06-10-2012, 05:56 PM
Motors convert electrical energy
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Motors convert electrical energy into mechanical energy by the interaction between the magnetic fields set up in the stator and rotor windings. Industrial electric motors can be broadly classified as induction motors, direct current motors or synchronous motors. All motor types have the same four operating components: stator (stationary windings), rotor (rotating windings), bearings, and frame (enclosure).
Motor Types
Induction Motors are the most commonly used prime mover for various equipments in industrial applications. In induction motors, the induced magnetic field of the stator winding induces a current in the rotor. This induced rotor current produces a second magnetic field, which tries to oppose the stator magnetic field, and this causes the rotor to rotate. The 3-phase squirrel cage motor is the workhorse of industry; it is rugged and reliable, and is by far the most common motor type used in industry. These motors drive pumps, blowers and fans, compressors, conveyers and production lines. The 3-phase induction motor has three windings each connected to a separate phase of the power supply.
Direct-Current Motors, as the name implies, use direct-unidirectional, current. Direct current motors are used in special applications- where high torque starting or where smooth acceleration over a broad speed range is required.
Synchronous Motors AC power is fed to the stator of the synchronous motor. The rotor is fed by DC from a separate source. The rotor magnetic field locks onto the stator rotating magnetic field and rotates at the same speed. The speed of the rotor is a function of the supply frequency and the number of magnetic poles in the stator. While induction motors rotate with a slip, i.e., rpm is less than the synchronous speed, the synchronous motor rotate with no slip, i.e., the RPM is same as the synchronous speed governed by supply frequency and number of poles. The slip energy is provided by the D.C. excitation power
Motor Rating Survey :
It is worthwhile that a motor rating survey is carried out in industrial plants as follows :
1) Select important motors by size and long running hours. Initially, ignore small motors (below 3.7 KW) as well as those running for few hours (less than 3000 hours/annum).
2) Measure normal running current and input power. Kindly note that even on no load, motors take 30% to 40% of the rated current. Hence % motor load is not given exactly by the ratio of motor input current to rated current. Fig.gives approximate curves for estimating motor load from motor current.
3) Identify motors with loads less than 40%, out of these again categorize frequently rewound motors.
4) Prepare a list of desired, properly sized motors for all important applications.
5) Interchange by properly sized motors (available in the plant) whenever possible.
Power Losses in Motors :
1. Core Losses :
This consists of hysteresis and eddy current losses mainly in the stator core. The frequency of rotation of the magnetic field within the rotor is small during running condition and hence the rotor core loss is negligible. This can be high under unbalanced voltage conditions.
2. Windage and Friction Losses :
These losses are due to bearing friction and rotation of the rotor and fan in air. Core loss and friction and windage losses are considered as fixed losses as they do not vary with load.
3. Stator Copper Losses :
These losses are due to flow of currents in stator conductors and are normally called I2Rlosses.
4. Rotor Copper Losses :
These losses are due to flow of currents in rotor conductors and end rings.
5. Stray Load Losses :
These losses are extra magnetic and I2R loss due to effects of slot openings, leakage flux and harmonic fields. These losses are difficult to measure or calculate. The losses from (3), (4) and (5) are taken to vary as square of the load current.