Speed Control of Synchronous Motor

Torque Production & Control of Speed in Synchronous Motor.
Speed of synchronous motors can be controlled using two methods called open loop and close loop control.
Open loop contol is the simplest scalar control method where motor speed is controlled by independent frequency control of the converter.
In case of close loop self control mode, instead of controlling the inverter frequency independentaly, the frequency and the phase of the output waveform are controlled by an absolute position encoder mounted on the machine shaft giving an account of position of the rotor.

1. ASHUTOSH JHA CUJ/I/2015/IEE/016 Centre For Energy Engineering Torque Production & Control of Speed in Synchronous Motor

2. Torque Production in Synchronous Motor A synchronous machine, as its name suggests, must rotate at synchronous speed; that is the speed is uniquely related to supply frequency using the following equation , There may be several types of synchronous machine depending on construction and type of field excitation used. Wound Field Machine Cylindrical Rotor Machine (Non-Salient Pole Machine) Salient Pole Machine Synchronous Reluctance Machine Permanent Magnet Machine Sinusoidal Surface Permanent Magnet Machine (SPM machine) Sinusoidal Interior Permanent Magnet Machine (IPM machine )

3. Non-Salient Cylindrical Pole Machine Torque equation of non salient pole machine is given as: T e can also be written as: Where, ѱ S = stator flux and, I T = I S cos ϕ is the in phase or torque component of stator current

4. According the torque eq n , if δ = 0 then, T e = 0 and if δ = ±π/2 then T e is maximum. Stability consideration dictates that machine should be oparated with δ angle within ±π/2 .

5. Salient Pole Synchronous Machine

6. The asymmetry in the direct and the quadrature axes magnetizing reactances causes the corresponding synchronous reactances to be unsymmetrical i.e., X ds ≠ X qs . The torque equation comes out to be: First term is contributed by field flux and is identical to torque equation of cylindrical rotor machine. The second term arises due to rotor saliency (i.e., X ds ≠ X qs ). Generally, L ds > L qs hence this is an additive component.

7. Speed Control of Synchronous Machine Drive In a synchronous machine drive, the speed of the machine is uniquely related to the frequency supplied by the inverter or cycloconverter. There are essentially two control modes for synchronous machine drive: Open loop, true synchronous mode Close loop, self control mode In open loop contol mode, the motor speed is controlled by independent frequency control of the converter. In the close loop self control mode, the variable frequency converter control pulses are derived from an absolute rotor position encoder mounted on the machine shaft.

8. Open loop Control This is the simplest scalar control method of synchronous machine. This method is particularly popular in multiple synchronous reluctance or PM machine drive, where close speed tracking is essential among a number of machines for application such as fibre spinning mills. In this method, all the machines are connected to the same inverter so that the move in synchronism corresponding to command frequency ω e * at the input. The phase voltage command V s * is generated through a function generator (FG), where the voltage is essentially maintained proportional to frequency (i.e., = constant) so that stator flux ψ s remains constant. The front end of voltage fed PWM inverter is supplied from the utility line the diode rectifier and the LC fiter. The machine is built with damper winding to prevent oscillatory behaviour during transient response.

9. Open loop Control This is the simplest scalar control method of synchronous machine. This method is particularly popular in multiple synchronous reluctance or PM machine drive, where close speed tracking is essential among a number of machines for application such as fibre spinning mills.

10. In this method, all the machines are connected to the same inverter so that they move in synchronism correspon-ding to command frequency ω e * at the input. The phase voltage command V s * is generated through a function generator (FG), where the voltage is essentially maintained proportional to frequency (i.e., = constant) so that stator flux ψ s remains constant. The front end of voltage fed PWM inverter is supplied from the utility line the diode rectifier and the LC fiter. The machine is built with damper winding to prevent oscillatory behaviour during transient response.

11. Assume for simplicity that load torque T L in machine was initially zero. Machine is started from standstill at point 0 to point A by increasing the frequency slowly. At steady state, T e = T L , the operating point will move vertically along AB in first quadrant. Operating point can be changed from B to C by slowly increasing the frequency command. It can be brought back to point D by gradually decreasing T L . At base speed ω b , voltage V s will saturate. Beyond this point, machine enters into field weakeening mode, therefore, ψ s decreases and hence toqrue T e also decreases. Figure showing the performance of the drive indicating motoring as well as braking mode in forward direction

12. Rate of change of ω e can be given as, Where, J = moment of inertia ω e = P/2 ω m is the synchronous electrical speed (rad/sec) P = number of poles At point A, if ω e * is ramped up, the developed T e will jump to point B and the machine will accelerate along the line BC until steady state is reached at point D. Similarly, the profile during deccleration will be D-E-F-A. The recovered electrical energy in decceleration is dissipated in the dynamic brake (DB) installed in the dc link. Speed reversal is possible by reversing the phase sequence of the inverter.

13. Absolute Position Encoder Synchronous machines have absolute position of rotor magnetic poles, therefore, an absolute position encoder is mandatory in self contol mode. In general, position encoder can be classified as: Optical-type and Resolver-type Optical Encoder Digital information about rotor position can be obtained directly from a coded disk that transmitts or interrupts the light beam. The figure given below shows a slotted (coded) disk. The encoder shown here has been designed for a four-pole machine. The disk has a large number of slots on the outer perimeter but two slots of π/2 angle at inner radius. There are four stationary sensors S 1 , S 2 , S 3 and S 4 . Sensor S 4 is mounted on outer perimeter and S 1 , S 2 and S 3 are at inner radius with π/3 angle spacing as shown in the fig.

14. A sensor is nothing but a pair of LED and photo-trnasistor pair. logic 1 is generated when the sensor is within the slot. Sensors S 1 and S 3 generate square wave at 2π/3 angle phase shift, and S 4 generate a high frequency pulse train. To understand absolute rotor position detection, consider that only S 1 and S 4 sensors are present, and the S 1 leading edge is aligned to the zero position of the rotor. A pulse at the leading edge of the S 1 wave can reset and trigger an “up” counter, which counts the pulses generated by S 4 . .

15. The counter resets at 360° by another pulse of S 1 . If the number of slots on the perimeter is 360, the mechanical angle resolution is 1°, i.e., 2° electrical for a four-pole machine.

16. Analog Resolver with Decoder This is a mechanical position encoder and therefore more robust than optical encoder. It has two parts: an analog resolver and a resolver-to-digital converter (RDC). The rotor winding of the resolver is excited by a revolving transformer whose primary receives the signal from an oscillator. The stator winding of the resolver give the amplitude modulated outputs given by V 1 = AV sinωt sinθ V 2 = AV sinωt cosθ Where, ω = oscillator frequency V = oscillator voltage A = effective transformation ratio θ = electrical orientation angle of the rotor excitation winding The analog output signals can be used directly or they can be processed through an RDC to obtain the digital signal

17. Vector Control For a non-salient sinusoidal SPM machine, Synchronous inductance L s and corresponding armature flux I s L s (=ψ a ) becomes negligible. then, ψ f = ψ s + ψ a ≈ ψ s Since, T e = For maximum torque sensitivity with stator current, we can set I ds = 0 and I qs = (as shown in phasor) then, T e = Where, is space vector magnitude (√2 ψ f ) and ψ s cosϕ = ψ s sinδ From the equation of T e it is clear that T e is directly proportional to I qs and p.f. angle is equal to torque angle (ϕ=δ).

18. In above block diagram, stator current i qs * is derived from speed control loop. Its polarity is positive for motoring mode and negative for regenerative mode. Rotating frame signal are converted to stator current command with the help of unit vector signal (cosθ e , sinθ e ). Magnetizing current i ds * = 0 because rotor flux is supplied by PM. Unit vector is generated from an absolute position sensor because poles are fixed on rotor.

19. Close loop Self Control Mode The stator frquency in this case is not independent of the rotor speed as in the case of open loop control. In this mode, the stator frequncy is decided by the rotor speed and therefore motor controls itself. Frequency and phase of the output wave are controlled by an absolute position encoder mounted on the machine shaft. Advantages of close loop contol mode: Since stator frequency is decided by rotorspeed hence, it cannot fall out of state w.r.t. the rotor. The rotor frequency and the stator frequency are locked hence, any possibility of hunting is ruled out. Since frequecy can be varied hence, such drive can be employed for variable speed application (without any fear of losing synchronism)

20. V a = V f + jI a X s + I a R a Phase angle of current I a w.r.t. phase-a axis = θ r + π/2 + γ From the phasor, V a cosθ = V f cosγ + I a R a Hence, V R I I – R dc I I 2 = 3V a I a cosϕ =3 I a ( V f cosγ + I a R a )

21. The inverter act as a current source due to large inductance in DC link. The fundamental component of motor phase current is shown. Using the Fourier series anlysis, I a ≈ I a1(rms) =  I I = I a Finally we get, This equation is used to derive the steady state equivalent circuit. Fig. Steady state equivalent circuit (per phase)

22. This circuit resembles that of a d.c.machine where there is a resistance and a back emf in the armature and the field is separately excited. Here as well, we can keep the field current constant and if we want to change the speed, we can change the inverter voltage V R . If V R increases, I a increases hence, there will be more torque and speed will increase. Output torque is given by T e = = If γ and field current (and hence cosγ and  af ) are kept constant then, torque can be controlled by varying the armature current I a .

23. C onclusion Speed of synchronous motors can be controlled using two methods called open loop and close loop control. Open loop contol is the simplest scalar control method where motor speed is controlled by independent frequency control of the converter. In case of close loop self control mode, instead of controlling the inverter frequency independentaly, the frequency and the phase of the output waveform are controlled by an absolute position encoder mounted on the machine shaft giving an account of position of the rotor.

24. Refrences P. Pillay (Ed.), Performance and Design of Permanent Magnet AC Motor Drives , IEEE-IA Society Tutorial Course, 1989 B. K. Bose, Modern Power Electronics and AC Drives , Prentice Hall, Upper Saddle River, NJ, 2002 D.P.Kothari, I.J.Nagrath, Electric Machines e/4  , Mc Graw Hill, 2010 Bimal Kumar Bose, Power Electronics and Motor Drives Advances and Trends, Academic Press, 2006 S.P.Das, Advanced Electric Drives (Video), http://nptel.ac.in/courses/108104011