All types of Motor & Generators - AC & DC, Starters:Induction Motor (3 Phase):The production of a rotating magnetic field (RMF) is essential for the operation of alternating current (a.c.) induction motors. The speed of rotation of an RMF is known as the synchronous speed and this will be governed by the value of the supply frequency and the number of poles wound within each phase winding. Each of the phase windings are 120 electrical degrees apart. Synchronous speed can be calculated using the following equation:-
The synchronous speed is magnetic flux speed within the windings (stator). The two major components of a three phase induction motor are the (Stator) which houses the phase windings and the (Rotor) the rotating part which reacts to the RMF. Energizing the stator winding causes the following to occur:• The RMF travels at synchronous speed. • Stator flux induces an emf into the rotor bars. • Rotor current flows in the rotor bars producing an opposing magnetic flux to the stator. • Rotor flux reacts with the stator flux which in turn causes motor rotation (torque). • Torque is maintained because the rotor runs at a slower speed than the RMF. This is referred to as the slip speed usually around 4%. In other words the stator is dragging the rotor along in a rotational force. The stator core is laminated to reduce eddy current loss. The operation of a three phase motor relies on:• Electromagnetic induction • Force on a current carrying conductor when under the influence of a magnetic field. • The stator which houses three phase windings and produces the RMF. • The rotor which has conductor bars that react to the RMF. Slip: - It is the term used to describe the differential between the synchronous speed (RMF) and the actual rotor speed. The larger the slip the larger the amount of torque developed.
Slip is determined by: nslip = nsyn - nrotor (rpm) Where: nslip = Slip speed nsyn = Synchronous speed nrotor = Rotor speed. The actual rotor speed is expressed as the difference between the synchronous speed and the slip speed.
Reversal of three phase induction motors: - Since the rotor follows the rotational magnetic force (RMF). To reverse the direction of a three phase induction motor you must reverse the
direction of the RMF. Directional change can be achieved by changing the phase sequence (swapping two supply lead connections).
Construction of a Three Phase Induction Motor:There are two main kinds of three phase induction motors: • Squirrel cage motor • Slip ring motor Both of these motors consist of two major parts. 1. Stator, (stationary part) 2. Rotor, (rotating part) Both of these motors have no electrical connectional or electrical differences between the stators and as well there are no electrical connection between the stators and rotors. However the wound rotor has a set of slip rings. Motor frames:The frame consists of cast or fabricated steel and the stator core is pressed directly into the frame. The type of frame used will be governed by the environment that it operates within: • Open type which allows free ventilation through the motor. • Drip proof that has a closed upper half, while allowing ventilation through the lower half. • Totally enclosed, that prevents air or moisture entry. Squirrel cage motor:The squirrel cage rotor consists of solid copper or aluminum bars embedded in the rotor slots. Each bar is short circuited by an end ring. Advantages of a squirrel cage motor include:• Simplicity and rugged construction • No sparking contacts in the motor so they can be used in explosive atmosphere (provided; the appropriate enclosure is used).
• Wide range of speed control (when used with electronic frequency controllers). Disadvantages of squirrel cage motor include:• Relatively poor starting torque • Fixed characteristics.
Slip ring or wound rotor type:- (Limited use in HVAC) This type of rotor has three – phase windings similar to its stator winding and wound with the same number...
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