Starting of Dc motor
Starting of DC motor … Dc motor has a very high starting current which makes the Dc motor unlike other types of motors.
And it also makes the starting of Dc motor different from the starting of all other types.
It’s the main reason which leads us to think in starting methods of Dc motor to limit the starting current and that happens by a starter or other devices containing variable resistance connected in series to the armature winding, but after we search in starting methods of Dc motor we need to find why the Dc motor has this high current and this make us start from scratch from the basic operational voltage equation:
We know that:
E: the supply voltage.
Ia: the armature current.
Ra: the armature resistance.
Eb: the back emf.
And the back emf will be:
If we focus we will find that Eb is directly proportional to the speed(N) and as we know at starting N=zero, so Eb=zero, so the voltage equation at starting will be:
So the starting current will be:
And as we usually keep the armature resistance very small and we have a constant voltage supply so we will have a starting current equals 440 A, and this high current creates two problems:
- The first that this high current has a potential of damaging the internal circuit of the armature winding.
- Secondly, we will have a high electromagnetic starting torque because as we know the torque is directly proportional to the current and that help producing a huge force capable of flying off the rotor winding.
After we know the basic problem in starting Dc motor we will mention the different types of starting of Dc motor, let’s go:
Starting of Dc motor:
To avoid this high starting current we add an external electrical resistance to the armature winding to increase the effective resistance to have a rated armature current described by:
And after a period of working a back EMF develops and increases so the current decreases to be:
To reach the armature current rated value we decrease the external resistance and that happens by a starter, which may be one of the next types:
3 point starter:
Construction of 3 point starter:
As shown in the figure it consists of sections called studs marked as off, 1, 2, 3, 4, 5, run, and there are also three main parts which are
- A line terminal connected to the positive of the supply(L).
- Armature terminal connected to the armature winding(A).
- And field terminal connected to the field winding(F).
There are also overload release and no volt coil which are protecting devices of the starter.
Working of 3 point starter:
When we switch on the supply to the Dc motor the spring-loaded starter handle moves from off to the first stud position and in this position we have a high starting resistance and as we slowly move the handle towards the run position the series resistance decreases and the motor gains speed and by the way the EMF increases.
When the current flows through the starter the no voltage coil shown in the figure magnetized.
So the handle remains in the run position, and during motor operation this no voltage coil acts as a safeguard and when any kind of supply failure happens the no voltage coil demagnetized and the handle spring returns to off position to effectively cut the motor.
But unfortunately, this 3 point starter suffers from a serious drawback with a large variation of speed which makes us think of other types of starters as the 4 point starter.
4 point starter:
The 4 point starter doesn’t have big different than 3 point start as we also use it with series wound Dc motor, shunt wound Dc motor and compound wound Dc motor and it mainly has the same construction except for an additional terminal(N) which links the supply to the no voltage coil.
Operation of 4 point starter:
In 4 point starter when the motor is supplied and a current flow through the starter this current will divide into 3 parts and flows through 3 points:
- Through the resistance(R1+R2+R3+…..) then to the armature.
- Through the field winding.
- And through the no voltage coil in series with the protective resistance(R).
And it’s different than the 3 point starter.
If there is any change in the shunt field circuit there won’t be any change in the no voltage coil because the two circuits are independent of each other, which means that the electromagnet pull subject upon the soft iron bar of the handle should be high enough to keep the handle at the run position or prevent the spring force to restore the handle at the original position off.
Series motor starter:
From the name, this starter is used with series Dc motor, and we can call this starter two-point starter because it has only two terminals:
- The line connecting the positive supply and the starting handle (L).
- The field/armature connection to the motor itself.
Operation of series starter:
The series starter is identical in operation to 3 point and 4 point starter the handle moves from off to run position to start the motor.
And the no-load release coil holds the starter arm to run position and the voltage supply lost it leaves the arm.
If we focused in the above starters; we will note that they are manually controlled and the starter handle moves by an operator so we think in an electronically controlled soft starter, which doesn’t contain moving parts and improve the time taken to ignite the motor.
In this starter, we use a microcontroller and thyristors to control the current flows through the motor and manage the speed.