Electrical engineering

1. Three Point Starter for DC
Motors
By : Dr. Atul R. Phadke
Associate Professor and Head of Electrical Engineering Department
Government College of Engineering, Jalgaon

2. NECESSITY OF STARTERS:
2
We know that armature current 𝐼𝑎 =
𝑉 − 𝐸𝑏
𝑅𝑎
At start, speed of the motor, N = 0.
𝐸𝑏 =
∅𝑃𝑁
60
×
𝑍
𝐴
, so if N = 0, 𝐸𝑏 = 0
Then, 𝐼𝑎 =
𝑉 − 0
𝑅𝑎
=
𝑉
𝑅𝑎
Resistance of armature is very small.
Hence, armature current Ia will be very large if the armature is directly
connected across the rated voltage supply.
For example – Let Ra = 1 , V = 200 V and power rating P = 5 kW = 5000 W
Then, rated current of the motor = 5000/200 = 25 A and
Starting current = V/ Ra = 200/1 = 200 A, which is eight times greater than the
rated current of motor.
F1
F2
IL
Ia
A1
A2
If
V
+
_
Eb
Ra

3. NECESSITY OF STARTERS:
3
Excessive starting current will blow out the fuses and before that it may
damage the commutator, brushes etc.
The performance of other loads connected to the same line may also get
affected due to the large voltage drop in the line.
To avoid this a variable resistance is introduced in the armature during starting
period.
With starting resistance Rs in series with armature, the starting current =
𝑉
𝑅𝑎+𝑅𝑠
.
Therefore, the high starting current is limited to a safe value.
Starting period is about 5 to 10 seconds depending on the size of motor.
Once the motor is started the speed of motor N increases. Therefore Eb
increases and armature current decreases.
The starting resistance Rs is gradually cut out of the armature circuit as the
motor gains speed and finally it is reduced to zero.
F1
F2
IL
Ia
A1
A2
Ish
V
+
_
Eb
Ra
M
F1
F2
A1
A2
V
Starting
Resistance
Ia
Ish Rs
+
_

4. NECESSITY OF STARTERS:
4
However, very small motors may be started directly by connecting them to supply
lines because of the following reasons:
1) These motors have a relatively higher resistance, hence their starting
current is not so high.
2) They have low moment of inertia, hence their speed increases quickly.
3) The starting current drawn by these motors is not sufficient to produce a
large drop in line voltage.
F1
F2
IL
Ia
A1
A2
If
V
+
_
Eb
Ra

5. THREE POINT STARTER 5
M
F1
F2
A1
A2
V
+
_
R
L A
F
OFF
1
2 3
4
5
6
H – Handle
S – Spring
I – Soft iron piece
P – Plunger
NVR – No Volt Release
OLR – Overload Release
M – Main switch
R – Field Rheostat
H
S
I
P
NVR
OLR
M
M
Starting Resistance
A A1 A2
L
+
_
F
V
F1 F2
OLR
NVR
R
Starting
Resistance
It has three terminals L, F and A.
L is connected to +ve line terminal.
F is connected to shunt field.
A rheostat is inserted in the field circuit to
control the speed of motor.
A is connected to armature.
Other terminals of armature and field are
connected to the negative of the line.

6. 6
H – Handle
S – Spring
I – Soft iron piece
P – Plunger
NVR – No Volt Release
OLR – Overload Release
M – Main switch
R – Field Rheostat
M
A A1 A2
L
+
_
F
V
F1 F2
OLR
NVR
R
Starting
Resistance
The starter arm or handle (H) is held in
the ‘OFF’ position by a strong spiral spring
(S) when the motor is at rest.
Graded starting resistance (in this case
graded in five steps) is connected in
series with armature.
NVR coil is connected in series with the
field winding and carries field current Ish.
OLR coil carries load current IL.
IL
Ish
THREE POINT STARTER
M
F1
F2
A1
A2
V
+
_
R
L A
F
OFF
1
2 3
4
5
6
H
S
I
P
NVR
OLR
M
Starting Resistance

7. 7
H – Handle
S – Spring
I – Soft iron piece
P – Plunger
NVR – No Volt Release
OLR – Overload Release
M – Main switch
R – Field Rheostat
M
A A1 A2
L
+
_
F
V
F1 F2
OLR
NVR
R
Starting
Resistance
To start the motor, the main switch (M) is
closed first.
Then the starter handle is moved on the
first stud.
Thus complete resistance of all the five
steps is included in the armature circuit.
Therefore, the starting current 𝐼𝑠𝑡 =
𝑉
𝑅𝑎+ 𝑅𝑠
,
where, Rs = resistance of all the steps.
Thus starting current is reduced to a safe
limit.
IL
Ish
THREE POINT STARTER
M
F1
F2
A1
A2
V
+
_
R
L A
F
OFF
1
2 3
4
5
6
H
S
I
P
NVR
OLR
M
Starting Resistance

8. 8
H – Handle
S – Spring
I – Soft iron piece
P – Plunger
NVR – No Volt Release
OLR – Overload Release
M – Main switch
R – Field Rheostat
M
A A1 A2
L
+
_
F
V
F1 F2
OLR
NVR
R
Starting
Resistance
When the motor gains sufficient speed, the
starter arm should be moved slowly on the
studs 2, 3, etc. up to the final position.
As the arm is moved, the starting
resistance is gradually cut out and finally it
is reduced to zero.
In this position, the soft iron piece (S)
attached to the handle is attracted and held
by NVR magnet.
It is known as LOW VOLTAGE or NO
VOLTAGE release.
Its coil is called as –
‘HOLD-ON’ coil.
IL
Ish
THREE POINT STARTER
M
F1
F2
A1
A2
V
+
_
R
L A
F
OFF
1
2 3
4
5
6
H
S
I
P
NVR
OLR
M
Starting Resistance

9. 9
H – Handle
S – Spring
I – Soft iron piece
P – Plunger
NVR – No Volt Release
OLR – Overload Release
M – Main switch
R – Field Rheostat
M
A A1 A2
L
+
_
F
V
F1 F2
OLR
NVR
R
Starting
Resistance
Function of NVR:
The normal function of the hold-on coil is to
hold on the starting arm in the final running
position when the motor is under normal
operation.
Whenever the supply fails or disconnected
or if the supply voltage becomes low, the
handle should return to its OFF position to
prevent direct connection of armature
when the supply restores again.
IL
Ish
THREE POINT STARTER
M
F1
F2
A1
A2
V
+
_
R
L A
F
OFF
1
2 3
4
5
6
H
S
I
P
NVR
OLR
M
Starting Resistance

10. 10
H – Handle
S – Spring
I – Soft iron piece
P – Plunger
NVR – No Volt Release
OLR – Overload Release
M – Main switch
R – Field Rheostat
M
A A1 A2
L
+
_
F
V
F1 F2
OLR
NVR
R
Starting
Resistance
Function of NVR:
If the field current becomes zero due to
opening of field circuit or if the field current
becomes very low, the motor will run at
dangerously high speed because, 𝑁 ∝
1
∅
.
Whenever the field circuit becomes open,
the starting arm immediately springs back
to the OFF position.
The motor gets disconnected from supply
and its operation of motor at dangerously
high speed is prevented.
IL
Ish
THREE POINT STARTER
M
F1
F2
A1
A2
V
+
_
R
L A
F
OFF
1
2 3
4
5
6
H
S
I
P
NVR
OLR
M
Starting Resistance

11. 11
H – Handle
S – Spring
I – Soft iron piece
P – Plunger
NVR – No Volt Release
OLR – Overload Release
M – Main switch
R – Field Rheostat
M
A A1 A2
L
+
_
F
V
F1 F2
OLR
NVR
R
Starting
Resistance
Function of OLR:
If the motor becomes overloaded beyond a
certain pre-determined value, the electro-
magnet of overload release gets more
magnetized.
It attracts the plunger P and the NVR coil
gets short circuited.
Due to demagnetization of NVR coil, the
starter arm returns to its OFF position and
the motor is protected against overload.
IL
Ish
THREE POINT STARTER
M
F1
F2
A1
A2
V
+
_
R
L A
F
OFF
1
2 3
4
5
6
H
S
I
P
NVR
OLR
M
Starting Resistance

12. 12
H – Handle
S – Spring
I – Soft iron piece
P – Plunger
NVR – No Volt Release
OLR – Overload Release
M – Main switch
R – Field Rheostat
M
A A1 A2
L
+
_
F
V
F1 F2
OLR
NVR
R
Starting
Resistance
If the motor is to be stopped, open only the
main switch.
If starter arm remains in the final position,
full voltage will be applied across the
armature when the supply is switched ‘ON’
again.
The current in holding coil will reduce to
zero and results in demagnetization of
NVR coil.
The starter arm returns to its OFF position
and the motor is disconnected from supply.
IL
Ish
THREE POINT STARTER
M
F1
F2
A1
A2
V
+
_
R
L A
F
OFF
1
2 3
4
5
6
H
S
I
P
NVR
OLR
M
Starting Resistance

13. 13
H – Handle
S – Spring
I – Soft iron piece
P – Plunger
NVR – No Volt Release
OLR – Overload Release
M – Main switch
R – Field Rheostat
M
A A1 A2
L
+
_
F
V
F1 F2
OLR
NVR
R
Starting
Resistance
To run the motor at high speed, the field
current must be reduced to a low value by
inserting high resistance in the series field
winding.
A very low field current will make the
holding magnet too weak to overcome the
force exerted by the spiral spring.
Then the holding magnet may release the
starter arm and the motor will stop.
This difficulty can be overcome by using a
four point starter.
IL
Ish
THREE POINT STARTER
M
F1
F2
A1
A2
V
+
_
R
L A
F
OFF
1
2 3
4
5
6
H
S
I
P
NVR
OLR
M
Starting Resistance