Why do we need starting methods of DC motor?
All DC motors are basically self-starting motors. Whenever the armature and the field winding of a DC motor receive a supply, motoring action takes place. So, we do not need any starting methods of DC motor but using the starting methods, it enables us to start the motor in a desirable safe manner. At the start of the DC motor, the armature draws a very high current of the order of 15 to 20A which blows out the fuses, disrupts the performance of other equipment connected to the same line which is for a very short time.
To restrict this high armature current, a variable resistance is connected in series with armature of the DC motor at the start and it is known as a starter or starting resistance. In addition to the starters, more protective devices are provided in the starters. The following are the starting methods or starters used for the DC motors explained.
The Top Common Four Starting Methods of DC Motor
. Three-Point Starting Method:
The starter used is basically a variable resistance divided into a number of sections. The contact points of these sections are called studs starting from the OFF position to the RUN position with many resistances within these positions. There are three main points of this starter:
- L line terminal which is to be connected to positive of supply.
- A to be connected to the armature winding.
- F to be connected to the field winding.
Point L is further connected to the electromagnet called Over-load Release (OLR). The second end of OLR is connected to a point where the handle of the starter is pivoted. This handle is free to move from its other side against the force of the spring. This spring brings back the handle to the OFF position under the influence of its own force. Another parallel path is derived from the stud adjacent to the OFF position given to another electromagnet called No Volt Coil (NVC). The NVC is further connected to terminal F causing the resistances in series with the armature winding. The OLR and NVC are the two protecting devices of this starter.
Initially, the handle is in the OFF position when here is no supply. When the motor is switched ON, the handle starts to move slowly against the spring force and it makes contact with stud one due to which limited current is passed through armature winding and field winding gets supply through the parallel path. As the handle is moved further gradually cutting out the starting resistance finally reaching to the RUN position with the removal of entire starting resistance from the armature circuit and motor starts operating at normal speed. While NVC being magnetized from the supply keeps the handle in the RUN position against the spring force.
. Four Point Starting Method:
The basic difference between the three-point starting method and the four-point starting method is the connection of No Volt Coil (NVC). In three-point starting method, NVC is in series with the field winding while in four-point starting method, NVC is connected independently across the supply through the fourth terminal called N in addition to the L, F and A. Hence, any change in the field current does not affect the performance and NVC holds the handle against the spring force using fixed resistance R in series with the NVC using fourth position N.
. Automatic Starting Method:
The starting methods discussed up till now are the manual starting methods. It becomes inconvenient when the DC motor is to be started and stopped frequently. In such a case, the automatic starting method is used.
The automatic starter uses the automatic switches called contactors which is a device whose operation depends on the solenoidal coil controlled electromagnetically. When the main supply is ON, the field winding gets supply as it gets directly connected across the supply which provides the required working flux.
The C1 is the line contactor which is in series with the armature and connects the armature to the line with R1 and R2 when closed. The coil which energizes C1 is O1. The contactors A1 and A2 are accelerating contractors which provide short circuit across R1 and R2 in sequence, to cut out from the armature circuit when motor gets accelerated. The control of the entire circuit is achieved by the push buttons S1 and S2, respectively. The O2 coil operates A1 while O3 operates the A2 contactor.
When S1 is pushed which is normally open, the O1 gets energized and operates C1 contactor. Thus, current Ia starts to flow through R1 and R2 as A1 and A2 are open and are less due to the R1 and R2. M1 is the auxiliary contactor which operates when C1 does. Thus, though S1 becomes open, O1 continues to be energized through M1 as S2 is usually off.
. Electronic Starting Method:
The thyristor is an electronic device that can work as a switch. It acts as a closed switch when conducts and becomes open when it is not conducting. Thus, the contactors in the automatic starter can be replaced by the thyristors due to which it is called an electronic starter.
When S1 is pushed, the positive voltages get applied to the line thyristor TL and it starts conducting due to which armature of the motor gets connected to the supply line through TL and the starting resistances R1 and R2. At the same time, capacitor C starts to charge. To cut-off R1 and R2, a tacho-generator is used to sense the back emf which produces the voltage proportional to the speed of the motor. The stopping of the motor can be achieved by pushing the S2 switch By pushing S2, it provides a positive voltage to the gate terminal of the third thyristor T3 due to which it conducts and current starts to flow through R1 and capacitor C starts to discharge. And this starts reducing the anode-cathode voltage across the TL and finally reverses its polarities and due to which Ia armature current becomes zero. The turning ON and OFF of the TL by capacitor C is done due to which it is known as the commutating capacitor.