Real Time FPGA Implementation of PWM Chopper Fed Capacitor Run Induction Motor

This paper presents the performance enhancement of capacitor run induction motor by pulse width modulated AC chopper.The phase angle control faces severe shortfall in the performance improvement for larger triggering angles. In this paper the comparison of phase angle control and sinusoidal pulse width modulation technique is encountered for effective speed control of single phase capacitor run induction motor. The necessary parameters are taken into considerations are higher efficiency, lesser total harmonic distortion and high input power factor. The results are compared by using the simulations using matlab Simulink environment. The validation of result in hardware is implemented using Field programmable gate array for sinusoidal pulse width modulation technique. All


INTRODUCTION
Single phase induction motor drives play a significant role in domestic and industrial applications. The speed control of induction motor is widely used in all applications. All the industries prefer single phase induction motor compared to other motor drives due to less cost, simpler control circuits and less maintenance. The conventional method utilizes integral cycle control and phase control for controlling the speed. In integral cycle control the single phase induction motor is controlled but due to sub harmonics causes lumbers in torque speed characteristics [1]. The merits of conventional control are low cost and easier control. The demerits of conventional control are poor power factor, high harmonic distortion and nonsinusoidal voltage at the input. The speed control of capacitor run induction motor drives fed from AC chopper utilizes different topologies of phase control methods. The semiconductor device TRIAC is connected in the main winding, TRIAC connected in auxiliary winding of motor and anti-parallel thyristors are connected to the input supply. The phase control is better compared to single pulse width modulation, it is shown that efficiency is slightly better in phase control than singe pulse width modulation technique. The literature shows that multiple pulse width modulation gives better quality output than phase control method. The performance is determined by good input power factor, less total harmonic distortion of current and voltage and improvement in efficiency.The sinusoidal pulse width modulation technique is utilized for effective speed control and improvement in performance parameters. In the literature TRIAC connected in auxiliary winding gives better output at lower triggering angles. At larger triggering angles the power factor is poor, harmonic distortion value is more and non-sinusoidal waveform at the input. TRIAC connected in auxiliary winding of motor is not suitable for speed control due to the unstable operation at certain triggering angles [8]. The merits of pulse width modulation AC chopper is high efficiency, less harmonic contents,  [2]. The commonly used configuration of AC-DC-AC which are used in variable speed drives, FACTS controller and wind turbines are replaced by AC-AC converter without DC source [3]. The turns ratio of single phase induction motor is treated as complex quantity and the corresponding performance parameters are computed [4]. The topology used is symmetrical AC voltage controller which utilizes three switches. The performance is effective for dimmer loads and induction heating, but the lower order harmonics is present in the output voltage [5]. PWM chopper requires second order filter for effective performance under closed loop control. The closed loop control with filter reduces the fluctuation due to load variations [6]. The capacitor run induction motor is used as braking using triac ac voltage controller. The inhibiting pulses of rectifier are a failure compared to disconnection of auxiliary winding [7].
AC voltage regulation is important aspect in industry and domestic applications. The new mathematical model is derived and low cost model. The controller is checked effectively with I and PI control mechanisms [9]. The drawback of AC to AC converter with phase control is at higher firing angles the output waveform is non-sinusoidal and poor power factor. This paper compares the conventional controller with extinction angle control and sinusoidal pulse width modulation [10]. PWM method is better than phase angle control and the converter produces low order harmonics at higher conduction angles [11]. The converter is connected with capacitor bank to reduce the total harmonic distortion. At higher conduction angles the distortion factor increased due to load [12]. The resonant dc link converter operates at zero voltage and zero current switching leads to more efficient. Compared to hard switching the delayed triggering angle control gives better output and utilizes the passive load at the output [13]. The harmonic elimination is done through sinusoidal pulse width modulation and gratifies through the amplitude and frequency modulation [14]. TRIAC control is utilized for attaining better torque characteristics for single phase induction motor and the results shows that the optimimum capacitor values determines the torque and efficiency. The lower capacitor shows better performance compared to high value of capacitor. The operation is noisy due to the harmonics present in the system [15]. The TRIAC controlled single phase induction motor experiences the pulsating torque at the output. The significant reason is production of even multiples of harmonics and these can be eliminated by connecting suitable shunt resistor. The limitation of this method is the power dissipation and losses are more due to the resistance [16]. Pulse width modulation of variable speed control of fan utilizes the ac chopper. The harmonics in the output voltage is minimized by employing low pass filters. The design of control circuit components is complex and in the present paer the advanced features of triggering is used [17].
In this paper phase control of speed control method is compared with sinusoidal pulse width modulation. The operation and working principle of sinusoidal pulse width modulation is explained along with the pulse generation obtained by field programmable gate array. In the corresponding chapter the simulation results are compared between phase control method and sinusoidal pulse width modulation method. The waveforms obtained from the hardware using field programmable gate array for the sinusoidal pulse width modulation.

AC VOLTAGE CONTROLLER 2.1. Phase Angle Control
In phase angle control, the two silicon controlled rectifier are connected in anti-parallel is used as the topology of AC voltage controller. The output of AC voltage controller is connected to the single phase capacitor run induction motor. The triggering pulses generated for the two thyristors are alternating in nature. For the period of positive half cycle thyristor T1 is in forward bias and starts conducting when the gate signal is available, while the thyristor T2 is in reversed bias condition. At 180 degrees thyristors need to be switched off by natural commutation. But due to the motor load the current continue to flow. For the period of negative half cycle thyristor T2 is in forward bias and starts conducting when the gate signal is available, while the thyristor T1 is in reversed bias condition. At 360 degrees thyristors need to be switched off by natural commutation. But due to the motor load the current continue to flow. Bidirestional AC voltage controller as shown in Figure 1.

Sinusoidal Pulse Width Modulation
The AC voltage controller uses different topology for the improvement in performance parameters of capacitor run induction motor. This topology uses four diodes and main device IGBT S1 is employed for conducting in both positive half and negative half cycle. For the freewheeling action it employs four diodes and one auxiliary device IGBT S2. The triggering signal is to be generated for the main switch and freewheeling switch. For the period of positive half cycle the current flows through diode D1, switch S1, diode D4 and connected to the capacitor run induction motor and return to the supply. For the period of negative half cycle the current flows through capacitor run induction motor, diode D3, switch S1, diode D2 and connected supply voltage. For the period of positive half cycle the freewheeling action takes through the path load, D3', switch S2, D2'and back to load. For the period of negative half cycle the freewheeling action takes through the path load, D1', switch S2, D4'and back to load.
The term L=time period/2 The term ao corresponds to dc offset component and the term an and bn corresponds to n th harmonic frequency component.
For our calculations the value of L is taken as 1. The PWM signal is defined as In the analysis the even harmonic components are removed by waveform symmetry and odd harmonic components are taken into account. The generalized amplitude of harmonic component an is illustrated by Where D is the duty cycle and odd harmonic components n=3, 5, 7 The total harmonic distortion of output voltage and current is given as

Implementation of Sinusoidal PWM using FPGA
The pulse generation of sinusoidal pulse width modulation is obtained by field programmable gate array. The VHDL language is used to program sinusoidal pulse width modulation for AC chopper. The pulses output from FPGA kit has to be isolated from the power circuit hence the optocoupler is used for this purpose. A suitable zero crossing circuit is used to sense the zero crossing of the waveform. The power switch requires a driver circuit to enable to get correct pulse output and the necessary amplication is done through it. Figure 3 shows control circuit using FPGA.

RESULTS AND ANALYSIS
This sections presents performance enhancement of capacitor run induction motor using different techniques like phase angle control and sinusoidal pulse width modulation technique. The comparative analysis obtained by simulation using matlab Simulink environment. The experimental results are computed for sinusoidal pulse width modulation technique and compared with simulation results. The capacitor run induction motor is used as the load. It is exemplified by 1 Hp, 230V, 5.2 A, 1440 rpm and 12.5 micro farad as running capacitor. The motor is coupled to the mechanical load to achieve variable speed operation. The motor parameters are Rm=7.55 Ω, Xm=5.1Ω, Rr=8.3Ω, Xr=5.1Ω, Ra=10Ω, Xa=10Ω and Xmm=0.324Ω. The results are obtained for bidirectional ac voltage controller and sinusoidal pulse width modulation. The waveforms which are depicted below are efficiency, input power factor, total current harmonic distortion and total voltage harmonic distortion.
The simulation is exhibited for the TRIAC control and the Figure 4 represents variation of firing angles with power factor. It is inferred that at lower firing angles from 0 to 60 degrees the power factor is 0.8. When the firing angle increases the power factor is low. The reason is due to un-symmetric voltage and current waveforms at higher firing angles.  The experimental results are obtained using for sinusoidal pulse width modulation technique using FPGA. The Figure 8 represents the simulated and experimental result of sinusoidal pulse width modulation. The simulated graph in marked in black and experimental graph is obtained in red. For different duty ratio the power factor is obtained at the input of capacitor run induction motor. The power factor variations are steady at all duty cycle and slightly low at 0.5 duty cycle. It is inferred that current and voltage waveforms have lesser phase difference. The Figure 6 is explained as motor input current of total harmonic distortion with variation in firing angles. The distortion is within the limit of IEEE standards and is exhibited till the firing angles of 60 degrees. The non-linearity in current harmonics is unveiled greater than 60 degrees. The power quality is poor due to capacitor run induction motor load, and the corresponding inductive energy requires another path to free wheel and dissipating the energy. The Figure 8 demonstrates the motor voltage harmonics of phase control method with different triggering angles. The harmonics is within the specified limits until 60 degrees and it rises to very high value due to dynamic motor load. It is subjective that current harmonics are high value due to forced commutation of the power switches in the converter. Conversely the higher order harmonics are moved to larger switching frequency and easily filtered by passive filters. The Figure 9 epitomizes the motor input voltage harmonics with respect to duty cycle of sinusoidal pulse width modulation. For higher duty cycle the amplitude of harmonic content is less.The PWM method shows better power quality than phase control. The Figure 10 portrays capacitor run induction motor efficiency for diverse triggering angles. At lower firing angles the efficiency is near 80%, but the value is lesser for higher triggering angles. The reason is due to increased copper losses at larger triggering angles.

CONCLUSION
The PWM chopper fed capacitor run induction motor is impelemented in hardware using FPGA. The proposed control has improved performance parameters compared to conventional phase angle control. The efficiency and power factor has improved compared to phase angle control. The total harmonic distortions of current and voltage are slightly higher value compared to IEEE standards. This can be enhanced by suitable harmonic eliminatic techniques for the effective operation of capacitor run induction motor.