Three-phase Synchronous Motor

October 30, 2020
Three-phase current

The three-phase motors work with a three-phase alternating current. They are more robust and therefore are the most used in industrial machinery.


Currently, most of the electric motors used in the industry are asynchronous, mainly due to their ease of construction, low maintenance, and good performance.

However, it has the disadvantage of the power factor that, although not very high, reduces the effective power of the motor while introducing a reactive load on the line that makes it necessary to eliminate it using capacitor banks.

The solution to this problem could be the use of synchronous motors.

The problem is that, with commercial models, the installation is complicated, their starting has disadvantages, and maintenance of these motors can be very expensive, that is why they are usually used as alternators and are only used as motors the few times in which an asynchronous motor would be difficult or impossible to use.

Three-phase Synchronous AC Motors

Synchronous motors own its name to the fact that the rotor’s speed and the speed of the stator’s magnetic field are equal.

Their startup has always been complicated; it is the main reason why they are hardly used.

Currently, there are new starting systems and modern designs of synchronous motors to correct the starting problem. This has made the industry see them as a good option again.


Starting Types for Three-Phase AC Synchronous Motors

  • Like an asynchronous motor.
  • Like an asynchronous motor but synchronized.
  • Using a secondary or auxiliary motor for starting.


Start as Asynchronous Motor

When the starting torque is small or with limited load, the starting is used as an asynchronous motor. This is a polar rotor synchronous motor with a short-circuit damper winding that joins the pole heads.

Under these conditions, the induced winding is connected to the grid, which, when traveling by the three-phase alternating current, will create a rotating magnetic field with the same speed as the synchronism.

The magnetic field will cut the damper cage’s conductors, inducing an electromotive force in them that will set it in motion.

The speed will almost catch up to synchro speed. By connecting the winding of the pole wheel to direct current excitation; after some speed oscillations, the moving member will reach synchronous speed.

This starting type also admits all starting methods for asynchronous motors to lower the value of the absorbed current when connected to the grid.

In the inductor winding, resistance is usually connected in series that limits the current absorbed at the moment of connection to the grid and helps with starting, since, being a starter as an asynchronous motor, the increase in resistance in the rotor favors starting.


Asynchronous Synchronized Motor

If you want to start with heavy loads, you will use the so-called synchronized asynchronous motor. This motor’s rotor is of the cylindrical type, and it has a three-phase winding with three slip rings as if it were a wound rotor induction motor.

The synchronous asynchronous motor can be started under load as with an asynchronous motor, having the rotor windings connected to the load’s variable resistance through the slip rings and the stator connected to the grid.

Once the motor is running at speed very close to the synchronization speed, it is switched, and the rotor is supplied with direct current, with which the synchronism speed is finally reached.


Starting by Driving Motor

This starting consists of coupling to the synchronous motor’s shaft, the shaft of a motor whose operating speed is higher than that of the synchronous motor to be started.

The drive motor must be adjusted so that its speed matches the synchronism of the motor we want to drive; after this, we only have to remove the drive motor, and the synchronous motor will work normally.


Most Common Types of Synchronous Motors

At present, synchronous motors have undergone a significant advance in design, mainly brushless synchronous motors.

At first, these types of motors were small and were only used in computer electronics, in modeling, in small industrial applications, and laboratories. Nowadays, brushless synchronous motors with high powers are now appearing, and even, due to the low friction they generate, they are replacing the classic alternators with brushless alternators for power generation.

The most common engines are:

  • Permanent magnet synchronous motor (PMSM)
  • Permanent magnet brushless or brushless motor (BLC)
  • Variable reluctance motor (VRM)
  • Stepper motor


Permanent Magnet Synchronous Motor (PMSM)

AC motors that use magnets to produce the air gap’s magnetic field are called Permanent Magnet Motors (PMSM).

In this motor, the rotation speed is directly proportional to the frequency of the three-phase alternating current grid that feeds it.

The synchronous motor uses the same concept of a rotating magnetic field produced by the stator, but now the rotor consists of electromagnets or permanent magnets (PM) that rotate synchronously with the stator field.

This motor is used when a constant speed is desired since the grid’s frequency imposes its speed.

However, if the number of electromagnets in the stator is doubled, the magnetic field will rotate at half the speed, which happens with the permanent magnet bipolar synchronous motor.


Brushless Permanent Magnet Motors (BLC)

The rotor has two magnets that cover approximately 180º of the rotor perimeter and produce a quasi-rectangular flux density in the gap.

The stator has a three-phase winding, where the conductors of each phase are uniformly distributed in arcs of 60º.

The power system connects a controlled current source to the stator windings so that at each moment, 2 phases of the winding are connected. Each rotor magnet interacts with two 60º arcs through which the current circulates.

When the rotor magnet’s edges reach the boundary between the stator phases, a detector, such as a Hall effect sensor mounted on the stator, will detect the reversal of the air-gap magnetic field and cause an appropriate switching sequence of transistors.


Variable Reluctance Motor

Reluctance motors eliminate permanent magnets (PM), brushes, and commutators. The stator consists of steel laminations that form projecting posts. A series of coils, independently connected in pairs for each phase, wrap around the stator posts.

If we remove the rotor coils, the rotor will basically be a piece of steel that forms projecting posts. The current is switched between the coils of each phase of the stator in a sequential pattern to develop a rotating magnetic field.

When a pair of stator pole coils is energized, the rotor moves to align with the stator posts.

Reluctance refers to the resistance characteristic of a magnetic circuit, also called magnetic resistance.


Stepper Motors

There are basically two types of stepper motors:

  • Variable reluctance motors.
  • Permanent magnet motors: unipolar motors, bipolar motors, and multiphase motors.

Basically, these motors usually are made up of a rotor on which different permanent magnets are applied, and a certain number of exciter coils wound on its stator.

Stepper motors can be viewed as electric motors without commutators. The coils are part of the stator, and the rotor is a permanent magnet, or, in the case of variable reluctance stepper motors, a toothed piece made of a magnetic material.

All the switching (or excitation of the coils) must be externally handled by a driver.

Permanent magnet unipolar stepper motors are wired with a center tap on each of the windings. Center taps are typically connected to the positive power source, and the ends of each winding are alternately grounded to reverse the direction of the field delivered by the winding.