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GRANT LAIDLAW
Grant Laidlaw is currently the owner of the Air Conditioning and Refrigeration Academy ( ACRA ) in Edenvale . He holds a Bachelor of Business Administration and an associate degree in educational administration . He has a National Technical Diploma and completed an apprenticeship with Transnet . He has dual-trades status : refrigeration and electrical . He has been involved with SAIRAC for over two decades and served on the Johannesburg committee as chairman and was also president between 2015 and 2018 . Currently he is the SAIRAC national treasurer .
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By Grant Laidlaw
Many people ask for assistance in the understanding of theoretical and practical aspects of the industry . I will endeavour to enlighten . I am going back to basics as I have questions coming in that indicate that the basic understanding necessary to work in industry is not in place .
Ryan asks : Would it be possible to assist with an understanding of three phase starters , in particular direct on line , star / delta and one we recently ran into , a part wind motor . Why and when do we use the different starting methods and possibly why are we using more and more VSD drives , thanks .
Hi Ryan , picking up from the past issue where we looked at direct on line and star delta , let us look into the Variable Speed Drives ( VSDs ) also known as a Variable Frequency Drive ( VFD ) and the reasons for their use .
A variable speed drive takes fixed-frequency AC supply and converts this to a variable-frequency AC supply through a DC link . Power use and mechanical power output is controlled so that the motor can run at the most efficient speed for the motor and the equipment being driven .
Control of the motor speed can be based on feedback from the equipment , for example , temperature , or pressure . This enables accurate control of the motor speed over a broad range to suit system demands . This provides precise electrical motor control so that motor speeds can be decreased or increased and therefore maintain the speeds and torque required . This is achieved by varying the voltage and frequency supplied to the electric motor . The result is that motors utilise only the energy required , thus increasing equipment efficiency .
Variable Speed Drives connect to standard AC induction motors with control capabilities for adjusting speed , torque and horsepower . Variable Speed Drives contain three key components which can be used to describe their basic working principle :
Image supplied
• The first step in this process is to convert the AC supply voltage into DC using a rectifier or converter .
• This rectifier is in turn connected to a DC filter ( DC Link or bus ). The DC circuit contains the capacitor and inductor used for filtering ( smoothing ) the DC power output from the previous step which contains voltage ripples .
• The next main element is the inverter . The basic working principle of an inverter is switching the DC on and off so rapidly that the motor receives a pulsating voltage similar to AC . The output voltage is turned on and off at a high frequency , with the duration of ontime or width of the pulse controlled to approximate a sinusoidal waveform . The switching rate is controlled to vary the frequency of the simulated AC that is applied to the motor .
This is all controlled by for example a logic controller : A standard AC across-the-line motor starter , line voltage and
A VSD drive flow block diagram .
www . refrigerationandaircon . co . za RACA Journal I March 2023 41