Synchronous motor

Control techniques

A permanent magnet synchronous motor and reluctance motor requires a control system for operating (VFD or servo drive).

There is a large number of control methods for synchronous machines, selected depending on the construction of the electric motor and the scope.

Control methods can be divided into:

• Scalar control
  • V/f control
• Vector control
  • Field oriented control
  • Direct torque control, a variant of VC
  • Feedback linearization

The PMSMs can also operate on open-loop control, which is sometimes used for start-up thus enabling the position sensing operation.

Synchronous speed

The synchronous speed of a synchronous motor is given:
in RPM, by:

${\displaystyle N_s=60\frac{f}{P}=120\frac{f}{p}}$

and in rad·s⁻¹, by:

${\displaystyle {\omega }_s=2\pi \frac{f}{P}=4\pi \frac{f}{p}}$

where:

• ${\displaystyle f}$ is the frequency of the AC supply current in Hz,
• ${\displaystyle p}$ is the number of magnetic poles,
• ${\displaystyle P}$ is the number of pole pairs (rarely, planes of commutation), ${\displaystyle P=p/2}$.

Examples

A single-phase, 4-pole (2-pole-pair) synchronous motor is operating at an AC supply frequency of 50 Hz. The number of pole-pairs is 2, so the synchronous speed is:

${\displaystyle N_s=60\frac{\text{rpm}}{\text{Hz}}\times \frac{50\text{ Hz}}{2}=1500\text{rpm}}$

A three-phase, 12-pole (6-pole-pair) synchronous motor is operating at an AC supply frequency of 60 Hz. The number of pole-pairs is 6, so the synchronous speed is:

${\displaystyle N_s=60\frac{\text{rpm}}{\text{Hz}}\times \frac{60\text{ Hz}}{6}=600\text{rpm}}$

The number of magnetic poles, ${\displaystyle p}$, is equal to the number of coil groups per phase. To determine the number of coil groups per phase in a 3-phase motor, count the number of coils, divide by the number of phases, which is 3. The coils may span several slots in the stator core, making it tedious to count them. For a 3-phase motor, if you count a total of 12 coil groups, it has 4 magnetic poles. For a 12-pole 3-phase machine, there will be 36 coils. The number of magnetic poles in the rotor is equal to the number of magnetic poles in the stator.

Construction

The principal components of electric motors are the stator and the rotor. Synchronous motor and induction motor stators are similar in construction. The construction of synchronous motor is similar to that of an alternator (also called synchronous generator). The stator frame contains wrapper plate (except for wound-rotor synchronous doubly fed electric machines). Circumferential ribs and keybars are attached to the wrapper plate. To carry the weight of the machine, frame mounts and footings are required. The synchronous stator winding consists of a 3 phase winding. It is provided with a 3 phase supply, and the rotor is provided with a DC supply.

DC excited motors require brushes and slip rings to connect to the excitation supply. The field winding can be excited by a brushless exciter. Cylindrical, round rotors, (also known as non-salient pole rotor) are used for up to six poles.

In some machines or when a large number of poles are needed, a salient pole rotor is used.

Most synchronous motor construction uses a stationary armature and rotating field winding. This type of construction has an advantage over DC motor type where the armature used is of rotating type.

Operation

Electric motors generate power due to the interaction of the magnetic fields of the stator and the rotor. In synchronous motors, the stator carries 3 phase currents and produces 3 phase rotating magnetic flux (and therefore a rotating magnetic field). The rotor eventually locks in with the rotating magnetic field and rotates along with it. Once the rotor field locks in with the rotating magnetic field, the motor is said to be synched. A single-phase (or two-phase derived from single phase) stator 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 startup arrangements.

Amortisseur winding

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 rotor no longer follows the rotating magnetic field.

Since the motor cannot produce torque if it falls out of synchronization, practical synchronous motors have a partial or complete squirrel-cage damper called an 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 cannot self-start. This property is due to rotor inertia; it cannot instantly follow the rotation of the stator's magnetic field. Since a synchronous motor produces no inherent average torque at standstill, it cannot accelerate to synchronous speed without a supplemental mechanism.

Large motors operating on commercial power include a squirrel-cage induction winding that provides sufficient torque for acceleration and also serves to damp motor speed oscillations. Once the rotor nears the synchronous speed, the field winding becomes 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. Electronically controlled motors can be accelerated from zero speed by changing the frequency of the stator current.

Small synchronous motors are commonly used in line-powered electric mechanical clocks or timers that use the power line frequency to run the gear mechanism at the correct speed. Such small synchronous motors are able to start without assistance if the moment of inertia of the rotor and its mechanical load are sufficiently small. The motor accelerates from slip speed to synchronous speed during an accelerating half cycle of the reluctance torque. Single-phase synchronous motors such as in electric wall clocks can freely rotate in either direction, unlike a shaded-pole type.

Costs are an important parameter for starters. Rotor excitation is a possible way to resolve the issue. In addition, starting methods for large synchronous machines include repetitive polarity inversion of the rotor poles during startup.

Applications, special properties, and advantages

Use as synchronous condenser

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 minimum 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 synchronous 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

${\displaystyle \mathbf{T}={\mathbf{T}}_{\text{max}}\sin (\delta )}$

where,

${\displaystyle \mathbf{T}}$ is the torque

${\displaystyle \delta }$ is the torque angle

${\displaystyle {\mathbf{T}}_{\text{max}}}$ is the maximum torque

here,

${\displaystyle {\mathbf{T}}_{\text{max}}=\frac{\mathbf{3}\mathbf{V}\mathbf{E}}{{\mathbf{X}}_{\mathbf{s}}{\omega }_s}}$

When load is applied, torque angle ${\displaystyle \delta }$ increases. When ${\displaystyle \delta }$ = 90° the torque will be maximum. If load is applied further then the motor will lose its synchronism, since 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 requiring precise speed or position control:

• Speed is independent of the load over the operating range of the motor.
• Speed and position 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 applications (e.g. ball mills).

Subtypes

• AC motor § Polyphase synchronous motor
• Stepper motor (may be synchronous or not)
• Synchronous brushless wound-rotor doubly-fed electric machine