Challenge Circuit 8
Star VS Delta Motor Connections
Solution and Practical Results
Electrical apprentices learn about Star VS Delta connections in mainly two units of competency of Certificate III in Electrotechnology (Systems Electrician) (UEE30811 or UEE30820) – UEENEEG102A or UEEEL0020 – Solve problems in low voltage a.c. circuits, and UEENEEG006A or UEEEL0024 – Test and connect alternating current (a.c.) rotating machines. Challenge circuit 8 asks questions related to the Star VS Delta Motor Connections, which is covered mainly in UEEEL0024.
In this post, I’ll discuss one of the ways of solving Challenge Circuit 8. There are six questions in this challenge circuit, let’s look at them one by one.
Challenge Circuit 8 details
A three-phase induction motor can be wired in two ways – Star or Delta. Both methods of connection have their purpose in the industry. Please answer the following questions about the Star VS Delta connection of Motors. Here are the questions
- Which type of motor connection will have different line and phase voltages?
- Which type of motor connection will have different line and phase currents?
- Which type of motor connection will draw more line current?
- Which type of motor connection will draw less starting current?
- Which type of motor connection will provide a higher motor torque?
- Which type of motor connection requires a neutral connection?
A1: Star Connection has Different Line and Phase voltages
We know that the line voltage will be the same whether the connection is Star or Delta. The change happens in the phase voltage when the motor terminals are connected in a certain way. If we recollect the formulas for Line and Phase voltages for Star and Delta connections, it becomes pretty easy to answer this question.
We can see from the formula above that the phase voltage of Delta connected motor is the same as the line voltage, whereas the phase voltage of the same motor, when connected in Star, will show a smaller voltage. So the answer to the question, “Which type of motor connection will have different line and phase voltages?” is Star Connection.
The video below shows you the voltage measurements for Star connected motor.
A1: Star Connection
A2: Delta Connection has Different Line and Phase Currents
Unlike Line voltage, which stays the same because it’s from the supply, the line currents for motors will change based on several factors. For example, the motor’s power, load on the motor, power factor, motor connection type, etc. If we are only discussing one motor running at no load, we narrow the difference down to mainly one factor – the type of motor connection, whether it’s Star or Delta.
The question is which of those two connections will have different line and phase currents. Looking at the formulas again might give us an insight into the answer to this question.
As you can see from the formulas above, the line and phase currents of the Star-connected motor will be the same, whereas, in the Delta-connected motor, they will be different. So the answer to this question is Delta Connection.
A2: Delta Connection
A3: Delta Connected Motor will draw More Line Current
According to the formula for line and phase voltages in the Delta connection, we understand that they are the same. Whereas, as discussed in the first question, the phase voltage in the Star connection is less than the line voltage. Therefore, if we use the same motor in both connections, the voltage across the phases (windings) of the motor will be higher in Delta as compared to that of Star.
Considering Ohm’s law, if the load doesn’t change and the voltage increases, the current drawn by the load also increases. So we can safely say that a motor connected in Delta will draw more phase current than the same motor connected in Star. However, the question asks about the line current.
The formula for line and phase currents of Delta connection describes that the line current will be SQRT(3) times higher than the phase current. So the answer to this question is Delta connection.
Line current comparison between Star and Delta connections are shown below.
A3: Delta Connection
A4: Star Connected Motor will draw Less Starting Current
Using the explanation from the previous question, since the motor is the same, but the voltage is less across the phases when connected in Star, the phase current will also be less. Contrary to the Delta connection, where the line current is higher than the phase current, in the Star connection, both line and phase currents are the same. One of the reasons why Star-Delta starters are used is because of the less starting current in the Star connection stage, which provides less starting torque and puts less mechanical and electrical stress on the system, thereby increasing its lifespan.
So if a motor is connected in Star, it’ll draw less starting current than if the same motor is connected in Delta. So the answer here is Star Connection.
Starting current comparison between Star and Delta connections is shown below.
A4: Star Connection
A5: Delta Connected Motor will Provide Higher Torque
The previous two questions discuss the currents of the Star and Delta connected motors. We understand that the line current of the Delta connection is higher than that of the Star connection. Since the line voltage stays the same and while keeping the same load, we can say that the motor torque is relative to the current drawn. Higher current means higher torque, and lower current means lower torque.
Delta connected motor will draw more line current, so the torque it produces will also be higher compared to the Star connection of the same motor. So the answer to this question is Delta connection.
A5: Delta Connection
The video below shows the connection of a motor in Star and Delta and the current measurement for both. It also compares the different motor torques produced in both connections.
A6: Neither Type of Motor connection Require Neutral
One of the functions of a Neutral conductor is to carry the out-of-balance current from a three-phase circuit. However, balanced three-phase loads consume the three-phase power evenly and allow the return current from each phase to pass through the other two phases, leaving no out-of-balance current. This is why balanced three-phase loads don’t need a neutral connection or conductor.
Since a three-phase motor is a balanced three-phase load, it also leaves any out-of-balance current and does not need a neutral connection. So there is no need for a Neutral, whether a three-phase motor is connected in Star or Delta.
A6: Neither Star nor Delta Connection
Star and Delta connections in motors is a crucial topic, but it’s also easy for the Electrical Apprentices to confuse between the two. However, connecting the motors safely on equipment like in our Motor Faults Pracbox makes it easy for them to understand it in a practical way.
If you would like to discuss other relevant points about the differences between Star and Delta connections, please share them in the comments so our readers and students can benefit from your input.
Thanks for reading.