Three-phase Induction motor
Three-phase induction motors are simple in construction as compared to DC Motors or synchronous motors. They are widely used in electric motors in the industry. The several features of induction motors are:
- A laminated stator core carrying a polyphase winding.
- A laminated rotor core carrying either a cage or polyphase winding, the latter with shaft-mounted slip rings polyphase winding, the latter with shaft-mounted slip rings.
- A stiff shaft to preserve the very short air gap.
- A frame to form the stator housing and carry the end covers bearings, and terminal box.
Construction of a Three-phase Induction motor
It is the outer body of the motor. The frame may be die-cast or fabricated. The frames for medium and large machines are almost exclusively fabricated. Machines up to about 50 KW may have a frame die cast in a strong silicon-aluminum alloy, sometimes with the stator core cast in.
For large motors rated from 250 KW to 10,000 KW, the frames are generally fabricated in the form of box-type enclosures with a detachable top cover. Frames are provided with feet by which they are fixed to the base plate.
The frames should be strong and rigid, as the air gap length in an induction motor is very small. If the frame is not rigid, the rotor will not remain concentric with the stator, giving rise to an unbalanced magnetic pull.
- To support the stator core and winding
- To protect the inner parts of the machine and serve as a ventilating housing or means of guiding the coolant into effective channels.
The stator of an induction motor is quite similar in construction to that of a 3-phase synchronous alternator or motor.
- Carry the alternating flux which produces Hysteresis losses and eddy current losses. In order to reduce eddy currents and hysteresis loss in the stator core it is assembled of high-grade,low-electrical loss, silicon steel punchings. The thickness of punching may vary from 0.35mm to 0.65mm.
Stator or Primary or Field winding
In a polyphase induction motor, the stator winding is usually 3-phase winding which is usually supplied from 3-phase supply mains. The three phases of the winding can be connected in either star or delta depending upon the methods of starting used.
The squirrel cage motors are usually started by star-delta starters and therefore their stators are designed for delta connections and six terminals are brought out to be connected to the starter.
The wound rotor motor is started by inserting resistance in the rotor circuit and therefore the stator winding can be connected in either star or delta as desired.
The double layer winding is mostly used in medium size motors because of its greater ease of manufacture, assembly, and repair. Stator winding is almost always short-pitched because of reduced copper weight and winding resistance as well as reduced leakage reactance and harmonic torque disturbances that result.
- small motors, operating at ordinary supply voltages with a small number of slots having a large number of turns per phase may use single-layer mush winding.
- For a large motor or high voltage motors the stator windings may be formed by single-layer concentric coils. Double-layer coils are placed in open slots so that form-wound coils can be used while mush winding is housed in semi-closed slots.
The rotor consists of a cylindrical laminated iron core with slots around the core carrying the rotor conductors. Like the stator, the rotor laminations are punched in one piece for small machines, but in large machines, the laminations are segmented and dovetailed to a fabricated spider. Equal numbers of ventilating ducts are provided on the stator core as well as the rotor core. To force circulating air through the motor for cooling, fan blades are usually employed on the ends of the rotor.
Rotor has a smaller number of slots than the stator and must be a non-integral multiple of stator slots so as to prevent magnetic locking of the rotor and stator teeth at the starting instant.
There are mainly two types of rotor employed in 3-phase induction motors:
- Squirrel cage Rotor
- Wound Rotor
Squirrel cage Rotor
Almost 90 percent of induction motors are provided with squirrel cage rotors because of their very simple, robust, and almost instructible construction. In cage construction, copper, brass, or aluminum bars are placed as the rotor conductors, parallel or approximately parallel to the shaft and close to the rotor surface. The conductors are not insulated from the core since the rotor currents naturally follow the path of least resistance, i.e., rotor conductors.
At both ends of the rotor, the rotor conductors are all short-circuited by continuous end rings of similar material to that of the rotor conductors. The rotor conductors and their end rings form a completely closed circuit in themselves, resembling a squirrel cage.
In motors with ratings up to 100 KW, the squirrel cage structure is formed by aluminum cast into the slots of the rotor. But in the case of large motors, the rotor bars, instead of being cast, are wedged into the rotor slots and are then welded securely to the end rings.
The squirrel cage rotor windings are perfectly symmetrical and have the advantage of being adaptable to any number of pole pairs.
Advantages of Squirrel Cage Induction Rotor
- It’s a very simple, robust, and almost instructible construction.
- As there are no brushes and slip rings, these motors require less maintenance.
Applications of Squirrel Cage Induction Rotor
Squirrel cage induction motors are used in lathes, drilling machines, fans, blower printing machines, etc
This type of rotor is wound with an insulated winding similar to that of the stator except that the number of slots is smaller and fewer turns per phase of a heavier conductor are used. Bar, strap, or wire is used for rotor windings, the last being used where many turns are desired. A large number of rotor turns increases the secondary voltage and reduces the current that flows through the slip rings.
The rotor is wound for the same number of poles as that of the stator. The rotor winding is always 3-phase winding even when the stator is wound for two phases. The rotor winding may be star and delta-connected but star connection is usually preferred.
The three finish terminals are connected together to form a star point and the three start terminals are connected to three phosphor-bronze(or brass) slip rings mounted on but insulated from the rotor shaft. The brushes, which carry the current form and to the rotor windings, are held in box-type holders mounted on insulated steel rods and securely bolted to the end shield. Each brush is fed forward by a lever held in tension by an adjustable spring. These brushes are further externally connected to a 3-phase star-connected rheostat for the purpose of starting and speed control.
At the time of starting, the entire resistance is included in the rotor circuit and this resistance is gradually cut out as the rotor picks up the speed. For the normal running condition, the entire external resistance is cut out and the rotor windings are short-circuited automatically through the slip rings by means of a metal collar which is pushed along the shaft and connects all the rings together.
Since the connection of the wound secondary to the external terminals is made through slip rings and brushes, so wound secondary motors often are called slip-ring induction motors.
Advantages of Slip Ring Induction Motor
- High starting torque and low starting current.
- Additional resistance can be added to control the speed.
Application of Slip Ring Induction Motor
- Used where high starting torque is required i.e in hoists, cranes, elevators, etc.
Shafts and Bearings
The rotor shaft is supported by bearings housed in the end shield. The air gap of an induction motor is kept necessarily small therefore the rotor shaft must be made short and stiff so that the rotor may not have any significant deflection. In case of minor deflection in the shaft would develop large irregularities in the air gap which would lead to the production of an unbalanced magnetic pull.
Ball-and-roller bearings are generally employed as their use makes accurate centering much simpler than with journal bearings.