Controlling the Speed of Induction Motor Using Variable Frequency Drive (Vfd)

IAETSD JOURNAL FOR ADVANCED RESEARCH IN APPLIED SCIENCES ISSN NO: 2394-8442

Ketul S. Kachhia, Bhavik Master, Janki Shah Gujarat Technological University, [email protected]

Abstract - This project discusses speed control of induction motor using Variable Frequency Drive (VFD). The speed of induction motors remains constant because it takes rated power from supply and therefore it causes problems when need less motor speed. The VFD mechanism provides an approach for variation in speed of induction motors. The project presents the working principle of VFD, its performance and the use of Pulse Width Modulation (PWM) in the three phase inverter to control or maintain the ratio of voltage to frequency. The model is simulated and the results are analyzed. This paper is intended to provide the basic understanding of VFD terms, VFD operations and Power factor improvement.

Keywords - Induction motor, rectifier, converter, drive, IGBTs, modulator, intermediate circuit etc.

I. INTRODUCTION

The motor get rated supply from the source but now if we want to increase the speed than what we do? At this time VFD comes in the play. The most obvious advantage of a VFD is the notable energy efficiency. The motor runs at a continuous speed regardless of the requirements placed on it by the compressor because the power supply is constant if the motor is controlled by a VFD, the frequency of the electricity powering the motor and hence the speed of the motor can be regulated according to the demands of the application. Drives are becoming more advanced, now offering exceptional feedback and monitoring capabilities. Optimizing motor and compressor performance can reduce stress on the mechanical and electrical components of a system, limiting component failure resulting from wear. Many drives now offer safety features such as short circuit protection and safety torque off (STO) functionality. In comparison to a mains-powered motor with direct starting, a VFD can reduce the starting current by up to 80% without affecting the starting torque. By enabling lower starting currents, the energy grid is placed under less stress when motors are powered up, reducing the impact on other local grid energy consumers. The resulting lower energy frequency fluctuations also mean that failures of electrical equipment can be reduced.

A. Rectifier:

Working principle of rectifier is changing the incoming alternating current (AC) supply to direct current (DC). Different designs are available and these are selected according to the performance required of the variable frequency drive. A three- phase diode rectifier converts a three-phase AC voltage at the input to a DC voltage at the output. To show the working principle of the circuit the source and load. The DC voltage is divided into six segments within one fundamental source period that corresponds to the different line-to-line source voltage combinations In each segment there is a minimum and maximum DC voltage.

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Fig.1 Rectifier

B. DC Link :

DC link is a connection which connects a rectifier and an inverter. These links are found in converter circuits and in VFD circuits. The AC supply of a specific frequency is converted into DC. This DC, in turn, is converted into AC voltage. The DC link is the connection between these two circuits. The DC link usually has a capacitor known as the DC link Capacitor. This capacitor is connected in parallel between the positive and the negative conductors. The DC capacitor helps prevent the transients from the load side from going back to the distributor side. It also serves to smoothen the pulses in the rectified DC.

Fig. 2 DC link

C. Inverter

An Inverter Drive (VFD) works by taking AC mains (single or three phase) and first rectifying it into DC, the DC is usually smoothed with Capacitors and often a DC choke before it is connected to a network of Power Transistors to turn it into three phases for the motor The control method is known as ‘PWM’ for 'Pulse Width Modulation'. This means the DC is switched on and off very quickly (chopped) by the Transistor switches. A sine wave of motor current is made by a series of DC pulses where the first has a very short ‘on’ period, followed by a longer on period, then longer until the widest pulse appears in the Centre of the positive sine wave, then smaller until the DC is inverted and the same pattern of pulses generate the negative part of the sine wave.

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Fig.3 Inverter

II. PROJECT DESIGN

VFDs are variable frequency and hence speed drives (motor controllers) which are suitable for use in constant torque and/or constant power loads by varying supply voltage and frequency. VFD are usually termed as inverter drives which are driven by pulse width modulation (PWM). Before the inverting stage, a converter is used.

Fig.4 circuit diagram of VFD

Many industrial drives require variable speed operation, for which frequency control is the most suitable method. A variable speed drive is basically a variable frequency drive where control over speed is accomplished by controlling the frequency of supply at the motor terminals. You have a front end converter for rectification ( AC to DC ). This is followed by a D.C link capacitor which maintains a ripple free D.C link voltage and finally there is a voltage source inverter (VSI) that converts D.C link voltage to variable frequency A.C. Typical VSIs have square wave outputs, and for the induction motor to drive a sinusoidal current PWM based VSIs are used.

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III ADVANTAGES OF VARIABLE

 Process temperature can be controlled without a separate controller.  Low maintenance.  Longer lifespan for the AC motor and other machinery.  Lower operating costs.  Equipment in the system that cannot handle excessive torque is protected.  Regenerative power capability.  Simple circuitry  Reliability (Current Limiting Operation).  Clean current waveform.  No motor cogging.  Efficiencies from 92% to 96%Excellent input power factor due to fixed DC bus voltage.  Low initial cost.

IV CONCLUSION

Providing high performance variable speed drives for maximum process productivity has always required complex engineering considerations. Rapid improvements in AC control technology, combined with the ready availability of standard fixed frequency AC motors has increased the number of possible solutions. However, a component approach will not lead to an optimal solution in many cases. In order to utilize the present generation of adjustable frequency controllers to meet application needs equal to or better than DC motors have in the past, a definite purpose AC motor is required. A square laminated-frame configuration with integral feet on the end brackets and adaptable electromagnetic designs is one approach that meets this objective.

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