SYNCHRONOUS MOTOR

 SYNCHRONOUS MOTOR 

Electrical motor in general is an electro-mechanical device that converts energy from electrical domain to mechanical domain. Based on the type of input we have classified it into single phase and 3 phase motors. Among 3 phase motors, induction motors and synchronous motors are more widely used. When a 3 phase electric conductors are placed in a certain geometrical positions (In certain angle from one another) then an electrical field is generated. Now the rotating magnetic field rotates at a certain speed, that speed is called synchronous speed. Now if an electromagnet is present in this rotating magnetic field, the electromagnet is magnetically locked with this rotating magnetic field and rotates with same speed of rotating field.

First we will discuss about Synchronous Motor:

Synchronous motors is called so because the speed of the rotor of this motor is same as the rotating magnetic field. It is basically a fixed speed motor because it has only one speed, which is synchronous speed and therefore no intermediate speed is there or in other words it’s in synchronism with the supply frequency. Synchronous speed is given byWhere, f = supply frequency and p = no. of poles

Synchronous motors are used where the constant speeds are required. eg timer, papermill, taperecorders.
Synchronous motors are work on leading and unity power factors.

Main parts of Synchronous Motor

The synchronous machine is used widely in industry both as a generator for power generation as well as a motor for driving industrial equipment such as conveyors, pumps, crushers etc.

The synchronous machine has the following parts

Stator

The Stator of the Synchronous Machine consists of the Core.  The core is made of electrical steel.  It is made in the form of insulated laminations which are stacked together.  This prevents the flow of eddy currents in the core.  The core has slots.  The windings of the core are placed in the slots. The windings are made of copper.  The winding are preformed and placed in the slots.

Rotor

The rotor of the Synchronous Generator consists of a number of poles.  The number of poles depends on the speed and frequency of the machine.  The relation between the number of poles and speed or the frequency is N=120 x f /p ( where N is the speed in rpm, f is the frequency and p is the number of poles).

The rotor of the synchronous machine can be either salient or non-salient in construction.  In salient pole rotors, the poles are protruding from the rotor while in non-salient pole construction, the rotor windings are placed in slots machined in the rotor (See article on Salient and Non-salient Rotor Construction).

Excitor

The excitor can be imagined to be a small generator placed in the rotor.  The excitor provides the excitation power for the the excitation.  The excitor consists of a field winding in its stator.  The armature winding of the excitor is placed in the the rotor of the machine. 

Rotating Rectifiers

The output of the excitor is a 3 phase AC supply.  This supply is rectified by means of the rotating rectifiers which are fitted on the rotor shaft.  This rectifier assembly converts the AC power into DC.  The rectified DC is then supplied to the main rotor windings. 

Operation

When the machine is first started, residual magnetism in the excitor field winding induces a voltage in the excitor stator.  This 3 phase supply is rectified and fed to the field windings of the main rotor poles.  These poles get magnetized and induce the output voltage in the stator.

The control of the machine output voltage is done by a device called the AVR ( Automatic Voltage Regulator).  The output voltage of the machine is connected through a potential transformer to the AVR.  The AVR then regulates the excitation input based on the feedback received.  This a closed loop control.

Construction:

Normally it's construction is almost similar to that of a 3 phase induction motor, except the fact that the rotor is given DC supply, the reason of which is explained later. Now, let us first go through the basic construction of this type of motor.
From the above picture, it is clear that how this type of motors are designed. The stator is given is given three phase supply and the rotor is given DC supply.

Some important facts about Synchronous Motor:

• Synchronous motors are inherently not self starting. They require some external means to bring their speed close to synchronous speed to before they are synchronized.
• The speed of operation of is in synchronism with the supply frequency and hence for constant supply frequency they behave as constant speed motor irrespective of load condition
• This motor has the unique characteristics of operating under any electrical power factor. This makes it being used in electrical power factor improvement.

Principle of Operation Synchronous Motor:

Synchronous motor is a doubly excited machine i.e two electrical inputs are provided to it. It’s stator winding which consists of a 3 phase winding is provided with 3 phase supply and rotor is provided with DC supply. The 3 phase stator winding carrying 3 phase currents produces 3 phase rotating magnetic flux. The rotor carrying DC supply also produces a constant flux. Considering the frequency to be 50 Hz, from the above relation we can see that the 3 phase rotating flux rotates about 3000 revolution in 1 min or 50 revolutions in 1 sec. At a particular instant rotor and stator poles might be of same polarity (N-N or S-S) causing repulsive force on rotor and the very next second it will be N-S causing attractive force. But due to inertia of the rotor, it is unable to rotate in any direction due to attractive or repulsive force and remain in standstill condition. Hence it is not self starting.
To overcome this inertia, rotor is initially fed some mechanical input which rotates it in same direction as magnetic field to a speed very close to synchronous speed. After some time magnetic locking occurs and the synchronous motor rotates in synchronism with the frequency.

Methods of Starting of Synchronous Motor:

• Motor starting with an external prime Mover: Synchronous motors are mechanically coupled with another motor. It could be either 3 phase induction motor or DC shunt motor DC excitation is not fed initially. It is rotated at speed very close to its synchronous speed and after that DC excitation is given. After some time when magnetic locking takes place supply to the external motor is cut off.
• Damper winding: In case, synchronous motor is of salient pole type, additional winding is placed in rotor pole face. Initially when rotor is standstill, relative speed between damper winding and rotating air gap flux in large and an emf is induced in it which produces the required starting torque. As speed approaches synchronous speed, emf and torque is reduced and finally when magnetic locking takes place, torque also reduces to zero. Hence in this case synchronous is first run as 3 phase induction motor using additional winding and finally it is synchronized with the frequency.

Application of Synchronous Motor

• Synchronous motor having no load connected to its shaft is used for power factor improvement. Owing to its characteristics to behave at any electrical power factor, it is used in power system in situations where static capacitors are expensive.
• Synchronous motor finds application where operating speed is less (around 500 rpm) and high power is required. For power requirement from 35 kW to 2500 KW, the size, weight and cost of the corresponding three phase induction motor is very high. Hence these motors are preferably used. Ex- Reciprocating pump, compressor, rolling mills etc.

Phasor diagrams of Synchronous motor

We will discuss here the simplest way of drawing the phasor diagram for synchronous motor and we will also discuss advantages of drawing the phasor diagram. Before we draw phasor diagram, let us write the various notations for each quantity at one place. Here we will use:
E_f to represent the excitation voltage
V_t to represent the terminal voltage
I_a to represent the armature current
Θ to represent the angle between terminal voltage and armature current
ᴪ to represent the angle between the excitation voltage and armature current 
δ to represent the angle between the excitation voltage and terminal voltage
r_a to represent the armature per phase resistance.

We will take V_t as the reference phasor in order to phasor diagram for synchronous motor. In order to draw the phasor diagram one should know these two important points which are written below:
(1) We know that if a machine is made to work as a asynchronous motor then direction of armature current will in phase opposition to that of the excitation emf.
(2) Phasor excitation emf is always behind the phasor terminal voltage.
Above two points are sufficient for drawing the phasor diagram for synchronous motor. The phasor diagram for the synchronous motor is given below,

In the phasor one the direction of the armature current is opposite in phase to that of the excitation emf.

It is usually customary to omit the negative sign of the armature current in the phasor of the synchronous motor so in the phasor two we have omitted the negative sign of the armature current. 

Prior to understanding this synchronous motor excitation, it should be remembered that any electromagnetic device must draw a magnetizing current from the AC source to produce the required working flux. This magnetizing current lags by almost 90^o to the supply voltage. In other words, the function of this magnetizing current or lagging VA drawn by the electromagnetic device is to set up the flux in the magnetic circuit of the device.The synchronous motor is doubly fed electrical motor Synchronous converts electrical energy to mechanical energy via magnetic circuit. Hence, it comes under electromagnetic device. It receives 3 phase AC electrical supply to its armature winding and DC supply is provided to rotor winding. Synchronous motor excitation refers to the DC supply given to rotor which is used to produce the required magnetic flux.
One of the major and unique characteristics of this motor is that it can be operated at any electrical power factor leading, lagging or unity and this feature is based on the excitation of the synchronous motor.

When the synchronous motor is working at constant applied voltage V, the resultant air gap flux as demanded by V remains substantially constant. This resultant air gap flux is established by the co operation of both AC supply of armature winding and DC supply of rotor winding.
CASE 1: When the field current is sufficient enough to produce the air gap flux, as demanded by the constant supply voltage V, then the magnetizing current or lagging reactive VA required from ac source is zero and the motor operate at unity power factor. The field current, which causes this unity power factor is called normal excitation or normal field current.
CASE 2: If the field current is not sufficient enough to produce the required air gap flux as demanded by V, additional magnetizing current or lagging reactive VA is drawn from the AC source. This magnetizing current produces the deficient flux (constant flux- flux set up by dc supply rotor winding). Hence in this case the motor is said to operate under lagging power factor and the is said to be under excited.
CASE 3: If the field current is more than the normal field current, motor is said to be over excited. This excess field current produces excess flux (flux set up by DC supply rotor winding – resultant air gap flux) which must be neutralized by the armature winding. Hence the armature winding draws leading reactive VA or demagnetizing current leading voltage by almost 90^o from the AC source. Hence in this case the motor operate under leading power factor.
This whole concept of excitation and power factor of synchronous motor can be summed up in the following graph. This is called V curve of synchronous motor.

NOTE: An overexcited synchronous motor operate at leading power factor, under-excited synchronous motor operate at lagging power factor and normal excited synchronous motor operate at unity power factor.

HUNTING

We come across the term HUNTING when we study about 3 phase synchronous motor operation. The word hunting is used because after sudden application of load the rotor has to search or hunt for its new equilibrium position. That phenomenon is referred to as hunting in synchronous motor. Now let us know what is the condition of equilibrium in synchronous motor.
A steady state operation of synchronous motor is a condition of equilibrium in which the electromagnetic torque is equal and opposite to load torque. In steady state, rotor runs at synchronous speed thereby maintaining constant value of torque angle (δ). If there is sudden change in load torque, the equilibrium is disturbed and there is resulting torque which changes speed of the motor.

Understanding about HUNTING

Unloaded synchronous machine has zero degree load angle. On increasing the shaft load gradually load angle will increase. Let us consider that load P₁ is applied suddenly to unloaded machine shaft so machine will slow down momentarily.
Also load angle (δ) increases from zero degree and becomes δ₁. During the first swing ELECTRICAL POWER developed is equal to mechanical load P₁. Equilibrium is not established so rotor swings further. Load angle exceeds δ₁ and becomes δ₂. Now electrical power generated is greater than the previous one. Rotor attains synchronous speed. But it does not stay in synchronous speed and it will continue to increase beyond synchronous speed. As a result of rotor acceleration above synchronous speed the load angle decreases. So once again no equilibrium is attained. Thus rotor swings or oscillates about new equilibrium position. This phenomenon is known as hunting or phase swinging. Hunting occurs not only in synchronous motors but also in synchronous generators upon abrupt change in load.

Causes of Hunting in Synchronous Motor

1. Sudden change in load.
2. Sudden change in field current.
3. A load containing harmonic torque.
4. Fault in supply system.

Effects of Hunting in Synchronous Motor

1. It may lead to loss of synchronism.
2. Produces mechanical stresses in the rotor shaft.
3. Increases machine losses and cause temperature rise.
4. Cause greater surges in current and power flow.
5. It increases possibility of resonance.

Reduction of Hunting in Synchronous Motor

Two techniques should be used to reduce hunting. These are –

Use of Damper Winding: It consists of low electrical resistance copper / aluminum brush embedded in slots of pole faces in salient pole machine. Damper winding damps out hunting by producing torque opposite to slip of rotor. The magnitude of damping torque is proportional to the slip speed.

Use of Flywheels: The prime mover is provided with a large and heavy flywheel. This increases the inertia of prime mover and helps in maintaining the rotor speed constant.

• Designing synchronous machine with suitable synchronizing power coefficients.