3 Phase Induction Motor
The three-phase induction motor is also called an asynchronous motor and it is the most commonly used type of motor in industrial applications. Specifically, the squirrel cage design is the most extensively used electric motor in industrial applications.
Three phase induction motors are run at basically constant speed from no-load to full-load. On the other hand, the speed is frequency dependent and thus these motors are not effectively adapted to speed control. They are simple, rugged, low-priced, easy to maintain and can be manufactured with characteristics to suit most industrial requirements.
Construction of a 3 Phase Induction Motor
It consists of a stator with stator windings and a rotor. The stator carries a 3-phase winding or stator winding while the rotor carries a short circuited winding or rotor winding. And the rotor is differentiated from the stator by a small air gap that ranges from 0.4mm to 4mm, relying on the power of the motor. When the three-phase voltages are applied to the stator windings, a rotating magnetic field is established. As the magnetic field rotates, currents are induced in the conductors of the squirrel cage rotor. The interaction of the induced currents and the magnetic field produces forces that cause the rotor to also rotate.
Principle of Operation
The 3 phase Induction motor works on the basis of Faraday’s law that an EMF is induced in the circuit due to the rate of change of magnetic flux through the circuit. The stator windings at 120 degrees phase from each other are given AC supply and hence a rotating magnetic field is produced in the coils. As rotor cuts through the rotating magnetic field (with relative velocity), an EMF is induced in the rotor, which causes electric current to flow in the rotor conductors. According to the Lenz law, the cause of production of electrical current will be opposed, which is the relative velocity of stator magnetic field, and hence the rotor will start rotating with a speed different from the synchronous speed of the stator magnetic field.
- It has simple and rugged construction
- It is relatively cheap
- It requires little maintenance
- It has high efficiency and reasonably good power factor
- It has self starting torque
As we know once a supply is connected to a three phase induction motor a rotating magnetic field will be set up in the stator, this will link and cut the rotor bars which in turn will induce rotor currents and create a rotor field which will interact with the stator field and produce rotation. Of course this means that the three phase induction motor is entirely capable of self starting.
The need for a starter therefore is not, conversely enough, to provide starting but to reduce heavy starting currents and provide overload and no-voltage protection. There are a number of different types of starter including the direct on-line starter, the star-delta starter and auto-transformer and rotor resistance. Each will be considered in turn. Here we are going to see star delta starter.
This is the most common form of starter used for three phase induction motors. It achieves an effective reduction of starting current by initially connecting the stator windings in star configuration which effectively places any two phases in series across the supply.
Starting in star not only has the effect of reducing the motor’s start current but also the starting torque. Once up to a particular running speed a double throw switch changes the winding arrangements from star to delta where upon full running torque is achieved. Such an arrangement means that the ends of all stator windings must be brought to terminations outside the casing of the motor.
Split Phase Motor
Normally the supply to homes is single phase, whereas the induction motors required to operate various electrical appliances require a multi phase motors. For this reason the induction motors consists of two windings to get two phases from the single phase supply.
The split-phase motor is a common single-phase motor. The split-phase motor also called an induction-start/induction-run motor, is most likely the most basic single-phase motor made for industrial use, though somewhat limited. It has two windings from a single phase arranged in the start. One is the main winding and other is the start or the auxiliary winding. The start winding is made with smaller gauge wire and fewer turns in respect to the main winding to make more resistance, thus putting the start winding’s field at a different electrical angle than that of the main winding and causing the motor to rotate. The main winding, of heavier wire, keeps the motor running the rest of the time. The main winding has low resistance but high reactance and the start winding has high resistance but low reactance.
A split-phase motor uses a switching mechanism that separates the start winding from the main winding when the motor comes up to something like 75% of evaluated speed. In most cases, it is a centrifugal switch on the motor shaft. The phase difference between start and main winding currents falls far short of 90 degrees.
The capacitor-start motor is used for creating a rotating stator field. This motor is a modification of the split-phase motor, uses a low reactance capacitor placed in series with the start winding of the stator to provide a phase shift of approximately 90 degrees for the start current.
Permanent-Split Capacitor Motor:
It has a run-type capacitor permanently connected in series with the start winding. This makes the start winding an auxiliary winding once the motor achieves running speed. Because the run capacitor must be designed for continuous use, it cannot provide the starting boost of a starting capacitor. The capacitor serves to shift the phase on one of the windings so that the voltage across the winding is at 90° from the other winding. Permanent split capacitor motors have a wide variety of applications depending on the design.
The split-phase motor is used for general-purpose loads. The loads are generally belt driven or small direct-drive loads. The applications for split-phase motors include small grinders, small fans and blowers and other low starting torque applications power needs from 1/20 to 1/3 hp. And these motors usually are designed for single voltage, limiting application flexibility.
The main feature of the split-phase motor is that it can be used in areas of the plant where three-phase has not been appropriated or on small loads on the plant floor where fractional-torque motors can handle the load. The motor does not provide a considerable measure of starting torque, so the load must be rather small or belt driven, where mechanical advantage can be utilized to help the motor start.
Working Example of Controlling a Split Phase Induction Motor
A split phase induction motor used in the exhaust fans consist of the two windings with one winding getting the Mains supply directly whereas the other winding getting the supply through a capacitor, which causes a lag in the voltage. The connection across these windings is done through relays. When one of the relay is energized, one of the windings gets the mains supply directly and other gets the supply through the capacitor. These relays are in turn operated by a relay driver which is controlled by a microcontroller according to the input from the user through a TV remote.