EducationSynchronous motor working principle - Synchronous motor construction and applications

Synchronous motor working principle – Synchronous motor construction and applications

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Synchronous motor working principle

If the windings of the stator are activated, a rotative magnetic field will develop within the stator. If we think of this field as a rotational pair of poles the unloaded rotor will be aligned to the field of the stator and will spin in synchrony with the field. The forces between poles depicted are radial and do not generate any torque. If the rotor has been loaded with a braking force, it will slow down slightly in relation to the field of rotation. In the end, the rotor’s axis won’t longer align with the stator’s axis, and the forces that are generated between the poles will create mechanical torque, which is opposed to the brake torque. Any changes in load will not result in a change in the speed of the rotor (as happens with an Asynchronous motor).

The rotor, both in idle (no loading) and when it is loaded, will rotate at a rate equal to that of the magnetic field rotating (synchronous velocity). But, if the torque of the load exceeds what is the electromagnetic maximum torque for the motor (if the angle between the rotor and stator the axes exceeds 90 degrees) then the machine will lose synchronization and stop for a period of period of.

Synchronous motor startup

One of the drawbacks of the time-synchronized motor is the fact that it is unable to begin on its own once the stator’s windings are powered by the mains. When the voltage is applied to the stator it creates a rotating force generated, which causes an alternating torque that acts on the motor’s rotor. The rotor cannot begin its operation due to the inertia created by the rotor as the frequency of torque fluctuations is too high.

The average value for the motor rotor’s synchronous motor the rotor’s starting torque will be zero.

Mean value of starting torque

Figure 1: Mean value of starting torque

There are many methods to handle this issue. One is to employ an additional machine to boost the rotor speed that is the part of the synchronous motor. The job of such a machine can be accomplished by an additional an asynchronous motor as well as a DC motor however this method is unlikely to be utilized in the real world. Another method to start an asynchronous motor is to utilize the same method used in Asynchronous motors. A cage that is usually composed of copper rods similar to the cage found in the rotor of a squirrel cage synchronous motor is installed in the poles of the rotor. The synchronous motor begins as an Asynchronous motor and eventually reaches similar to synchronous speed. When it reaches this speed, the circuit for excitation is charged with DC current this will enable the motor to enter synchronization as it continues to operate at a synchronized speed.

The best option currently for starting synchronous motors is believed to be using electronic frequency converters (inverters) that permit an ongoing increase in the voltage of the stator winding supply that permits gradual acceleration for the rotating. For permanent magnet motors, it is the only option.

Synchronous motor construction

Similar to the Asynchronous motor This motor usually comes with an alternating stator winding which creates a circular rotational field. There are some differences in their rotors. The older synchronous motor designs presume that the rotor is constructed by winding it around an inner core. It is powered by brushes and slip rings with an DC or AC source. Rotors can be made in two ways cylindrical rotors (with poles that are latent) or rotors that have poles that are open.

Four-pole open impeller

Figure 2: Four-pole open impeller

Each pole has its own winding around the pole’s core. By forming the poles, an induction distribution that is appropriate around the rotor’s circumference is made.

Open pole rotors are typically employed in high-power machines due to their design (significantly less mechanical strength in the face of gravitational forces). They are used in machines that do not have high speeds. The most popular uses for this type of machine are generators and motors powered through hydro turbines (hydrogenerators).

The excitation winding in a cylindrical rotating rotor is inserted into the grooves that are milled in the body of the steel and is secured to prevent it from being pulled through the grooves with wedges. The winding is only a small part of the circuit for the rotor (about 1/3 part of it).

These rotors cost more than open-ended ones, however due to their superior mechanical strength, they are employed in machines with greater speeds of rotation. This model is employed in, for instance, generators with high speeds (turbogenerators) that typically reach speeds of 3000 rpm. They are powered by water or steam turbines.

Two-pole cylindrical rotor (with hidden poles)

Figure 3: Two-pole cylindrical rotor (with hidden poles)

In recent times, however, devices that use magneto-electric excitation, in where the rotor is outfitted with permanent magnets, instead of windings, are being increasingly utilized.

Specially designed magnets made from appropriate metal alloys are positioned onto the rotating. Because of this, the torque is quite high. can be achieved with a smaller moment of inertia for the rotating rotor.

Four-pole rotor with radially bonded magnets

Figure 4. Four-pole rotor with radially bonded magnets

Synchronous motor excitation

Rotor excitation

The installation of a winding in the rotor to create an unchanging magnetic field that makes the rotor spin requires an excitation current is applied to the rotating. In the stator, on contrary there is the AC winding is positioned. This creates an electric field that rotates and “pulls” the rotor in an arranged direction. Of of course, the rotor must be designed in a way that the amount of magnetic poles that are created is equal to the magnetic poles that are created by the stator.

The current that is excitation comes from a different source, usually , from a particular circuit that draws power directly by a rectifier or via an DC generator known as an exciter. An exciter could include, for instance, a generator placed on a common shaft along with an exciter. It is connected to the motor rotor. The amount of power needed to stimulate an synchronous motor generally is not more than one percent of the recommended motor power. In smaller synchronous motors, it is typical to find designs that has the excitation winding located in the stator, thus, the additional excitation circuit is the magneto and the rotor is the armature.

In a lot of new motors that are synchronous, to make their designs simpler permanent magnets, also called permanent magnets are placed on the rotor rather than the winding, which needs direct current to be supplied Most often, they are Neodymium magnets. These magnets are placed (bonded) onto the exterior of the motor. This construction ensures the rotor has a high percentage of the rotational torque can be attained, and a minimal value of that moment of inertia of rotating rotor. Motors with this design are referred to as PMSM (Permanent Magnetic Synchronous Motor), i.e. permanent magnet synchronous motors.

Another type of synchronous motors are the reluctance motors. They are constructed with open poles but , unlike PMSM motors their rotors have absence of excitation wires. To move the rotor they make use of their reluctance torque that occurs in the absence excitation. It is generated by applying an electric field to a rotor that is magnetically asymmetrical composed from ferromagnetic material. The rotor that is subjected to the torque of reluctance “tries to find” such an optimum position in relation to the stator, in which the reluctance is at its lowest.

In the field of industrialization it is common to see motors referred to as hysteresis, where the principle of magnetic hysteresis is utilized. The rotors of these motors are composed entirely of materials with a large loop of hysteresis. The torque of motors motor is determined by the size of the rotor, as well as losses in the unit hysteresis. Hysteresis motors offer the benefits of quiet operation, high beginning torque even at low currents and seamless sync. But, they are becoming less popular because of their inefficiency.

Starting a synchronous motor

The most significant drawback to synchronous motors is their absence of a starting torque. This is due to the voltage that is which is applied to the motor produces an electromagnetic field that rotates and produces an oscillating torque. This is because the voltage that is that is applied to the stator creates an electromagnetic field that rotates, which produces an alternating torque that causes the rotor to be “pulled” to one side before moving to the opposite. Because of the frequency of this variation in torque the rotor is unable to move, and the average torque is zero.

There are many solutions to this issue. The easiest solution is to utilize another starting motor however, in the industrial setting, this option isn’t practical. In reality, the method employed in asynchronous motors is utilized to initiate an synchronous motor. A second starter cage comprised of copper rods similar to the cages that is used in the squirrel-cage rotors of Asynchronous motors, is put within the poles of the rotor. The synchronous motor begins like an Asynchronous motor and, when it is at the speed that is close to synchronous speed and it is activated. DC power supply for the circuit that drives it is switched on. Because of this, the rotor is synchronized with the magnetic field rotating (enters synchronization) which continues to move in synchronization.

The second, and most frequently employed in automation systems, method for the synchronous motor beginning can be described as frequency starting-up. Frequency converters (inverters) are employed in this case, which allow for a gradual increasing the frequency of the supply voltage within the windings of stator. This permits gradual and systematic acceleration of the rotating. In the instance of PMSM motors that have permanent magnets, it is the only method that works in the industrial setting.

A different, somewhat older method of controlling frequency is to use synchronous generators that supply the armature windings of the motor that is being initiated. The speed of rotation and, consequently, that of the generator is gradually increased from zero to synchronous speeds, that allows the motor to begin the motor similar to the operation the inverter.

Synchronous motor protection

Electric high-power induction and synchronous machines are typically protected from the long term (thermal) overload as well as the maximum (momentary) overload, and other supply voltage fluctuations that could harm their drive system or them. The necessary protections as well as the ranges for setting the tripping parameters must be identified on the page “Machine operating data” attached to the documents supplied by the manufacturer along with the machine. The protection should cover the machine from:

  • Current overloads, and over the temperature of operation that is allowed,
  • Automatic start after the temporary power outage and the returning power
  • asymmetrical voltage e.g. after phase loss,
  • A decrease in resistance of the windings’ insulation system to lower than the limit.

It is apparent from the above , that there are numerous requirements to protecting electrical devices. They are not always effective in operating situations, since not enough focus is given to the diagnostics, maintenance and checking the proper the operation of the protection system.

This protection could not function if the relay contacts in the protection system become dirty or oxidized, and when they do this because of a short circuit, they are unable to provide a signal to turn off the switch. In one steel mill one of them, a DC rolling mill motor that had a power of 1 MW and 1000 rpm, failed and all sections of the commutator went out. The motor was in operation for 10 years before the breakdown. The motor was protected by three operating safeguards that were in place.

  • at a rotational speed of 1.2 numax,
  • at 2.5IN and an armature current of 2.5IN and
  • when there is a the loss of current excitation.

The motor broke up because of an emergency interruption in excitation current. If one of the protections listed been activated and this motor would have stopped. There is also a documented instance of a brand new 40 MVA 110 kV/(514,304) V furnace transformer that was completely destroyed after just three months of use. The cause of the failure was the control winding breakers being switched on, which created the internal circuit short to the winding. The relay intended to stop this switch’s activation turned on during this situation of the transformer’s circuit had contacts that were contaminated by the chamotte dust found on the casting line of continuous steel. The interlocking relay wasfunctional, however, not able to fulfill its job. For drive system, electrical machines operate in steady state and electromechanical transients. The characteristic transients are the synchronous starting for AC motors. Each electric motor is designed to structurally handle the transients in electromagnetic energy that occur in the beginning of. Under the conditions of steady state of operation, there can be disturbances in the voltage of mains, e.g. caused by lightning, trigger the functioning on AVR or APV. APV as well as the AVR. These disruptions are extremely hazardous due to the fact that the time for the power loss is around one second. Turning off the voltage and back on creates electromagnetically unstable conditions in motors, which is far more hazardous than the beginning of the process.

Synchronous motor applications

Synchronous motors that servo are mostly sprinters designed for specific jobs. A low moment of inertia translates to the highest level of dynamics and efficient and efficient control. These parameters enable synchronous motors to serve as manipulators and end-effectors of machines, which is typically multi-axis machines.

Recreational and sports vehicles are vehicles in which the mass of the entire vehicle is an important factor. PMSM motors are much smaller mass as compared to DC and induction motors that have similar traction characteristics that make them perfect to be used in these types of vehicles. Electric-powered recreational and sport vehicles are increasing in popularity and are gaining more applications, which is due to their benefits.

The losses of the synchronous motor are around two times as much as an Asynchronous motor. When it comes to residential uses, the level of noise produced by the motor while in operation is an important factor that defines the motor. The tests show that the noise produced by an open synchronous motor is around 10 dB less than the noise produced by an air-cooled Asynchronous motor. A comparison of the same (with similar motor power) permanent magnet-driven synchronous motors with gearless asynchronous motors reveals that they are 50 percent heavier, weigh 20% more volume and around 3% lower efficiency than the synchronous motors. The advantage they have over reducer asynchronous motors in contrast is the lack of mechanical gearboxes, and the resultant small design, compact construction and longer life of the drive.


Michal Pukala
Electronics and Telecommunications engineer with Electro-energetics Master degree graduation. Lightning designer experienced engineer. Currently working in IT industry.