Experiment No.: 07
Aim of the Experiment: –
To study D.C. motor starters three-point & four-point starter.
Apparatus Required: –
| Sl. No. | Name | Specification | Quantity |
|---|---|---|---|
| 01 | 3- point starter | 1 no. | |
| 02 | 4- point starter | 1 no. |
Circuit Diagram: –
Three-Point Starter
Theory: –
The starting of DC motor is somewhat difference from the starting of all other types of electrical motors. This difference is credited to the fact that a dc motor unlike other types of motor has a very high starting current that has the potential of damaging the internal circuit of the armature winding of dc motor if not restricted to some limited value. This limitation to the starting current of dc motor is brought about by means of the starter. Thus the distinguishing fact about the starting methods of dc motor is that it is facilitated by means of a starter. Or rather a device containing a variable resistance connected in series to the armature winding so as to limit the starting current of dc motor to a desired optimum value taking into consideration the safety aspect of the motor.
A three-point starter in simple words is a device that helps in the starting and running of a shunt wound DC motor. Now the question is why these type of DC motors require the assistance of the starter in the first case. The only explanation to that is given by the presence of back emf Eb, 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. This also essentially means, that the back emf at the starting is zero and develops gradually as the motor gathers speed.
the general motor emf equation E = Eb + IaRa,
at starting is modified to E = IaRa as the starting Eb= 0.
- Ia= E/Ra
Thus we can well understand from the above equation that the current will be dangerously high at starting (as armature resistance Ra is small) and hence its important that we make use of a device like the three-point starter to limit the starting current to an allowable lower value.
Let us now look into the construction and working of three point starter to understand how the starting current is restricted to the desired value. For that let’s consider the diagram showing all the essential parts of the three-point starter.
Construction of three-point starter:
Construction wise a starter is a variable resistance, integrated into number of section as shown in the figure beside. The contact point of these sections are called studs and are shown in fig. namely OFF, 1, 2, 3, 4, 5, RUN. Other than that there are 3 main points, refer to as
- “L” Line terminal. (Connected to positive of supply)
- “A” Armature terminal. (Connected to the armature winding)
- “F” Field terminal. (Connected to the field winding)
and from there is gets the name 3 point starter.
Now studying 3 point starter in further details reveals that, the point “L” is connected to an electromagnet called overload release (OLR) as shown in 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” give 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 the Three-Point Starter:
At starting 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 Volt 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 mother 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.
Working of No Voltage Coil of three-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 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 through NVC is affected and it immediately losses 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.
Four-Point Starter:
The four 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, which naturally brings about remains same.
Construction and operation of four-point starter:
the 4 main operational points are,
- “L” Line terminal. (Connected to the positive of supply)
- “A” Armature terminal. (Connected to the armature winding)
- “F” Field terminal. ( Connected to the field winding)
- “N” Connected to the No Voltage Coil.
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 brings 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 figure.
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 moved manually to regulate the starting current with gathering speed.
Operation of Four-point starter:
Look into the above diagram of four-point starter, considering that supply is given and the handle is taken stud no. 1, then the circuit is complete and line 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.
ii). a 2nd part flowing through the field winding F.
iii). 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 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 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 acts as a protective device.
Conclusion:
Written by student.