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:

  • 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 on-time 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:

Image supplied

Image supplied

A standard AC across-the-line motor starter, line voltage and frequency are applied to the motor and the motor speed is solely dependent on the number of motor stator poles.

In comparison, a VSD delivers a varying voltage and frequency to the motor, which then in turn determines the motor’s speed. The higher the frequency supplied to the motor, the faster it will run. Power applied to the motor through the VSD can therefore have the motor working speed lower than the nameplate base speed or increase the speed. This is set up to match the equipment requirements.

VSDs come in different sizes ranging from 0.18kW through to Mega Watts and are available as stand-alone devices, as part of supplied control systems or integrated into a motor.

A VSD can be used for simple applications, such as controlling a pump or a fan where variable air flow is required. They can be interfaced with a transducer, such as a pressure sensor, and programmed to operate within specific parameters. VSDs are used in a wide variety of systems including most kinds of ventilation systems, air extraction systems, cooling systems, unitary air conditioning systems, water pumps and central air conditioning systems to name a few. Unitary ‘Inverter systems’ are now widely used and offer considerable energy savings.

VSD versus soft starters

VSDs offer all the benefits of a soft starter but add much more flexibility in terms of features and settings and in most applications, bigger energy cost savings. Not only do VSDs provide an extremely smooth and controlled start, but they also efficiently control motors to nearly any desired level of speed, torque, or position, including over-speed and full torque from start.

As I said before, in most applications VSDs, controlling speed and torque to demand greatly improves efficiency and yields considerable energy savings.

The biggest advantage of a soft starter over a VSD is the lower initial cost. If matching of the speed of the process is not required, then the initial cost of the product tends to become a bigger factor, which might make a soft starter a more optimal choice. If the system can be optimised or additional costs can be saved on electricity and/or by reducing maintenance costs of the overall system then a VSD may be the optimal choice.

Energy conservation utilising VSDs In HERVAC systems

Among the advantages of using a VSD, the biggest is the energy usage reduction and the consequent cost savings. Energy savings of between 25 and 70% can be realised on a three phase motor. VSDs are, at the moment, possibly the most effective method for controlling motor speed in response to varying system operating requirements. Since motors consume energy, the importance of motor control matching load demand increases as energy supply becomes more expensive and our supply more and more strained. A small decrease in the speed of a pump or fans can result in large decreases in energy usage.

VSDs are ideal for equipment with varying load conditions such as HVAC systems and even in the most energy-efficient HVAC systems, VSDs will help to further reduce energy usage.

The following points give an idea as to possible usage:

  • There exists a broad range of VSDs available specifically for heating, refrigeration, ventilation and air conditioning applications. Many of these drives can be integrated into a building management system to improve flexibility and optimise energy efficiency.
  • The effect of the addition of VSDs results in that when a system requires less cooling or heating than the maximum load, VSDs allow compressors, fans or pumps to operate at lower speed and as a consequence, use less energy.
  • In ventilation systems for large buildings, VSDs on fans save energy by allowing the volume of air moved to match the system’s demand.
  • VSDs can vary the output of your HVAC system to meet an installation’s actual requirements, again offering decreased load on the energy supply and increased energy savings.
  • VSDs are one of the most cost-effective methods to increase efficiency and reduce the costs of HVAC systems – they cut power usage and prevent energy wastage by precisely matching motor speed with cooling and circulatory demand.
  • When looking at the bigger picture we find that by virtue of their energy intensiveness, heating, ventilation and air conditioning systems account for a large percentage of buildings’ energy costs.
  • HVAC systems are responsible for 26% of the electricity consumed in the commercial sector, with motors that drive compressors, pumps and fans in these systems accounting for more than 98% of that energy.
  • VSDs are potentially the most under-utilised tools for optimising the energy efficiency of electrical equipment. Using them in suitable applications such as HVAC systems can save energy, cut operating costs and extend the lifespan of equipment.

In reality when considering installing a VSD:

  • Most VSDs are dust sensitive, an appropriate dust filter needs to be installed when operating in dusty conditions – they also need to operate within specified temperature and humidity parameters.
  • Conduct a proper feasibility study before investing in a VSD to ensure that it is the best solution to optimise your HVAC system.
  • VSDs can increase harmonics in the electricity supply, which disturb the sine curve of the alternating current and cause motors to run warmer than they are designed for, reducing their life expectancy. Harmonics can also decrease the life expectancy of computers and negatively influence the operation and accuracy of electronic measuring devices. The appropriate harmonic filters and necessary chokes must be installed along with the VSD to filter out the harmonics and protect your equipment.
  • Once installed, a VSD must be properly programmed – incorrect setting of parameters will result in poor control and energy wastage.
  • Regular maintenance of VSDs and associated motors is obviously essential to maintain energy savings. Preventive maintenance is less expensive than correcting faults, repairs and having to deal with unanticipated breakdowns, loss of production and unhappy clients.

To sum up, these are the benefits when using a VSD:

  • Lower energy usage
  • Achieve cost savings
  • Operate lower speeds resulting in quieter systems
  • Control starting current and acceleration
  • Increase motor and equipment longevity
  • Eliminate expensive mechanical drive components
  • Limits and controls torque
  • Adjust and control operating speed
  • Controlled motor stopping
  • Achieve reverse operation
  • Reduced start up inrush current
  • Achieve more efficient HERVAC systems
  • Increase motor starts per hour
  • Benefit from built-in motor protection

Thank you for your question Ryan, I hope that we helped you with motor starter understanding.

Thank you for all your questions. Send your problems (and sometimes your creative solutions) to with ‘Solutions Page’ in the subject line. You may include pictures.


  1. ACRA
  2. Myelectricalsolutions 

About Grant Laidlaw

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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