Synchronous motor
Encyclopedia
A synchronous electric motor is an AC motor
distinguished by a rotor
spinning with coils passing magnets at the same rate as the power supply frequency and resulting rotating magnetic field
which drives it.
Another way of saying this is that it does not rely on slip under usual operating conditions and as a result, produces torque at synchronous speed. Synchronous motors can be contrasted with an induction motor
, which must slip in order to produce torque
. They operate synchronously with line frequency. As with squirrel-cage induction motors, speed is determined by the number of pairs of poles and the line frequency.
Synchronous motors are available in sub-fractional self-excited sizes to high-horsepower direct-current excited industrial sizes. In the fractional horsepower range, most synchronous motors are used where precise constant speed is required. In high-horsepower industrial sizes, the synchronous motor provides two important functions. First, it is a highly efficient means of converting ac energy to work. Second, it can operate at leading or unity power factor and thereby provide power-factor correction.
With recent advances in independent brushless excitation control of the rotor winding set that eliminates reliance on slip for operation, the 'brushless wound-rotor doubly fed electric machine' is the third type of synchronous motor with all the theoretical qualities of the synchronous motor and the wound-rotor doubly fed motor combined, such as power factor correction, highest power density, highest potential torque density, low cost electronic controller, highest efficiency, etc.
, reluctance
and hysteresis
designs. Reluctance and hysterisis designs employ a self-starting circuit and require no external excitation supply. Permanent magnet designs require electronic control for practical operation (see Permanent magnet synchronous generator
).
Reluctance motor designs have ratings that range from sub-fractional to about 30 hp
. Sub-fractional horsepower motors have low torque, and are generally used for instrumentation applications. Moderate torque, integral horsepower motors use squirrel cage construction with toothed rotors. When used with an adjustable frequency power supply, all motors in the drive system can be controlled at exactly the same speed. The power supply frequency determines motor operating speed.
Hysteresis motors are manufactured in sub-fractional horsepower ratings, primarily as servomotors and timing motors. More expensive than the reluctance type, hysteresis motors are used where precise constant speed is required.
Slip rings and brushes are used to conduct current to the rotor. The rotor poles connect to each other and move at the same speed - hence the name synchronous motor.
Synchronous motors fall under the category of synchronous machines which also includes the alternator (synchronous generator). These machines are commonly used in analog electric clocks, timers and other devices where correct time is required.
where
is the speed of the rotor (in rpm
),
is the frequency of the AC supply
(in Hz
) and
is the number of magnetic poles. Different from all other synchronous motors, the synchronous brushless wound-rotor doubly fed electric machine operates from sub-synchronous to super-synchronous speeds or twice synchronous speed.
Large machines include additional parts for cooling the machine, supporting the rotor, lubricating and cooling the bearings, and various protection and measurement devices.
Once the motor is in operation, the speed of the motor is dependent only on the supply frequency. When the motor load is increased beyond the breakdown load, the motor falls out of synchronization i.e., the applied load is large enough to pull out the field winding from following the rotating magnetic field. The motor immediately stalls after it falls out of synchronization.
The following techniques are employed to start a synchronous motor:
The leading power factor operation of synchronous motor finds application in power factor correction. Normally, all the loads connected to the power supply grid run in lagging power factor, which increases reactive power consumption in the grid, thus contributing to additional losses. In such cases, a synchronous motor with no load is connected to the grid and is run over-excited, so that the leading power factor created by synchronous motor compensates the existing lagging power factor in the grid and the overall power factor is brought close to 1 (unity power factor). If unity power factor is maintained in a grid, reactive power losses diminish to zero, increasing the efficiency of the grid. This operation of synchronous motor in over-excited mode to correct the power factor is sometimes called as Synchronous condenser
.
The following have synchronous types, but are frequently not synchronous:
AC motor
An AC motor is an electric motor driven by an alternating current.It commonly consists of two basic parts, an outside stationary stator having coils supplied with alternating current to produce a rotating magnetic field, and an inside rotor attached to the output shaft that is given a torque by the...
distinguished by a rotor
Rotor (electric)
The rotor is the non-stationary part of a rotary electric motor, electric generator or alternator, which rotates because the wires and magnetic field of the motor are arranged so that a torque is developed about the rotor's axis. In some designs, the rotor can act to serve as the motor's armature,...
spinning with coils passing magnets at the same rate as the power supply frequency and resulting rotating magnetic field
Rotating magnetic field
A rotating magnetic field is a magnetic field which changes direction at a constant angular rate. This is a key principle in the operation of the alternating-current motor. Nikola Tesla claimed in his autobiography that he identified the concept of the rotating magnetic field in 1882. In 1885,...
which drives it.
Another way of saying this is that it does not rely on slip under usual operating conditions and as a result, produces torque at synchronous speed. Synchronous motors can be contrasted with an induction motor
Induction motor
An induction or asynchronous motor is a type of AC motor where power is supplied to the rotor by means of electromagnetic induction. These motors are widely used in industrial drives, particularly polyphase induction motors, because they are robust and have no brushes...
, which must slip in order to produce torque
Torque
Torque, moment or moment of force , is the tendency of a force to rotate an object about an axis, fulcrum, or pivot. Just as a force is a push or a pull, a torque can be thought of as a twist....
. They operate synchronously with line frequency. As with squirrel-cage induction motors, speed is determined by the number of pairs of poles and the line frequency.
Synchronous motors are available in sub-fractional self-excited sizes to high-horsepower direct-current excited industrial sizes. In the fractional horsepower range, most synchronous motors are used where precise constant speed is required. In high-horsepower industrial sizes, the synchronous motor provides two important functions. First, it is a highly efficient means of converting ac energy to work. Second, it can operate at leading or unity power factor and thereby provide power-factor correction.
Type
There are two major types of synchronous motors: 'non-excited' and 'direct-current excited', which have no self-starting capability to reach synchronism without extra excitation means, such as electronic control or induction.With recent advances in independent brushless excitation control of the rotor winding set that eliminates reliance on slip for operation, the 'brushless wound-rotor doubly fed electric machine' is the third type of synchronous motor with all the theoretical qualities of the synchronous motor and the wound-rotor doubly fed motor combined, such as power factor correction, highest power density, highest potential torque density, low cost electronic controller, highest efficiency, etc.
Non-excited motors
These are manufactured in permanent magnetMagnet
A magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, and attracts or repels other magnets.A permanent magnet is an object...
, reluctance
Reluctance motor
A reluctance motor is a type of electric motor that induces non-permanent magnetic poles on the ferromagnetic rotor. Torque is generated through the phenomenon of magnetic reluctance.A reluctance motor, in its various incarnations, may be known as a:...
and hysteresis
Hysteresis
Hysteresis is the dependence of a system not just on its current environment but also on its past. This dependence arises because the system can be in more than one internal state. To predict its future evolution, either its internal state or its history must be known. If a given input alternately...
designs. Reluctance and hysterisis designs employ a self-starting circuit and require no external excitation supply. Permanent magnet designs require electronic control for practical operation (see Permanent magnet synchronous generator
Permanent magnet synchronous generator
A permanent magnet synchronous generator is a generator where the excitation field is provided by a permanent magnet instead of a coil.Synchronous generators are the majority source of commercial electrical energy...
).
Reluctance motor designs have ratings that range from sub-fractional to about 30 hp
Horsepower
Horsepower is the name of several units of measurement of power. The most common definitions equal between 735.5 and 750 watts.Horsepower was originally defined to compare the output of steam engines with the power of draft horses in continuous operation. The unit was widely adopted to measure the...
. Sub-fractional horsepower motors have low torque, and are generally used for instrumentation applications. Moderate torque, integral horsepower motors use squirrel cage construction with toothed rotors. When used with an adjustable frequency power supply, all motors in the drive system can be controlled at exactly the same speed. The power supply frequency determines motor operating speed.
Hysteresis motors are manufactured in sub-fractional horsepower ratings, primarily as servomotors and timing motors. More expensive than the reluctance type, hysteresis motors are used where precise constant speed is required.
DC-excited motors
Made in sizes larger than 1 hp, these motors require direct current supplied through slip rings for excitation. The direct current can be supplied from a separate source or from a dc generator directly connected to the motor shaftSlip rings and brushes are used to conduct current to the rotor. The rotor poles connect to each other and move at the same speed - hence the name synchronous motor.
Synchronous motors fall under the category of synchronous machines which also includes the alternator (synchronous generator). These machines are commonly used in analog electric clocks, timers and other devices where correct time is required.
Synchronous speed
The "synchronous speed" of a synchronous motor is determined by the following formula:where
is the speed of the rotor (in rpmRevolutions per minute
Revolutions per minute is a measure of the frequency of a rotation. It annotates the number of full rotations completed in one minute around a fixed axis...
),
is the frequency of the AC supplyUtility frequency
The utility frequency, line frequency or mains frequency is the frequency at which alternating current is transmitted from a power plant to the end-user. In most parts of the world this is 50 Hz, although in the Americas it is typically 60 Hz...
(in Hz
Hertz
The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....
) and
is the number of magnetic poles. Different from all other synchronous motors, the synchronous brushless wound-rotor doubly fed electric machine operates from sub-synchronous to super-synchronous speeds or twice synchronous speed.Parts
A typical synchronous motor has the following parts:- A stator winding, connected to the AC supply, which creates a rotating magnetic field.
- A rotor, carrying a field winding supplied by a DC source. This winding is an electromagnetElectromagnetAn electromagnet is a type of magnet in which the magnetic field is produced by the flow of electric current. The magnetic field disappears when the current is turned off...
. Some machines use permanent magnetMagnetA magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, and attracts or repels other magnets.A permanent magnet is an object...
s in the rotor. - The slip ringSlip ringA slip ring is a method of making an electrical connection through a rotating assembly. Slip rings, also called rotary electrical interfaces, rotating electrical connectors, collectors, swivels, or electrical rotary joints, are commonly found in electric motors, electrical generators for AC...
s on the rotor, supply the DC to the field winding from an external supply (some motors have other arrangements). - The stator frame contains and supports the other parts and may include bearing housings.
Large machines include additional parts for cooling the machine, supporting the rotor, lubricating and cooling the bearings, and various protection and measurement devices.
Operation
The operation of a synchronous motor is simple to imagine. The 'Stator' winding, when excited by a poly-phase (usually 3-phase) supply, creates a rotating magnetic field inside the motor. The rotor winding, which acts as a permanent magnet, simply locks in with the rotating magnetic field and rotates along with it. During operation, as the rotor locks in with the rotating magnetic field, the motor is said to be in synchronization.Once the motor is in operation, the speed of the motor is dependent only on the supply frequency. When the motor load is increased beyond the breakdown load, the motor falls out of synchronization i.e., the applied load is large enough to pull out the field winding from following the rotating magnetic field. The motor immediately stalls after it falls out of synchronization.
Starting methods
Synchronous motors are not self-starting motors. This property is due to the inertia of the rotor. When the power supply is switched on, the armature winding and field windings are excited. Instantaneously, the armature winding creates a rotating magnetic field, which revolves at the designated motor speed. The rotor, due to inertia, will not follow the revolving magnetic field. In practice, the rotor should be rotated by some other means near to the motor's synchronous speed to overcome the inertia. Once the rotor nears the synchronous speed, the field winding is excited, and the motor pulls into synchronization.The following techniques are employed to start a synchronous motor:
- A separate motor (called pony motor) is used to drive the rotor before it locks in into synchronization.
- The field winding is shunted or induction motor like arrangements are made so that the synchronous motor starts as an induction motor and locks in to synchronization once it reaches speeds near its synchronous speed.
- Reducing the input electrical frequency to get the motor starting slowly, variable-frequency drives can be used here which have rectifierRectifierA rectifier is an electrical device that converts alternating current , which periodically reverses direction, to direct current , which flows in only one direction. The process is known as rectification...
-inverterInverter (electrical)An inverter is an electrical device that converts direct current to alternating current ; the converted AC can be at any required voltage and frequency with the use of appropriate transformers, switching, and control circuits....
circuits or cycloconverterCycloconverterA cycloconverter or a cycloinverter converts an AC waveform, such as the mains supply, to another AC waveform of a lower frequency, synthesizing the output waveform from segments of the AC supply without an intermediate direct-current link . They are most commonly used in three-phase...
circuits.
Special Properties
Synchronous motors show some interesting properties, which finds applications in power factor correction. The synchronous motor can be run at lagging, unity or leading power factor. The control is with the field excitation, as described below:- When the field excitation voltage is decreased, the motor runs in lagging power factor. The power factor by which the motor lags varies directly with the drop in excitation voltage. This condition is called under-excitation.
- When the field excitation voltage is made equal to the rated voltage, the motor runs at unity power factor.
- When the field excitation voltage is increased above the rated voltage, the motor runs at leading power factor. And the power factor by which the motor leads varies directly with the increase in field excitation voltage. This condition is called over-excitation.
- The most basic property of synchro motor is that it can be used both as a capacitorCapacitorA capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...
or inductorInductorAn inductor is a passive two-terminal electrical component used to store energy in a magnetic field. An inductor's ability to store magnetic energy is measured by its inductance, in units of henries...
. Hence in turn it improves the power factor of system.
The leading power factor operation of synchronous motor finds application in power factor correction. Normally, all the loads connected to the power supply grid run in lagging power factor, which increases reactive power consumption in the grid, thus contributing to additional losses. In such cases, a synchronous motor with no load is connected to the grid and is run over-excited, so that the leading power factor created by synchronous motor compensates the existing lagging power factor in the grid and the overall power factor is brought close to 1 (unity power factor). If unity power factor is maintained in a grid, reactive power losses diminish to zero, increasing the efficiency of the grid. This operation of synchronous motor in over-excited mode to correct the power factor is sometimes called as Synchronous condenser
Synchronous condenser
In electrical engineering, a synchronous condenser is adevice identical to a synchronous motor, whose shaft is not connected to anything but spins freely. Its purpose is not to convert electric power to mechanical power or vice versa, but to adjust conditions on the electric power transmission grid...
.
Uses
- Synchronous motors find applications in all industrial applications where constant speed is necessary.
- Improving the power factor as synchronous condenserSynchronous condenserIn electrical engineering, a synchronous condenser is adevice identical to a synchronous motor, whose shaft is not connected to anything but spins freely. Its purpose is not to convert electric power to mechanical power or vice versa, but to adjust conditions on the electric power transmission grid...
s. - Low power applications include positioning machines, where high precision is required, and robotRobotA robot is a mechanical or virtual intelligent agent that can perform tasks automatically or with guidance, typically by remote control. In practice a robot is usually an electro-mechanical machine that is guided by computer and electronic programming. Robots can be autonomous, semi-autonomous or...
actuators. - Mains synchronous motors are used for electric clocks.
- Record player turntables
Advantages
Synchronous motors have the following advantages over non-synchronous motors:- Speed is independent of the load, provided an adequate field current is applied.
- Accurate control in speed and position using open loop controls, e.g. stepper motors.
- They will hold their position when a DC current is applied to both the stator and the rotor windings.
- Their power factorPower factorThe power factor of an AC electric power system is defined as the ratio of the real power flowing to the load over the apparent power in the circuit, and is a dimensionless number between 0 and 1 . Real power is the capacity of the circuit for performing work in a particular time...
can be adjusted to unity by using a proper field current relative to the load. Also, a "capacitive" power factor, (current phase leads voltage phase), can be obtained by increasing this current slightly, which can help achieve a better power factor correction for the whole installation. - Their construction allows for increased electrical efficiency when a low speed is required (as in ball millBall millA ball mill is a type of grinder used to grind materials into extremely fine powder for use in mineral dressing processes, paints, pyrotechnics, and ceramics.-Description:...
s and similar apparatus). - They run either at the synchronous speed or they do not run at all.
Examples
- Three-phase AC synchronous motors.
- Synchronous brushless wound-rotor doubly fed electric machine.
The following have synchronous types, but are frequently not synchronous:
- stepper motorStepper motorA stepper motor is a brushless, electric motor that can divide a full rotation into a large number of steps. The motor's position can be controlled precisely without any feedback mechanism , as long as the motor is carefully sized to the application...
. - reluctance motorReluctance motorA reluctance motor is a type of electric motor that induces non-permanent magnetic poles on the ferromagnetic rotor. Torque is generated through the phenomenon of magnetic reluctance.A reluctance motor, in its various incarnations, may be known as a:...
.