Presented by-
MOINUDDIN M SHIRBADGI

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Abstract - Some form of simple control is needed for small induction motors. This paper proposes a simple speed control method for small induction motors using a simple calculation without the need for a speed sensor. In this method, the load torque is estimated from the detected power factor of the induction motor and the speed of the induction motor is compensated for by a frequency-boost technique. The boost frequency is selected from a data table measured before hand. Satisfactory experimental results are obtained with 9OW induction motors.
I .INTRODUCTION
The induction motor is one of the most widely used actuators for industrial applications today mostly because of its low cost, ruggedness and reliability. The important parameters of an induction motor that are susceptible to uncertainties on operating conditions are the load torque and rotor resistance. Induction motor is basically a simple, cheap, and reliable machine which can provide excellent characteristics. In recent years, with the development of the technology for the adjustable speed drive of induction motors, the research in field-oriented control has been advanced on the control of induction motors. Especially, the speed sensor less control is greatly developed. When a rapid response is not required, and in order to lower the cost of controllers for small induction motors, some form of simple speed control is needed that does not require an expensive pulse-encoder and DSP. Because the stator current of small induction motors is small and the variations of current are very small when the load fluctuates, and since the noise of inverter current is large, the detecting of the variations of current is quite difficult. Although detecting the stator currents is difficult, the variation of the phase difference can be easily detected when the load is fluctuating. This paper presents a new speed sensor less control method based on the phase difference of the stator current for small induction motors. In this system, the speed fluctuations are estimated by the phase difference of the stator currents. The inverter frequency is calculated using a data table that has been measured previously, and adjusted to compensate for the fluctuation of rotational speed. The steady-state and the transient torque-speed characteristics are measured to confirm that this method can be easily applied to 9OW induction motors. Single phase induction motors are widely used, due to their simplicity, strength and high performance. They are used in household appliances, such as refrigerators, freezers, air conditioners, hermetic compressors, washing machines, pumps, fans, as well as in some industrial applications.
PRINCIPELE
A. Calculating the Phase Difference Using an Equivalent Circuit
In order to control the speed of the induction motor without a speed sensor, it is necessary to estimate the torque. For estimating rotor speed, the power factor is an important parameter. The relationship between torque and phase difference can be easily understood from the measured values shown in Fig.1. Here, the phase difference is converted to a time difference. If the load torque changes, the phase difference must also changes under the condition that the motor rotates at the preset speed. The fluctuation of the torque is presumed by detecting the phase difference between the stator voltage and the stator current of the motor. Motor slip can then be estimated.
Figure 2 shows an L-type equivalent circuit, where V1 is the stator voltage, I1 is the stator current, r1 is the stator resistance, f is the operating frequency, x (=2*3.142*FL) is the leakage reactance of both the stator and rotor, r2 is the rotor resistance referred to on the primary side of the motor and s is the slip. The power factor of the induction motor can be obtained as follows using the parameters shown in Fig.2:
B Relations Between Operating Frequency and Phase Difference
In this control system, the relations between operating frequency and phase difference of the primary circuit are measured previously under the condition that the rotational speed of the induction motor is held constant, even if the load torque varies. Figure 3 shows the characteristics of operating frequency versus the phase difference when the rotational speed of the motor is 300rpm, 600rpm, 900 and 1200rpm respectively. The solid line shows the measured value and the dotted line shows the calculated value. Furthermore, the phase difference is converted to a time difference in order to measure the phase difference more easily using a digital counter. Next, the results of these characteristics are stored in a microprocessor as a data table. When the load torque changes, the rotational speed of the motor also changes. For the motor to maintain a constant speed, the operating frequencies are calculated by the microprocessor using the characteristics in Fig.3 to increase or reduce the frequency. Although only four kinds rotational speed are shown in Fig.3, linear interpolation can be used to calculate the values at another speed.