PWM AC Chopper Control of Single-Phase for Variable-Speed Fan Application

Deniz Yildirim Murat Bilgic Istanbul Technical University Istanbul Technical University Faculty of Electrical and Electronics Engineering Faculty of Electrical and Electronics Engineering Department of Electrical Engineering Department of Electrical Engineering 34469 Istanbul, Turkey 34469 Istanbul, Turkey Email: [email protected] Email: [email protected]

torque Abstract—This work presents a variable speed control method operating for fan applications. A pulse-width-modulated (PWM) AC chop- V1 V >V >V point per changes the effective value of the supply applied to a 1 2 3 single-phase induction motor. This variable supply voltage gives the ability to control the speed of the motor. Harmonics generated V2 by the speed control unit are filtered by an input filter according motor to EN 61000-3-2 limits for harmonic current emissions standards. Experimental results on a 230V, 210W in-line centrifugal fan for V exhaust ventilation application shows that PWM AC chopper 3 is simple and cost effective control method compared to other methods in terms of simplicity and input harmonic content. fan speed I. INTRODUCTION n n1 s Fractional horse power permanent-split-capacitor (PSC) single-phase induction motors are widely used in blower and Fig. 1. Torque-speed characteristics of an induction motor and fan load with fan applications. These motors mostly have external variable voltage. types resulting in compact structure and installation easiness of fan propeller to the rotating part of the motor. When rated The is turned on at a desired phase angle allowing voltage is applied, fan motors typically run at a constant some portions of the supply voltage be applied to the main and speed determined by the capacitance value that gives optimum auxiliary windings of the motor [3], [4]. In another method, running performance with a proper starting torque. only main winding voltage is varied by a triac AC chop- Depending on the application area, variable air flow rate, per while keeping the auxiliary winding voltage constant at therefore, a variable speed operation may be advantageous rated value [5]. Phase control method results in discontinuous in terms of energy efficiency [1]. In a typical application input current waveform which consists of higher order odd where low air flow rate is required, some form of mechanical harmonics of supply frequency. Multi-speed operation at two- flow reducers are employed while motor is running at rated thirds and three-thirds of the supply frequency is proposed in speed. This type of control is not energy efficient since [6] using a triac bridge where the current waveform contains motor is consuming rated power even though lower operating odd and even harmonics as well as subharmonics of supply speed may give the same air flow resulting in smaller power frequency which exceeds the harmonic limit standards. AC consumption. choppers employing integral-cycle control method uses certain Several methods exist for variable speed operation of a number of complete cycles be applied to load followed by single-phase induction motor. Considering simplicity and low certain number of zero voltage periods [7]. Subharmonics of cost, most common type is the control of applied voltage to the supply frequency also occur in this type of control which the motor. Typical torque-speed characteristics of an induction is very difficult to filter. motor with variable voltage along with the fan load curve Operation of pulse-width-modulated (PWM) AC choppers are shown in Fig. 1 indicating the adjustable speed operation. at high chopping frequencies will result in harmonics appear- The voltage applied to the motor can be varied by an au- ing at higher frequencies where small sized filters can easily totransformer or a tapped winding arrangement [2]. Since tap eliminate them. A four-quadrant high-frequency AC chopper changing is performed by a mechanical switch and considering operation is given in [8] where chopper feeds an inductor- size and weight of the , this type of voltage control capacitor load. In another application, PWM AC chopper is may not be favorable. Another approach is to employ an AC used to control the speed of a [9] and a chopper using triac semiconductor switches. single-phase induction motor [10] at a switching frequency

k,(((  of 1.8kHz.

II. SYSTEM DESCRIPTION Typical application for a variable speed fan operation em- ploying PWM AC chopper is illustrated in Fig. 2 where closed loop operation permits to keep the temperature of a process at desired level by adjusting the rate of air flow. The input filter removes high frequency switching present in the input line current. (a) BDTX−200B 120

PWM input 100 AC filter chopper A 80 supply motor and fan voltage control signal 60 sensor gating input power (W) circuit sensor 40 gain A: mechanical control B 20 controller B: variable speed control - + 0 reference 0 100 200 300 400 500 600 700 800 air flow rate (m3 /h)

Fig. 2. Typical schematic of a variable-speed fan application. (b)

Fig. 3. (a) Experimental test setup and (b) power consumption of fan motor An experimental test is performed to compare the consumed at various air flow rates obtained by a mechanical louver and variable speed power by a fan motor operating at various air flow rates control. with mechanical louver and variable speed controls. An in- line centrifugal fan operated at rated speed is connected to Vm one end of a long pipe as shown in Fig. 3a and air flow is varied by an adjustable diameter opening located at the other end. While power consumption is approximately same for all 2π air flow rates with mechanical control, considerable energy π ωt savings occur for variable speed operation where reduction of 1 DTs fs= input electrical power especially at low air flow rates is larger d(t) detailed Ts view as is evident from Fig. 3b. Ts

III. PWM AC CHOPPER motor vs(t) and fan PWM control technique simply chops the supply voltage at high frequencies as shown Fig. 4 where duty cycle D is ωt defined as the ratio of on-time to total switching period. The DTs Ts line voltage is chopped by bidirectional switches. The change in the duty cycle of the switch changes the effective value of Fig. 4. PWM chopping of an AC sinusoidal voltage. the load voltage and load current. The increase in duty cycle will allow the load current and load voltage to increase while frequency. a0, an and bn can be calculated using (2) to (4). decreasing the duty cycle will do the opposite. ton ton The chopped voltage can be expressed by multiplying the a0 = = = D (2) T ton + toff sinusoidal supply voltage with the switching signal d(t) as 1 depicted in Fig. 4. Switching function d(t) can be expressed an = sin(n2D) (3) n by opening the Fourier series of the pulse for one switching 1 bn = [1 + cos(n2D)] (4) period as in (1) n ∞ 2 2 cn = pan + bn (5) d(t) = a0 + X(an cos nωst + bn sin nωst) (1) n=1 The load voltage can then be calculated by multiplication of supply voltage and switching function. where a0 is the DC component, an and bn are the Fourier coefficients, n is the harmonic order and ωs is the switching vs(t) = Vm sin ωt

k,(((  vL(t) = vs(t) d(t) = Vm sin ωt d(t) A. Realization of AC Chopper vL(t) = a0Vm sin ωt Fig. 6 shows the realized PWM AC Chopper with opto- ∞ couplers where UC3525 is employed as a PWM generator + X [anVm (cos nωst sin ωt) =1 n M1 M2 Lf + bnVm (sin nωst sin ωt)] (6) induction motor The terms in square brackets of (6) are the high frequency M3 auxiliary vs winding terms. When these are filtered, the load voltage can be Cd expressed according to the fundamental component of supply Cf R main frequency. d winding M4

vL(t) = a0Vm sin ωt = D Vm sin ωt (7) C

The effective value of load voltage can now be calculated as isolated V auxiliary a2 power LED in (8) TLP250 supply LED

V TLP250 D Vm a3 V vLrms = (8) a1 √2 V =+15V a1 The PWM AC Chopper is actually a Buck converter operating in AC mode. It consists of two bidirectional switches with complementary switching patterns. The upper switch (SW1) is employed for voltage chopping (Fig. 5a) and the lower switch TSC428 (SW2) is used to provide an alternative path for the current UC3525 of the motor when SW1 is turned off (Fig. 5b). Mechanical

auxiliary winding Fig. 6. Realized PWM AC Chopper with optocouplers. SW1 fan operating at operating frequency of 25kHz. In open-loop con-

SW2 figuration the saw tooth waveform is compared to a reference

vs DC voltage derived by a variable resistor. The output signal main winding induction motor is then applied to a MOSFET driver which generates two complementary PWM signals. These signals are applied to the gates of the MOSFETs by two separate optocouplers. C (a) B. Input Filter Design auxiliary winding SW1 The input filter stage filters the high frequency switching fan harmonics from entering the utility. It is nearly always required that a filter must be added at the power input of a switching

SW2 converter for improving power quality and interface issues.

vs By attenuating the switching harmonics that are present in the main winding induction motor converter input waveform, the input filter allows compliance with regulations that limit conducted electromagnetic interfer- ence (EMI) and harmonic issues. C The PWM AC Chopper injects the pulsating current into (b) the power source at harmonics multiples of the switching fre- Fig. 5. A simple Single-Phase AC Chopper showing current flow path (a) quency fs and duty cycle (D) affects the magnitudes of these when SW1 switch is ON and (b) when SW2 switch is ON. harmonics. Equations (3) and (4) show that the harmonics are a function of duty cycle (D). The input current is(t) and input switches in Fig. 5 can be realized by MOSFETs. Shorted gate voltage vs(t) have similar degrees of harmonics as it is in load and source terminals of two MOSFETS allows bidirectional voltage. controlled switch operation. Both MOSFETs in the switching Fig. 7 shows the experimental results of input voltage and network are floating devices, i.e., the converter stage and input current waveforms when no input filter is employed. the control network must be isolated to drive these switches. Corresponding FFT results are illustrated in Fig. 8a along with This isolation can be obtained by either implementing a pulse the computed ones using (5) in Fig. 8b. It is seen that the first transformer or an optocoupler. harmonic starts at 25kHz which is the switching frequency

k,(((  The addition of an input filter affects the dynamics of the power electronic converters, often in a manner that degrades the regulator performance. The input filter affects all transfer functions of converter. Moreover, the influence of this input filter on these transfer functions can be quite severe [5]. The Bode diagram shows an asymptotic peak occurring near the corner frequency causing the gain of the filter to go to infinity. This rise would cause extreme current peaks which would make the system worse than it was before. The output impedance of the LC filter tends to infinity at frequencies near corner frequency f0 [12]. 1 (a) f0 = (9) 2pLf Cf Therefore, low pass input LC filter needs to be damped at the corner frequency. The damping is obtained by placing a series connected capacitor and resistor in parallel with original capacitor. Fig. 9 shows the structure of a undamped and damped LC filter and corresponding magnitude plots. The

PWM PWM Lf Lf AC C AC chopper d chopper Cf C vs and vs f and R induction d induction motor motor undamped input filter damped input filter (b) (a) Undamped Input Filter Fig. 7. Input waveforms at 110V, 50Hz, D = 75%; (a) input voltage 60 (100V/div, 2.5ms/div), (b) input current (0.75A/div, 2.5ms/div). 40 20 compared with the 50Hz fundamental component. Magnitudes 0

of multiples of this harmonic content change depending on Magnitude (dB) −20 −40 the duty cycle (D). The fundamental component is at 50Hz 2 3 4 5 6 10 10 10 10 10

Damped Input Filter

60 40 20 0

Magnitude (dB) −20 −40 2 3 4 5 6 10 10 10 10 10 Frequency (rad/sec) (b)

(a) (b) Fig. 9. (a) Damped LC input filter and (b) frequency response.

Fig. 8. FFT of input voltage in Fig. 7a (110V, 50Hz, D = 75%); (a) experimental (25 kHz/div), (b) calculated using cn computed from (5), n = 1 optimum value of damping resistor Rd is calculated based is the fundamental component at 25 kHz. on the value of Cd using a procedure given in [12]. which is the supply voltage frequency. These high switching C. Experimental Results of the PWM AC Chopper with harmonics cause two main problems. The first one is that Damped Input Filter high frequency switching causes electromagnetic disturbances PWM AC chopper is operated at various duty cycle values affecting nearby electronic equipment and the second one with damped input filter. The effect of the filter can easily is that it draws harmonic currents form the power supply be seen in the input current and voltage at the supply side decreasing the power quality. International standards have been as shown in Fig. 10 for a duty ratio of D = 75%. FFT developed in order to bring some limits to electromagnetic spectrum of these waveforms in Fig. 11 indicate that higher compatibility (EMC) and power quality issues [11]. order harmonics are completely eliminated from the supply

k,(((  (a) (b)

(a)

(c) (d)

Fig. 11. FFT of unfiltered (a) input current and (b) voltage of Fig. 10 (25kHz/div, supply voltage 110V, 50Hz) and FFT of (c) input current and (d) voltage with damped input filter (25Hz/div, supply voltage 220V, 50Hz).

M1 M2 Lf

induction motor M3 auxiliary vs winding Cd

Cf R main d winding (b) R M g1 Rg2 4

Fig. 10. Input current, input voltage and converter input current for a duty C ratio of (a) D = 75% and (b) D = 50%, 5ms/div, 400V/div, 1.5A/div.

auxiliary side. Only 50Hz fundamental component is present in the power supply supply voltage and current. As explained earlier, power circuit +15V Cs of AC Chopper must be isolated from driving network and this isolation was realized using optocouplers shown in Fig. 6. The optocouplers work both well in isolation and signal transfer but these isolated drivers need two isolated DC power supplies which increases complexity of the total circuit structure. TSC428 A secondary PWM AC chopper circuit was also developed UC3525 using a pulse transformer to isolate the two stages instead of optocouplers as illustrated in Fig 12. The advantage of a pulse transformer is that there is no need for two extra isolated power Fig. 12. Realized PWM AC chopper with pulse transformer isolation. supplies. The pulse transformer has an important disadvantage in which the designer must keep in mind. A transformer is not capable of transferring DC signals, therefore the drive network performance with the initial circuit given in Fig. 6. must be designed to work in the duty cycle range between Even though a pulse transformer is employed for gate drive 0.1 and 0.9. In any case of a duty cycle of 0 or 1 would isolation in Fig. 12, there is still need for a DC voltage source force to open both of the bidirectional switches at the same used in the PWM generating circuitry. The DC voltage(s) time. The input filter for this new circuit has the same filter can be obtained from the supply voltage by a low power designed for the chopper shown in Fig. 6 and PWM generating (about 2W) isolated flyback converter. These power supplies section is also the same. Fig. 13 shows the output voltage and are implemented into the circuits given in Figs. 6 and 12 such output current for the PWM AC Chopper shown in Fig. 12 at that both choppers work independently without the need for two different duty cycle values. The chopper shows the same external DC power supplies.

k,(((  (a)

Fig. 14. Comparison of measured torque-speed characteristics for two different obtained by PWM chopper and regulated sinusoidal voltage source.

been eliminated. It can be concluded that PWM AC chopper is simple and cost effective control method compared to other methods in terms of simplicity and input harmonic content.

ACKNOWLEDGMENT The authors would like to thank Mr. Omer Faruk Bahcivan from Bahcivan Company for his kind support in performing this study.

(b) REFERENCES

Fig. 13. Output voltage and output current (a) D = 1 and (b) D = 0.5, [1] M. M. Morcos, J. A. Mowry, and A. J. Heber, “A solid-state speed con- 5ms/div, 200V/div, 0.75A/div troller for capacitor motors driving ventilation fans,” IEEE Transactions on Industry Applications, Vol. 30, No. 3, May-June 1994, pp. 656-664. [2] T. J. E. Miller, J, H, Gliemann, C. B. Rasmussen, D. M. Ionel, “Analysis IV. MOTOR PERFORMANCE TESTS of a tapped-winding capacitor motor,” International Conference on Electrical Machines (ICEM 98), Istanbul, Turkey, Sep. 2-4, 1998, pp. Final performance tests are performed on the external-rotor 581-585. single-phase induction motor to determine how the torque- [3] K. Sundareswaran and P. S. Manujith, “Analysis and performance speed curves vary when motor is fed from a PWM AC chopper. evaluation of triac-voltage controlled capacitor run induction motor,” Electric Power Components and Systems, 2004, pp. 913-925. Two reduced voltage values are used in the test. Induction [4] C. Kim, C. Choi, D. Lee, G. Choi, S. Baek, “Torque characteristics motor is first tested with pure sinusoidal voltage obtained of single phase induction motor for phase control method,” Sixth In- from a well regulated AC power source and then same voltage ternational Conference Electrical Machines and Systems, ICEMS 2003, Beijing, China, Nov 9-11, pp. 510-513. values are applied through PWM AC chopper. The resulting [5] M. B. M. Hamid,. “New method for speed control of single phase torque-speed curves shown in Fig. 14 are obtained along with induction motor with improved performance”, Energy Conversion and the fan characteristic curve. Although the curves are close to Management, 2000, pp. 941-950. [6] A. R. Julian, R. S. Wallace and P. K. Sod, “Multi-speed control of single- each other, PWM AC fed curves are somewhat lower than phase induction motors for blower applications,” IEEE Transaction On the sinusoidal voltage case for speeds up to maximum torque Power Electronics, Vol. 10, No.1, 1995, pp. 72-77. speed. [7] M. S. J. Asghar, “Smooth speed control of single-phase induction motors by integral-cycle switching,” IEEE Transactions on Energy Conversion, V. CONCLUSION Vol. 14, Issue 4, Dec. 1999, pp. 1094-1099. [8] S. B. Yaakov and Y. Hadad, ”A Four Quadrants HF AC Chopper with no A 25kHz PWM AC Chopper used in a single-phase in- Deadtime”, IEEE Applied Power Electronics Conference and Exposition, duction was designed and realized in this study APEC ’06. 19-23 March 2006. [9] H. Bodur, A. F. Bakan, and M. H. Sarul, ”Universal Motor Speed Control for both domestic and industrial fan applications. This circuit with Current Controlled PWM AC Chopper by using a Microcontroller”, can be used to control the motor speed according to the Proceedings of IEEE International Conference on Industrial Technology, temperature reference. Due to high frequency switching, the Jan. 19-22 2000, pp. 394-398. [10] N. A. Ahmed, K. Amei and M. Sakui, ”AC chopper voltage controller- PWM AC chopper does not generate low frequency harmonics fed single-phase induction motor employing symmetrical PWM control which are multiples of the 50Hz component. The harmonic technique,” Electric Power System Research, 2000, pp. 15-25. distortions appear at higher frequencies that are actually [11] EN 61000-3-2, ”Limits for harmonic current emissions (equipment input current up to and including 16A per phase),” European Standard, 2000. multiples of the switching frequency and very easy to filter. [12] R. W. Erickson, ”Optimal Single Resistor Damping of Input Filter”, Experimental results of the damped input filter have shown that IEEE Applied Power Electronics Conference and Exposition, APEC ’99, the input voltage and input current harmonic distortions have Vol. 2, 14-18 March 1999, pp.1073 - 1079.

k,(((