An Induction motor It is similar to Poly-phase transformer In the case of a secondary, it is short circuited. So, at a normal supply voltages, as in transformers that are short circuited, the initial current absorbed by the primary can be very massive for a brief time. Contrary to DC motors A large current at the beginning is caused by an absence of back EMF. When an induction motor is directly powered by the source, it will consume five to seven times the load current, and then develops the torque of 1.5 up to 2.5 times the load torque. The large start-up current results in an enormous voltage drop on this line that could interfere with the performance of other devices that are connected to that line. Therefore, it’s not advised to start induction motors with higher ratings (generally over 25 kW) straight from the power source.
Various Induction motors are able to start using different methods of starting. They are described below.
Direct-On-Line (DOL) Starters
Small Three phase induction motors It is possible to start the motor directly-on-line, meaning that the supply that is rated is directly connected directly to the motor. However, as we’ve mentioned earlier this time, the starting current could be extremely high generally between 5 and 7 times rating current. The torque at the start is expected to range from 1.5 up to 2.5 times the load torque. Induction motors can start immediately on-line with the DOL starter, which typically comprises a contactor as well as protection for the motor, like circuit breakers. A DOL starter is made up of a contactor operated by coil that can be controlled using the stop and start buttons. When the start button is hit, the contactor is activated, and it shuts down all three phases the motor to supply phases simultaneously. Stop push button disenergises the contactor and shuts off all three phases for the stop of the motor.
In order to prevent excessive voltage drops within the power line because of the large current of starting the DOL starter is typically employed for motors not rated at less than 5kW.
Starting Of Squirrel Cage Motors
The inrush current that starts in motors with squirrel cages is controlled through applying a lower tension to the stator. These techniques are often referred to as Methods to reduce voltage for the beginning squirrel cage motors. . In this regard, the using the following techniques:
- Utilizing primary resistors
- Star delta switches
1. Utilizing Primary Resistors:
The reason for primary resistors is to lower the voltage of the stator and then apply lower amount of voltage on the stator. For instance, the voltage at which the stator is activated is decreased by 50 percent. According to Ohm’s Law (V=I/Z) the starting current will be also decreased by the same amount. This is derived from the the torque equation of the torque equation of a three-phase induction motor The torque at the beginning is equivalent to the size of the voltage applied. In other words, when the voltage applied is half of its rated value, then the initial torque will be just 25% of the usual voltage. This technique is typically employed to calculate a easy starting of induction motors . It is not advised to use primary resistors for starting method in motors that have significant torque requirements to start.
Resistors are usually selected in a way they can provide 70% nominal voltage is applied on the motor. When the motor starts the motor, the entire resistance will be connected into line with the stator winding and gradually decreases when the motor speed increases. Once the motor is at an acceptable speed, the resistances are removed from the circuit, and the stator phases are connected directly to supply lines.
Auto-transformers They are also referred to as auto-starters. They are able to be utilized for delta or star connected squirrel cage motors . It’s basically an three-phase step down transformer, with a variety of taps available that allow users to begin the motor at, for instance 50%, 65 percent or 80% the line voltage. Auto-transformer starts, and the power drawn from the supply line is always lower than the motor’s current by a value equivalent to the Transformation ratio . For instance, if the motor is started at the 65% line the voltage that is applied for the motor would be approximately 65% of the line voltage. The applied current will be approximately 65% of line voltage’s starting value while the line voltage is 65% the 65 percent (i.e. 42 percent) from the line voltage’s beginning value. The difference between the line current and motor current is caused by transformer action. Internal connections for an auto-starter work as illustrated in the figure. At the time of starting, the switch is in “start” position and a low voltage (which is set using tapping) is applied to the stator. If the motor is able to reach the appropriate speed, for instance to 80% or more of its nominal rate, then the automatic transformer disconnects from the circuit when the switch moves from “start” position to “run” state.
The switch that changes the connection from run to start position can be one that is air-break (small motors) or oil-immersed (large motors) type. There is also a provision for overload and no-voltage with time delay circuits in autostarters.
3. Star-Delta Starter:
This technique is employed in motors that are specifically designed to run on a delta-connected stator. A two-way switch is utilized to connect the winding of the stator to the star during start-up and operating at normal speed. When the stator’s winding is connected in star to the motor, the voltage for each motor phase is reduced by a ratio of 1/(sqrt. 3) of which is the case for windings with delta connections. The torque at the start will be 1/3 of what it would be for winding connected to delta. Thus, a star-delta starter is similar to an auto-transformer with a ratio of 1/(sqrt. 3.) which is 58% lower voltage.
The Start Of Slip-Ring Motors
Motors with slip-rings Start by supplying the full line voltage since external resistance is easily added to the circuit for the rotor with the aid of slip-rings. A rheostat that is star-connected is connected to the rotor by means of slip-rings as illustrated in the figure. The introduction of resistance to the rotor’s current can reduce the initial current in the rotor (and therefore in the stator). In addition, it enhances the power factor, and the power factor is improved. The connected rheostat can be operated by hand or automated.
In addition, the increase in resistance to the rotor increases the torque of the start and slip-ring motors are initiated upon loads.
The external resistance that is introduced is for only the purpose of starting and will gradually be eliminated as the motor gains speed.