Chapter 6 Synchronous Motors Edit by Chi-Shan Yu Electric Machinery

Synchronous Machine Electric Machinery

Chapter 6 Synchronous Motors Basic Principles of Motor Operation Steady-State Synchronous Motor Operation Starting Synchronous Motors Synchronous Generators and Synchronous Motors Synchronous Motor Rating Summary Electric Machinery

Basic Principles of Motor Operation Two dependent magnetic fields exist in the machine The dc field current IF produces a rotor field BR The three-phase ac current produce a stator field BS Electric Machinery

The equivalent circuit The equivalent circuit of a synchronous motor is the same as the equivalent circuit of a synchronous generator. (Besides, the current flow direction is reverse) Since the current flow direction is reverse, the relation between the terminal and internal voltage is Electric Machinery

The phasor diagram of a synchronous generator – with lagging power factor load The induced torque is opposing the applied torque from prime mover Electric Machinery

The phasor diagram of a synchronous motor The induced torque drives the motor rotation. Electric Machinery

Steady-state synchronous motor operation – Torque speed curve The steady-state speed is constant from no-load to full-load. Electric Machinery

The maximum torque or pull-out torque While the voltages are constant, the induced torque of the synchronous motor only depends on the power angle d. Thus, the pull-out torque is maximum induced torque. That is, d = 90 degrees. Electric Machinery

The effect of load changes on a synchronous motor (what is the meaning of pull-out torque) At steady-state the torque that the synchronous motor supports is equal to the load torque. When the load torque increases ? The motor will slow down (rotor mechanical speed) The stator rotation flux still remain its constant speed (stator flux speed) Thus, the power angle d increases Finally, the synchronous motor’s will generate a large torque that is equal to the load torque. However, if the maximum induced torque is still lower than the load torque ? Electric Machinery

The phasor diagram of increasing load torque When the load increases, all the armature current IA, power angle d, and the power factor angle q are increase (from leading to lagging). Electric Machinery

Example 6-1 Electric Machinery

The effect of field current changes on a synchronous motor An increase in field current increase the magnitude of EA but does not affect the real power supplied by the motor. Why ? The power supply by the motor is (Pm = w×tload) The terminal voltage Vf supplies to the motor is constant Electric Machinery

The phasor diagram – increase the field current Electric Machinery

What is the effect of increasing field current What is the meaning of the power factor of the motor changes from lagging to leading can support Q to the electrical system The motor is now acting like a capacitor-resistor combination load, and the magnitude of the capacitor can be changed by the field current Electric Machinery

Synchronous motor V curve A plot of IA versus IF for a synchronous motor is the V curve. Electric Machinery

Underexcited and overexcited The internal voltage can be smaller or larger than the terminal voltage Electric Machinery

Example 6-2 Electric Machinery

Homework - 2 Use the MATLAB to complete the Figure 6.12 Electric Machinery

EX 6-3 Power factor correction Why the power factor is so important in power system ? Electric Machinery

EX 6-3 Power factor correction Electric Machinery

The reason for power factor correction 4. Meanwhile, the harmonic content is also important reason Electric Machinery

The synchronous capacitor or synchronous condenser A synchronous motor can be operated overexcited to supply reactive power Q for a power system. No real power have been drawn from the load and the shaft of the motor is removed Electric Machinery

The V curve of the synchronous capacitor Electric Machinery

Starting the synchronous motors How to start the synchronous motor ? What is the problem of the synchronous motor in starting? Electric Machinery

The problem in starting The induced torque between rotor flux and stator flux The directions of the induced torque depends on the angle relation between the BS and BR Electric Machinery

The problem in starting At starting, the motor speed is increased from zero speed The speed of the rotor is slower than the speed of the stator flux Electric Machinery

The problem and how to solve The motor will vibrate but not rotate !! Electric Machinery

Reducing the stator electrical frequency At starting, the motor operates at a low enough speed. The speed of the stator magnetic field can then be increased gradually up to 60Hz Notably The stator voltage must also be reduced to prevent the over-current in stator winding. (EA = Kfw) The power electronics can build the inverter to achieve the variable frequency and voltage drive Electric Machinery

Motor starting by using damper windings The damper or amortisseur windings are special bars laid into notches carved in the face of a synchronous motor’s rotor, and then shorted out on each end by a large shorting ring. Electric Machinery

The simplified diagram of the damper winding The rotor shows an damper winding with the shorting bars on the ends of the two rotor pole faces connected by wires Electric Machinery

The reason of adding the damper windings At starting, the bars of damper winding are shorted, and the field winding is disconnected from the field voltage The motor will act as an induction motor Electric Machinery

Starting the synchronous motor by damper winding Electric Machinery

The effect of damper windings on motor stability The damper windings speeds up slow machines and slows down fast machines Thus, the stability can be increased by adding the damper windings Electric Machinery

The differences between the synchronous generators and the synchronous motors The differences between them are the direction of the induced torque and the angle relation between the internal and terminal voltage Electric Machinery

Phasor diagram

Synchronous motor rating Electric Machinery

Recap The synchronous speed The field current and the power factor of the synchronous motor Synchronous condenser The starting problem of the synchronous motor Electric Machinery

Appendix C Salient-Pole Theory of Synchronous Machines Edit by Chi-Shan Yu Electric Machinery

Why we discuss the salient-pole effect ? Salient pole rotor and cylindrical rotor Electric Machinery

Until now the result is The following result is only valid for cylindrical rotor The salient pole rotor has another reluctance torque Electric Machinery

The effect of armature reaction in a salient-pole synchronous generator The rotor magnetic field and the induced voltage on the stator conductor Electric Machinery

The effect of armature reaction in a salient-pole synchronous generator If a lagging load is connected to the terminals of this generator Electric Machinery

The effect of armature reaction in a salient-pole synchronous generator The stator magnetic field is no more 90 degrees behind the armature current Electric Machinery

The stator voltage Each component of the stator magnetic field produces a voltage in the stator winding by armature reaction. The total voltage in the stator is thus Electric Machinery

Now we may include the armature self- resistance and reactance The armature self-reactance is independent of the rotor angle Electric Machinery

How to plot the phasor diagram ? The armature current IA is broken into Id and Iq by angle d+q Usually, the torque angle d is unknown, and the power factor angle is q known. Without the knowledge of d, how to plot the phasor diagram ? Once the angle d is known, the armature current IA can be broken into Id and Iq. Electric Machinery

How to plot the phasor diagram ? The EA’’ has the same angle as the EA Electric Machinery

Example C -1 Electric Machinery

Torque and Power equation of salient-pole machine The power produced by d and q axis current Electric Machinery

Power equation The power is The d-axis current is The q-axis current is Electric Machinery

Power equation The power is Electric Machinery

Torque equation Electric Machinery