Medium Frequency Induction Power Supply

1. 1. General Description

Power Supply source is a static line frequency to medium/high frequency converter. Usually the industrial power is available at line frequency, 3-phase, where as 1- high frequency power is required to be fed to Induction Coil. The static power supply conditions the incoming power suitable to feed heating/melting application. The power supply also has an ability to regulate the power delivered to the coil besides switching ON/OFF and protection of vital semiconductor components used.

As shown in the block diagram, the three phase incoming supply is fed to 3-phase full wave thyristorized controlled rectifier to convert AC supply into DC supply. A L/C filter, filters out the high frequency ripples going back to mains. The Transistorized Inverter converts the DC supply into AC. Varying the Inverter switching frequency will change the output power. The Inverter switching frequency is automatically adjusted to a required value depending on the power demanded by the operator. When maximum power is demanded, the switching frequency will be at its optimum value and is only limited by Inverter Voltage and Current.

1. 1. The Operation of Converter

The 3- input supply is fed to the full wave Thyristorised Converter Bridge through an Breaker/Isolator and Fuse Assembly. The Rectifier unit converts the input AC supply into D.C. and supplies to Inverter through DC choke.

During operation, Converter Thyristors remains fired at a maximum firing angle and hence full D.C. supply remains up to Inverter in Power On state of the Panel.

When the Isolator/Main Breaker is closed, a steady D.C. bus voltage ramps up to an Intermediate value provided that all interlocks are healthy. Gradual ramping is done by phase control of input rectifier when inverter is made ON. It remains fully ‘ON’ till any interlock fails or any trip signal becomes unhealthy.

The DC choke filters the Rectified supply to further smooth D.C. supply.

1. 1. The Operation of Inverter

Pressing the ON button will enable oscillator to release the pulse to the Inverter Transistors. The tank circuit starts oscillating at inverter switching frequency and delivers power to coil. Various feedback signals start appearing to respective control section. The Inverter switching frequency will be set appropriately till the required power is obtained. A small time delay is provided to limit current surges while starting or stopping.

HEAT ON indication lamp glows to indicate that the High frequency power is being applied to the coil. When full power is demanded, the switching frequency sets to its optimum value and is only limited by Inverter voltage and current limits.