A dc motor is basically self-starting, it doesn't require any starting device. But in order to start the motor in a desired safe way, there is a need for an external device called 'Starter'. A dc motor can start without a starter, but when it is connected across the supply.
We know that, from the equation of back emf of a dc motor. The relation between speed and back emf of a dc motor is Eb ∝ N. At starting speed N = 0, and hence Eb. The voltage equation of a dc motor is,
Here, Ia Ra is the armature drop due to armature resistance Ra. Generally, the armature resistance of a dc motor is kept small. When the dc motor is switched on with rated supply voltage. From the above voltage equation at starting, due to small Ra, the armature current at starting will be very high. This mainly because at starting the back emf will be zero. For a detailed explanation check out the article need for a starter for starting a dc motor.
For limiting this starting current there are two types of starters 3-point and 4-point starters. In this article let us see about working of 3-point starter with its construction.
Construction of 3-point Starter :
A starter basically consists of a set of resistors connected in series. The terminals of each resistance are brought out separately as shown below i.e., OFF, 1, 2, 3, 4, 5, or RUN.
A handle attached with spring, which is movable against the spring force is pivoted as shown below. The 3-point starter is named for its 3 terminals to connect with the motor. The three terminals of the 3-point starter are,
- Terminal-L (positive terminal supply),
- Terminal-F (field winding terminal), and
- Terminal-A (armature winding terminal).
The dc supply is given to the positive terminal which is connected to the over-load release (OLR). The OLR is then connected to the starter handle. The handle is made to move in such a way that it maintains contact with different resistances accordingly with the movement. The terminal of the final resistance i.e., 5th terminal, is connected to the armature winding (terminal A) of the motor.
An arc made up of conducting material is placed below resistors, maintain continuous contact with the handle. The arc supplies current to the series connected no-volt coil (NVC) and field winding (terminal F). Before knowing the function of NVC, OLR, and arc let see the working of the 3-point starter.
Working of 3-point Starter :
The purpose of using a starter is to limit the starting current of a motor. Initially, the starter handle is moved to terminal 1 of the resistances set, connecting the motor across the supply.
Here we can see that when the handle is at terminal 1. The whole resistances (R1 + R2 + R3 + R4 + R5) get connected in series with the armature circuit. Due to this, the effective resistance of the armature circuit increases, thereby reducing starting current drawn by the armature.
As the motor picks up speed, the handle is gradually moved to the final terminal (5th or ON). When the handle comes in contact with the final terminal i.e., at ON position, the entire resistances are disconnected from the armature circuit.
Thus motor will be protected from the high rush in currents at the time of starting. Once the motor pickups its speed the back emf in the motor builds that regulate armature current.
One can get a question that, there is no usage of NVC, OLR, and arc in this entire operation and how the handle can hold its position at ON position ? let us see them.
Function of No-volt Coil (NVC) :
It is a coil wound with a number of turns that behave like an electromagnet. In the above diagram, we can see that field winding is connected through NVC (series connection). This means that current through both field and NVC are equal.
When the handle moved to the ON position, due to the electromagnetic effect of NVC it attracts the soft iron piece fixed to the handle. Due to this magnetic force of attraction, the NVC holds the handle in RUN position maintaining the supply. It should ensure that the force of attraction must be greater than the spring restoring force or else the handle is pulled back to OFF position.
Also, in the case, if field winding is disconnected due to internal winding cut-off or field-rheostat failure the shunt motor will over speed. The causes current to becomes zero in both field and NVC. The NVC loses its magnetic property and the magnetic force of attraction between NVC and handle is devastated. This makes the handle return to its OFF position by spring restoring force.
It also protects the motor by moving the handle to the OFF position in case of supply failure. Otherwise, when the supply is restored, the armature gets directly connected across the supply and causes a flow of heavy inrush currents. It also operates under low supply voltage and other abnormal conditions.
Function of Overload Release (OLR) :
It is also a coil wound with a number of turns that behave like an electromagnet. The overload protection of the motor is done with OLR. It is incorporated by a series connection with the armature circuit. When the motor is overloaded the armature will start drawing large currents, which also flow through OLR.
When the current through the OLR increases the magnetic strength of the OLR increases. This tends to attract the movable iron lever placed below the OLR by pulling upwards. This movement causes the short-circuiting of two terminals that are connected to NVC.
The short-circuiting of NVC causes current to become zero in it and gets de-energized. Therefore the starter handle is pulled to OFF position by spring restoring force, thereby protecting the motor against overload.
The instant of attraction force (pulling upward) by the OLR on the iron lever is in such a way that. Under normal conditions, for normal armature currents, the gravitational force balances the magnetic force without causing any movement in the iron lever. At overload conditions, the magnetic force becomes greater than the gravitational force.
Use of Arc :
We know that supply to the NVC and field winding is given through arc. Also, without an arc, we can supply current by directly connecting across the first resistance i.e., to the terminal-1, as shown by a dotted line. But once the handle is moved to the ON position. The entire resistance which is connected in the armature circuit during starting will comes under the field circuit. This increase in field resistance will affect its performance.
This problem can be overcome by the use of an arc made up of brass or copper. It enables the direct connection of the field circuit with the supply, without any starter resistances affect. The arc maintains continuous contact with the handle once it is moved from the OFF position.
