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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 .
UNDERSTANDING STARTERS
By Grant Laidlaw
The reasons for the selection of which starting method would be the best option for any particular application revolve around various factors , such as what type of equipment the motor is connected to , the size of the motor , costs , and the maximum permissible starting current .
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 partwind motor . Why and when do we use the different starting methods ? Thanks .
Hi Ryan , these are the common three phase motor starting methods used in our industry together with variable frequency drives ( VFDs ).
A direct on line connected motor connected in delta will produce very high starting torque and can reach its operating speed ( rpm ) in under a second . At the same time the motor will draw a very high starting current .
For a small motor with a robust connection , say for example a water pump , this type of starter is effective . However as we increase the motor size the size and cost of the switch gear will exponentially increase to deal with the higher starting current drawn .
In addition , if we connect a direct on line motor to a v-belt driving a drum fan with a spider hub or an axial fan we find two possible issues . Firstly , the belts will not be able to transmit the sudden almost instant high starting torque up to the maximum speed of the motor . The belts will slip , burn and break . Secondly , if the belts do manage to overcome the high starting torque the inertia created by the outer drum of the fan or fins versus the centre of the fan almost instantly running at maximum speed can and will snap off the outer section of a drum fan or the blades of an axial fan .
Think about the latest electric cars , their 0-100km / hour times can be impressively low , Why ? Because an electric motor connected direct on line will reach its maximum speed producing
All image credits : Supplied high torque almost instantly . So if the motor is strong enough , there is sufficient power available and we can get the tyres to maintain grip and nothing breaks , we can theoretically achieve 0 - 100 in under a second . Of course the G-forces will most likely kill you though .
But in reality the car standing still has mass , and as a result inertia . Should the wheels instantly accelerate from standstill to 100 km / h the tyres will simply lose grip . Put on really really sticky tyres and something will break . We need to control the motor ’ s startup in the same way as we do in refrigeration by using , for example variable frequency drives or in fact a star delta starter .
DIRECT ON LINE ( DOL ) STARTING Direct on line or ( DOL ) is the simplest method and start-up procedure of three-phase motors , as only one contactor is required .
The three phase motor is started by the DOL method when 400 volts is applied to all the windings . ( It can be delta or star connected ).
DOL start-up method connection .
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RACA Journal I January 2023 www . refrigerationandaircon . co . za