Synchronous motor

A synchronous motor-generator set for AC to DC conversion.

A synchronous electric motor is an AC motor in which, at steady state, the rotation of the shaft is synchronized with the frequency of the supply current; the rotation period is exactly equal to an integral number of AC cycles. Synchronous motors contain electromagnets on the stator of the motor that create a magnetic field which rotates in time with the oscillations of the line current. The rotor turns in step with this field, at the same rate.

The motor does not rely on "slip" under usual operating conditions, and as a result produces torque at synchronous speed. Synchronous motors can be contrasted with induction motors, which must slip in order to produce the rotor magnetic field and to provide torque. The speed of the synchronous motor is determined by the number of magnetic poles and the line frequency.

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

These machines are commonly used in analog electric clocks, timers and other devices where correct time is required.

Type

Synchronous motors fall under the more general category of synchronous machines which also includes the synchronous generator. Generator action will be observed if the field poles are "driven ahead of the resultant air-gap flux by the forward motion of the prime mover". Motor action will be observed if the field poles are "dragged behind the resultant air-gap flux by the retarding torque of a shaft load".

Diagram showing assembly of rotor and stator with their windings

There are two major types of synchronous motors depending on how the rotor is magnetized: non-excited and direct-current excited.

Non-excited motors

In non-excited motors, the rotor is made of steel. At synchronous speed it rotates in step with the rotating magnetic field of the stator, so it has an almost-constant magnetic field through it. The external stator field magnetizes the rotor, inducing the magnetic poles needed to turn it. The rotor is made of a high-retentivity steel such as cobalt steel, These are manufactured in permanent magnet, reluctance and hysteresis designs:

• Reluctance motors: These have a rotor consisting of a solid steel casting with projecting (salient) toothed poles, the same number as the stator poles. The size of the air gap in the magnetic circuit and thus the reluctance is minimum when the poles are aligned with the (rotating) magnetic field of the stator, and increases with the angle between them. This creates a torque pulling the rotor into alignment with the nearest pole of the stator field. Thus at synchronous speed the rotor is "locked" to the rotating stator field. This cannot start the motor, so the rotor poles usually have squirrel-cage windings embedded in them, to provide torque below synchronous speed. The machine starts as an induction motor until it approaches synchronous speed, when the rotor "pulls in" and locks to the rotating stator field.

Reluctance motor designs have ratings that range from fractional horsepower (a few watts) to about 22 kW. Very small reluctance 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: These have a solid smooth cylindrical rotor, cast of a high coercivity magnetically "hard" cobalt steel. This material has a wide hysteresis loop (high retentivity), meaning once it is magnetized in a given direction, it requires a large reverse magnetic field to reverse the magnetization. The rotating stator field causes each small volume of the rotor to experience a reversing magnetic field. Because of hysteresis the phase of the magnetization lags behind the phase of the applied field. The result of this is that the axis of the magnetic field induced in the rotor lags behind the axis of the stator field by a constant angle δ, producing a torque as the rotor tries to "catch up" with the stator field. As long as the rotor is below synchronous speed, each particle of the rotor experiences a reversing magnetic field at the "slip" frequency which drives it around its hysteresis loop, causing the rotor field to lag and create torque. There is a 2-pole low reluctance bar structure in the rotor. As the rotor approaches synchronous speed and slip goes to zero, this magnetizes and aligns with the stator field, causing the rotor to "lock" to the rotating stator field.

A major advantage of the hysteresis motor is that since the lag angle δ is independent of speed, it develops constant torque from startup to synchronous speed. Therefore it is self-starting and doesn't need an induction winding to start it, although many designs do have a squirrel-cage conductive winding structure embedded in the rotor to provide extra torque at start-up.

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.

• Permanent-magnet motors: These have permanent magnets embedded in the steel rotor to create a constant magnetic field. At synchronous speed these poles lock to the rotating magnetic field. They are not self-starting. Because of the constant magnetic field in the rotor these cannot use induction windings for starting, and must have electronically controlled variable frequency stator drive.

DC-excited motors

DC-excited motor, 1917. The exciter is clearly seen at the rear of the machine.

Made in sizes larger than 735 W, these motors require direct current supplied to the rotor for excitation. This is most straightforwardly supplied through slip rings, but a brushless AC induction and rectifier arrangement may also be used. The direct current may be supplied from a separate DC source or from a DC generator directly connected to the motor shaft.

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.

Synchronous speed

The synchronous speed $N_{s}\,$ of a synchronous motor is given by,

$\mathbf {N_{s}} ={\frac {\mathbf {120} \mathbf {f} }{\mathbf {p} }}$

where,
$\mathbf {f}$ is the frequency of the AC supply current in Hz.
$\mathbf {p}$ is the number of magnetic poles per phase.

Synchronous speed can also be expressed in terms of angular speed,

$\mathbf {\omega _{s}} ={\frac {\mathbf {4} \pi \mathbf {f} }{\mathbf {p} }}$

Here,
$\mathbf {\omega _{s}}$ is the angular speed expressed in rad·s⁻¹.

Example

A 3-phase synchronous motor is running with 6 poles is operating at 50 Hz. The synchronous speed will be,

$N_{s}={120\times {50} \over {6}}$ = 1000 rpm

Construction

Rotor of a large water pump. The slip rings can be seen below the rotor drum.

Stator winding of a large water pump

The construction of synchronous motor is similar to that of an alternator. The principal components of a synchonous motor are the stator and the rotor. The stator of synchronous motor and stator of induction motor are similar in construction. The stator frame contains wrapper plate. Circumferential ribs and keybars are attached to the wrapper plate. To carry the weight of the machine, frame mounts and footings are required. The field system is excited by DC excitation. So brushes and slip rings are required. The field system can also be excited via brushless exciter. The rotor may be a cylindrical type (also known as round rotor, non salient pole rotor) or a salient pole type. For designing a motor with large number of poles, salient pole rotor is used and for motors up to six poles cylindrical rotor is used.

Operation

The rotating magnetic field is formed from the sum of the magnetic field vectors of the three phases of the stator windings

The operation of a synchronous motor is due to the interaction of the magnetic fields of the stator and the rotor. The stator winding, when excited by a poly-phase (usually 3-phase) supply, creates a rotating magnetic field inside the motor. The rotor locks in with the rotating magnetic field and rotates along with it. Once the rotor locks in with the rotating magnetic field, the motor is said to be in synchronization. A single-phase (or two-phase derived from single phase) stator winding is possible, but in this case the direction of rotation is not defined and the machine may start in either direction unless prevented from doing so by the starting arrangements.

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 and the field winding no longer follows the rotating magnetic field. Since the motor cannot produce (synchronous) torque if it falls out of synchronization, practical synchronous motors have a partial or complete squirrel-cage damper (amortisseur) winding to stabilize operation and facilitate starting. Because this winding is smaller than that of an equivalent induction motor and can overheat on long operation, and because large slip-frequency voltages are induced in the rotor excitation winding, synchronous motor protection devices sense this condition and interrupt the power supply (out of step protection).

Starting methods

Above a certain size, synchronous motors are not self-starting motors. This property is due to the inertia of the rotor; it cannot instantly follow the rotation of the magnetic field of the stator. Since a synchronous motor produces no inherent average torque at standstill, it cannot accelerate to synchronous speed without some supplemental mechanism.

Large motors operating on commercial power frequency include a "squirrel cage" induction winding which provides sufficient torque for acceleration and which also serves to damp oscillations in motor speed in operation. Once the rotor nears the synchronous speed, the field winding is excited, and the motor pulls into synchronization. Very large motor systems may include a "pony" motor that accelerates the unloaded synchronous machine before load is applied. Motors that are electronically controlled can be accelerated from zero speed by changing the frequency of the stator current.

Very small synchronous motors are able to start without assistance if the "moment of inertia of the rotor and its mechanical load is sufficiently small [because the motor] will be accelerated from slip speed up to synchronous speed during an accelerating half cycle of the reluctance torque."

Polyphase synchronous motors rotate in a direction determined by the sequence of the currents in the phase windings. Single-phase synchronous motors such as in electric wall clocks can freely rotate in either direction unless a shaded-pole type.

Small synchronous motors are commonly used in line-powered electric mechanical clocks or timers that use the powerline frequency to run the gear mechanism at the correct speed. Synchronous motors in clocks typically use an anti-reversing mechanism to ensure starting in the correct direction.

Applications, special properties, and advantages

Use as synchronous condenser

Main article: Synchronous condenser

Synchron­masch­in­es-V-Kurv­en, zeig­nung das unter- und uber-ex­ci­ta­tions­feld­volt­age­glock­en­spiel­ge­dingdorf­schnitz­el, in Mittel ist erregt natuer­lich, wo Ein­heit des Fak­tor dem Kraft ge­fund­et hab­en sind ge­wes­en ge­habt hab­en ge­word­en sein, also Not­en Sie Kom­mas for Dec­i­mal­punkten Um­steig­en. Gewesen.

By varying the excitation of a synchronous motor, it can be made to operate at lagging, leading and unity power factor. Excitation at which the power factor is unity is termed normal excitation voltage. The magnitude of current at this excitation is minimum. Excitation voltage more than normal excitation is called over excitation voltage, excitation voltage less than normal excitation is called under excitation. When the motor is over excited, the back emf will be greater than the motor terminal voltage. This causes a demagnetizing effect due to armature reaction.

The V curve of a synchronous machine shows armature current as a function of field current. With increasing field current armature current at first decreases, then reaches a minimum, then increases. The mimimum point is also the point at which power factor is unity.

This ability to selectively control power factor can be exploited for power factor correction of the power system to which the motor is connected. Since most power systems of any significant size have a net lagging power factor, the presence of overexcited synchonous motors moves the system's net power factor closer to unity, improving efficiency. Such power-factor correction is usually a side effect of motors already present in the system to provide mechanical work, although motors can be run without mechanical load simply to provide power-factor correction. In large industrial plants such as factories the interaction between synchronous motors and other, lagging, loads may be an explicit consideration in the plant's electrical design.

Steady state stability limit

$\mathbf {T} ={\mathbf {T_{max}} }{\sin \delta }$

where,

$\mathbf {T}$ is the torque

$\delta$ is the torque angle

$\mathbf {T_{max}}$ is the maximum torque

here,

$\mathbf {T_{max}} ={\frac {{\mathbf {3} }{\mathbf {V} }{\mathbf {E} }}{{\mathbf {X_{s}} }{\omega _{s}}}}$

When load is applied, torque angle $\delta$ increases. When $\delta$ = 90° the torque will be maximum. If load is applied further then the motor will loose its synchronism, as motor torque will be less than load torque. The maximum load torque that can be applied to a motor without losing its synchronism is called steady state stability limit of a synchronous motor.

Other

Synchronous motors are especially useful in applications requring precise speed and/or position control.

• Speed is independent of the load over the operating range of the motor.
• Speed and postion may be accurately controlled using open loop controls, e.g. stepper motors.
• Low-power applications include positioning machines, where high precision is required, and robot actuators.
• They will hold their position when a DC current is applied to both the stator and the rotor windings.
• A clock driven by a synchronous motor is in principle as accurate as the line frequency of its power source. (Although small frequency drifts will occur over any given several hours, grid operators actively adjust line frequency in later periods to compensate, thereby keeping motor-driven clocks accurate (see Utility frequency#Stability).)
• Record player turntables
• Increased efficiency in low-speed spplications (e.g. ball mills).

Subtypes

• Three-phase AC synchronous motors
• Synchronous brushless wound-rotor doubly fed electric machine
• Stepper motor (may be synchronous or not)
• Reluctance motor (may be synchronous or not)

See also

• Short circuit ratio
• Doubly fed electric machine

External links

• Synchronous motor animation