In this topic, you study Starting of DC Motor and Starters for DC Motors.
The armature current of a dc motor is given by the relation
\[{{\text{I}}_{\text{a}}}=\frac{\text{V – }{{\text{E}}_{\text{b}}}}{{{\text{R}}_{\text{a}}}}\]
In the running condition, back emf (Eb) being nearly equal to the voltage applied across the armature (V), armature current is very small. But when the motor is at rest at the beginning, the armature conductors do not cut any flux lines. Hence,
\[{{\text{E}}_{\text{b}}}=0\]
Thus,
\[{{\text{I}}_{\text{a}}}=\frac{\text{V}}{{{\text{R}}_{\text{a}}}}\]
As the resistance of the armature is normally very small. Therefore, if the motor is switched directly across the supply, the starting armature current will be excessively high in the absence of the back emf This excessive current blows out the fuses and prior to that may damage the armature. The large voltage drop caused by this current in the line also affects the performance of the other equipments connected to the same line. It is, therefore, absolutely necessary to limit this current. This is achieved by connecting a variable resistance called the starting resistor in series with the armature at starting (Fig. 1). As the speed and the back emf increase, the starting resistor is gradually cut off from the armature circuit. The complete starting resistor assembly is called a starter. In addition to a starting resistor which limits the starting current, the (l.c. motor starter usually contains some protective devices. These devices provide protection to the motor in case the field circuit becomes open or the applied voltage becomes too low or load exceeds its predetermined value. The starter also ensures that the starting resistance is automatically reconnected into the armature circuit every time the motor stops.
Fig. 1: Starting of DC Motor
Starting of DC Motor
All dc motors except very small motors need starters. Small fractional-kilowatt dc motors normally do not require starters because of the following reasons
- In comparison with large motors, they have high armature resistance. This helps in limiting the starting current.
- Being small with low moment of inertia, they pick-up speed very quickly and thereby fastly develop the back emf
- Disturbance caused due to their momentary starting current to the other equipments connected to the lines is minimum.
Various types of manually operated face-plate starters are commonly used for starting dc motors. All these starters have a face-plate fitted with a rotary type of switch connected to a group of current limiting resistors. Push-button type automatic starters are also available now-a-days. These starters use electromagnetic contactors and time-delay relays for automatically cutting out the starting resistors from the armature circuit in a predetermined time sequence or when the armature current drops to a preset value.
Shunt Motor Starters
Following two varieties of manually operated face-plate type starters are in common use for starting dc shunt motors.
- Three-point starter,
- Four-point starter.