THREE PHASE MOTORS

Three Phase Motors

Kinds:

Three phase induction motor

Squirrel cage motor

Wound rotor

Three phase synchronous motor

Uses a three Phase AC power source to operate
Vary form fractional-horsepower size to several thousand horsepower
Practically, made for every standard voltage and frequency (very often dual voltage motors)
Extremely rugged and required little maintenance
Use to drive machine tools, pumps, elevators, fans, cranes, hoists, blowers, conveyors, and many others…

Enclosure

The enclosure is consist of a frame and two end brackets or bearing housing. The stator is mounted inside the frame, the rotor fits inside the stator with slight air gap separating it from the stator. There is no direct physical connection between the rotor and the stator. The enclosure also protects the electrical and operating parts of the motor from harmful effects of the environment in which the motor operates.
Bearings, mounted on the shaft, support the rotor and allow it to turn. A fan is also mounted on the shaft and is used on the motor for cooling.

Rotor

The rotor is the rotating part of the electromagnetic circuit. The most common type of rotor is the “squirrel cage rotor”. It is consist of a stacked of steel laminations with evenly spaced conductor bars around the circumference to form a rotor core. Current flow thru the conductor bars are electrically connected with end rings and mounted on a steel shaft to form a rotor assembly.

Wound – Rotor Motors

Has an insulated wye-connected three-phase winding connected to slip rings mounted on t he shaft.
Runs essentially as a squirrel cage rotor if the three brushes are short-circuited
If resistance is connected to the brushes we can alter the torque of the motor.
Also known as a slip-ring motor

Construction:

Stator

The stator and the rotor are electrical circuits that perform as electromagnet. It is the stationary part of the motor. It is made up of several hundred thin laminations stacked together to form hallow cylinder. Coils of insulated wires are inserted into each slot of the stator core. Each grouping of coils, together with the steel core it surrounds form an electromagnetic, electromagnetism is the principle behind motor operation. The stator windings are connected directly to the power source.

Principle of operation

The coils in the slots of the motor are connected to form three separate windings called phases. These windings / phases are then supplied by a three phase AC power so that a (rotating) magnetic field is formed inside a stator that causes the rotor to turn at a certain speed ( current flows thru that windings). The magnetic field developed in the phase winding depends on the direction of current flow.

Synchronous motors:

It has two supply voltage. A three phase AC, connected to the stator and DC voltage connected to the rotor by slip ring and brushes.
The rotor turns at the same speed as synchronous speed ( speed of rotating magnetic field)
A variation of synchronous motors include a permanent magnet rotor, no external DC source is required.
There is no slip

The difference between the synchronous speed and the actual rotor RPM is called Slip. Slip is found by subtracting the rotor speed from the synchronous speed. The percentage of slip can be found by using the formula:

% slip = Ss – Sr x 100

Ss

Ss = The synchronous speed in RPM

Sr = The actual rotor speed in RPM

The full load slip in most induction motors varies between 4 and 6 percent. Should and induction motor become heavily overloaded or stalled ( the slip would be 100%.
Induction motors also have wire wound rotors. In this case, the coil ends are shorted together, and the operation of the motor is the same as for the squirrel cage rotor.

Steps in three phase rewinding

TAKING THE DATA. THE FOLLOWING DATA ARE RECORDED: NAMEPLATE DATA, NO. OF COILS, NO. OF SLOTS, TYPE OF CONNECTION, NO. OF TRURNS PER COIL, SIZE OF WIRE, PITCH OF POLES AND WHETHER LAP OR CONCENTRIC WINDING.

STRIPPING THE WINDING. BEFORE THE WIRES ARE REMOVED FROM THE STATOR, THE TYPE OF CONNECTION MUST BE RECORDED. THIS CAN BE OBTAINED IF ONE IS FAMILIAR WITH THE METHODS OF WINDING THE THREE PHASE MOTOR AND CONNECTING THE PHASES AND POLES TO ONE ANOTHER. A PAIR OF PLIERS CAN BE USED TO PULL OUT THE WIRES. CARE MUST BE TAKEN NOT TO BEND THE LAMINATIONS. AFTER THE WIRES HAVE BEEN REMOVED, CHECK CAREFULLY FOR SHARP BURRS, FUSED COPPER, BENT STATOR TEETH OR ANYTHING THAT CAN PUNCTURE THE SLOT LINER. ONE CORE MUST BE SAVED TO PROVIDE THE DIMENSIONS FOR THE NEW COILS.

INSULATING THE STATOR. THE STATOR INSULATION MAYBE REPLACED WITH THE SAME THICKNESS AND TYPE USED IN THE OLD WINDING. FISH PAPER OR MILAR (HARDENED PLASTIC) IS USUALLY USED IN SLOT INSULATIONS OF STATOR. IT IS IMPORTANT THAT THE LINER FIT THE SLOT, 3/16 IN. ON SMALL MOTORS, 1/8 FOR MEDIUM SIZE UP TO 3/8 ON LARGE MOTORS.

WINDING THE COILS. THREE PHASE MOTORS ARE ALWAYS WOUND ON FORMS. GROUP WINDING. MOST THREE PHASE MOTORS, WITH THE EXCEPTION OF VERY LARGE ONE AND THSE WITH OPEN SLOTS USE COIL WOUND IN GROUPS. THE NO. OF COILS IN EACH GROUP DEPENDS ON THE NO. OF SLOTS OR GANG WINDING. IN GROUP WINDING, SEVERAL COILS ARE WOUND BEFORE THE WIRE IS CUT. THIS SAVES TIME BY ELIMINATING THE NECESSITY OF CONNECTING COILS TO ONE ANOTHER OR STUBBING.

PLACING THE COIL IN THE SLOTS. THE TURNS OF THE COILS ARE INSERTED ONE BY ONE INTO THE SEMI CLOSED SLOTS. THE ENDS ARE SOMETIMES TAPED AFTER EACH COIL IS PLACED IN THE SLOT.

IN INSERTING THE COIL, SPREAD OR FAN OUT THE TURNS ON ONE SIDE OF THE COIL AND HOLD THE COIL AT ONE ANGLE SO THAT ALL THE TURNS CAN BE FED INTO THE SLOT. MAKE SURE THAT EACH TURN IS PLACED INSIDE THE INSULATION, OTHERWISE GROUND MAY RESULT IF WIRES ARE MISTAKENLY PLACED BETWEEN THE SLOT UNTIL ALL THE TURNS ARE IN THE SLOT. THE OTHER SIDE OF THE COILS REMAIN FREE. CONTINUE BY PLACING ONE SIDE OF THE SECOND COIL IN THE SLOT BEYOND THE FIRST COIL UNTIL THE BOTTOM HALF OF THE SLOT IS OCCUPIED BY ALL THE SIDE OF THE COILS. THE SECOND SIDE OF THE COILS IS THEN FITTED ON TOP OF THE FIRST SIDE OF THE COIL SEVERAL SLOTS AWAY ACCORDING TO THE PITCH OR SPAN OF THE COIL.

NOTE THAT COIL SIDE OCCUPIES HALF A LOT, SO THAT THERE ARE TWO SIDES OF COILS IN A SLOT. MAKE CERTAIN THAT EACH COIL SIDE EXTEND BEYOND THE SLOT AT BOTH ENDS AND DOES NOT PRESS AGAINST THE IRON CORE AT THE CORNERS. BEFORE INSULATE IT FROM THE COIL ALREADY IN THE SLOT. THIS IS BECAUSE EACH GROUP BELONGS TO A DIFFERENT PHASE. THE VOLTAGE BETWEEN EACH GROUP IS VERY HIGH. TO INSULATE, SLIDE A SEPARATOR OVER THE BOTTOM SIDES OF THE COIL IN THE SLOT BEFORE INSTALLING THE TOP SIDE. IT SHOULD EXTEND ABOUT 3/8 in. BEYOND THE SLOT LINER AS EACH GROUP OF COILS IS PLACED IN THE SLOTS, SLIDE OR SLIP A FORMED FIBER WEDGE, WOODEN WEDGE, OR BAMBOO WEDGE OVER THE TOP CLOSED COIL. THIS WEDGE SHOULD EXTEND ABOUT 1/8 in. BEYOND THE SLOT ENDS AS EACH GROUP COILS IS PLACED IN THE SLOTS, PHASE INSULATION MUST BE USED OR PLACED BETWEEN GROUPS.

CONNECTING THE COILS. ALL THREE PHASE MOTORS ARE WOUND WITH THE NO. OF COILS USUALLY AS MANY COILS AS THE SLOTS. THESE COILS ARE CONNECTED AS TO PRODUCE THREE SEPARATE WINDINGS CALLED PHASES, EACH OF WHICH MUST HAVE THE SAME NO. OF COILS. THE NO. OF COILS IN EACH PHASE MUST BE ONE-THIRD THE TOTAL NO. OF COILS IN THE STATOR.