SOLUTION

Sources of high frequency for induction heating

Basically, there are three types of equipment used for converting the electrical energy at a low frequency to one at a high frequency suitable for induction heating

1. The motor-generator set
2. The spark gap converter
3. The vacuum tube oscillator.

Motor generator Set:- The motor-generator set consists of an induction motor coupled to a specially designed generator having both the armature and field windings on the stator. The change in reluctance produces a corresponding change in the magnetic flux flowing in the iron circuit which induces a voltage in the armature winding. The voltage is proportional to the rate of change of total flux and the frequency is determined by the number of complete flux reversals per second.

Spark Gap Converter:- The basic principle of operation of a spark gap converter is the alternate charging and discharging of a capacitor.

Although no longer produced commercially, the spark-gap converter was one of the earliest means of providing high-frequency ac power in the radio frequency region. It was used primarily in metal melting, in particular for precious metals.

In its simplest form, the spark-gap converter includes a step-up transformer (T₁) which increases the line voltage to a higher level, which is subsequently impressed across the spark gap. The spark gap is in parallel with a capacitor (C₁) and the primary transformer T₂. This is an output transformer; its secondary is connected to the work coil.

In operation, as the primary voltage rises in each half-swing of the 60-Hz line current, the voltage across the gap also rises until it exceeds the breakdown level. This voltage is also equal at that instant to the voltage impressed across the series circuit formed by the capacitor and the output transformer primary. The capacitor charges to this peak voltage. When the voltage exceeds the breakdown level of the gap, the low-resistance arc that has formed discharges the capacitor through the gap, passing an oscillating current through the combination of the capacitor and the transformer primary. As the capacitor discharges, the voltage across the gap decreases until the arc can no longer be sustained. On the reverse half-cycle of the 60-Hz line current, the cycle is repeated with the current in the circuit going in the reverse direction.

In later developments of the system, vacuum tubes were used to increase stability, and the gaps were contained in a hydrogen atmosphere to reduce the need for constant maintenance.

Frequencies generated by this system are generally in the range of 80 to 200 kHz. However, many frequency components are produced simultaneously so that the output is not purely sinusoidal. The efficiency of these systems is rather low—on the order 15 to 50% line-to-load.

Vacuum tube Converter:- The vacuum tube oscillator or electronic heater as it is sometimes called. produces large power outputs at the higher frequency of the order of 600 kHz. The 50 HZ supply is first converted into d.c. and then inverted into high-frequency currents.