- Starting Current Definition: The starting current in a DC motor is defined as the initial high current that flows when the motor starts, which needs to be limited to prevent damage.
- Role of Back EMF: Back emf is the voltage generated by the motor’s rotation that opposes the supply voltage and helps regulate the starting current.
- Use of Starters: Starters are essential devices that help control the high starting current in DC motors by adding external resistance.
- Types of Starters: There are different types of starters, such as 3-point and 4-point starters, each designed for specific motor types.
- Starting Methods of DC Motor: The primary method to limit starting current involves using a starter with variable resistance to ensure safe motor operation.
Starting of DC Motor
Starting a DC motor is different from starting other types of electrical motors because DC motors have a very high starting current. This high current can damage the motor’s internal circuit if not controlled. To limit the starting current, a starter is used. The starter contains a variable resistance connected in series with the armature winding, which helps limit the current to a safe level.
Now the immediate question is why does a DC motor have such a high starting current?
To answer this question, let us take into consideration the basic operational voltage equation of the DC motor which is given by,
Where, E is the supply voltage, Ia is the armature current, Ra is the armature resistance. And the back emf is given by Eb.
Now the back emf, in case of a DC motor, is very similar to the generated emf of a DC generator as it’s produced by the rotational motion of the current carrying armature conductor in presence of the field. This back emf of DC motor is given by
and has a major role to play in case of the starting of DC motor.
From this equation we can see that Eb is directly proportional to the speed N of the motor.
Now since at starting N = 0, Eb is also zero, and under this circumstance the voltage equation is modified to
In practice, the armature resistance of a DC motor is kept very low, usually around 0.5 ohms, with a minimum supply voltage of 220 volts. This setup results in a starting current of 440 amps (220V / 0.5Ω).
Such high starting current of DC motor creates two major problems.
- Firstly, current of the order of 400 A has the potential of damaging the internal circuit of the armature winding of DC motor at the very onset.
- Secondly, since the torque equation of DC motor is given by
- Very high electromagnetic starting torque of DC motor is produced by virtue of the high starting current, which has the potential of producing huge centrifugal force capable of flying off the rotor winding from the slots.
Starting Methods of DC Motor
High starting current and torque can disrupt the motor system, leading to potential damage. To prevent this, several starting methods for DC motors have been developed. The main method involves adding external electrical resistance (Rext) to the armature winding, increasing the total resistance to (Ra + Rext), and thus limiting the armature current to a safe level.
Now as the motor continues to run and gather speed, the back emf successively develops and increases, countering the supply voltage, resulting in the decrease of the net working voltage. Thus now,
At this moment to maintain the armature current to its rated value, Rext is progressively decreased unless its made zero, when the back emf produced is at its maximum. This regulation of the external electrical resistance in case of the starting of DC motor is facilitated by means of the starter.
Starters can be of several types and requires a great deal of explanation and some intricate level understanding. But on a brief over-view the main types of starters used in the industry today can be illustrated as:-
3 point starter and 4 point starter are used for the starting of shunt wound DC motor and compound wound DC motor. Series wound DC motor’s starter using no load release coil.
All of these play a very significant role in limiting starting current of DC motor for proper starting and running of the DC motor, and are described vividly under their respective sub-headings.
