An induction motor is a 3 phase motor consisting of the 3 phase winding as the stator with a permanent magnet and the rotor as another 3 phase winding. It works on the principle of rotating magnetic field, i.e. the formation of the magnetic flux from 3 phase winding fluxes, which rotates about its axis, causing the rotor to rotate. An Induction motor has the ability to self start owing to the interaction between the rotating magnetic field flux and the rotor winding flux, causing a high rotor current as torque is increased. As a result the stator draws high current and by the time the motor reaches to full speed, a large amount of current (greater than the rated current) is drawn and this can cause heating up of the motor, eventually damaging it. To prevent this, motor starters are needed.
Need for Motor Starting
In an induction motor, when supply is given to the stator windings, the rotating magnetic field flux and the produced flux in the rotor windings due to the back emf, causes the motor torque to increase, causing a high rotor current. During the time between the application of electric supply to the motor and the actual acceleration of the motor to its full speed, a large amount of current is drawn by the stator from the supply. This starting current is about 5 to 6 times more than the full load current. This time duration can be for few seconds or longer. This causes the electrical equipment’s to damage because of the increasing voltage drop in electrical systems due to flow of larger currents across the cable. For this reason, a definite method of starting the motor is needed.
Definition of Motor Starter
It is the device connected in series with the motor to decrease its starting current and then increase it as the motor starts rotating gradually. It consists of a connector which acts as a switch to control flow of current to the motor and a overload unit which measures the flow of current through the motor and controls the halting of motor in case of large current being drawn.
Principle of Motor Starter
The current drawn by the motor can be controlled by reducing the back emf (possible by reducing the supply voltage) or by increasing the rotor resistance during the motor starting.
Types of Motor Starters
Direct On line: It consists of simple push button as the controller. When the start button is pressed, the switch connecting the motor and the main supply is closed and the motor gets the supply current. In case of over current, the stop button is pressed and the bypass auxiliary contact is opened.
Star Delta: The 3 windings are first connected in star connection and then after some time (decided by the timer or other controller circuit) the windings are connected in delta connection. In star connection, the current drawn is 0.58% of the normal current and also the phase voltage is reduced to 0.58%. Thus the torque is reduced.
Auto Transformer starter: It consists of an auto transformer (A Transformer with a single winding tapped at different points to supply percentage of it primary voltage across secondary) in star connection, which reduces the voltage applied to the motor terminals. It consists of 3 tapped secondary coils connected to the three phases. In the starting period the transformer allows application of lower voltages to the three windings.
Stator Resistance Starter: It consists of three resistors in series with each phase of the stator windings, which causes a voltage drop across each resistor and as a result a low voltage is applied to each phase.
Rotor Resistance Starter: It consists of 3 resistances connected in series with the rotor windings, thus reducing the rotor current, but increasing the torque.
Application of Star Delta Starter to Control Starting of Induction Motor
The star delta starter is the cheapest among all the starters and is suitable for applications like machine tools, pumps, motor generators etc. A star delta starter can be used in starting an induction motor by using 2 relays as the connector and the timer as the controller. 1 connector is used to provide the mains supply while the other connector controls the connection of motor in star or delta connection.
Transformers are used, primaries of which are connected to the 3 phase supply and the secondaries are connected to the relays and the timer in such a way that failure of any 1 phase will stop the supply to the timer. The two relays are used to trigger the timer, which develops a high logic output at pin 3, thus switching on the relay 4, causing the supply in star connection, which offers low power intensity to the load by isolating the load from the normal 3 phase supply through relay 3 (operated by the two triggering relays). After some time, the timer (operating in mono stable mode) output goes low (time being decided by the RC combination at pins 2 and 6) and the relay 4 is switched off, thus causing the 3 phase supply to be given to the motor and the motor operates in delta mode.
Some more regarding this starting of induction discussed below.
Soft Start of Induction Motor by Stepped Delay of Reducing Firing Angle
Soft Start and Soft Stop:
In normal start up of the induction motor, more torque is developed, which causes the stress to be transferred to the mechanical transmission system resulting in excessive wear and failure of the mechanical parts. Also, as acceleration increases, high current is drawn which is about 600% of the normal run current. This can be seldom solved using a star delta starter.
Soft start provides a reliable and economical solution to these problems by delivering a controlled release of power to the motor, thereby providing smooth, steeples acceleration and deceleration. The damage to windings and bearings are reduced, resulting in an extended motor life.
With this technique, a controlled starting and stopping is achieved with proper selection of ramp times and setting of the current limit.
- Less mechanical stress.
- Improved power factor.
- Lower maximum demand.
- Less mechanical maintenance.
This technique is suitable for applications where torque transients are frequent, such as in pumping fluids, which can ultimately lead to rupture pipes and couplings.
Technology followed in Soft Starter:
A soft starter is a type of reduced voltage starter for AC induction motors. The soft starter is similar to a primary resistance or primary reactant starter in that it is in series with the supply to the motor. The input current to the started equals its output current. It consists of solid-state devices to control the current flow and the voltage applied to the motors. The soft starters can be connected in series with the line voltage or connected inside the delta loop.
Solid state AC switches are arranged in series with one or more phases to achieve voltage control.
Use of Solid-state switches:
1 x Triac per phase
1 x SCR and 1 x Diode reverse parallel connected per phase.
2 x SCRs reverse parallel connected per phase.
Varying the conduction angle of the switches can control the average voltage as increasing conduction angle can increase the average output voltage. This process proves advantageous with improved efficiency and lesser power dissipation. Also the average voltage can be easily altered using control electronics.