Cement, Energy and Environment

' ' t I product family was announced by Westinghouse in 1892. In 1888, Mikhail Davila Dobrovolsky created a 3-phase induction with a squirrel cage motor. GE began developing 3-phase AC induction motors beginning in 1891 under Charles Steinmetz's leadership. Induction motors continued to evolve in form and use. The 3-phase induction motor is used in the larger horsepower application above 1 hp while the single-phase AC motors were used in the smaller, below 1 hp applications. Construction The AC induction motor consists of two major assemblies - rotor and stator. ACpower in the form of 60Hz sinusoidal signals are fed into the 3- phase stator windings. This rotating magnetic field induces a rotor field in the rotor's shorted (closed circuit) squirrel cage windings. The established rotor field runs at a frequency and rotor shaft speed that is not in synchronism with the rotating stator frequency. This condition, defined as slip, results in the rotor shaft speed to be tens of rpm below the induction motor's synchronous speed (frequency) developed by the stator. The slip value varies between one percent and five percent on most AC induction motors. The 3-phase stator signals establish a series of rotating magnetic vector fields that allow the induction motor to continue to rotate freely without external support. Fig. 1 For the design A, B, C and D NEMA Mg-1 standard-rated torque-speed curves, only the design D AC induction motor curves can re-start and move up the curve back to the rated torque region. The price for the Design D's higher starting torque performance is much lower power efficiency. The other three design curves require extra help to return to the rated torque region. Current is non-linear; the most current for all four The single-phase AC induction motor works in a similar manner except it possesses a second winding-or auxiliary winding-that allows one to use a capacitor to establish a near 90 electric degrees between the stator winding and auxiliary winding. This action supports continuous rotor and shaft rotation . The housing, end caps and bearings complete the AC motor construction that suppo•1s the motor's rotation (Fig. 1). Performance The mechanical output is represented by the motor's torque vs. speed curves, based first on 60Hz input voltage and current inputs, and later on a range of input frequencies. The motor's shaft speed is a function of the number of magnetic poles within the motor. It runs at a no-load speed of almost 3,600 rpm for a 2-pole and 1,800 rpm for a 4-pole. Remember the impact of slip. The AC induction motor's torque vs. speed curve is highly non-linear. There are three different regions on an AC induction motor's torque vs. speed curves. They are the near-vertical-rated portions of the curve (Fig. 2). Once the torque speed curve begins to bend horizontally, one reaches the maximum percent torque point - the breakdown point - the AC induction motors speed falls rapidly to stall or zero speed . ~ w 0::: 0::: ::> u A B CorD PERCENT OF SYNCHRONOUS SPEED Fig. 2 design torque-speed curves is drawn at stall (Fig. 3); the Design B curve is the most popular one today. The AC induction motor was originally designed for appl ications that have a near– constant load. One just plugs the AC induction motor's power cord directly into the 60Hz wall outlet for constant speed applications. The motor can self-regulate its speed within +20 percent of rated load.