The AC induction motor is the most widely used motor technology today, accounting for over 90% of installed motor capacity. Induction motors are available in single-phase (1ϕ) and three-phase (3ϕ) configurations and can range in size from fractions of a horsepower to tens of thousands of horsepower. They may run at fixed speeds, most commonly 900, 1200, 1800, or 3600 rpm, or be equipped with an adjustable-speed drive1.
The most commonly used AC motors have a squirrel-cage configuration, so named because of the aluminum or copper squirrel cage embedded within the iron laminates of the rotor. There is no physical electrical connection to the squirrel cage. Current in the rotor is induced by the rotating magnetic field of the stator. Wound rotor models, in which coils of wire in the rotor windings, are also available. These are expensive but offer greater control of the motor’s performance characteristics, so they are most often used for special torque and acceleration applications and for adjustable-speed applications.
SINGLE-PHASE MOTOR CONNECTIONS
The majority of single-phase AC induction motors are designed for 120 to 240 V, 60 Hz power sources and are available in fractional horsepower sizes. Although there are several types of single-phase motors, they are essentially identical except for the means of starting. The most commonly used type is the split-phase motor, which is widely employed in medium starting applications.
The operation of a split-phase motor can be summarized as follows:
- The motor has a start and a main winding, both of which are energized when the motor is started.
- The start winding produces a phase difference to initiate motor rotation and is switched out by a centrifugal switch as the running speed is approached. When the motor reaches approximately 75% of its rated full load speed, the starting winding is disconnected from the circuit.
- Split-phase motors are available in sizes up to about ½ horsepower and find popular applications in fans, blowers, home appliances such as washers and dryers, and tools like small saws or drill presses where the load is applied after the motor has reached its operating speed.
It is possible to reverse the motor by reversing the leads to either the start-winding or main winding, but not both. Generally, the industry standard is to reverse the start winding leads.
The nominal rating of a dual-voltage split-phase motor is 120/240 V. When a choice between voltages is available, the higher voltage is preferred. The motor uses the same amount of power and produces the same amount of horsepower when operating from a 120 V or 240 V supply. However, as the voltage is doubled from 120 V to 240 V, the current is halved. This reduced current level allows you to use smaller circuit conductors and reduces line power losses.
Many single-phase motors use a capacitor in series with one of the stator windings to optimize the phase difference between the start and run windings for starting. This results in a higher starting torque than a split-phase motor can produce. There are three types of capacitor motors: capacitor start, permanent-split capacitor, and two-value capacitor. The permanent-split capacitor motor uses a capacitor permanently connected in series with one of the stator windings. This design is lower in cost than the capacitor-start motors that incorporate capacitor switching systems. Capacitor start motors have the capacitor phase in the circuit only during starting, while permanent-split capacitor motors have it in the circuit for both starting and running. Two-value capacitor motors have different capacitance values for starting and running. Capacitor motors are used in compressors, pumps, machine tools, air conditioners, conveyors, blowers, fans, and other hard-to-start applications.
THREE-PHASE MOTOR CONNECTIONS
Three-phase AC induction motors are the most common type of motor used in commercial and industrial applications. They are more efficient and self-starting than single-phase motors, and they can handle higher horsepower loads. All three-phase motors have three windings: phase A, phase B, and phase C. These windings can be connected in either wye (Y) or delta configuration. The connection type depends on the application and the power supply requirements.
- Y-connected motors are typically used for lower voltage applications. In a Y-connected motor, the three windings are connected in a star pattern, with one end of each winding connected to a common neutral point. The other end of each winding is connected to a power line.
- Delta-connected motors are typically used for higher voltage applications. In a delta-connected motor, the three windings are connected in a triangle pattern, with one end of each winding connected to the other end of the next winding.
DUAL-VOLTAGE MOTOR CONNECTIONS
Three-phase motors are commonly manufactured to operate at different voltage levels. The most common multiple-voltage rating for three-phase motors is 208/230/460 V. Always check the motor specifications or nameplate for the proper voltage rating and wiring diagram for method of connection to the voltage source.
Each phase coil (A, B, C) is divided into two equal parts and connected in either series for high-voltage operation or parallel for low-voltage operation. One end of each phase is internally permanently connected to the other phases. According to NEMA nomenclature, these leads are marked T1 through T9. High-voltage and low-voltage connections are given in the accompanying connection table and motor terminal board. The same principle of series (high-voltage) and parallel (low-voltage) coil connections is applied for dual-voltage wye–delta connected three-phase motors. In all cases, refer to the wiring diagram supplied with the motor to ensure proper connection for the desired voltage level.
MULTISPEED MOTOR CONNECTIONS
Some three-phase motors, referred to as multispeed motors, are designed to provide two separate speed ranges. The speed of an induction motor depends on the number of poles built into the motor and the frequency of the electrical power supply. Changing the number of poles provides specific speeds that correspond to the number of poles selected. The more poles per phase, the slower the operating rpm of the motor.
RPM = 120 × (Frequency / Number of poles)
Two-speed motors with single windings can be reconnected, using a controller, to obtain different speeds. The controller circuitry serves to change the connections of the stator windings. These motors are wound for one speed but when the winding is reconnected, the number of magnetic poles within the stator is doubled and the motor speed is reduced to one-half the original speed. This type of reconnection should not be confused with the reconnection of dual-voltage three-phase motors. In the case of multispeed motors, the reconnection results in a motor with a different number of magnetic poles. Three types of single-winding two-speed motors are available: constant horsepower, constant torque, and variable torque.
To reverse the direction of rotation of any three-phase wye or delta-connected motor, simply reverse or interchange any two of the three main power leads to the motor. When you are connecting a motor, the direction of rotation is usually not known until the motor is started. In this case, the motor may be tem-porarily connected to determine the direction of rotation before making permanent connections.
In certain applications, unintentional reversal of motor rotation can result in serious damage. When this is the case, phase failure and phase reversal relays are used to protect motors, machines, and personnel from the hazards of open-phase or reversed phase conditions. The operation of this circuit can be summarized as follows:
- The relay is designed to continuously monitor phase rotation of the three-phase lines.
- A solid-state sensing circuit within the relay con-trols an electromechanical relay coil, the normally open (NO) contact, that closes when power with correct phase rotation is applied.
- The relay coil will not energize if the applied phases are reversed and will deenergize if phase rotation is reversed while the motor is running.
The speed of an AC induction motor depends on the number of motor poles and the frequency of the applied power. Inverter duty and vector duty motors are a class of motors that are designed to operate with variable-frequency drives. These motors have improved insulation systems, additional active material, and/or external fans to keep them cool at low speeds.
Variable-frequency motor drives work by varying the frequency of the voltage applied to the motor. This allows the motor to be operated at different speeds. Standard induction motors can be damaged when operated with variable-frequency drives, so inverter duty and vector duty motors are used in applications where variable speed operation is required.