A linear induction motor provides linear force and motion rather than rotational torque. The shape and operation of a linear induction motor can be visualized as depicted in the figure by making a radial cut in a rotating induction machine and flattening it out. The result is a flat “stator,” or upper section, of iron laminations that carry a three-phase, multipole winding with conductors perpendicular to the direction of motion. The “rotor,” or lower section, could consist of iron laminations and a squirrel-cage winding but more normally consists of a continuous copper or aluminum sheet placed over a solid or laminated iron backing.

One application of linear motors is in rapid-transit vehicles for public transportation. The “stator” is carried on the underside of the vehicle, and the “rotor” is located between the rails on the track. An advantage of this type of propulsion is that high acceleration and braking can be obtained without dependence on adhesion of the steel wheels to the steel rails in the presence of rain, ice, or a steep slope.

Electrical power is supplied to such a rapid-transit vehicle through sliding connections to an energized rail or overhead wire. To provide speed control and braking, an electronic power converter on board the vehicle produces a three-phase output of the desired voltage and frequency.

In an alternative arrangement for vehicle propulsion, the copper and iron sheets of the figure can be placed on the underside of the vehicle and sections of stator can be placed at intervals along the track. This has the advantage that no electric power need be supplied to the vehicle itself.

Linear induction motors also are used to drive conveyors, sliding doors, textile shuttles, and machine tools. Their advantage is that no physical contact is required and thus wear and maintenance are minimized. In another form, linear motors are used as electromagnetic pumps where the rotor consists of a conducting fluid, such as a liquid metal (say, mercury or sodium-potassium alloy).

The efficiency of linear motors is somewhat less than that of rotating motors because of end effects. Its “rotor” must be magnetized as it comes under the “stator.” This reduces the effectiveness of the first one or two pole spans. The input current is also relatively high because the air gap is usually larger than in rotating machines and more current is required to produce the magnetic field across it.

Induction motors for speed and position control

On a constant-frequency supply, an induction motor is essentially a near-constant speed drive. Induction motors, however, can be used to provide accurate speed and position control in either direction of rotation by the use of a controllable-voltage, controllable-frequency three-phase supply. This is produced by means of an electronic inverter. Using semiconductor switches, the utility supply is converted into a set of three near-sinusoidal inputs of controlled voltage and frequency to the stator windings. The speed of the motor will then approach the synchronous value of 120 f/p revolutions per minute for a controlled frequency of f cycles per second. Reversal of the phase sequence from abc to acb reverses the direction of the torque. For accurate control of speed or position, the speed of the shaft can be monitored by a tachometer or position sensor and compared with a signal representing the desired value. The difference is then used to control the inverter frequency. Generally, the voltage varies directly with the frequency to keep the magnitude of the magnetic field constant.

Synchronous motors

A synchronous motor is one in which the rotor normally rotates at the same speed as the revolving field in the machine. The stator is similar to that of an induction machine consisting of a cylindrical iron frame with windings, usually three-phase, located in slots around the inner periphery. The difference is in the rotor, which normally contains an insulated winding connected through slip rings or other means to a source of direct current (see figure).

The principle of operation of a synchronous motor can be understood by considering the stator windings to be connected to a three-phase alternating-current supply. The effect of the stator current is to establish a magnetic field rotating at 120 f/p revolutions per minute for a frequency of f hertz and for p poles. A direct current in a p-pole field winding on the rotor will also produce a magnetic field rotating at rotor speed. If the rotor speed is made equal to that of the stator field and there is no load torque, these two magnetic fields will tend to align with each other. As mechanical load is applied, the rotor slips back a number of degrees with respect to the rotating field of the stator, developing torque and continuing to be drawn around by this rotating field. The angle between the fields increases as load torque is increased. The maximum available torque is achieved when the angle by which the rotor field lags the stator field is 90°. Application of more load torque will stall the motor.

One advantage of the synchronous motor is that the magnetic field of the machine can be produced by the direct current in the field winding, so that the stator windings need to provide only a power component of current in phase with the applied stator voltage—i.e., the motor can operate at unity power factor. This condition minimizes the losses and heating in the stator windings.

The power factor of the stator electrical input can be directly controlled by adjustment of the field current. If the field current is increased beyond the value required to provide the magnetic field, the stator current changes to include a component to compensate for this overmagnetization. The result will be a total stator current that leads the stator voltage in phase, thus providing to the power system reactive volt-amperes needed to magnetize other apparatuses connected to the system such as transformers and induction motors. Operation of a large synchronous motor at such a leading power factor may be an effective way of improving the overall power factor of the electrical loads in a manufacturing plant to avoid additional electric supply rates that may otherwise be charged for low power-factor loads.

Three-phase synchronous motors find their major application in industrial situations where there is a large, reasonably steady mechanical load, usually in excess of 300 kilowatts, and where the ability to operate at leading power factor is of value. Below this power level, synchronous machines are generally more expensive than induction machines.

The field current may be supplied from an externally controlled rectifier through slip rings, or, in larger motors, it may be provided by a shaft-mounted rectifier with a rotating transformer or generator.

A synchronous motor with only a field winding carrying a direct current would not be self-starting. At any speed other than synchronous speed, its rotor would experience an oscillating torque of zero average value as the rotating magnetic field repeatedly passes the slower moving rotor. Normally, a short-circuited winding similar to that of an induction machine is added to the rotor to provide starting torque. The motor is started, either with full or reduced stator voltage, and brought up to about 95 percent of synchronous speed, usually with the field winding short-circuited to protect it from excessive induced voltage. The field current is then applied and the rotor pulls into synchronism with the revolving field.

This additional rotor winding is usually referred to as a damper winding because of its additional property of damping out any oscillation that might be caused by sudden changes in the load on the rotor when in synchronism. Adjustment to load changes involves changes in the angle by which the rotor field lags the stator field and thus involves short-term changes in instantaneous speed. These cause currents to be induced in the damper windings, producing a torque that acts to oppose the speed change.

Protection for synchronous motors is similar to that employed with large induction motors. Temperature may be sensed in both the stator and field windings and used to switch off the electric supply. Considerable heating occurs in the rotor-damper winding during starting, and a timer is frequently installed to prevent repeated starts within a limited time interval.