Synchronous motor

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Synchronous motor

Electric motor is an electromechanical device that converts electrical energy into mechanical energy. There are two major types of motors based on the type of input: three phase motors and single phase motor. Among three phase motors induction motors and synchronous motors are mostly used. In synchronous motors, the rotating magnetic field rotates at a certain speed which is known as synchronous speed. If an electromagnet is present in this rotating magnetic field, the electromagnet will get magnetically locked with this rotating magnetic field and will rotate at the same speed as the rotating magnetic field. Synchronous motor is so called because the speed of rotation of rotor of this motor is same as that of rotating magnetic field. It is a fixed speed motor because it has only one speed that is the synchronous speed and no intermediate speed. In other words the motor is in synchronism with the supply frequency. Synchronous speed is given by:

Synchronous speed of synchronous motor

Construction of synchronous motor

Normally the construction of synchronous motor is similar to that of 3 phase induction motor except for the fact that in synchronous motor the rotor is given the dc supply. Now firstly we will go through the basic construction of synchronous motor. In the below figure we can see that in the synchronous motor the stator is given three phase supply and the rotor is given dc supply.

Synchronous speed of synchronous motor

Fig (1): Synchronous motor construction.

Operating principle of Synchronous motor

The operation of a synchronous motor is caused due to the interaction between the magnetic field of stator and the magnetic field of rotor. In Synchronous motor there are two electrical inputs. The stator winding (three phase winding) is supplied with 3 phase supply whereas the rotor is supplied with the DC input. The three phase stator windings having the three phase currents produces the 3 phase rotating magnetic flux. The rotor locks in with the rotating magnetic field and rotates along with it. When the rotor locks in with rotating magnetic field, the motor is said to be in synchronisation. A single phase stator winding is possible but in this case the direction of rotation is not fixed and the machine may start in the either direction unless prevented from other starting arrangements.

Once the motor is running, the speed of motor is only dependent on the supply frequency as shown above. When the motor load is increased beyond the breakdown load, the motor goes out of synchronisation and the field winding no longer follows the rotating magnetic field. The motor cannot produce the torque if it falls out of the synchronisation, hence there are partial or complete squirrel cage damper windings to stabilize the operation and facilitate starting. This winding is smaller than that of an equivalent induction motor hence can overheat on long operations and large slip-frequency voltages are induced in the rotor excitation windings, the synchronous motor protection devices senses this condition and interrupts the power supply.

Starting methods of synchronous motor

In synchronous motor, there is inertia of the rotor due to which it cannot instantly follow the rotation of the magnetic field of the stator. Since the synchronous motor produces zero torque when it is still, so it cannot come up to the synchronous speed without some external supplemental mechanism. Large motors operate on commercial power frequency also includes a “squirrel cage” induction windings providing sufficient torque for acceleration and serving to damp the oscillations in motor speed operation. Once the rotor nears the synchronous speed, the field winding is excited resulting in the motor pulling into synchronization. Some very large motor system may include a “pony” motor that accelerates the unloaded synchronous machine before the load is applied. Motors that are electronically controlled can be accelerated from zero speed by changing the frequency of the stator current.

Very small motors are able to start without assistance. That is, only if the moment of inertia of the mechanical load and the rotor is very small. This is so because the motor will accelerate from slip speed to synchronous speed during an accelerating half cycle of the reluctant torque. The sequence of currents in the phase windings determine the direction of rotation of polyphase synchronous motor. Single phase synchronous motors can rotate freely in either direction for example as in electric wall clock until and unless a shaded-pole type. Anti reversing mechanism is used in Synchronous motors in clocks to ensure starting in the correct direction. Line powered electric mechanical clocks or timers use small synchronous motors and use the powerline frequency to run the gear mechanism at correct speed.

Application, special properties and advantages

Synchronous motor can be made to operate at leading, lagging and unity power factor. Normal excitation voltage is the voltage at unity power factor, also the current is minimum at this excitation. Excitation voltage less than normal excitation is called under excitation and excitation voltage more than normal voltage is known as over excitation voltage. The back emf will be generated more than the motor terminal voltage when the motor is over excited which causes the demagnetizing effect due to armature reaction. The V curve of a synchronous machine shows armature current as a function of field current. The armature current first decreases, then reaches a minimum and then increases with increase in the field current. The minimum point is the point where power factor is unity.

V curve of a synchronous machine

Fig (2): V-curve of a synchronous machine.

This ability of Synchronous motors to selectively control power factor can be used for power factor correction of the power system to which the motor is connected. Synchronous motors improved the efficiency of most of the power systems as most power systems have net lagging power factor hence the presence of overexcited synchronous motors moves the system’s net power factor close to unity. Such power factor correction is generally a side effect of motors already present in the system to provide mechanical work. Although the synchronous motors can be attached to the systems without running mechanical load simple for the power factor correction. In factories or large industrial plants the interaction between other lagging loads and synchronous motor is an explicit consideration while designing the plant’s electrical design.

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