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Stardelta Forward and Reverse starter.

A stardelta forward and reverse starter is widely used in Industries, as an electrician it is important to understand the starter.

The starter has two circuits, the power and control circuit which work together for electric motor to run.

The starter enables the electric motor to run either in forward direction or in reverse direction.

The stardelta forward and reverse starter is usually found on Industrial machines which are supposed to change direction of operation regularly.

A good example of application of a stardelta forward and reverse starter is on conveyor belt used to move objects from one place to the other.

The forward and reverse starter anables the conveyor belt to change the direction of rotation without reconnecting the phases to the electric motor.

The diagrams below shows a forward and reverse stardelta starter power and control circuit.

Power Circuit.

Control Circuit.

Key

• TP Breaker - Tripple pole breaker
• SP Breaker - Single pole breaker
• K - Contactor
• btn - Button
• NC - Normal close
• N - Neutral
• O/L - Overload relay
• NO - Normal open
• F-K - Forward contactor
• F-R - Rervese contactor

How does a stardelta forward and reverse starter works.

As in the previous diagrams, the starter consist of:

1.Start buttons (forward and reverse)- a start button is a normal open contact button which is pressed for the electric motor to run. 2.Stop button - is a normal close contact button which is pressed to stop the electric motor. 3.Circuit breakers - a circuit breaker is a device which is manually operated to switch off the circuit for maintenance or they automatically trip off, breaking the circuit in event of a fault. 4.Overload relay - is a device which breaks off the circuit in event of overload which may cause overheating of the electric motor winding.

Overload is a condition in which current flowing in a circuit exceeds the amount required. Some overloads may not cause tripping of circuit breakers, so they is a need of an overload relay. 5.Contactors (Forward, reverse, main, delta and star) - A contactor is a device which when current flow through its coil, closes the contacts of power circuit for electric motor to run and vice versa. 6.Timer - is a device which anables the electric motor to switch from star to delta. 7.Electric motor - is a device which convert electrical energy into mechanical energy.

Explanation

With the circuit breakers in ON position, to start the electric motor in forward, the forward start button is pressed. When the forward start button is pressed, current flow through the forward contactor coil.

As current flows through the forward contactor coil, the coil became energized and cause the power contact of a forward contactor and other normal open contacts of the forward contactor to close including the normal open contact labeled "F-K(NO)" and normal close contacts of the forward contactor to open.

The contact labeled "F-K(NO)" anables current to reach the stardelta part of the control circuit, which cause the current to first flow through the main contactor coil and the star contactor coil.

As current flow through the main contactor and star contactor coils, it cause their power contacts and normal open contacts to close and normal close contacts to open, resulting in power reaching the electric motor and the electric motor begin to run in forward direction.

As soon as the preseted time on the timer is reached, the timer open the control circuit contact connecting the star contactor coil and closes the control circuit contact connecting the delta contactor coil, thus stopping current from flowing through the star contactor coil and enabling current to flow through the delta contactor coil.

This de-energise the star contactor coil and energise the delta contactor coil causing the power contact and other normal open contact of star contactor in close position to open and normal close contacts of a star contactor in open position to close.

As we said the delta contactor coil is energised, this cause its power contacts and normal open contacts to close and normal close contacts to open, cause power to reach to the electric motor through the delta contactor and the electric motor begin to run in delta in the forward direction.

We have already covered how a stardelta starter works, check it out for further explanations.

To start the electric motor in reverse, we first need to stop the electric motor using the stop button if it is still running in forward and then press the reverse start button.

When the reverse start button is pressed, what happen is just the same as what we have said in forward direction except that the reverse components e.g reverse start button and contactor are now in action.

When the reverse start button is pressed, current flow through the reverse contactor coil.

As current flows through the reverse contactor coil, the coil became energized and cause the power contact of a reverse contactor and other normal open contacts of the reverse contactor to close including the normal open contact labeled "R(NO)" and normal close contacts of reverse contactor to open.

The contact labeled "R-K(NO)" anables current to reach the stardelta part of the control circuit and the electric motor begin to run in reverse direction, we have explained what happen in the stardelta part previously.

The contacts labeled R-K(NC) and F-K(NC) are for interlock, they prevent the electric motor from accidentally switching between forward to reverse direction.

And also on the diagram they are contacts which prevents the electric motor from accidentally switching between star and delta.

When designing a forward and reverse star delta starter and other starters it is important to put into account how the electric motor will run and also to keep the number of components as few as possible.