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Induction Motor Drive • Why induction motor (IM)? – – – – Robust; No brushes. No contacts on rotor shaft High Power/Weight, Lower Cost/Power ratios Easy to manufacture Almost maintenance-free, except for bearing and other “external” mechanical parts • Disadvantages – Essentially a “fixed-speed” machine – Speed is determined by the supply frequency – To vary its speed need a variable frequency supply • Motivation for variable-speed AC drives – – – – Inverter configuration improved Fast switching, high power switches Sophisticated control strategy Microprocessor/DSP implementation • Applications – Conveyer line (belt) drives, Roller table, Paper mills, Traction, Electric vehicles, Elevators, pulleys, Airconditioning and any industrial process that requires variable-speed operation. • The state-of-the-art in IM drives is such that most of the DC drives will be replaced with IM in very near future. Dr. Zainal salam; Power Electronics and Drives (Version 2),2002, UTMJB 1 Torque production (1) • Only “squirrel-cage” IM (SCIM) is considered in this module • Neglecting all harmonics, the stator establishes a spatially distributed magnetic flux density in the air-gap that rotate at a synchronous speed, w1 : w1 we p where we : supply frequency (in Hz) p: pole pairs (p=1for 2 pole motor, p=2 for 4 pole motor etc) • If the rotor is initially stationary, its conductor is subjected to a sweeping magnetic field, inducing rotor current at synchronous speed. • If the rotor is rotating at synchronous speed (i.e. equals to f1), then the rotor experience no induction. No current is induced in the rotor. Dr. Zainal salam; Power Electronics and Drives (Version 2),2002, UTMJB 2 Torque production (2) • At any other rotor speed, say wm, the speed differential wi-w2 creates slip. Per-unit slip is defined as: w wm s 1 ; w1 w1 we p where : we : supply frequency wm : rotor frequency p : pole pair • Slip frequency is defined as: w2=w1-wm. • When rotor is rotating at wm., rotor current at slip frequency will be induced. • The interaction between rotor current and air-gap flux produces torque. Dr. Zainal salam; Power Electronics and Drives (Version 2),2002, UTMJB 3 Single-phase Equivalent Circuit (SPEC) 1:nS R1 V1 L1 L2 Rm Lm Vm STATOR SIDE V2 = nSVm R2 ROTOR SIDE R1 : Stator resistance L1 : Stator leakage inductance R2 : Rotor resistance L2 : Rotor leakage inductance Lm : Magnetisin g inductance v1 : Supply vol tage (phase voltage) Dr. Zainal salam; Power Electronics and Drives (Version 2),2002, UTMJB 4 SPEC, referred to stator I1 V1 R1 L1 I2 Rm L2 Lm R2 S • From previous diagram, SPEC is a dual frequency circuit. On the stator is w1 and on the rotor wm • Difficult to do calculations. • We can make the circuit a single frequency type, by referring the quantities to the stator Dr. Zainal salam; Power Electronics and Drives (Version 2),2002, UTMJB 5 Rotor current If E1 is the back EMF in the stator phase, then the back EMF in an equivalent rotor phase with the same effective turns ratio will be E 2 where : E2 sE1 At standstill, i.e when w m 0, E2 sE1 1E1 E1 At synchronous speed, i.e whenw m 1, E2 (0) E1 0 Hence the current in the rotor phase, E2 sE1 R2 jsX 2 R2 jsX 2 E1 R2 jX 2 s Note that the quantities are now referred to the stator, but with the rotor resistance alteration. I2 Dr. Zainal salam; Power Electronics and Drives (Version 2),2002, UTMJB 6 Performance calculation using SPEC I1 R1 V1 L1 Rm I2 L2 Lm R2 S Pin 3V1I1 cos Input Power : Note : V1 and I1 must be phase voltage and current Stator copper loss : Core loss : Power across the air - gap : Rotor copper loss : Pls 3I12 R1 3V12 Plc Rm 3I 2 2 R2 Pg s Pin Pls Plc Plr 3I 2 2 R2 Dr. Zainal salam; Power Electronics and Drives (Version 2),2002, UTMJB 7 Performance calculation (2) Gross output power : Po Pg Plr 2 3I 2 2 R2 3 I R (1 s ) 3I 2 2 2 2 Pg (1 s ) s s Power at the shaft : Psh Po PFW ; PFW : friction and windage loss. Developed (electromagnetic) torque : Po 3I 2 2 R2 (1 s ) Te wm sw m Since w1 w m s w m (1 s )w1 , w1 3I 2 2 R2 Te sw1 But w1 we p ; w e is the supply frequency. Then, 3 pI 2 2 R2 Te sw e Dr. Zainal salam; Power Electronics and Drives (Version 2),2002, UTMJB 8 Example calculation • A single phase equivalent circuit of a 6-pole SCIM that operates from a 220V line voltage at 60Hz is given below. Calculate the stator current, output power, torque and efficiency at a slip of 2.5%. The fixed winding and friction losses is 350W. Neglect the core loss. I1 R1 X1 0.2 0.5 V1 I2 X2 0.2 Xm R2 20 0.1 V1 220V line-to line 3 220V 127V 3 2.5% 0.025 Dr. Zainal salam; Power Electronics and Drives (Version 2),2002, UTMJB 9 Calculation (solution) X 1 0.5, X 2 0.2, X m 20 R Z in ( R1 jX 1 ) jX m // 2 jX 2 s 0. 1 j 0. 2 4.220 o 0.2 j 0.5 j 20 0.025 0.1 j 0.2 j 20 0.025 V 220 3 o I1 1 30 . 0 20 A Z in 4.220 o Pin 3V1I1 cos 3(220 3)(30)(cos 20 o ) 10,758W Pls 3I12 R1 3(30 2 )(0.2) 540W Power tran sferred to rotor (neglectin g core loss) Pg Pin Pls 10,758 540 10,216W Gross power Po Pg (1 s ) 10,216 (1 0.025 ) 9,961W Power at the shaft Psh Po PFW 9,961 350 9,611W Output power 9611 89.3% Input power 10758 Electromag netic Torque P Po 9611 Te o wm (we1 p ) (1 s ) 2 (60 / 3)(1 0.025) Efficiency 78.4 N .m Dr. Zainal salam; Power Electronics and Drives (Version 2),2002, UTMJB 10 Starting current • For the previous example, Calculate the stating current when motor is first switched on to rated applied voltage. Solution : At standstill, s 1 0.1 j 0.2 Z in 0.2 j 0.5 j 20 0.1 j 0.2 j 20 0.76 V 220 3 I1 1 167 A Z in 0.76 Note that thestarting current is about 5 times than full load current. This is common for induction motors.Care should be taken when starting induction motors. Dr. Zainal salam; Power Electronics and Drives (Version 2),2002, UTMJB 11 Approximate SPEC + R1 L1 L2 R2 s LM V1 - Since L m is large, the circuit above can be drawn I2 V1 2 R2 2 2 R1 w1 L1 L2 s Power at the rotor (per phase), R Po I 2 2 2 s Electromag netic (developed ) torque, Te 3Po w1 3R2V12 2 R2 2 2 sw1 R1 w1 L1 L2 s Dr. Zainal salam; Power Electronics and Drives (Version 2),2002, UTMJB 12 Single (fixed supply) frequency characteristics For a give frequency w1 , the torque (versus slip) characteri stics can be shown as below. Note that : w1 wm s ; at standsill s 1, at sync speed, s 0. w1 TORQUE(+) wm we PLUGGING MOTORING we we wm wm Tem (max torque or pull-out torque) GENERATING zero slip w e (sync.speed) Tes (starting torque) 0 unity slip (standstill) rated slip SPEED SLIP,s TORQUE(-) Dr. Zainal salam; Power Electronics and Drives (Version 2),2002, UTMJB 13 Single frequency characteristics CURRENT TORQUE operating point (rated torque) EFFICIENCY POWER FACTOR rated current rated slip SLIP 0 1.0 Standstill synchronous speed Dr. Zainal salam; Power Electronics and Drives (Version 2),2002, UTMJB 14 Single frequency characteristic • As slip is increased from zero (synchronous), the torque rapidly reaches the maximum. Then it decreases to standstill when the slip is unity. • At synchronous speed, torque is almost zero. • At standstill, torque is not too high, but the current is very high. Thus the VA requirement of the IM is several times than the full load. Not economic to operate at this condition. • Only at “low slip”, the motor current is low and efficiency and power factor are high. Dr. Zainal salam; Power Electronics and Drives (Version 2),2002, UTMJB 15 Typical IM Drive System + IDC + VDC IM Modulation Index, BLOCK DIAGRAM Supply Rectifier and Filter 3-phase Inverter n CIRCUIT Dr. Zainal salam; Power Electronics and Drives (Version 2),2002, UTMJB IM 16 Variable speed characteristics • For variable speed operation, the supply is an inverter. • The frequency of the fundamental AC voltage will determine the speed of IM. To vary the speed of IM, the inverter fundamental frequency need to be changed. • The inverter output frequency must be kept close to the required motor speed. This is necessary as the IM operates under low slip conditions. • To maintain constant torque, the slip frequency has to be maintained over the range of supply frequencies. Dr. Zainal salam; Power Electronics and Drives (Version 2),2002, UTMJB 17 Variable voltage, variable frequency (VVVF) operation • In order for maximum torque production, motor flux should be maintained at its rated value. m sin w1t But the back emf is : d e1 N Nw1 m cos w1t dt In RMS, 1 E1 Nw11 m 4.44 f1 N1 m 2 E1 or 4.44 N1 m f1 Therefore, in order to maintain t he motor flux, the E1 f1 ratio has to be kept constant. This is popularly known as the constant Volt/Hertz operation Dr. Zainal salam; Power Electronics and Drives (Version 2),2002, UTMJB 18 Constant Torque region • Hence for VVVF operation, there is a need to control the fundamental voltage of the inverter if its frequency (and therefore the frequency of the IM) need to be varied. • To vary the fundamental component of the inverter, the MODULATION INDEX can be changed. • The rated supply frequency is normally used as the base speed • At frequencies below the base speed, the supply magnitude need to be reduced so as to maintain a constant Volt/Hertz. • The motor is operated at rated slip at all supply frequencies. Hence a “constant torque” region is obtained. Dr. Zainal salam; Power Electronics and Drives (Version 2),2002, UTMJB 19 Constant Torque Region f1 f 2 TORQUE(+) f1 f 2 f 3 f 4 f 5 f3 f4 f5 rated torque 0 SPEED rated slip SLIP,s TORQUE(+) rated torque 0 SLIP,s SPEED Dr. Zainal salam; Power Electronics and Drives (Version 2),2002, UTMJB 20 Constant Power region • Above base speed, the stator voltage reaches the rated value and the motor enters a constant power region. • In this region, the air-gap flux decreases. This is due to increase in frequency frequency while maintaining fixed voltage. • However, the stator current is maintained constant by increasing the slip. This is equivalent to field weakening mode of a separately excited DC motor. Dr. Zainal salam; Power Electronics and Drives (Version 2),2002, UTMJB 21 Constant Power region TORQUE(+) rated torque 0 SLIP,s Base speed SPEED TORQUE(+) "FIELD WEAKENING" CONSTANT TORQUE REGION 0 SLIP,s Base speed Dr. Zainal salam; Power Electronics and Drives (Version 2),2002, UTMJB SPEED 22 VVVF Summary CONSTANT POWER CONSTANT TORQUE Electromagnetic torque, T e Terminal (supply) voltage, V 1 slip frequency,fs slip,s 0 Base speed Dr. Zainal salam; Power Electronics and Drives (Version 2),2002, UTMJB SPEED 23 Examples • A three-phase 4-pole, 10 horsepower, 460V rms/60Hz (line-to line) runs at full-load speed of 1746 rpm. The motor is fed from an inverter. The flux is made to be constsnt. Plot the torque-speed graphs for the following frequency: 60Hz, 45 Hz, 30Hz, 15Hz. • A three-phase induction motor is using a three-phase VSI for VVVF operation. The IM has the following rated parameters: • • • • voltage: frequency: slip (p.u) pole pair 415V (RMS) 50Hz 5% 2 – If the inverter gives 415V (RMS) with modulation index of 0.8, calculate the required modulation index if the motor need to be operated at rotor mechanical speed of 10Hz. Dr. Zainal salam; Power Electronics and Drives (Version 2),2002, UTMJB 24