Anatomy of an ac induction motor

12/01/1999


O ften referred to as the "workhorse" of industrial electric motors, ac induction motors offer users simple, rugged construction and easy maintenance. These factors have promoted standardized motor designs and development of a manufacturing infrastructure leading to a vast installed base worldwide. Cost-effective pricing is a further advantage.


An ac induction motor consists of two basic assemblies- stator and rotor -and is analogous to an ac transformer with a rotating secondary. The stator structure is composed of steel laminations (or stampings) shaped to form poles around which are wound copper wire coils. These primary windings connect to, and are energized by, the voltage source to produce a rotating magnetic field. Three-phase windings spaced 120 electrical degrees apart are popular in industry.
The rotor (or rotating secondary ) is another assembly of laminations over a steel shaft core. Radial slots around the laminations' periphery house rotor bars-cast-aluminum or copper conductors shorted at one end and positioned parallel to the shaft (see photo). Arrangement of the rotor bars, viewed on end, looks like a "squirrel cage," hence the colloquial reference: squirrel-cage induction motor.

The motor's name comes from the alternating current (ac) "induced" into the rotor by the rotating magnetic flux produced in the stator. Motor torque is developed from interaction of currents flowing in the rotor bars and the stator's rotating magnetic field.

Synchronous and actual speeds
The magnetic field rotates at synchronous speed , V S -the motor's theoretical top speed that would result in no torque output. In actual operation, rotor speed always lags the magnetic field's speed, allowing the rotor bars to cut magnetic lines of force and produce useful torque. This speed difference is called slip speed. Typical slip values range 2-5% of V S at running speed, but can be large at motor startup. Slip also increases with load, so for accurate control of speed, closed-loop control or feedback is needed.

In equation form, V S =120(f/p), where f is supply frequency and p is the number of poles. Thus, for a 60-Hz motor with 4 poles, and 3% slip, V S =1,800 rpm and actual speed, V a =1,746 rpm. Other common synchronous speeds (aka, base speeds ) are 600, 900, 1,200, and 3,600 rpm. For 50-Hz motors, V S has proportionately lower values. The rotating magnetic field's direction and activation sequence of the phase voltages determine direction of motor rotation.

Induction motors are available in an extremely wide size range from very small units to hundreds of kW-even thousands of kW for custom designs. Some common input voltages are 230, 460, and up to 575 V for 60-Hz operation (up to 690 V for 50-Hz-rated units).

Design, construction varieties
Standardized motor designs have evolved based on such motor characteristics as current, torque, and slip. Examples are NEMA (National Electrical Manufacturers Association) design A, B, C, and D motor types. (See CE, July 1998, pp. 91-99 and OnLine Extra at www.controleng.com, for more details.) IEC-style induction motors made for European and international markets are also available.

Motor frames can be cast iron, rolled steel, cast aluminum, etc. Variable torque refers to motors intended for near zero to base speed operation. Constant torque motors provide full torque over a specific speed range, for example, 4:1 or 10:1, even 1,000:1 with special cooling. Enclosure varieties include ODP (open drip proof), TENV (totally enclosed, non-ventilated), and TEFC (totally enclosed, fan cooled).

Other design varieties include premium efficiency motors built with higher quality electrical materials and higher grades of magnet wire insulation.




Frank J. Bartos, executive editor
fbartos@cahners.com





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