School of Electrical Systems Engineering EET 421 Power

School of Electrical Systems Engineering EET 421 Power Electronic Drives – 2) synchronous motor Abdul Rahim Abdul Razak 1 ABD RAHIM 2008

2) SYNCHRONOUS MOTORS School of Electrical Systems Engineering Figure 1: Revolving-field synchronous motor. Synchronous motors have the characteristic of constant speed between no load and full load. Their speed ns (synchronous speed) only depends on AC line frequency f, and the # of motor poles per phase, P. 2 ABD RAHIM 2008

Example 1: School of Electrical Systems Engineering 1) 2) 24 p What pole number would be needed for a synchronous motor to run at a speed of 300 rpm from a 60 Hz supply? 3 ABD RAHIM 2008

SYNCHRONOUS MOTORS Advantages… School of Electrical Systems Engineering Synchronous motors have the following advantages over non-synchronous motors: • Speed is independent of the load, provided an adequate field current is applied. • Accurate control in speed and position (stepper motors). • They will hold their position when a DC current is applied to both the stator and the rotor windings. • Their power factor can be adjusted to unity by using a proper field current relative to the load. Also, a "capacitive" power factor, (current phase leads voltage phase), can be obtained by increasing this current slightly, which can help achieve a better power factor correction for the whole installation. • Their construction allows for increased electrical efficiency when a low speed is required (as in ball mills and similar apparatus). • More efficient compared to induction motor (large industrial scale) 4 ABD RAHIM 2008

SYNCHRONOUS MOTORS They are capable of correcting the low power factor of an inductive loads when they are operated under certain conditions (overexcited). School of Electrical Systems Engineering Often used to drive DC generators or as replacement of a capacitor banks to improve power factor. Synchronous motors are designed in sizes up to thousands of horsepower. They may be designed as either single-phase or multiphase machines. 5 ABD RAHIM 2008

SYNCHRONOUS MOTORS Examples School of Electrical Systems Engineering 1) Brushless permanent magnet DC motor – Toyota Prius, Honda Civic Hybrid car 2) Stepper motor – printer, floppy drive etc 3) Slow speed AC synchronous motor – cement factory 4) Switched reluctance motor. – washing machine, electric car 6 ABD RAHIM 2008

SYNCHRONOUS MOTORS Steady states conditions… School of Electrical Systems Engineering Figure 2 : Equivalent circuit of a synchronous motor KVL Equivalent equation : or 7 ABD RAHIM 2008

SYNCHRONOUS MOTORS Steady states conditions… School of Electrical Systems Engineering Figure 3 : Equivalent phasor diagram of a synchronous motor Torque equation : or Where δ is the load angle. 8 ABD RAHIM 2008

SYNCHRONOUS MOTORS School of Electrical Systems Engineering Figure 4 : Torque-speed characteristic of a synchronous motor • Loads are basically constant-Speed devices. • Terminal voltage and the system frequency will be constant regardless of the amount of power drawn by the motor. 9 ABD RAHIM 2008

The steady-states speed of the motor is constant from no load all the way up to the maximum torque that the motor can supply (called the pullout torque). • • School of Electrical Systems Engineering • Thus the speed regulation SR, of this motor is 0 percent. The torque equation can be stated as: • • Thus, The maximum or pullout torque Tmax, occurs when δ =90º. The maximum power can be produce is : • Normal full-load torques are much less than that, however. In fact, it would be indicated at 1/3 rd of the pullout torque. • when the torque on the shaft of a synchronous motor exceeds the pullout torque, the rotor can no longer remain locked to the stator and net magnetic fields. Instead, the motor will slows down and at last will vibrate severely. • The loss of synchronization after the pullout torque is exceeded is known 10 as slipping poles. ABD RAHIM 2008

SYNCHRONOUS MOTORS When the field current IF, changes … how does it affect the synchronous motor? School of Electrical Systems Engineering 1) Consider a synchronous motor operating at lagging power factor 11 ABD RAHIM 2008

SYNCHRONOUS MOTORS School of Electrical Systems Engineering 2) Increasing IF will result magnitude EA to increase. But it does not effect the real power supplied from to the motor (power will only increase when torque increased). Thus, P=constant. 3) Input power to motor is given by: 4) 4) or So the distance proportional to power on the phasor diagram (EA sin δ and IA cos θ) must therefore be constant. Thus, when EA to increase, it will only slide along the constant power line (EA 1 – EA 4). 12 ABD RAHIM 2008

SYNCHRONOUS MOTORS School of Electrical Systems Engineering 6) When value of EA increased, magnitude of IA will slightly decrease then increased. But it can only do so within the constant power line (IA 1 – IA 4). 7) Note: At low EA, the armature current is lagging, and the motor is an inductive load (consuming reactive power Q). As the field current is increased, the armature current eventually lines up with VØ, and the motor looks purely resistive. As the field current is increased further, the armature current becomes leading, and the motor becomes a capacitive load. It is now acting like a capacitor-resistor combination. Consuming negative reactive power Q or, alternatively, supplying reactive power Q to the system. 13 ABD RAHIM 2008

SYNCHRONOUS MOTORS The V curves… School of Electrical Systems Engineering The several V curves drowns represent a different motor power levels. For each curve, the minimum armature current occurs at unity power factor (when only real power is being supplied to the motor) For field currents less than the value giving minimum IA, the armature current is lagging. Consuming Q. For field currents greater than the value giving the minimum IA. , the armature current is leading, supplying Q to the power system as a capacitor would. Therefore, by controlling the field curent IF of a synchronous motor, the reactive power consumed or supplied to by the motor can be controlled. 14 ABD RAHIM 2008

SYNCHRONOUS MOTORS School of Electrical Systems Engineering Underexcited overexcited 15 ABD RAHIM 2008

Example 2: School of Electrical Systems Engineering A 208 V 45 -k. VA, 0. 8 -PF-leading, Δ- connected, 60 Hz synchronous motor has a synchronous reactance of 2. 5Ω and a negligible armature resistance. Its friction and windage losses are 1. 5 k. W, and its core losses are 1. 0 k. W. Initially the shaft is supplying a 15 HP load with initial power factor of 0. 85 PF lagging. The field current IF at these conditions is 4. 0 A. a) Sketch the initial phasor diagram of this motor, and find the values l. A and EA. 25. 8 L-31. 8 deg 182 L-17. 5 V b) lf the motor's flux is increased by 25 percent, sketch the new phasor diagram of the motor. What are values l. A and EA and the power factor of the motor now? 227. 5 L-13. 9 22. 5 L 13. 2 leading 16 ABD RAHIM 2008

Example 3: School of Electrical Systems Engineering Develop a table showing the speed of magnetic field rotation in ac machines of 2, 4, 6, 8, 10, 12, and 14 poles operating at frequencies of 50, 60, and 400 Hz. 17 ABD RAHIM 2008

Example 4: School of Electrical Systems Engineering At a location X, it is necessary to supply 300 k. W of 60 -Hz power. But, the only power sources available at the site is 50 Hz. It is decided to generate the power by means of a motor-generator set consisting of a synchronous motor driving a synchronous generator. How many poles should each of the two machines have in order to convert 50 -Hz power to 60 -Hz power? 10 -12 18 ABD RAHIM 2008

Synchronous motor & power factor correction School of Electrical Systems Engineering The following figure shows a large power system whose output is connected through a transmission line to an industrial plant at a distant point. The industrial plant shown consists of three loads. Two of the loads are induction motors with lagging power factors, and the third load is a synchronous motor with a variable power factor. What does the ability to set the power factor of one of the loads do for the power system? 19 ABD RAHIM 2008

Synchronous motor & power factor correction Example 5: School of Electrical Systems Engineering The power system in Figure 5 -39 operates at 480 V. Load 1 is an induction motor consuming 0. 78 PF isload lagging, and 2 consuming 200 k. W 100 k. W at an induction motor at 0. 8 PF lagging. Load 3 is a synchronous motor whose real power consumption is 150 k. W. a. lf the synchronous motor is adjusted to operate at 0. 85 PF lagging, what is the transmission line current in this system? 667 A b. lf the synchronous motor is adjusted to operate at 0, 85 PF leading, what is the transmission line current in this system? 566 A c. d. Assume that transmission line losses PLL given as ; PLL = 3 IL 2 RL how do the transmission losses compare in the two cases? 28% less 20 ABD RAHIM 2008

SYNCHRONOUS MOTORS How does it works? . . School of Electrical Systems Engineering Assume that the application of three-phase AC power to the stator causes a rotating magnetic field to be set up around the rotor. The rotor is energized with DC (it acts like a bar magnet). The strong rotating magnetic field attracts the strong rotor field activated by the dc. This results in a strong turning force on the rotor shaft. The rotor is therefore able to turn a load as it rotates in step with the rotating magnetic field. It works this way once it’s started. N S However, one of the disadvantages of a synchronous motor is that it cannot be started from a standstill by applying three-phase ac power to the stator… Why ? ? 21 ABD RAHIM 2008

SYNCHRONOUS MOTORS Because. . School of Electrical Systems Engineering …when ac is applied to the stator, a high-speed rotating magnetic field appears immediately. This rotating field rushes past the rotor poles so quickly that the rotor does not have a chance to get started. Since the field is rotating at synchronous speed, the motor must be accelerated before it can pull into synchronism. Therefore, separate starting means must be employed. A synchronous motor in its purest form has no starting torque. It has torque only when it is running at synchronous speed. 22 ABD RAHIM 2008

SYNCHRONOUS MOTORS – drive & control Starting Methods : School of Electrical Systems Engineering 1) Variable AC frequency 2) Mechanical drive - Turn the rotor into Synchronize speed 3) DC motor drive – coupled on the common shaft 4) Embedded Squirrel cage winding on rotor poles. 23 ABD RAHIM 2008

SYNCHRONOUS MOTORS – drive & control 1) Variable AC frequency School of Electrical Systems Engineering Note : Electronic speed control – constrain: Modern solid state electronics has able to increase the options for speed control. By changing the 50 or 60 Hz line frequency to higher or lower values, the synchronous speed of the motor can be changed. However, decreasing the frequency of the AC current fed to the motor also decreases reactance Xs which increases the stator current. This may cause the stator magnetic circuit to saturate with disastrous results. Thus in practice, the voltage to the motor needs to be decreased when frequency is decreased. 24 ABD RAHIM 2008

SYNCHRONOUS MOTORS – drive & control Example 6: School of Electrical Systems Engineering If a 60 -Hz synchronous motor is to be operated at 50 Hz, will its synchronous reactance be the same as at 60 Hz, or will it change? (Hint: Think about the derivation of XS. ) SOLUTION: The synchronous reactance Xs represents the effects of the armature reaction voltage Estat and the armature self-inductance Ls. The Estat is caused by the armature magnetic field Bs , and the amount of voltage is directly proportional to the speed with which the magnetic field sweeps over the stator surface. The higher the frequency, the faster Bs sweeps over the stator, and the higher the armature reaction voltage Estat is. Therefore, the armature reaction voltage is directly proportional to frequency (EA ~ f). Similarly, the reactance of the armature self-inductance is directly proportional to frequency so the total synchronous reactance Xs is directly proportional to frequency (XS ~ f), . If the frequency is changed from 60 Hz to 50 Hz, the synchronous reactance will be decreased by a factor of 5/6 as well. 25 ABD RAHIM 2008

SYNCHRONOUS MOTORS – drive & control Example 7: School of Electrical Systems Engineering What voltage should be used to allow a 420 V, 60 Hz, 4 -pole synchronous motor to be used on a 50 Hz supply? 350 v 26 ABD RAHIM 2008

4) Embedded Squirrel cage winding on rotor poles. Self starting method. . School of Electrical Systems Engineering A squirrel-cage type of winding is added to the rotor of a synchronous motor to cause it to start. The squirrel cage is shown as the outer part of the rotor in figure 4 -7. It is so named because it is shaped and looks something like a turnable squirrel cage. Simply, the windings are heavy copper bars shorted together by copper rings. 27 ABD RAHIM 2008

SYNCHRONOUS MOTORS Self starting process. . School of Electrical Systems Engineering Fleming′s Right Hand Rule Also known as the Generator Rule this is a way of determining the direction of the induced emf of a conductor moving in a magnetic field. Fleming′s Left Hand Rule Also known as the Motor Rule this is a way of determining the direction of a force on a current carrying conductor in a magnetic field. 28 ABD RAHIM 2008

Starting process… School of Electrical Systems Engineering A low voltage is induced in these shorted windings by the rotating three -phase stator field. Because of the short circuit, a relatively large current flows (induced emf) in the squirrel cage. This causes a magnetic field that interacts with the rotating field of the stator. Because of the interaction (left hand rule), the rotor begins to turn, following the stator field; the motor starts. We will run into squirrel cages again in other applications, in more detail. To start a practical synchronous motor, the stator is energized, but the dc supply to the rotor field is not yet energized. The squirrel-cage windings bring the rotor to near synchronous speed. At that point, the dc field is energized. This locks the rotor in step with the rotating stator field. Full torque is developed, and the load is driven. 29 ABD RAHIM 2008

Starting process… School of Electrical Systems Engineering A mechanical switching device that operates on centrifugal force is often used to apply dc to the rotor as synchronous speed is reached. The practical synchronous motor has the disadvantage of requiring a dc exciter voltage for the rotor. This voltage may be obtained either externally or internally, depending on the design of the motor. Constant speed 30 ABD RAHIM 2008

Questions to ponder… School of Electrical Systems Engineering 1. What requirement is the synchronous motor specifically designed to meet? 2. What is the speed regulation of a synchronous motor? 3. When would a synchronous motor be used eventhough its constant speed characteristic is not needed? 4. Why can't a synchronous motor start by itself? 5. What techniques are available to start a synchronous motor? 6. What happens to a synchronous motor as its field current is varied? 7. A synchronous motor is operating at a fixed load. Once the field current is increased, the armature current falls, was the motor initially operating at a lagging or a leading Power factor? 31 ABD RAHIM 2008

School of Electrical Systems Engineering The end on synchronous motor 32 ABD RAHIM 2008