Three Point Starter for DC Motor have three point for electrical connection namely Line (L) to which positive / negative terminal of DC Voltage is connected, Field (F) which is connected to the field winding of DC Motor and Armature which is connected to the Armature of DC Motor. As it is having three points for external connection therefore it is called Three Point Starter. The diagram of Three Point Starter for DC Motor is shown below.
To summarize the connection of Terminals,
A – Shall be connected to either terminal of Armature
L – Shall be connected to either positive or negative terminal of DC supply
F – Shall be connected to either terminal of Field winding.
Working Principle of Three Point Starter:
As discussed in earlier post DC Motor Starter, during starting DC Motor takes high inrush current which may damage the winding of the machine. Therefore to limit the starting current we make use of Resistance which is connected in series with the Armature and as the Motor accelerates the Series Resistance is cut out of the circuit in step. This principle is used in the Three Point Starter of DC Motor.
When DC Motor is at rest, obviously the position of the handle of starter should be at OFF position where the whole resistance is out of the armature circuit of DC Motor. At OFF position the handle of starter is kept because of strong force of spiral spring as shown in figure above.
For starting the DC Motor, the handle is brought at position 1. Carefully see that when handle is at position 1, the Field winding and holding coil HC get connected in series with the applied DC Voltage whereas whole resistance (R1+R2+R3+R4+R5) get connected with the Armature circuit of the DC Motor. As current now start flowing in both the Armature circuit and the Field circuit, the DC Motor will start rotating.
After the Armature has picked up sufficient speed, handle is brought to position 2. (While starting DC Motor using starter, the practice shall be to start slowly not rapidly, therefore when the Armature or Rotor pick up enough speed Handle shall be brought to position 2). Observe that at position 2, resistance R1 is cut out of the Armature but at the same time R1 is added in series with the field circuit. Note this point carefully.
Thus when we move the Handle from position 1 to ON, series resistance from the Armature circuit is cut out and the same is added to the Field Circuit. The Handle is moved from position 1 to position ON during the starting of DC Motor. At position ON, the soft Iron on the Handle touches the Holding Coil (It is nothing but an electromagnet as current is flowing in the coil wounded on HC). As soft Iron touches the HC, Handle remain at ON position because of magnetic force of HC. The Holding Coil is sometimes called No Release Coil or Low Voltage Release Coil.
HC plays a great role in Three Point Starter of DC Motor. It does the following functions:
Suppose DC Motor is running and suddenly power supply is lost. What will happen without HC?
If there were no HC, the Handle will remain there at ON position and when power supply restores, no resistance is there in series with the Armature for starting so DC Motor may get damage. Thus it is because of HC that when power supply is lost, the Handle comes back to OFF position as HC is no more Electromagnet (As no current through the HC).
Again suppose the Field winding get open circuited because of some reason and Handle of starter remains at ON position then what will happen?
As we know that Ea = KaØωm
In our case, Ea = Vt -Iara= Constant
Where, Vt = Supplied DC Voltage V
Ia = Armature Current
ra = Armature Resistance
Therefore when Field circuit get open, the magnetic flux Ø start reducing and hence
ωm = Ea / KaØ
starts increasing. At small residual field flux, the DC Motor may get dangerously high speed which in turn will damage the DC Motor.
Also, to supply constant load torque, Te = KaØIa
As Ø is decreasing, the Armature current must increase to supply constant Torque Load. At residual magnetic flux Ø, the Ia may be excessive which will burn the Armature winding of the DC Motor.
Now, see from Starter point of view. When field circuit get open, the current through the HC stops which de-energizes the Holding Coil HC because of which its magnetic property is lost and starter Handle id pulled back to OFF position by spring force. Thus see how HC plays such a great role.
Can we put the starter Handle at any intermediate position?
Answer is NO, we can’t. As we have only two choice either to put the Handle at OFF where spring force will keep Handle at OFF position or at ON position where HC will keep Handle there against spring pull.
As we observed, when we move the Handle from position 1 to ON, series resistance from the Armature circuit is cut out and the same is added to the Field Circuit. Why such an arrangement in which series resistance is added to field circuit? What would happen if no series resistance added to Field Circuit?
Suppose we put the DC Motor off by bringing the Handle from ON to OFF position then the Armature of DC Motor, Field coil and HC get connected in series as shown in figure below.
As the stored energy in Field coil = LI2 /2 where L is Inductance and I is current, therefore as soon as we put the DC Motor OFF, this much of stored energy in Field coil is dissipated in the series resistance. If there would have been no series resistance in the field circuit then there would have been heavy sparking while bringing the Handle from position 1 to OFF. Hope you understand, if not write in comment box, I will be happy to answer.
Why Over Load Release is given?
Observe the figure carefully; we note that Over Load Release is in series with the Armature circuit. It is so designed that it will pick up when the value of Armature current exceeds a preset value. When the Armature current exceeds the preset value the Over Load Release (OR) picks up and attracts the movable soft iron pivoted below the OR. As soon as soft iron get attracted by the OR, the two terminals of HC get short circuited and hence no current will flow through the HC and therefore no attractive force by HC to the starter Handle due to which spring force will pull back the Handle to OFF position and hence DC Motor will be switched OFF. Thus OR protects the DC Motor from excessive Armature current.
Hope you enjoyed the topic. If any query, write in comment box. It will be my pleasure to answer. Thank you!