS
• Speed control of compound motors may be obtained by any one of the
methods described for shunt motors.
• Speed control cannot be obtained through adjustment of the series field
since such adjustment would radically change the performance
characteristics of the motor.

Speed control of dc motors

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