Zhengzhou Gou's analysis vacuum tube high frequency oscillator

The function of the oscillator is to convert DC power into high-frequency AC power and transmit it to the metal workpiece through the inductor to complete high-temperature heating.

1. Oscillation circuit

(1) Three-circuit oscillation circuit. Figure 2-38 is a circuit of a high-frequency induction heating power supply with a three-loop oscillator. Only the three-phase AC power supply, three-phase AC voltage regulator, anode transformer, high-voltage silicon stack three-phase rectifier, and vacuum tube high-frequency oscillation are shown. The main circuit of the converter and high-frequency load is omitted; the grid circuit, the filament power supply circuit of the vacuum tube, and the trigger circuit and low-voltage control circuit of the three-phase AC voltage regulator are omitted. Lz is a high-frequency choke, and Cg is a DC-blocking capacitor. They complete the separation of AC and DC circuits. The vacuum tube VE is a transducer element, and its anode power supply is a parallel power supply; inductors L1, L2, capacitors C1, C2, TP0 and C, together form a three-circuit oscillation circuit; inductors L2 and Ls are feedback links, this link is to complete the automatic It is essential to stimulate oscillation, and feedback regulation can be achieved by adjusting the coupling degree between them. The structure of this oscillation circuit has three resonant frequencies, but only one frequency can operate reliably, so it is called a three-loop oscillation circuit. The old vacuum tube high-frequency induction heating power supply equipment GP100-C3 is a typical three-circuit oscillating power supply.

(2) Single-loop oscillation circuit. This kind of circuit is simpler than the three-loop oscillation circuit, has fewer transmission circuits, higher output efficiency, and no coupling link, so there is no "coupling" adjustment, only "feedback" adjustment. The vacuum tube single-loop oscillation circuit is shown in Figure 2-39.

(3) Dual-frequency oscillation circuit This circuit can produce two frequencies of oscillation to meet the requirements of different heat treatment processes: one is super audio frequency (30-50kHz), and the other is high-frequency frequency (200-300kHZ) . The circuit shown in Figure 2-40 is actually two independent single-loop oscillation circuits. They achieve super-audio or high-frequency oscillation by switching positions 1 and 2.

2. Working status of vacuum tube oscillator

The working status of high-power vacuum tube oscillators is mainly classified by the size and presence of gate current, and by the ratio of gate current to anode current, the ratio of the maximum gate voltage to the minimum anode voltage, and the fundamental wave of the anode voltage. Measured as the ratio of maximum to anode DC voltage. When the gate current is negligibly smaller than the anode current, this working state is called a small gate current state or an undervoltage state; when the gate current is large and accounts for a considerable part of the vacuum tube anode current, it is called a large gate current. state or overvoltage state; states in between are called critical states. If the gate feedback voltage is too small, an undervoltage state will occur; if the gate feedback voltage is too large, an overvoltage state will occur. They will all reduce the oscillation power and anode efficiency, which are not the conditions we require.

The critical state is the state we require. When adjusting the oscillator, as long as the positive current/grid current = 4-7 times (5-10 times for some oscillator tubes), the oscillator is considered to be in a critical state. For this reason, for a three-loop oscillator, "coupling" and "feedback" adjustments must be performed; for a single-loop oscillator, only "feedback" adjustment must be performed. When the oscillator is in a critical state, the maximum power output at a certain positive voltage value can be obtained.

3. Efficiency of vacuum tube oscillators

Vacuum tube oscillators have the following two concepts of efficiency:

(1) Anode efficiency of vacuum tube. It indicates how much of the DC power supplied to the oscillator from the DC power supply is converted into high-frequency oscillation power (fundamental wave power).

(2)Output efficiency. It shows how much of the DC power is absorbed by the load after the high-frequency oscillation power is lost by various factors such as high-frequency transmission circuits. Obviously, the output efficiency is smaller than the anode efficiency. Efficiency is often expressed as a percentage. The theoretical value of vacuum tube anode efficiency is 78 ohms, and the output efficiency is 40% - 45 ohms. For example: Ua=12kV, Ia=12A, then DC input power=12kV x 12A = 144KW; take anode efficiency = 70%, then oscillation power = 144KW x70%≈10OKW; take output efficiency = 45%, then the power absorbed by the load =144KW x45%=65kW. The output efficiency of a single-loop oscillator is higher than that of a three-loop oscillator.