Starters

STARTERS

Introduction

DC motors are self-starting motors, whenever the armature and field winding of a DC motor receives supply, motoring action takes place. So, DC motors do not require any additional device to start it.

Necessity of Starter

When a de motor is started with full voltage applied across its armature terminals during the starting period, it will draw more current than its rated current. This excessive current will overheat the armature winding and may even damage the winding. During starting period, a variable resistance called a starter is connected in series with the armature circuit to limit the starting current.

Starting of DC Motors

Voltage equation E_b = V - I_a R_a

It speed is zero, back emf is also zero. For example, we assume that R_a value is 0.5 Ω. The supply voltage is 220 V. Power rating of the motor is 7500 W.

Then, the armature current

I_a = V / R_a = 220 / 0.5 = 440 A.

But the full-load current:

I_L = 7500 / 220 = 34 A.

Without using starter, if the motor is started directly, the starting current is around 13 times of the full-load current.

Thus the absence of back emf causes the armature current as the time of starting to shoot up to about (13 to 15) times the normal armature current.

Sudden drawing of this large current from the supply system is unwarranted. It causes sudden drop in voltage of the supply system.

This voltage drop affects the other loads connected in the system. However, this large current exists only for a brief period.

Just at the time of closing the supply to the armature, as and when the armature pick its speed, the back emf starts increasing and the armature current will come down.

The difficulty is that the circuit breaker may not be able to withstand this large current and it may open the supply connection.

In order to start and accelerate the motor within a reasonable time, it may be enough to have 2 to 3 times the rated current as the starting current.

Therefore, to limit the starting current, an additional resistance R_st should be added in series with the armature circuit.

Two-point Starter

A two-point starter is used for starting de motor which has the problem of over- speeding due to loss of load from its shaft. Such a starter is shown in fig.2.37

Here for starting the motor, the control arm is moved clockwise from its OFF position to the ON position against the spring tension. The control arm is held in the ON position by an electromagnet. The hold-on electromagnet is connected in series with the armature circuit. If the motor losses its load, current decreases and hence the strength of the electromagnet also decreases. The control arm returns to the OFF position due to spring tension, this preventing the motor from overspending. The starter arm also returns to the OFF-position when the supply voltage decreases appreciably. L and F are two points of the starter which are connected with the supply and motor terminals.

3 Point Starter

A 3 point starter is a device that helps in the starting and running of a DC shunt motor or compound wound DC motor (similar to a 4 point starter). Now the question is why these types of DC motors require the assistance of the starter in the first place? Well, it's due to the presence of back emf (E_b), which plays a critical role in governing the operation of the motor. The back emf develops as the motor armature starts to rotate in presence of the magnetic field, by generating action and counters the supply voltage. Hence the back emf at the starting of the motor is zero, but it develops gradually as the motor gathers speed.

The general motor emf equation is:

E = E_b + I_a R_a

Where E - Supply Voltage; E_b - Back EMF; I_a - Armature Current; and R_a = Armature Resistance. Since at starting E_b = 0, then E =I_a R_a. Hence we can rearrange for the armature current I_a:

I_a = E / R_a

We can see from the above equation that the current will be dangerously high at ort or we can starting (as the armature resistance R_a is small). This is why it's important that we make use of a device like the 3 point starter to limit the starting current to acceptably low value.

To understand how the starting current is restricted to the desired value, we need to look at the construction and working of three-point starter. The electrical symbols in the diagram below show all the essential parts of a three-point starter.

Construction of 3 Point Starter

Construction wise a starter is a variable resistance, integrated into the 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 three 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 spring is also attached with it, and the starter handle also contains a soft iron piece housed on it. This handle is free to 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 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 act as the two protecting devices of the Starter

Working of Three Point Starter

Having studied its construction, let us now go into the working of the 3 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 contact with stud No. 1. At this point, field compound motor gets supply through the parallel path provided to starting the 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:

I_a = E / (R_a + R_st)

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 the development of speed. Now the obvious question is once the handle is taken to the RUN position how it is supposed to stay there, as long as the motor is running.

To find the answer to this question 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, a soft iron piece is 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 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 loses 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 problem. Thus it also acts as a protective device safeguarding the motor from any kind of abnormality.

Drawbacks of a Three Point Starter

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 field resistance can be increased. Therefore current through the shunt field is reduced.

Field current becomes very low which results in 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 desirable. A 4 point starter is thus used instead, which does not have this drawback.

4 Point Starter

A 4 point starter protects the armature of a DC shunt motor or compound wound DC motor against the initially high starting current of the DC motor. The 4 point starter has a lot of constructional and functional similarity to a 3 point starter, but this special device has an additional point and coil in its construction (as the name suggests). This brings about some difference in its functionality, though the basic operational characteristic remains the same. The basic difference in the circuit of a 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.

Now to go into the details of the operation of 4 point starter, let's have a look at its construction diagram. This will help demonstrate the difference between a 4 vs 3 point starter.

Construction and Operation of Four Point Starter

A 4 point starter as the name suggests has 4 main operational points, namely

i. '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 the 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 let's 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 the 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.

i. 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 3_rd part flowing through the no voltage coil in series with the protective resistance R.

4. 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.

5. 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. As otherwise both are almost similar and are used for limiting the starting current to a shunt wound DC motor or compound wound DC motor, and thus act as a protective device.