MEBS Utilities services M.Sc.(Eng) in building services Department of Electrical & Electronic Engineering University of Hong Kong

Transcription

1 MEBS Utilitie ervice Joint peed torque characteritic of electric motor and mechanical load Electric motor exhibit a variety of peed-torque characteritic that are uitable for a wide range of load demand. Mechanical load alo have a wide range of peed-torque characteritic depending on their mechanical propertie. For example, the load torque of a hoit or a conveyor i largely independent of it peed. Load torque of a fan or a pump i proportional to the quare of it peed. When an electric motor i connected to a mechanical load, the ytem operate at a peed-torque tatu that matche the characteritic of the motor a well a the mechanical load. Braking If no braking i applied to a motor, after electric power i removed, it only top when all the kinetic energy i diipated. Braking i a generic term ued to decribe a et of operating condition for electric drive ytem. It include rapid topping of the electric motor, holding the motor haft, thu the driven uipment, to a pecific poition, etc. Beide mechanical braking, all thee apect of braking can alo be done electrically. During braking, the energy flow change it direction and utilization of the braking energy enhance ytem efficiency. Conider a PWM fruency inverter, in term of power, the power input to the motor i: P 3VI coφ where V i the phae to neutral voltage, I the phae current and φ the power factor. In motoring operation, φ <90 o, power i then poitive and flow from the inverter to the motor. A reduction in fruency of the upply reduce the ynchronou peed, before the rotor peed i reduced to the deired value, the rotor peed i higher than the ynchronou peed. Thi reveral of relative peed revere the rotor induced emf, rotor current and component of tator current which balance the rotor ampere turn. Conuently, the power factor φ >90 o, power become negative and flow from motor to the ource. There are everal form of electric braking method, collectively known a:. regenerative feeding energy back to the power ource,. dynamic by diipating the energy in an electrical reitance, 3. countercurrent braking by revering the direction of the tator field. We will dicu, briefly, dynamic braking and, more in detail, regenerative braking of ac induction motor. K.F. Chan (Mr.) Page of 88 July 00

2 MEBS Utilitie ervice Bi-directional drive ytem Conider the cae of an electric motor driving an elevator. Let u aume that the lift car i fully loaded with weight larger than the counterweight. When the lift car i moving upward, the motor torque i in the ame direction a the motor rotation. When the lift car i moving downward, the motor torque mut be in oppoite direction to the rotation to keep the lift car from falling overpeed. Four quadrant electric drive ytem The following convention govern the power flow analyi of electric drive ytem: When the torque of the electric motor i in the ame direction a the ytem rotation, the motor conume electric power from the power ource and deliver mechanical energy to the load When the motor torque and the ytem rotation are in oppoite direction, the motor i in fact driven by the mechanical power and delivering electric power to the electric power ource. K.F. Chan (Mr.) Page of 88 July 00

3 MEBS Utilitie ervice (Adopted from EL-SHARKAWI, M.A., Fundamental of Electric Drive) (Adopted from DUBEY, G.K. Fundamental of Electrical Drive) The above how the four quadrant of peed-torque characteritic to cover all poible combination of any electric drive ytem. K.F. Chan (Mr.) Page 3 of 88 July 00

4 MEBS Utilitie ervice Quad Direction of Load Power flow Operating Example rant rotation torque and a rotation t Same a nd Oppoite From motor Motor Fully loaded quadrant but direction to load elevator moving oppoite to 3 rd upward nd Same a t Same From load Generator Empty lift car quadrant direction to motor moving upward 3 rd Oppoite to t Oppoite From motor Motor Empty lift car quadrant direction to load moving downward 4 th Oppoite to t Same From load Generator Fully loaded quadrant but direction to motor elevator moving ame a 3 rd downward A bi-directional grinding machine, a horizontal conveyor, a bi-directional travellator, are all example of machine ruired to operate in the t and 3 rd quadrant. Example of drive ytem operating in all four quadrant (Adopted from DUBEY, G.K. Fundamental of Electrical Drive) K.F. Chan (Mr.) Page 4 of 88 July 00

5 MEBS Utilitie ervice A veratile motor drive ytem operate in all 4 quadrant. An elevator carrying paenger or a hoit with counterweight are typical example. For maximum efficiency, the power electronic circuit mut then be deigned to allow the electric power to flow in both direction. Four quadrant operation can be obtained by any drive ytem with braking capability. A reduction of the inverter fruency, to make ynchronou peed le than the pinning rotor peed, tranfer the operation from quadrant (forward motoring) to quadrant (forward braking). The inverter fruency and voltage are progreively reduced a peed fall to brake the machine to deired peed. Next phae uence of the inverter output voltage i revered by interchanging the firing pule between any leg of the inverter. Thi tranfer the operation to quadrant 3 (revere motoring). The inverter fruency and voltage are increaed to get the ruired peed in the revere direction. Then a reduction of inverter fruency will make the ynchronou peed le than the pinning rotor peed, tranferring operation from quadrant 3 to 4 (revere braking). Dynamic braking (Adopted from Dubey, G.K., Fundamental of Electrical Drive) The above how a implified circuit for dynamic braking. A witch, SW, a braking reitor R, and a elf-commutated witch S, a tranitor hown here, i added to the dc link of a PWM fruency inverter circuit. During motor operation, the witch SW i cloed and the tranitor S i opened, o the circuit act a a normal PWM fruency inverter. During braking operation, the witch SW i opened, generated energy from the motor flowing into the dc link charge the capacitor and o it voltage rie. When the voltage rie above a et value, the tranitor i cloed, allowing the energy to be diipated in the reitor. K.F. Chan (Mr.) Page 5 of 88 July 00

6 Regenerative braking MEBS Utilitie ervice Regenerative braking occur when the motor peed exceed the ynchronou peed. Thi may happen when the load torque drive the motor beyond it ynchronou peed. In thi cae, the load i the ource of energy and the induction motor i converting the mechanical power into electrical power, which i delivered back to the electrical ytem. If the power ource of a fruency inverter come from a rechargeable dc battery like the one hown below, the circuit by it nature ha the capability of regenerative braking. (Adopted from Dubey, G.K., Fundamental of Electrical Drive) However, if the power ource come from a ac/dc diode bridge like the one below, ome mean hall be built into the fruency inverter drive to allow the regenerated power during braking to return to the ac ource. (Adopted from Dubey, G.K., Fundamental of Electrical Drive) K.F. Chan (Mr.) Page 6 of 88 July 00

7 MEBS Utilitie ervice One method i to ue dual converter in the power ource ection, one converting the ource ac to the dc link, the other one inverting the regenerated power in the dc link back into the ac power ource. (Adopted from MURPHY, JMD, & TURNBULL, FG. Power electronic control of AC motor) K.F. Chan (Mr.) Page 7 of 88 July 00

8 MEBS Utilitie ervice Another method i to ue a ynchronou link converter. (Adopted from Dubey, G.K., Fundamental of Electrical Drive) Recall that diode are typically intalled acro the tranitor in the dc/ac inverter ection of a fruency drive to allow energy to flow back to the dc link. So one can imagine that PWM inverter I, hown above in the power upply ection, i an inverter intalled a a mirror image to PWM inverter II. PWM I and the inductor L contitute a SLC. PWM I i operated to produce voltage V I of ruired magnitude and phae in phae with V for motoring, and 80 o out of phae with V for braking. A SLI inverter chematic K.F. Chan (Mr.) Page 8 of 88 July 00

9 MEBS Utilitie ervice Wind turbine uing induction machine are good example of regenerative braking. Induction machine are popular in wind application becaue they are ideally uited for variable power profile application. Unlike the ynchronou or dc machine, induction machine become automatically ynchronized with the external power ytem. The above figure how the baic component of a wind turbine. At moderate wind peed (called cutoff peed) the gearbox i deigned to rotate the high peed haft near the ynchronou peed of the induction machine. If the wind increae beyond the cutoff peed, the induction machine rotate at a peed higher than it ynchronou peed. Thi i regenerative braking operation. The houing box can wivel at the top of the tower to point the blade at the direction of maximum wind effect. When the wind peed become exceive, the blade can be locked to prevent any mechanical damage to the ytem. Regenerative braking can be explained by the torque uation: T d 3V R R ω R + + X where V i the voltage acro each tator winding. K.F. Chan (Mr.) Page 9 of 88 July 00

10 MEBS Utilitie ervice The negative lip occur when the peed of the machine exceed the ynchronou peed: n n n When the turbine blade drive the electric machine to a peed fater than the ynchronou peed, the lip become negative. Since the load torque i in the ame direction a the machine, it operate in the nd quadrant a a generator. In fact, induction machine of the wind ytem i deigned to operate at regenerative braking in the nd quadrant only. When the wind peed i low o that the rotor peed i near or below the ynchronou peed, the blade are locked and the motor i diconnected from the electrical upply to prevent the machine from running a a motor. When ued in wind application, induction machine demand a ignificant amount of reactive power from the utility ytem, mainly becaue they do not have their own field circuit. When ued in regenerative mode, the induction machine conume reactive power from the ytem while delivering real power. The inductive reactive power Q i conumed in the magnetizing inductive reactance X m and the uivalent winding reactance X. K.F. Chan (Mr.) Page 0 of 88 July 00

11 MEBS Utilitie ervice K.F. Chan (Mr.) Page of 88 July 00 m X I X V Q m X R R X X V Q where V i the phae to neutral voltage. The reactive power Q i plotted in the following diagram:

12 MEBS Utilitie ervice Q i at it minimum at ynchronou peed but i then almot linearly proportional to peed when n>n. To alleviate, a reactive power controller can be intalled at the wind farm. In more general drive ytem, the induction machine may operate in the firt or econd quadrant only. The above figure how a contant but reverible load torque. When the load torque change from T to T, the motor change from t to nd quadrant operation, bear in the mind that the motor i till rotating in the ame direction. K.F. Chan (Mr.) Page of 88 July 00

13 MEBS Utilitie ervice The following figure how another example. The figure how characteritic for two different value of v/f control. The load torque i aumed to be contant. When a peed reduction i deired from point, before the peed i changed, the motor temporarily move to operation point in the nd quadrant under regenerative braking then ettle at point 3, the deired lower peed. K.F. Chan (Mr.) Page 3 of 88 July 00

14 Example MEBS Utilitie ervice A 400V, tar connected, 3-phae, 6-pole, 50Hz induction motor ha following parameter referred to the tator: R R Ω, X X Ω For regenerative braking operation of thi motor: a) Determine maximum overhauling torque it can hold and range of peed for afe operation. b) Determine peed at which it will hold an overhauling load with a torque of 00Nm. c) If a capacitive reactance of Ω i inerted in each phae of the tator, calculate maximum overhauling torque the motor can hold a a ratio of maximum overhauling torque without capacitor. Anwer a) Synchronou peed 000RPM π 04.7rad/ 3 3 max ± X R + R 4 ± ± max For regenerative braking operation, the lip i negative, thu For generator action T T max max 3V ω R + + R X π 3 [ ] -44.5Nm Maximum torque occur at max of -0.43, i.e. at (+0.43)000 43RPM Stable operation during regenerative braking occur from ynchronou peed to thi peed at which the torque i maximum. Thu range of peed for afe operation will be from 000 to 43RPM. K.F. Chan (Mr.) Page 4 of 88 July 00

15 MEBS Utilitie ervice b) T d 3V R R ω + R + X Re-arranging, the above uation become 3V R ( R + X ) + R R + R 0 Tdω Subtituting the numerical value, the uation become Note that for a quadratic uation: ax + bx + c 0 the olution can be written a: b ± x b 4ac a So or However, will give untable operation, therefore Motor peed i thu (+0.063) RPM c) If a capacitor i added per phae, the net effect i reducing the inductive reactance. A max ± X R + R o max ± ± ( 4 ) For generator action K.F. Chan (Mr.) Page 5 of 88 July 00

16 MEBS Utilitie ervice T max 3V ω R + + R X T max π ( 4 ) -68Nm Thi i the maximum torque if a capacitor of Ω i added to each phae in erie with the tator winding. The ratio i [Thi example adopted from Dubey, Gopal.K., Fundamental of Electrical Drive] K.F. Chan (Mr.) Page 6 of 88 July 00

17 Example MEBS Utilitie ervice A 380V, 50Hz, ix pole, tar connected induction motor ha the following parameter : R 0.6Ω, R 0.4Ω, X 5Ω The motor i loaded by a 30 Nm bi-directional contant torque. revered, neglecting mechanical lo calculate a) Motor peed b) Power delivered to the electrical upply ytem If the load torque i Anwer a) Phae to neutral voltage i V Synchronou peed i T d 3V R R ω + R RPM 6 + X Re-arranging, the above uation become 3V R ( R + X ) + R R + R 0 Tdω Subtituting the numerical value, the uation become ( 0) ( 0.4) π ( 5.36) + ( 8.97) Note that for a quadratic uation: ax + bx + c 0 The olution can be written a: b ± b 4ac x a So can be expreed a K.F. Chan (Mr.) Page 7 of 88 July 00

18 MEBS Utilitie ervice 3V R RR ± T dω Thu or RR d ( R + X ) 3V R T ω 4 ( R + X )( R ) However, max R ± ± X R + Obviouly, if the lip i -0.74, motor will run in untable braking condition. The regenerative peed i n n 000( ) 009 ( ) RPM b) P d Tω 009 ( 30)(π ) 370Watt 60 To calculate the tator and rotor copper lo we mut firt calculate the current Recall I P ( ) d P g R 3I ( ) 0.4 thu 370 3I ( ) ( ) I 4. 7Amp So rotor copper lo i 3I R 3(4.7) (0.4) 6. 7W Stator copper lo i 3I R 3(4.7) (0.6) 40. W Therefore power delivered to the electrical ytem i Watt [Unle otherwie tated, text and figure adopted from EL-SHARKAWI, Mohamed A., Fundamental of Electric Drive] K.F. Chan (Mr.) Page 8 of 88 July 00