1. EEE223 Energy Conversion II Md. Asif Uddin Khan Lecturer, EEE BRAC University

2. Synchronous Motor Materials taken from: Stephen J. Chapman: Electric Machinery Fundamentals, McGraw-Hill, 5 Th Edition Chapters: 5

3. Synchronous motor: Synchronous motors are synchronous machines used to convert electrical power to mechanical power. They are used to supply power to constant- speed loads. Same as synchronous generator construction, just current flow reversed in the stator winding; and all the basic speed, torque and power relationships for a generator also hold good for the motor.

4. Basic Operation Principles: A three –phase set of voltages V is applied to stator that results in three phase current flow in the windings and a rotating mmf B S. The field current IF produces a steady ‐ state rotor field B R which will tend to line up with stator field. Since the stator magnetic field is rotating, the rotor magnetic field (and the rotor itself) will constantly chase it but never quite catch up with it i.e. will maintain an angle δ termed torque angle. The larger the angle between the two fields (up to a certain maximum), the greater is the torque developed on the rotor of the machine.

5. Equivalent Circuit of a Synchronous Motor: A synchronous motor is the same in all respects as a synchronous generator, except that the direction of power flow is reversed. Therefore, the equivalent circuit of a synchronous motor is exactly the same as the equivalent circuit of a synchronous generator, except that the reference direction of I A is reversed.

6. The Synchronous Motor Torque-Speed Characteristic Curve: Synchronous motors are usually connected to power systems which are very much larger than individual motors i.e. appear as constant voltage source to the motors. So the terminal voltage and system frequency will be constant regardless of the power drawn by the motors. The motor’s speed will be locked to this constant frequency so that it will remain constant regardless of the load up to a limit termed pullout torque. If I F ↑ then E A ↑ so that pullout torque also increases i.e. motor’s stability limit increases If loading exceeds pullout torque the locking between rotor and stator fields is lost eventually resulting in vibration of the whole motor and loss of synchronism. δ≈20° at full load so that is about one ‐ third of pullout torque

7. Effects of load change at constant supply voltage and field excitation: The rotor initially slows down with increase in shaft load so that torque angle δ increases and hence induced torque increases. This recovers back the synchronous speed provided the load increase is below the stability limit. As mechanical load increases, I A ↑ and becomes leading to lagging, δ ↑, P in ↑ while frequency f, supply voltage Vφ, motor’s field current I f and hence E A remains constant

8. A 208-V, 45-kVA, O.8-PF-leading, ∆ -connected, 60-Hz synchronous machine has a synchronous reactance of 2.5 Ω and a negligible armature resistance. Its friction and windage losses are 1.5 kW, and its core losses are 1.0 kW. Initially, the shaft is supplying a 15-hp load, and the motor's power factor is O.8O leading. (a) Sketch the phasor diagram of this motor, and find the values of IA, I L and E A. (b) Assume that the shaft load is now increased to 30 hp. Sketch the behavior of the phasor diagram in response to this change. (c) Find I A, I L and E A after the load change. What is the new motor power factor?

9. Effects of field current change at constant supply voltage and load (i.e. power input): As field current I f ↑ E A ↑ but δ ↓ since Vφ and P in is to remain constant and armature current I A ↑ and becomes lagging to Leading while passing through unity power factor when I A becomes minimum.

10. Synchronous Motor V curve: At low field current (under excitation) a synchronous motor acts as a lagging(inductive) load and with increase of I f as unity pf (resistive) and with further increase as leading (capacitive) load. This phenomenon can be used to improve power factor of a plant. Also over excitation increases stability limit i.e. pullout torque. However, I f can not be increased beyond the heating limit of the field winding.

11. The 208-V, 45-kVA, O.8-PF-leading, ∆ -connected, 60-Hz synchronous motor of the previous example is supplying a 15-hp load with an initial power factor of 0.85 PF lagging. The field current I F at these conditions is 4.0 A. (a) Sketch the initial phasor diagram of this motor, and find the values I A and E A. (b) If the motor's flux is increased by 25 percent, sketch the new phasor diagram of the motor. What are E A, I A, and the power factor of the motor now?

12. two cases?

13. The Synchronous Motor and Power-Factor Correction: The use of synchronous motors or other equipment to increase the overall power factor of a power system is called power-factor correction. The ability to adjust the power factor of one or more loads in a power system can significantly affect the operating efficiency of the power system. The lower the power factor of a system, the greater the losses in the power lines feeding it. Most loads on a typical power system are induction motors, so power systems are almost invariably lagging in power factor. Having one or more leading loads (overexcited synchronous motors) on the system can be useful for the following reasons: A leading load can supply some reactive power Q for nearby lagging loads, instead of it coming from the generator. Since the reactive power does not have to travel over the long and fairly high-resistance transmission lines, the transmission line current is reduced and the power system losses are much lower. Since the transmission lines carry less current, they can be smaller for a given rated power flow. A lower equipment current rating reduces the cost of a power system significantly. In addition, requiring a synchronous motor to operate with a leading power factor means that the motor must be run overexcited. This mode of operation increases the motor 's maximum torque and reduces the chance of accidentally exceeding the pullout torque.

14. The Synchronous Capacitor or Synchronous Condenser: An overexcited synchronous motor at no load looks just like a large capacitor to the power system. Hence, it can be used for power-factor correction in a power system. At some times in the past some synchronous motors were purchased and run without a load, simply for power factor correction. These machines had shafts that did not even come through the frame of the motor- no load could be connected to them even if one wanted to do so. Such special- purpose synchronous motors were often called synchronous condensers or synchronous capacitors. Today, conventional static capacitors are more economical to buy and use than synchronous capacitors. However, some synchronous capacitors may still be in use in older industrial plants.

15. Starting Problems in a Synchronous Motor: A synchronous motor has no net starting torque and so cannot start by itself. The rotor is already energized by DC. The moment power is supplied to stator of the motor, the rotor of the motor and hence the rotor magnetic field B R are stationary. But stator magnetic field Bs is starting to sweep around the air gap at synchronous frequency speed (here f = 60 Hz considered). During one electrical cycle, the torque is first CCW and then CW, and the average torque over the complete cycle is zero. As a result the motor vibrates heavily with each electrical cycle and finally overheats.

16. Three basic approaches can be used to safely start a synchronous motor: 1. Use of low frequency during starting: Here stator is initially supplied a frequency much less than 50 or 60 Hz using an inverter so that stator field sweeps slowly and the rotor field can chase and lock in with it during one half cycle of stator field’s rotation. 2. Run first as generator and then as motor: Here the synchronous motor is rotated using an external prime mover up to synchronous speed and paralleled with the utility supply to run as a generator. Then the prime mover is declutched from the motor shaft so that it will now take power supply from the utility and run as motor. 3.Use of a third winding termed amortisseur or damper or starting winding : This is the most widely used method. Here a separate winding in the form of special bars are laid into notches carved with the ends shorted by a large ring in the rotor pole face. The following steps are to be followed. The field windings are disconnected from their dc power source and shorted out. A three phase voltage is applied to the stator of the motor, and the rotor accelerates up to near synchronous speed. The motor should have no load on its shaft, so that its speed can approach n sync as closely as possible. The dc field circuit is connected to its power source. After this is done, the motor will lock into step at synchronous speed, and loads may then be added to its shaft.

17. As the field windings are disconnected from the DC power source, B R will be zero. The three-phase set of voltages applied to the stator windings develops a rotating magnetic field, B S. As the magnetic field B S sweeps along in a counterclockwise direction, it induces a voltage in the bars of the damper winding given by, = ( × ) ∙ The induced voltage produces a current flow in the damper windings resulting in a winding magnetic field Bw. Hence, induced torque on the bars (and the rotor) is given by: = ×