Download 4 Point Starter

Starting Methods of A DC Motor
Basic operational voltage equation of a DC motor is given as
V = Eb + IaRa and hence,
Ia = (V - Eb) / Ra
Now, when the motor is at rest, obviously, the back emf Eb = 0. Hence, armature current at the
moment of starting can be given as Ia = V / Ra. In practical DC machines, armature resistance is
basically very low, generally about 0.5 Ω. Therefore, a large current flows through the armature
during starting. This current is large enough to damage the armature circuit.
Due to this excessive starting current –
The fuses may blow out and the armature winding and/or commutator brush arrangement may
get damaged.
Very high starting torque will be produced (as torque is directly proportional to the armature
current), and this high starting torque may cause huge centrifugal force which may throw off the
armature winding.
Other loads connected to the same source may experience a dip in the terminal voltage.
A large DC motor will pick up speed rather slowly due to its large rotor inertia. Hence, building
up the back emf slowly causing the level of high starting current maintained for quite some time.
This may cause severe damage. To avoid this, a suitable DC motor starter must be used. Very
small dc motors, however, may be started directly by connecting them to the supply with the help
of a contactor or a switch. It does not result in any harm because they gather speed quickly due to
small rotor inertia. In this case, the large starting current will die down quickly because of the
fast rise in the back emf.
DC Motor Starters
To avoid the above dangers while starting a DC motor, it is necessary to limit the starting
current. So, a DC motor is started by using a starter. There are various types of dc motor
starters, such as 3 point starter, 4 point starter, no-load release coil starter, thyristor controller
starter.
The basic concept behind every DC motor starter is adding external resistance to the armature
winding during starting. From the followings, 3 point starters and 4 point starters are used for
starting shunt wound motors and compound wound motors
3 Point Starter:
.
Construction of 3 Point Starter:
Construction wise a starter is a variable resistance, integrated into number of sections as shown
in the figure beside. The contact points of these sections are called studs and are shown
separately as OFF, 1, 2, 3, 4, 5, RUN. Other than that there are 3 main points, referred to as
1. 'L' Line terminal. (Connected to positive of supply.)
2. 'A' Armature terminal. (Connected to the armature winding.)
3. 'F' Field terminal. (Connected to the field winding.)
And from there it gets the name 3 point starter. Now studying the construction of 3 point starter
in further details reveals that, the point 'L' is connected to an electromagnet called overload
release (OLR) as shown in the figure. The other end of 'OLR' is connected to the lower end of
conducting lever of starter handle where a spring is also attached with it and the starter handle
contains also a soft iron piece housed on it. This handle is free to move to the other side RUN
against the force of the spring. This spring brings back the handle to its original OFF position
under the influence of its own force. Another parallel path is derived from the stud '1', given to
the another electromagnet called No Volt Coil (NVC) which is further connected to terminal 'F'.
The starting resistance at starting is entirely in series with the armature. The OLR and NVC acts
as the two protecting devices of the starter.
Working of Three Point Starter:
To start with the handle is in the OFF position when the supply to the DC motor is switched on.
Then handle is slowly moved against the spring force to make a contact with stud No. 1. At this
point, field winding of the shunt or the compound motor gets supply through the parallel path
provided to starting resistance, through No Voltage Coil. While entire starting resistance comes
in series with the armature. The high starting armature current thus gets limited as the current
equation at this stage becomes Ia = E/(Ra+Rst). As the handle is moved further, it goes on making
contact with studs 2, 3, 4 etc., thus gradually cutting off the series resistance from the armature
circuit as the motor gathers speed. Finally when the starter handle is in 'RUN' position, the entire
starting resistance is eliminated and the motor runs with normal speed. This is because back emf
is developed consequently with speed to counter the supply voltage and reduce the armature
current. So the external electrical resistance is not required anymore, and is removed for
optimum operation. The handle is moved manually from OFF to the RUN position with
development of speed.
Now once the handle is taken to the RUN position how is it supposed to stay there, as long as
motor is running ? To know this let us look into the working of No Voltage Coil.
Working of No Voltage Coil of 3 Point Starter
The supply to the field winding is derived through no voltage coil. So when field current flows,
the NVC is magnetized. Now when the handle is in the 'RUN' position, soft iron piece connected
to the handle and gets attracted by the magnetic force produced by NVC, because of flow of
current through it. The NVC is designed in such a way that it holds the handle in 'RUN' position
against the force of the spring as long as supply is given to the motor. Thus NVC holds the
handle in the 'RUN' position and hence also called hold on coil.
Now when there is any kind of supply failure, the current flow through NVC is affected and it
immediately looses its magnetic property and is unable to keep the soft iron piece on the handle,
attracted. At this point under the action of the spring force, the handle comes back to OFF
position, opening the circuit and thus switching off the motor. So due to the combination of NVC
and the spring, the starter handle always comes back to OFF position whenever there is any
supply problems. Thus it also acts as a protective device safeguarding the motor from any kind
of abnormality.
Drawbacks / Disadvantages of a Three Point Starter:
The following drawbacks of a 3 point starter are as follows:
The 3 point starter suffers from a serious drawback for motors with a large variation of
speed by adjustment of the field rheostat.

To increase the speed of the motor, the field resistance should be increased. Therefore,
the current through the shunt field is reduced.

The field current may become very low because of the addition of high resistance to
obtain a high speed.

A very low field current will make the holding electromagnet too weak to overcome the
force exerted by the spring.

The holding magnet may release the arm of the starter during the normal operation of the
motor and thus, disconnect the motor from the line. This is not a desirable action.
4 Point Starter:
A 4 Point Starter is almost similar in functional characteristics like 3 Point Starter. In the
absence of back EMF, the 4 Point Starter acts as a current limiting device while starting of the
DC motor. 4 Point Starter also acts a protecting device.
Construction and Operation of Four Point Starter:
A 4 point starter as the name suggests has 4 main operational points, namely
1. 'L' Line terminal. (Connected to positive of supply.)
2. 'A' Armature terminal. (Connected to the armature winding.)
3. 'F' Field terminal. (Connected to the field winding.)
4. Like in the case of the 3 point starter, and in addition to it there is, A 4th point N.
(Connected to the No Voltage Coil NVC)
The remarkable difference in case of a 4 point starter is that the No Voltage Coil is connected
independently across the supply through the fourth terminal called 'N' in addition to the 'L', 'F'
and 'A'. As a direct consequence of that, any change in the field supply current does not bring
about any difference in the performance of the NVC. Thus it must be ensured that no voltage
coil always produce a force which is strong enough to hold the handle in its 'RUN' position,
against force of the spring, under all the operational conditions. Such a current is adjusted
through No Voltage Coil with the help of fixed resistance R connected in series with the NVC
using fourth point 'N' as shown in the figure above.
Apart from this above mentioned fact, the 4 point and 3 point starters are similar in all other
ways like possessing is a variable resistance, integrated into number of sections as shown in the
figure above. The contact points of these sections are called studs and are shown separately as
OFF, 1, 2, 3, 4, 5, RUN, over which the handle is free to be maneuvered manually to regulate the
starting current with gathering speed.
Now to understand its way of operating lets have a closer look at the diagram given above.
Considering that supply is given and the handle is taken stud No.1, then the circuit is complete
and line current that starts flowing through the starter. In this situation we can see that the current
will be divided into 3 parts, flowing through 3 different points.
1. 1 part flows through the starting resistance (R1+ R2+ R3…..) and then to the armature.
2. A 2nd part flowing through the field winding F.
3. And a 3rd part flowing through the no voltage coil in series with the protective resistance
R.
So the point to be noted here is that with this particular arrangement any change in the shunt field
circuit does not bring about any change in the no voltage coil as the two circuits are independent
of each other. This essentially means that the electromagnet pull subjected upon the soft iron bar
of the handle by the no voltage coil at all points of time should be high enough to keep the
handle at its RUN position, or rather prevent the spring force from restoring the handle at its
original OFF position, irrespective of how the field rheostat is adjusted. This marks the
operational difference between a 4 point starter and a 3 point starter.
Working Principle of Four Point Starter:
The 4 point starter like in the case of a 3 point starter also acts as a protective device that helps
in safeguarding the armature of the shunt or compound excited DC motor against the high
starting current produced in the absence of back emf at starting. The 4 point starter has a lot of
constructional and functional similarity to a three point starter, but this special device has an
additional point and a coil in its construction. This naturally brings about some difference in its
functionality, though the basic operational characteristic remains the same. The basic difference
in circuit of 4 point starter as compared to 3 point starter is that the holding coil is removed from
the shunt field current and is connected directly across the line with current limiting resistance in
series.
Electrical Braking of DC Motor
1. Dynamic Braking or Rheostatic Braking of DC Motor:
In Dynamic Braking, a braking resistor Rb is connected across the armature as soon as the DC
motor is disconnected from the supply mains. The motor now works as a generator, producing
the braking torque.
For the braking operation in Dynamic Braking, the motor is connected in two ways. Firstly the
separately excited or shunt motor can be connected either as a separately excited generator,
where the flux is kept constant. The second way is that it can be connected as a self-excited shunt
generator, with the field winding in parallel with the armature.
For Shunt DC Motor: The connection diagram of Dynamic Braking of separately excited DC
motor is shown below.
When the machine works in the motoring mode:
The Connection diagram is shown below when braking with separate excitation is done,
The Connection diagram is shown below when braking with self-excitation is performed,
This method is also known as Rheostatic Braking because an external braking resistance Rb is
connected across the armature terminals for electric braking. During an electric braking, the
kinetic energy stored in the rotating parts of the machine and the connected load is converted into
electric energy, when the motor is working as a generator. The energy is dissipated as heat in the
braking resistance Rb and armature circuit resistance Ra.
The connection diagram of Dynamic Braking of DC Shunt Motor is shown below,
When the machine is working in the motoring mode,
The Connection diagram of shunt motor Braking with self and separate excitation is shown in the
figure below.
For Series DC Motor: For Dynamic Braking, the series motor is disconnected from the supply.
A variable resistance Rb as shown in the figure below is connected in series, and the connections
of the field windings are reversed.
The field connections are reversed so that the current through the field winding flows in the same
direction as before i.e. from S1 to S2 in order that the back EMF produces the residual flux. The
machine now starts working as a self-excited series generator.
The Dynamic or Rheostatic Braking is an insufficient method of braking because all the energy
which is generated is dissipated in the form of heat in the resistance.
2. Regenerative Braking:
In Regenerative Braking, the power or energy of the driven machinery which is in kinetic form
is returned back to the power supply mains. This type of braking is possible when the driven load
or machinery forces the motor to run at a speed higher than no load speed with a constant
excitation.
Under this condition, the back emf Eb of the motor is greater than the supply voltage V, which
reverses the direction of motor armature current. The machine now begins to operate as a
generator and the energy generated is supplied to the source.
Regenerative braking can also be performed at very low speeds if the motor is connected as a
separately excited generator. The excitation of the motor is increased as the speed is reduced so
that the two equations shown below are satisfied.
The motor does not enter into saturation on increasing excitation.
Regenerative braking is possible with shunt and separately excited motors.In compound motors,
braking is possible only with weak series compounding.
The necessary condition for regeneration is that the back EMF Eb should be greater than the
supply voltage so that the armature current is reversed and the mode of operation changes from
motoring to generating.
Regenerative Braking in DC Shunt Motors:
Under normal operating conditions the armature current is given by the equation shown below.
When the load is lowered by a crane, hoist or lift causes the motor speed to be greater than the
no-load speed. The back EMF becomes greater than the supply voltage. Consequently, armature
current Ia becomes negative. The machines now begin to operate as a generator.
Regenerative Braking in DC Series Motors:
In case of DC Series Motor an increase in speed is followed by a decrease in the armature current
and field flux. The back EMF Eb cannot be greater than the supply voltage. Regeneration is
possible in DC Series Motor since the field current cannot be made greater than the armature
current.
Regeneration is required where DC Series Motor is used extensively such as in traction, elevator
hoists etc. For example – In an Electro-locomotive moving down the gradient, a constant speed
may be necessary. In hoist drives the speed is to be limited whenever it becomes dangerously
high.
One commonly used method of regenerative braking of DC Series Motor is to connect it as a
shunt motor. Since the resistance of the field winding is low, a series resistance is connected in
the field circuit to limit the current within the safe value.
Applications of Regenerative Braking:

Regenerative braking is used especially where frequent braking and slowing of drives is
required.

It is most useful in holding a descending load of high potential energy at a constant
speed.

Regenerative braking is used to control the speed of motors driving loads such as in
electric locomotives, elevators, cranes and hoists.

Regenerative braking cannot be used for stopping the motor. It is used for controlling the
speed above the no-load speed of the motor driving.
3. Plugging or Reverse Current Braking:
In Plugging or Reverse Current Braking the armature terminals or the supply polarity of a
separately excited or shunt motor when running are reversed. Therefore, in Plugging the supply
voltage V and the induced voltage Eb which is also called back EMF will act in the same
direction.
Thus, during plugging the effective voltage across the armature will be (V + Eb) which is almost
twice the supply voltage. The armature current is reversed, and a high braking torque is
produced. An external current limiting resistor is connected in series with the armature to limit
the armature current to a safe value.
The connection diagram of DC separately excited motor and its characteristics is shown in the
figure below.
Where,
V is the supply voltage, Rb is the external resistance, Ia is the armature current and If is the field
current.
Similarly, the connection diagram and the characteristic of the series motor in plugging mode is
shown in the figure below.
For braking, a series motor either the armature terminals or field terminals are reversed. But both
armature and field terminals are not reversed together. Reversing of both the terminals will give
only normal working operation.
At the zero speed, the braking torque is not zero. The motor must be disconnected from the
supply at or near zero speed when the motor is used for stopping a load. If the motor is not
disconnected from the supply mains, the motor will speed up in the reverse direction. For
disconnecting the supply centrifugal switches are used.
The method of braking, known as Plugging or Reverse Current Braking is a highly insufficient
method because, in addition to the power supplied by the load, power supplied by the source is
also wasted in resistance.
Applications of Plugging:
The Plugging is commonly used for the following purposes listed below.

In controlling elevators

Rolling Mills

Printing Presses

Machine tools, etc.