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Single-phase Synchronous Motor

November 1, 2020
Single-phase Current

These are the motors that run on single-phase alternating current.

It should be clarified that some of these motors have extremely low starting torque and sometimes need an extra pair of coils just for starting as they cannot start rotating on their own.

This is arguably the same as the DC motor with series excitation but with single-phase alternating current.

The single-phase system is that of distribution, production, and electricity consumption for a single phase. Therefore the tension varies equally together.


To understand how these motors work, we must know that a single-phase current through a coil will produce a fluctuating magnetic field.

The operation of the motor is based on the coupling of magnetic fields that rotate at the same time, which means they are synchronous.

Synchronous motors contain multiphase AC electromagnets in the motor’s stator that create a magnetic field that rotates in time with oscillations in the line current.

For this purpose, the rotor has coils connected to a collector, which are connected in series with the inductor coils.

[This video can help you understand better]

Therefore, the same motor can operate with both alternating current (AC) and direct current (DC). We can also say that it is a type of universal motor.

Synchronous motors are available in sizes from self-excited sub-fractional horsepower to high power industrial sizes.

In the fractional power range, they are used when precise constant speed is required. Some applications of synchronous motors are:

  • Analog Electric Clocks
  • Timers
  • Chronometers
  • Alarms
  • Other devices where exact time is required

In higher power industrial sizes, the synchronous motor provides two essential functions.

First, it is a highly efficient means of converting AC energy into work.

Second, it can operate at unity or leading power factor and thus provide power factor correction.

Synchronous motors belong to the most straightforward general synchronous machines, where synchronous generators are also found.

What distinguishes a motor from a generator in the field pole’s action and the resulting air gap flux by the retarding torque of a shaft load.

If the field poles are dragged behind the resulting air gap flux by the retarding torque of a shaft load, then we are talking about a motor.

If the field poles are dragged ahead of the resulting air gap flow by the retarding torque of a shaft load, then we are talking about a generator.

Types of Synchronous Single-phase Motors

There are two main types of synchronous motors, depending on how the rotor is magnetized:

  • Excited by DC
  • Unexcited

Excited by DC Motors

These motors are generally manufactured relatively large + 1hp.

These motors’ general characteristic is that they require direct current (DC) in the rotor for their excitation.

DC is quickly supplied through slip rings, but rectifiers and brushless AC induction can also be used.

Direct current can be supplied from a separate DC source or a DC generator connected directly to the rotor.

Unexcited Motors

In unexcited motors, the rotor is made of steel.

At synchronous speed, the rotor rotates at the same time as the rotating magnetic field of the stator, so it has a nearly constant magnetic field across it.

The field from the external stator magnetizes the rotor, inducing the magnetic poles necessary to make it turn.

The rotor is made of high retention steel, such as cobalt steel.

Unexcited Motor Types

There are different types of unexcited motors, depending on the design in their manufacture.

  • Hysteresis
  • Permanent magnet
  • Reluctance

Hysteresis Engine

Hysteresis motors are mainly manufactured as servo motors and timing motors.

They are more expensive than reluctance.

They are used when a high precision constant speed is required.

An essential advantage of the hysteresis motor is that since the delay angle is independent of speed; it develops a constant torque from start to synchronous speed.

So it is self-starting and does not need a secondary winding to start (although some have a squirrel cage structure on the rotor to have more starting torque).

Its rotor is a solid and smooth cylinder of some high coercivity steel alloy (this means, it has the ability to resist an external magnetic field without demagnetizing). This material has a wide hysteresis loop (meaning that when it is magnetized in a specific direction, it requires a large inverse magnetic field to reverse its magnetization).

The rotating field of the stator causes absolutely the entire rotor to experience a reverse magnetic field.

Due to hysteresis, the phase of magnetization is delayed in direct proportion to the phase of the applied field. The result of this is that the axis of the induced magnetic field in the rotor lags behind the axis of the stator field at a constant angle, producing torque when the rotor tries to catch up with the stator field.

As long as the rotor is below synchronous speed, absolutely, the entire rotor experiences a magnetic field inverse to the slip frequency that drives it around its hysteresis loop, causing the rotor field to lag and create torque.

There is a two-pole low reluctance bar-shaped structure in the rotor; As the rotor approaches synchronous speed and the slip reaches zero, it becomes magnetized and aligned with the stator field, eventually causing the rotor to remain synchronous in the rotating field of the stator.

Permanent Magnet Motor

These motors are similar to brushless DC motors.

Permanent magnets are generally made of neodymium.

They are mostly used for gearless elevators and as more efficient replacements for induction motors.

Most require a variable frequency drive to get started. However, some incorporate a squirrel cage in the rotor for starting; These are known as inline-boot or autostart PMSM.

This motor uses permanent magnets (PM) in the rotor to create a constant magnetic field. The stator has windings connected to AC to produce a rotating magnetic field. At synchronous speed, the rotor poles are kept synchronous in the rotating magnetic field.

Permanent magnet synchronous motors are mainly controlled by direct torque control and field-oriented control. However, these methods have the disadvantage of requiring relatively high torque and ripples in the stator flux. But today, there are controllers (PIDs) designed to reverse these problems.

Reluctance Motor

These motors have designs from 1/8 hp to 20kw.

Small engines are generally used in instrumentation applications.

A great advantage is that if you use a power supply with adjustable frequency; All motors in the system can be controlled at precisely the same speed.

Its rotor is made of solid steel that forms protruding toothed poles.

In general, the poles of the rotor are less than those of the stator; with the intention of minimizing torque ripple and preventing all poles from aligning perfectly simultaneously, ­­since this position does not generate torque.

Magnetic resistance is minimal when the poles are aligned with the rotating magnetic field of the stator, and the angle between them increases. This creates a torque that pulls the rotor into line with the closest pole of the stator.

At synchronous speed, the rotor locks in the rotating field of the stator; this cannot start the motor, so the rotor poles usually have squirrel-cage windings; with the intention of providing torque below synchronous speed. This causes the motor to start as an induction (asynchronous) motor until it approaches synchro speed and remains synchronous in the rotating field of the stator.