University of New South Wales School of Electrical Engineering & Telecommunications ELEC4613 - ELECTRIC DRIE SYSTEMS Experiment 2 IM drive with slip power recovery 1. Introduction This experiment introduces the method of speed control of a slip-ring induction machine via control of its slip power. This scheme, which is widely used in pumping installations, requires only a low rated power converter to return some of the slip power in the rotor to the AC mains. The experiment demonstrates the controllability of the method and the energy saving opportunity that this method offers. A variant of this scheme is widely used wind power systems employing induction generators. 2. Brief Theory Slip power control (The Static Scherbius Scheme) This static Scherbius scheme is indicated in figure 1. In this scheme, the rotor terminals of a slipring induction motor are connected to a three-phase diode bridge which rectifies the rotor voltages, as in figures 1. This rectified output DC voltage is then inverted into mains frequency AC by a fully-controlled thyristor converter operating in the naturally commutated inversion mode. A stepup transformer of turns-ratio n allows the low AC voltage at the output of the inverter to be stepped up and connected to the AC mains which also supplies the stator of the motor. In this way, some of the rotor (or slip) power can be returned to the AC mains thereby adjusting the speed of the motor. Note that no power is dissipated in any external rotor resistance. AC mains I d 1 1:n 1 2 d di Wound rotor IM Slip rings 3-phase Diode Bridge 3-phase Thyristor Bridge 3-phase Step-up Transformer Figure 1. Slip-power recovery by static Scherbius scheme Experiment 2 Slip power recovery scheme for IM 1 F. Rahman
It should be appreciated that the diode rectifier and the thyristor inverter only handles the slip power which is a small fraction of the total rotor power P 2. This alone usually leads to considerable saving of converter size than an inverter in the stator side which must handle the full input power of the motor. If the power returned to the AC source via the thyristor inverter is P ret, for total power balance in the rotor, P2 Po Psl Pret (1) P 1 s P, from (1), Using o 2 s P P P sl ret (2) 2 Here, P ret is positive for power returned to the mains. By setting the firing angle of the thyristor converter, the motor can be operated at any speed. Slip s will also vary with the load. Adjustment of the firing angle also adjusts the DC voltage di at the input of the inverter. This has the effect of adjusting the DC link current I d and hence torque T d developed by the motor. The purpose of the DC link inductor is to ensure continuous conduction of current through the inverter over the operating speed range of the drive. By ignoring the stator impedance, the RMS voltage per phase in the rotor circuit is given by E 2 s a 1 (3) where a is the stator-rotor turns ratio. The DC-link voltage d at the output terminals of the diode bridge rectifier is given by d 3sE 3s a 2 max 1max (4) Assuming that the transformer interposed between the inverter and the mains AC supply has the primary-secondary turns ratio n, the primary winding being on the AC mains side. 3 2 3 2 n (5) 2 1 di cos cos The negative sign arises due to the fact that the thyristor converter develops negative DC voltage in the inverter mode of operation. Assuming that the DC link current is smooth, in the steady-state the DC voltage on either side of it must be equal. Thus, d = di. Combining equations (3) and (4) a s cos (6) n Thus, the motor speed can be controlled by adjusting the firing angle. By varying between 180 and 90, the speed of the motor can be varied from zero to full speed, respectively, as shown in figure 2. Normally, a is equal to n. If the voltage drops in the diode and the thyristor converters and the resistance of the inductor are negligible, a should be equal to n. If these voltage drops are taken into account, n should be slightly larger than a. Experiment 2 Slip power recovery scheme for IM 2 F. Rahman
Speed, Rev/min Torque T d, Nm Figure 2. T- characteristics with a static Scherbius drive. For a motor with low rotor resistance and with the assumptions taken earlier, it can be shown that the developed torque of the motor is given by T d Ki Nm (7) d where i d is the DC-link current. Thus, the inner torque control loop of a variable-speed drive using the Scherbius scheme normally employs a DC-link current loop as the inner-most torque control loop. The drive is normally started with a short-time rated liquid resistor and the thyristor speed controller takes hold when the drive reaches a certain speed for which the slip is reasonably small. The DC link current, smoothed by the inductor, can be continuously regulated by controlling the firing angle of the converter in order to maintain the developed torque at the level required by the load in order to rotate the load at the desired speed. 3. Equipment The equipment required for the experiment are: The three-phase thyristor converter One 3-phase firing angle controller One 3-phase diode bridge rectifier One power analyser/meter One 50mH, 30A inductor AC and DC meters One 95/380 step-up transformer One 3.5kW, -connected, 3-phase slip-ring induction machine One DC generator for loading One wall mounted load bank One storage oscilloscope Isolated current and voltage sensors One variac (auto-transformer) Experiment 2 Slip power recovery scheme for IM 3 F. Rahman
4.0 Experiment 4.1 The experimental set-up is indicated in Figure 3. The slip-ring (wound-rotor) induction motor is Y-connected in both stator and rotor, and is supplied from an AC source via an auto-transformer (ariac) and a power meter. The line-line output voltage of ariac should always be slowly increased to 380 from zero. Caution! Before switching on the AC supply to the auto-transformer at any time, care must be taken so that the auto-transformer (ariac) adjustment is at minimum at start. The DC generator is for applying load to the AC motor. The DC generator, and hence the induction motor, is loaded by connecting a (wall mounted) load resistor bank across it. The electrical power output of the DC generator (the product of its output voltage and current) is also the power applied to the shaft of the induction motor if the DC machine losses are assumed negligible. Figure 3. The experimental set-up The three-phase outputs at the rotor terminals at slip frequency are rectified by a three-phase diode bridge rectifier, smoothed by an inductor, and inverted into AC source at mains frequency. The inverter output power is returned to the AC mains via a step-up transformer. Identify and familiarise yourself with all components of the experimental set-up and the AC and DC current and voltmeters, and the speed sensor. The firing control unit has an adjustment for knob for firing angle and a switch for enabling and disabling the firing pulses. If the firing pulses are disabled, the rotor windings of the motor are open and the motor will not rotate. 4.2 With the firing pulses disabled, adjust the input voltage to the motor to 380 line-line. Measure the line-line AC voltage at the input of the rectifier. The motor should not be running now, because of the open circuited rotor. Hence find the stator-rotor winding turns ratio of the motor and note it in your log book. Adjust the ariac to zero and switch it off. 4.3 Connect a short circuiting wire link between terminals R1 and R2, thereby setting the external rotor resistance to zero. Set all he load resistor switches OPEN, switch ON the variac and its output voltage slowly to 380 (line-line). The machine should now be Experiment 2 Slip power recovery scheme for IM 4 F. Rahman
running at full speed without any load connected to the DC generator. Prepare the following table in your logbook for data entry, one table for each section, for all remaining sections of this experiment. (Note that in section 4.3, the rotor terminals are short circuited at the rectifier end, so that the firing angle may be taken as 90). For each section, you must keep the AC input voltage at 380 for all conditions of load and speed by adjustment of the variac. Table I Load 1 Load 2 AC Input oltage AC Input Current A AC input power W DC Gen oltage DC Gen Current A DC-link voltage DC link current A Speed in Rev/ min Converter Firing Angle Enter data in a table for several speeds as the DC generator is loaded via the wall-mounted load banks in steps. Do not exceed 7A in the stator winding of the induction machine. Note that the DC generator output power in Watts divided by the operating speed in rad/sec is roughly the developed torque of the induction machine except for the losses in the DC generator. Adjust ariac to zero and switch off. 4.4 Remove the short circuiting link from R1 and R2 and adjust the potentiometer on the firing controller more than half-way between its control ranges. Enable the firing pulses and close the rotor- side circuit breaker CB2. Turn on the ariac and slowly adjust it to 380 lineline. Adjust the firing control know slowly to make sure that the motor speed can be adjusted by this control. Set the CRO to line trigger mode and adjust the time base such that exactly six complete voltage pulses across the inverter terminals (C1 and R2) are displayed on the full screen of the CRO. Adjust the firing angle control so that the motor runs at the highest speed. For this condition of operation, the firing angle of the thyristor bridge should be taken as 90 and the DC-link voltmeter should now read zero. The voltage waveform across C1 and R2 should now all have equal positive and negative areas. Note: Ask the demonstrator to set this condition of operation. Throughout this experiment, you must keep the firing angle higher than 90. Any firing angle less than 90 may cause large circulating current between the rectifier and the thyristor inverter. This condition should be avoided. Also, make sure that the induction motor current, the DC generator current and the rectifier DC link current never exceed 7A (rms), 10A (DC) and 20A (DC), respectively. Set two vertical cursors on the CRO, one on the vertical transition of the output voltage waveform. With respect to this cursor, the other cursor will allow you to measure the firing angle as you increase the firing angle using the firing control knob, as clarified in Figure 2. Experiment 2 Slip power recovery scheme for IM 5 F. Rahman
= 90 0 Cursor 1 Cursor 2 360 = 90 + 0 Figure 2 4.5 With = 90, load the DC generator via the wall mounted resistors (switches) and enter data in a table for several speeds for each load. The product of these two meters is the power returned to the AC mains from the rotor. Sketch in your log book the rectifier output voltage waveform for only one load condition for this firing angle. 4.6 Repeat 4.5 for firing angles of 100-140, in steps of 10, for several loads for each firing angle. Adjust the ariac to zero and switch off the AC mains. 5. Results and Report 5.1 Plot the rotor speed versus firing angle characteristic of the drive for = 90 to 145. Comment on how speed varies with 5.2 Plot the variation of the DC link current versus torque characteristic of the drive. Comment on how the load torque varies with the DC link current I d. 5.3 Calculate and plot the operating slip s, and the range of its variation for each of the firing angles. The slip is the synchronous speed minus the actual speed expressed as a ratio of the synchronous speed of the motor. 5.4 Comment on the efficiency of the drive for one particular load condition for each of the firing angles you have used. The efficiency at any speed and load is the ratio of the DC generator shaft input power and the AC input power to the induction motor. Experiment 2 Slip power recovery scheme for IM 6 F. Rahman
Appendix: Machine parameters TABLE II THE INDUCTION MOTOR PARAMETERS Rated line-line voltage 415 Continuous current 7.8 A/phase Rated power 3.7 kw Number of Poles 4 Rated Torque 22 Nm Stator-rotor turns ratio 4 R s 1.54 /phase L m 184.7 mh/phase L ls 11.56 mh/phase R 2 2.7 /phase L lr 11.56 mh/phase TABLE III - PERMANENT MAGNET DC GENERATOR PARAMETERS Rated Armature oltage 180 Rated Armature Amps 21. 6A Rated Output Power 3.6 kw Rated speed 1750 rev/min Armature Resistance 0.6 Experiment 2 Slip power recovery scheme for IM 7 F. Rahman