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 part-wind 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.
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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 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.
Ryan, let us move on to star delta starting:
- A star delta starter starts the motor in the star connection. As a consequence the starting torque and current are reduced. The motor gradually gathers momentum and only once the motor gains around 80% of its maximum speed will the starter switch over to delta.
- It is interesting that even then the final 20% increase is achieved very quickly in delta and at that time slightly loose v belts will squeal. The change over is determined by the setting on a timer which has to be correctly set for the application on commissioning or replacement.
- When a three-phase motor is started on the star-delta method, it means that when the motor is started, a reduced voltage is applied to all the windings (lower starting torque).
- This is done by connecting two windings in series in the star configuration. By doing this, the voltage across each winding is only 230 volts.
- It must be noted that the motor construction is the same as the motor for DOL starting.
- For this method of starting a six terminal motor is essential. A motor with only three terminals cannot be started with the star-delta method.
- In order to start a star-delta motor it must be changed over from the star connection to the delta connection, either manually or as in most applications, automatically.
- This change over must happen when the motor has accelerated to approximately 80% of its final speed.
- The automatic timed change over is achieved by means of three contactors.
The legend for the starter below
The power and control circuits for a Star-Delta started motor.
The figures below show both stages of starting, ie stage 1 (star connection) and stage 2 (delta connection).
Ryan, next up is the part-wind method for starting a motor.
This method is somewhat special to our industry and I have only seen it used in refrigeration applications. Electricians do not cover this in their training.
To understand part-wind starting you have to understand what a part-wind motor is.
A part-wind motor is two motors in one casing.
Both motors are DOL started and, as such, will behave like DOL started motors.
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When the first “half” motor is connected to the power supply it starts like a DOL started motor; being only “half a motor” it will only draw half the starting current of a whole motor started DOL.
The torque produced will also be only half but this is still more than the torque of a “whole” motor started with star-delta starting. This is necessary as we have to overcome some inertia when starting a refrigeration compressor.
This approach is widely used in many refrigeration compressors.
The time setting is crucial as the motor will rapidly overheat if the setting is too long.
A part-wind motor showing the two sets of windings.
The part-wind motor has six terminals as below. From the diagram, the voltage across each winding is 400 volts.
Power circuit and control circuit for a part-wind motor.
The part-wind motor has six terminals as below. From the diagram, the voltage across each winding is 400 volts.
Because the part-wind motor has six terminals and looks like the star delta starter motor, it can be incorrectly identified and treated like a star-delta starter motor. This results in the time delay on the starter being set too long, which defeats the whole purpose of part-wind starting.
Thank you for your question Ryan, we are using more and more variable speed drives which I will look at in a future publication.
References:
- ACRA
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About 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.
