The basic principle of an AC induction motor is that one or more out-of-phase AC (sinusoidal) currents energize sets of electromagnet coils (called stator coils or windings) arranged around the circumference of a circle. As these currents alternately energize the coils, a magnetic field is produced which “appears” to rotate around the circle. This rotating magnetic field is not unlike the appearance of motion produced by a linear array of light bulbs blinking on and off in sequence: although the bulbs themselves are stationary, the out-of-phase sequence of their on-and-off blinking makes it appear as though a pattern of light “moves” along the length of the array. Likewise, the superposition of magnetic fields created by the out-of-phase coils resembles a magnetic field of constant intensity revolving around the circle. The following twelve images show how the magnetic field vector (the red arrow) is generated by a superposition of magnetic poles through one complete cycle (1 revolution), viewing the images from left to right, top to bottom (the same order as you would read words in an English sentence):
If a permanent magnet is placed within the circle on a shaft so that it is free to rotate, the magnet will attempt to spin at the exact same speed as the rotating magnetic field. If the magnetic field completes one full revolution in 1/60 of a second, the rotating speed of the magnet will be 3600 revolutions per minute (3600 RPM). Since the magnet follows in lockstep with the rotating magnetic field, its rotational speed is said to be synchronous. We would thus identify this motor as a synchronous AC motor.
If an electrically conductive object is placed within the circle on a shaft so that it is free to rotate, the relative motion between the rotating magnetic field and the conductive object (rotor) induces electric currents in the conductive object, which produce magnetic fields of their own. Lenz’s Law tells us that the effect of these induced magnetic fields will be to try to oppose change: in other words, the induced fields react against the rotating magnetic field of the stator coils in such a way as to minimize the relative motion. This means the conductive object will try to rotate in sync with the stator’s rotating magnetic field. However, if the rotor ever did achieve synchronous speed, there would no longer be any relative motion between the rotor and the rotating magnetic field, which means the induction will cease. No induction means no electric currents in the rotor, which means no reactive magnetic field, which means no torque to keep the rotor spinning. Thus, the electrically conductive rotor fails to fully achieve synchronous speed. Instead, it lags (“slips”) behind the rotating magnetic field’s synchronous speed just a bit. We would call this motor an induction AC motor.
It may come as a surprise for some to learn that any conductive object – ferromagnetic or not – will experience a torque when placed inside the rotating magnetic field generated by the stator coils. So long as the object is electrically conductive, electromagnetic induction will ensure the creation of electric currents in the rotor, and these currents will produce their own magnetic fields which react against the stator’s rotating magnetic field to produce a torque on the rotor.
Induction motors are by far the most popular design in the industry. The most common variant of the induction motor is the so-called squirrel-cage design, where the rotor is made up of aluminum bars joining two aluminum “shorting rings,” one at either end of the rotor. Iron fills the spaces between the rotor bars to provide a lower-reluctance magnetic “circuit” between stator poles than would be otherwise formed if the rotor were simply made of aluminum. If the iron were removed from the rotor, the remaining aluminum bars and shorting rings would resemble the cage-wheel exercise machine used by hamsters and other pet rodents, hence the name.
A photograph of a small, disassembled three-phase AC induction “squirrel-cage” motor is shown here, revealing the construction of the stator coils and the rotor:
Given the extremely simple construction of AC induction motors, they tend to be very reliable. So long as the stator coil insulation is not damaged by excessive moisture, heat, or chemical exposure, these motors will continue to operate indefinitely. The only “wearing” components are the bearings supporting the rotor shaft, and those are easily replaced.
Starting a three-phase induction motor is as simple as applying full power to the stator windings. The stator coils will instantly produce a magnetic field rotating at a speed determined by the frequency of the applied AC power, and the rotor will experience a large torque as this high-speed (relative to the rotor’s stand-still speed of zero) magnetic field induces large electric currents in it. As the rotor comes up to speed, the relative speed between the rotating magnetic field and the rotating rotor diminishes, weakening the induced currents and the rotor’s torque.
One way to “model” an AC induction motor is to think of it as an AC transformer with a short-circuited, movable secondary winding. When full power is first applied, the initial amount of current drawn by the stator (primary) windings will be very large, because it “sees” a short-circuit in the rotor (secondary) winding. As the rotor begins to turn, however, this short-circuit draws less and less current until the motor reaches full speed and the line current approaches normal. Just like a transformer, where a reduction in secondary current (from a load change) results in a reduction in primary current, the reduction in induced rotor current (from reduced slip speed) results in a reduction in the stator winding current.
The huge surge of current at start-up time (as much as ten times the normal running current!) is called inrush current, causing the rotor to produce a large mechanical torque. As the rotor gains speed, the current reduces to a normal level, with the speed approaching the “synchronous” speed of the rotating magnetic field. If somehow the rotor achieves synchronous speed (i.e. the slip speed becomes zero), the stator current will fall to an absolute minimum.
Any mechanical load causing the motor to spin slower likewise causes the stator windings to draw more current from the power supply. This is due to the greater slip speed causing stronger currents to be induced in the rotor. Stronger rotor currents equate to stronger stator currents, just like a transformer where a heavier load on the secondary winding causes greater currents in both secondary and primary windings.
Reversing the rotational direction of a three-phase motor is as simple as swapping any two out of three power conductor connections. This has the effect of reversing the phase sequence of the power “seen” by the motor. The flip-book animation beginning in Appendix A.1 beginning on page 2253 shows how reversing two of the three lines has the effect of reversing the phase sequence.
An interesting problem to consider is whether it is possible to make an AC induction motor run on single-phase power rather than polyphase power. After all, it is the unambiguous phase sequence of three-phase AC power that gives the stator windings’ magnetic field its rotation. If we have only one sine wave supplied by the AC power source, how is it possible to generate a rotating magnetic field? At best, all we could produce is a pulsing or “blinking” magnetic field. If you imagine a string of light bulbs blinking on and off 180o out of phase (i.e. ABABABAB), one could argue the sequence is marching from A to B, or alternatively from B to A – there is no definite direction to the lights’ “motion.”
This limitation of single-phase AC power is a problem for induction motors, and the solution to this problem is to artificially create a second phase within the motor itself to produce a magnetic field with a definite rotation. One common way to do this for large single-phase AC motors is to add a second set of stator windings offset from the first and energize those windings through a high-voltage capacitor, which creates a leading phase shift in the winding current. This phase shift creates an out-of-step magnetic field in the second winding set, providing a definite direction of rotation. Once the motor comes up to speed, this auxiliary winding may be disconnected if desired, since a spinning motor will happily run16 on single-phase AC. This is called a capacitor-start induction motor, and it is the design used for most single-phase AC induction motors requiring a high starting torque (e.g. pumps, shop grinders, drill presses, etc.). This strategy of “polyphase start, single-phase run” means a three-phase motor may continue to run if one or more of its phases are “lost” due to an open wire connection or a blown fuse. The motor cannot deliver full-rated mechanical power in this condition, but if the mechanical load is light enough the motor will continue to spin even though it no longer has multiple phases powering it! A three-phase motor, however, cannot start from a standstill on just one phase of AC power. The loss of phases to an AC induction motor is called single-phasing, and it may cause a great deal of trouble in an industrial facility. Three-phase electric motors that become “single-phased” from a fault in one of the three-phase power lines will refuse to start. Those that were already running under heavy (high torque) mechanical load will stall. In either case, the stopped motors will simply “hum” and draw excessive current until (hopefully!) the thermal overloads trip or thermal damage occurs.
Instrumentation Basics 