Starting Methods to limit Starting Current and Torque of DC Motor

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, I_a is the armature current, R_a is the armature resistance. And the back emf is given by E_b.
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 E_b is directly proportional to the speed N of the motor.

Now since at starting N = 0, E_b 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.

1. 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.
2. Secondly, since the torque equation of DC motor is given by
3. 
4. 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 (R_ext) to the armature winding, increasing the total resistance to (R_a + R_ext), 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, R_ext 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.

FAQ

How do you limit the starting current of a DC motor?

The main method involves adding external electrical resistance (R_ext) to the armature winding, increasing the total resistance to (R_a + R_ext), and thus limiting the armature current to a safe level

How do starters limit the starting current?

The only component that limits the starting current is the armature resistance. In most DC motors the armature resistance is a very low value (approximately one ohm or less). In order to reduce this very high starting current, an external resistance must be placed in series with the armature during the starting period

What are the starting methods of DC motors?

So, a DC motor is started by csing a starter. There are variocs types of dc motor starters, scch as 3 point starter, 4 point starter, no-load release coil starter, thyristor controller starter etc. The basic concept behind every DC motor starter is adding external resistance to the armatcre winding dcring startng

How starting current is reduced in DC motors?

We connect at large resistance in armature circuit and gradually starts decreasing or cutting out its value as speed increases. Due high resistance at starting, the value of starting current decreases

How to reduce motor starting current?

To compensate for the reactive power during starting then a parallel capacitor bank can be used with the motor, either star of delta connected. The capacitances can be calculated for the starting transient and disconnected when the machine has run up to speed. This will help reduce the starting current

What limits current in a DC motor?

Current flow in a DC motor is limited partially by the resistance of the coils, but also by the counter electromotive force or CEMF. Not only can DC motors operate as generators, they always act as generators to some degree. As the armature of a motor turns, its coils cut the magnetic flux lines of the field

Which starting method of a motor takes maximum starting current?

Direct-on-line Starting:
The impedance of the motor at standstill is relatively low and when it is directly connected to the supply system, the starting current will be high (4 to 10 times the full-load current) and at a low power factor

Which motor starting method provides the highest starting torque?

DC series motor – DC motor is used for high starting torque as well as for wide range of speed control is required. DC series motor gives the maximum torque at starting. With the help of Ward Leonard method of speed control, we can achieve various range of speeds. AC motors only run when an alternating current is given as input