- 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
At standstill, a DC motor has no rotational back emf. Unlike many other electrical motors, a directly connected DC armature can therefore draw a high starting current limited mainly by circuit resistance and inductance. A traditional starter adds resistance in series with the armature winding; a modern DC drive can limit current electronically by controlling armature voltage.
The reason is clear from the answer to this question: why does a DC motor have such a high starting current?
If armature inductance and brush drop are neglected after the first electrical transient, the operating voltage equation is as follows.
Here, E is armature supply voltage, Ia is armature current, Ra is armature-circuit resistance and Eb is back emf.
In a DC motor, armature conductors moving through the field induce back emf by the same electromagnetic conversion principle used in a DC generator. The ideal back-emf relation is shown below.
Back emf is therefore central to the starting of DC motor.
For constant flux, Eb is proportional to motor speed N.
At standstill N = 0, so Eb is zero. After the initial inductive current rise, the simplified voltage equation becomes the following.
For an example armature resistance of 0.5 Ω and a 220 V armature voltage, the resistance-only calculation gives a 440 A starting current. These are example values, not typical minimum ratings. Actual current rise also depends on inductance, brush drop, supply impedance and starter or drive action.
An uncontrolled starting current of a DC motor can create the two problems described in the protected list.
- 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
A traditional resistance starter limits armature current by adding external electrical resistance Rext in series with Ra. This makes the starting resistance Rext plus the armature resistance. The permitted current and torque limits must come from the motor, driven load and supply requirements.
As the motor accelerates, rising back emf opposes the applied armature voltage and reduces the current for a fixed circuit resistance.
A manual or automatic starter progressively removes Rext as speed builds, while keeping current within the intended acceleration band. The external electrical resistance is normally removed after the DC motor reaches operating speed. A semiconductor DC drive instead regulates voltage and current without dissipating starting energy in stepped resistors.
Traditional starter types are summarised below.
A 3 point starter or 4 point starter can start a shunt wound DC motor or compound wound DC motor. A Series wound DC motor’s starter uses series resistance and protective release functions suited to its field connection and load.
Starter selection must account for motor connection, rated armature current, acceleration time, starting frequency, field-loss protection and the mechanical load on the DC motor.
