Switchboard

Switchboard : 

• Monitors incoming voltage and current along with a few other things
• Protects the motor by shutting down power if voltage or current wander outside acceptable parameters
• Direct on Line (DoL) device, this means that the line power coming in is the same as the power going out.  The switchboard does not modify the power

Components and Function :

1- Basic Circuit :

The basic circuit can be divided into two parts:

1. The control circuit which operates at 120 Volts, and up to 5 Amps.
2. The power circuit which operates at the voltage and current of the ESP (allowing for cable loss).

2- Power Circuit :

1. Disconnect  Switch: Incoming power to the controller is applied to the manually operated disconnect switch.
2. Vacuum Contactor: The controller controls the opening and closing operation of the contactor. Auxiliary contacts provide electrical indication of the position of the contactor.
3. Power/Control Fuses: The current limiting power fuses selected can withstand repeated starting of the motor while maintaining short circuit protection. These particular fuses were selected for motor instantaneous overcurrent protection.
4. Lightning Arrestor: The lightning arrestors provide protection of control and motor circuits from lightning strikes. 

3- Control Circuit :

1. Control Power Transformer: The transformer primary accepts voltages of 220 to 5050 AC and converts down to approximately 120 volts AC used to run all the control circuitry in the Switchboard.
2. Current Module It is used to convert the high primary current to a low secondary current that is suitable for input to the controller.
3. Instrument Potential Transformers The primary taps provide different voltage ranges “selected based on the input”  to provide approximately 120 volts AC at the secondary terminals. The secondary voltage is input to the controller for monitoring, controlling and display purposes.

Potential Transformers :

1. Used to step-down the high voltage (from between 330 to 4900 volts) to a control voltage of 120 volts.
2. The 120 volt control voltage is used to run all the control circuitry in the Switchboard.
3. Motor controllers will monitor the control voltage and using the P.T. ratio calculate the line voltage in the switchboard.
4. Accuracy is tested to a tolerance of +/- 2%.
5. Two types: Control PT & Instrument PT.

Current Transformers :

1. To monitor the motor current it has to be reduced to a level which is easier to measure.
2. If this was not reduced then measuring devices would be large and cumbersome in order to cope with the high currents.
3. To reduce the level to a more manageable one current transformers are used
4. Since most control circuits operate in a 0 – 5 amp range, the CT is used to convert actual amperage to a value useful to the controller.