Classification of AC Motors

The AC motors works by rotating the stator field, they make use of the natural alternating nature of the AC wave to turn the field coils on and off sequentially. Generally, AC motors can be classified into two groups: single-phase and polyphase, with each group being further subdivided into induction and synchronous motors. Single-phase motors are typically used for low-power requirements while poly-phase/three-phase motors are used for high-power needs. Induction motors tend to be cheaper than synchronous motors and are thus very widely used.

Induction Motors

Induction motors are the most commonly used AC motors. They are simple, reliable and power most domestic and industrial machines. AC induction motors are grouped into classifications depending on the type of power they use: single-phase or three-phase (two-phase motors are actually supplied with single-phase power).

Related: Single-phase and Three-phase Power Systems

Single-phase Squirrel-cage Induction Motor

This motor consists of a squirrel-cage rotor, i.e. copper or aluminium bars that fit into slots in end rings to form complete electrical circuits.

There are no external electrical connections to the rotor. A basic motor consists of this rotor with a stator having a set of windings. The squirrel cage rotor has no magnetic properties when the power is off. However, when AC power is applied to the stator windings and the stator field starts rotating, the rotor becomes magnetized by induction.

Operation of an Induction Motor

As the rotor field rotates past an individual bar, the field strength in the bar rises and falls. This changing magnetic field induces a voltage in the bar, and the voltage causes a current to flow. The current flows through the bar, through the end rings, and back to the other bars. This current causes the bar to have a magnetic field, and it is this field, interacting with the rotating field that produces the mechanical torque.

Initially, when the motor is stationary, the forces on the current-carrying conductors of the rotor in the magnetic field of the stator are such as to result in no net torque. A number of techniques are used to make the motor self-starting and give this initial impetus to start it; the most common way to start a single-phase motor is to use a second set of windings, called the start windings, which are only energized during the start-up period. In effect, the motor temporarily becomes a two-phase motor, which is self-starting. This type of motor is known by the name split-phase. The AC in the start winding should ideally be 90° out of phase with the run winding. The simple split-phase motor shown below relies on the fact that the start winding, which consists of a few turns of thin wire, has much less inductance and much more resistance than the run winding windings and so creates a phase shift with respect to the run winding. When the motor is up to about 80% speed, a switch opens automatically by centrifugal force, disconnecting the start windings. The motor then continues running on only the run windings. This starting method provides about 40-50° of phase shift, enough to start the motor but not enough to provide a lot of start-up torque.

Another way to create the required phase shift between the run and start windings is with non-polarized capacitor. This motor illustrated in Figure 1.2 is a capacitor start motor. Capacitor motors have a capacitor in series with the auxiliary winding.

When the motor is started, the centrifugal switch is closed, and the capacitor causes the current in the start winding to lead the current in the run winding. Once the motor is running, the switch opens, and the motor continues to operate in single-phase mode on the run windings. The capacitor start motor can achieve a full 90° phase shift between run and start windings, which result in high starting torque.

Capacitor motors come in three varieties:

• Capacitor start
• Two-value capacitor
• Permanent-split capacitor

The first two, use a centrifugal switch or relay to open the circuit or reduce the size of the starting capacitor when the motor comes up to speed. A two-value capacitor motor with one value for starting and one for running, can be designed for optimum starting and running performance; The motor uses a large value capacitor for starting and then switches to a lower value for running; the starting capacitor is disconnected after the motor starts. In Permanent-split capacitor motor, the start windings (with capacitor) are permanently connected, eliminating the need for the centrifugal switch. This design increases the torque of the motor, simplifies the hardware, increases the reliability, and improves the power factor. These motors also run more quietly with less vibration.

The single-phase motors we have discussed so far will start off turning in the same direction with each power up. By reversing the polarity of one of the windings, the motor will start and run in the opposite direction.

The last form of a single-phase induction motors based on how they are started, are the shaded-pole motors. These are the least expensive of the fractional-horsepower motors, generally rated up to 1/20 hp, they have salient stator poles, with one coil-per-pole main windings. Shaded-pole motors do not require a spate start winding for them start. Instead, a small portion of each pole is separated from the rest and encircled by a copper band as illustrated below:

When power is applied to the motor, a current is induced in this copper band, which results in the magnetic field being delayed in its vicinity. This unbalances creates a weak rotating component to the stator field, which is enough to start the motor turning. This motor design is simple, and easy to construct but is inefficient and has low torque. As a result, it is usually used where only low power and a nearly constant load is needed, such as small blowers.

The rotor rotates at a speed determined by the frequency of the alternating current applied to the stator.

Three-phase Induction Motor

The three-phase induction motor is similar to the single-phase induction motor but has a stator with three windings located 120° apart, each winding being connected to one of the three lines of the supply. Since the three phases reach the maximum currents at different times, the magnetic field can be considered to rotate round the stator poles, completing one rotation in one full cycle of the current. The rotation of the field is much smoother than the single-phase motor. The three-phase motor has great advantage over the single-phase motor of being self-starting. The direction of rotation is reversed by interchanging any two of the line connections, which changes the direction of the magnetic field.

The figure below shows a diagram of the three-phase motor [it can either be a wye or a delta connection]

It has three sets of stator windings, with each set of windings being powered by one of the phase voltages. The natural timing sequence of the three individual phase voltages produces the rotating stator field that pulls the rotor around.

The three-phase motor can be wired to run either direction by simply reversing any of the three leads. A three-phase motor, once started, will continue to run even when one of the phases is disconnected, because two-thirds of the rotating fields is still working and the mechanical inertia of the spinning rotor will carry it over the ‘dead spot’ caused by the missing wire. However, vibration and noise will increase, torque will decrease, and the motor may overheat due to greater current in the active field windings.

Also Read: Features of Stepper Motors & How they are applied in Industrial Control

Split-phase Control Motors

The split-phase control motor is technically a two-phase motor because it has two sets of windings. It is not as common as the three-phase or single-phase types aforementioned. These motors do find application in control systems. The operating parameters that make them desirable are:

• They are self-starting.
• They can easily be controlled to turn in either direction.
• Typically they are small (less than 1 hp) and are used to move something back-and-forth, such as opening and closing a valve.

The main challenge is that two-phase AC is not directly available from the power we normally get supplied with; it must be created, usually from single-phase AC. This adds some complexity to the system, but it does provide the opportunity to control the direction of rotation of the motor and hence its value as a back-and-forth type of actuator in a control system.

The required two-phase power is created from single-phase AC by placing a capacitor in series with one of the windings as demonstrated below:

The capacitor causes the current in winding 2 to lead the line current in winding 1 by almost 90°. To change the direction of rotation, the capacitor must be able to switch so that it is in series with the other winding. A special case of the two-phase motor is the AC servomotor. This is a high-slip, high torque motor, that is designed for control systems, and it has a relatively linear torque-speed curve.

Synchronous Motors

The synchronous motor is similar to the induction motor but with one important difference: The rotor in the synchronous motor rotates at exactly the speed of the rotating field, there is no slip i.e. the speed of the synchronous motor is always an exact multiple of the line frequency. This feature is extremely desirable in industrial applications, for example, when several motors along a conveyor belt must all be going exactly the speed.

Operation of Synchronous Motors

In induction motors, the rotor receives its power through induction, which requires a difference (slip) between the speed of the rotor and rotating field. To make a synchronous motor work, the power to form a magnetic field in the rotor must come from another source. Usually, this is done by supplying DC power into the rotor via slip rings and brushes. Slip rings and brushes on the synchronous motor are similar to the commutator assembly used in DC motors, with one key difference; here the electrical contact from stator to rotor is made through a smooth ring and not the multiple contacts of the DC motors’ commutator. The action is smoother, the components last far longer, and less electrical noise is generated. The rotor of the synchronous motor utilizes DC power (termed to as excitation) to energize electromagnets around its parameter. These magnets tend to lock on to the rotating magnetic field in the stator and cause the rotor to rotate at the exact speed of the rotating field, that is, the synchronous speed.

The synchronous motor consists of the following:

• A stator, which supports the magnetic field poles.
• The rotor, which is basically an electromagnetic supplied from a dc source via slip rings.

The rotor will follow the rotating magnetic field at synchronous speed.

This type of motor is not self-starting and has to be brought up to or near to synchronous speed by some means, after which it will continue to rotate of its own accord. Any of the following methods can be used to start a synchronous motor:

• Provide for the frequency to start slow and then speed up. As long as the rotating field does not get too far ahead of the rotor, the rotor will continue to be tugged in one direction. This requires a means to provide the variable frequency. Therefore, if you can control the frequency, you can control the speed.
• Use another motor called a pony motor to bring the synchronous motor up to speed. This method has a disadvantage of requiring significant additional hardware.
• Insert some squirrel cage rotor bars (as used in induction motors) in the rotor of the synchronous motor. This allows the motor to get started as if it were an induction motor.
• Use hysteresis-synchronous motor.

Synchronous-induction Motor

This is essentially an induction motor with a wound rotor. It starts as an induction motor, and when its speed has almost reached synchronous speed the dc supply is switched on and the motor will then continue to function as a synchronous motor.

This type of motor has various applications, for example, if the dc supply to the rotor is increased, the motor can be made to run at a leading power factor. This effect may be used to correct the overall power factor of an installation.

As it is a constant-speed machine, it is often used in motor-generator sets, larger industrial fans and pumps.

A great advantage of the synchronous-induction type is its ability to sustain heavy mechanical overloads. Such an overload pulls the motor out of synchronism, but it continues to run as an induction motor until the overload is removed, at which time it pulls back into synchronism again.

Related: Electrical Motors

Small Synchronous Motors

Small synchronous motors are employed as a source of rotary power in instruments and small machines for examples as timing or clock motors. They are available to be used as direct drive or with an attached gear train.

Small synchronous motors do not use electromagnets in their rotors and therefore do not require slip rings. Instead, the rotor is magnetized in other ways: We have three designs:

• Reluctance motor
• Hysteresis motor
• Permanent magnet motor

Universal Motors

The universal motor can be powered with either AC or DC. Basically, it is a series-wound DC motor that has been specifically designed to operate on AC. Like its DC counterpart, it is reversible by changing the polarity of either the field or the rotor windings, but not both.

Typically, universal motors are designed to operate at high speeds from 3600 to 20000 rpm but since they use a commutator and brushes (which wear out), they have a limited lifetime. Given that they are series-wound motors, they have high starting torque, and therefore they are widely used for handheld power tools like a hand drill motor.

Bottom Line

AC motor have a key advantage over DC motors of being inexpensive, more rugged, reliable and maintenance free. Nonetheless, speed control is generally more complex with DC motors and as a result a speed-controlled DC drive works out less costly than a speed controlled AC drive; however the price difference is decreasing with technological advancements and dropping prices of solid-state devices.

Related articles:

• How to Wire an Industrial Control Motor
• AC vs DC Power & Motors –  Differences and Benefits
• How cycloconverter is used as a direct frequency converter
• Single-Phase & Three-Phase Inverters: Function and Operation
• Key Facts about Thyristor DC Motor Drives Operation