UNIT-II

SYNCHRONOUS MACHINES-IISynchronous Motors

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BY:Mohammad ImranASST. PROFESSOR-EEJahangirabad Educational Trust Group of Institutions, Barabanki

SYNCHRONOUS MOTOR

• INTRODUCTION: ELECTRICAL MOTOR in general is an electro-

mechanical device Which converts electrical energy to mechanical energy. Based on the type of input motors are classified into single phase and 3-phase motors. Among them 3 Phase induction motor and synchronous motors are more widely used.

Principle of Operation• When a 3 phase electric conductors are

placed at a certain geometrical positions there is an electrical field generated.

• The rotating magnetic field rotates at a certain speed, that speed is called synchronous speed.

• 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.

• It is called synchronous because the speed of the rotor of this motor and rotating magnetic field are same.

• It is basically a fixed speed motor because it has only one speed, which is synchronous speed and therefore no intermediate speed .other words it’s in synchronism with the supply frequency.

• Synchronous speed is given by :

Conti...

MAIN FEATURES

1. 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.

2. The speed of operation of it is in synchronism with the supply frequency and hence for constant supply frequency they behave as constant speed motor irrespective of load condition

3. This motor has the unique characteristics of operating under any electrical power factor

Methods of starting the Motor

There are 03 methods of starting:1. Using a 3-phase cage induction motor or

pony motor2. Using damper winding3. By operating the pilot exciter as a DC motor

Methods of starting the Motor

• Synchronous motors are mechanically coupled with another motor. It could be either DC SHUNT MOTOR or 3 Phase Induction 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.

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Effect of Field Change (Load constant)

Note: Er same as Ef

Va same as Vt

Ra has been neglected

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V curves

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Conclusion for effect of field change with constant load

•For motor with increased excitation power factor becomes leading.

•For motor with decreased excitation power factor becomes lagging.

•For generator with increased excitation power factor becomes lagging.•For generator with decreased excitation power factor becomes leading.

•Unloaded overexcited synchronous motors are sometimes used to improve power factor. They are known as synchronous condensers.

Equilibrium

• A steady state operation of synchronous motor is a condition of equilibrium in which the electromagnetic torque is equal and opposite to load torque.

• If there is sudden change in load torque , the equilibrium is disturbed.

• Then the resulting torque will change the speed.

Hunting

• The word hunting is used because after sudden application of load the rotor has to search or hunt for it’s equilibrium position. That phenomenon is known as Hunting in synchronous motor.

Causes of Hunting

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

Effects of Hunting

• It may lead to loss of synchronism.• Produces mechanical stresses at the rotor

shaft.• Increases machine losses and causes temp.

rise.• Causes greater surge in current and power

flow.• Resonance .

Damping

• Damping is the energy dissipation properties of a material or system under a cyclic stress.

• Damping in machine tools basically is derived from two sources.

• The material damping & the interfacial slip damping.• Material damping depends upon the constructional

material.• It’s magnitude is small as compared to total damping.• Interfacial damping depends upon the joints or bolted

joints.

Applications• Synchronous motor having no load connected to its shaft is used for power

factor improvement. Because of its characteristics to behave at any electrical power factor.

• 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 3 phase induction motor is very high. Hence Synchronous motors are preferably used. Example- Reciprocating pump, compressor, rolling mills etc.

• These Motors are also used for Voltage-regulation, where a heavy voltage dip/rise occurs. when a heavy inductive load in put on/off at the end of the long transmission lines.

References

• www.mfg.mtu.edu/cyberman/mactool/machtool/vibration/damping.html

• www.electrical4u.com• Electric Machines by Ashfaq Hussain.• Induction & Synchronous Machines by

Murugesh Kumar vikas publishing huse pvt. Ltd.