08-16-2017, 10:11 PM
A New Induction Motor V/f Control Method Capable of High-Performance Regulation at Low Speeds
Abstract
A novel open-loop speed control method for induction motors that provides high output torque and nearly zero steady-state speed error at any frequency is presented. The control scheme is based on the popular constant volts per hertz (V/f) method using low-cost open-loop current sensors. Only stator current measurements are needed to compensate for both stator resistance drop and slip frequency. The scheme proposed fully compensates for the current resistance (IR) voltage drop by vectorially modifying the stator voltage and keeping the magnitude of the stator flux constant, regardless of changes in frequency or load. A novel slip frequency compensation, based on a nonlinear torque speed estimate, is also introduced. This method reduces the steady-state speed error to almost zero. It is also shown that a linear torque speed approximation is a special case of the nonlinear estimate and that it leads to large speed errors for loads greater than rated. It is shown that, by using the proposed method, the speed can be accurately controlled down to, at least, 1.2 Hz with load torques of more than 150% of rated value. Since the only machine parameter required, the stator resistance, is automatically measured at startup time, using the same pulsewidth modulated voltage-source inverter without additional hardware, the proposed drive also exhibits self-commissioning capability.
Index Terms Constant volts per hertz, induction machine, low speed, slip compensation.
I. INTRODUCTION
THE operation of induction motors in the so-called constant volts per hertz (V/f) mode has been known for many decades, and its principle is well understood [1], [7]. With the introduction of solid-state inverters, the constant V/f control became popular [2] [4], and the great majority of variablespeed drives in operation today are of this type [5]. However, since the introduction of vector control theory by Blaschke [6], almost all research has been concentrated in this area, and little has been published about constant V/f operation. Its practical application at low frequency is still challenging, due to the influence of the stator resistance and the necessary rotor slip to produce torque. In addition, the nonlinear behavior of the modern pulsewidth modulated voltage-source inverter (PWMVSI) in the low voltage range [8] [10] makes it difficult to use constant V/f drives at frequencies below 3 Hz [11]. The simplest stator resistance compensation method consists of boosting the stator voltage by the magnitude of the current resistance (IR) drop [12]. Improved techniques using the in-phase component of the stator current and a compensation proportional to a slip signal have also been proposed [7]. A vector compensation was proposed in [13], but it required both voltage and current sensors and accurate knowledge of machine inductances. More recently, a scalar control scheme was proposed [14]. In this scheme, the flux magnitude is derived from the current estimation. In [15], using the dc-link voltage and current, both flux and torque loops are introduced. Its use at low frequency is limited by the flux estimation. Also, the slip compensation was based on a linear torque speed assumption which led to large steady-state errors in speed for high load torques. A linearized frequency compensation control based on an ideal induction motor was proposed in [16]. In this paper, a new stator resistance and frequency compensation technique requiring minimum knowledge of the motor s parameters is presented. The only measured quantity is the stator current. The stator resistance voltage drop is fully compensated for by vectorially adding it to the commanded voltage using both in-phase and quadrature components of the stator current. The frequency compensation is based on an estimation of the airgap power and a nonlinear relationship between slip frequency and airgap power. This method predicts the correct slip frequency for any load at any frequency. The proposed control scheme requires only nameplate data, the stator resistance value, and a reasonable estimation of the breakdown torque. The proposed method is validated by simulation and experimental results. It is shown that large torques are obtainable, even in the low speed range, with almost no steady-state error in speed.
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