Abstract: Proposed one kind of ultrasonic frequency induction heating power source’s phase-lock control technology, the use phase-locked loop’s phase-lock filter function, enables the invertor to have the steady separate excitation and the autoexcitation switching process; Introduces the detention link in the phase-locked loop, not only has compensated control circuit’s inherent delay, moreover enables the invertor to have the precise triggering time in advance.
Key word: Current mode invertor; Induction heating; Phase-lock control
1 outline
Because the induction heating power source is the heat treatment important equipment, its control plan always receives the attention. Because the heat treatment field operation condition is complex, the disturbance factor are many, when design must reduce the noise source as far as possible and weaken or eliminates the external interference to the system the influence, therefore, does not stop according to the actual situation control plan in the improvement.
The induction heating power source invertor the position which locates according to its load compensation electric capacity is different, may divide into the current mode invertor and the voltage invertor. The current mode invertor has the circuit structure to be simple, the power source movement is reliable, is strong to the load adaptiveness and the overflow protection easy and so on merits, Figure 1 is the current mode inversion electric circuit’s analysis situs. Speaking of the current mode electric circuit, must first prevent invertor’s instantaneous opening; Next is the selection suitable fixed time or the constant angle triggering way in advance; Finally, requests the invertor to have the wide start frequency range.
2 control plan principle and improvement
Invertor’s control diagram as shown in Figure 2. And Vo is invertor’s output voltage signal, undergoes the peak detection, produces with the control given value comparison cuts the installment cut signal X1, when X1 is the high level, cuts installment output signal X2 to stir up the signal connection with it, the invertor work in its violent condition, the control signal stirs up the signal generating device from it to send out, the electric circuit operating frequency is fixed, and stirs up the signal generating device control by it; When X1 is the low level, X2 and the autoexcitation signal connection, the invertor work in the autoexcitation condition, the electric circuit operating frequency decided by load’s natural frequency. According to the phase-locked loop closed loop filter function, carries on the time delay in the phase-locked loop feedback circuit, uses for compensating system’s inherent delay, control lag time td, the invertor already may work in the perceptual condition, may also work Yu Rongxing the condition.
3 invertor instantaneous opening preventing with transformation smooth transition
By all controls the component usually to use the separate excitation as switch’s invertor’s control to transfer the autoexcitation the control policy, namely when starting perhaps the load voltage is lower than threshold value Vco uses the split-ring to decide the frequency control, works in the separate excitation condition; But when the output load voltage is bigger than valve value Vco carries on the automatic cut over, causes the invertor work in the frequency closed loop, the track load frequency change.
But this kind of control plan has such problem: Because it stirs up the signal and the autoexcitation signal is impossible always the synchronization, therefore, in the most situations will produce the narrow pulse in the cut process (low level), this narrow pulse will create invertor’s instantaneous opening inevitably; Moreover, scene actual movement environment is bad, usually is works in the bad electromagnetic environment, this kind of control plan is very bad regarding the outside resistance to interference, cannot satisfy system’s antijamming request.
In view of this kind of situation, after cutting the electric circuit the level inserts a phase-lock filter circuit, the narrow pulse which with filters when transformation produces, similarly, this kind of electric circuit the peak which produces to the external interference also has the very strong inhibiting ability. Figure 3 has given key point X1 and X3 around the cut profile. May see from Figure 3, as a result of the phase-lock characteristic, cut in the process the narrow pulse to filter out by the phase-locked loop. Figure 3 1 group is the X1 profile, 2 groups are the X2 profiles, 3 groups are the X3 profiles.
Figure 4 has given invertor’s output terminal voltage when stirs up from it cuts the autoexcitation the profile. Figure 5 has given the invertor changes to it from the autoexcitation to stir up when the oscillogram. May see by these two profiles, the cut process is a smooth transient process, the contrast may know with Figure 3, system’s stability enhances greatly, the first level narrow pulse is filtered by the phase locking circuit.
4 precise realizations fixed time
As a result of the invertor output lead inductance’s existence, to reduce the contravariant tube’s line of current compressed stress, generally requests the invertor to work Yu Rongxing to trade the class condition. This requests before the circuit of the cells voltage zero crossing some time (some angle) to trade the class. This divides into the control circuit fixed time with the constant angle two triggering ways. In this control circuit uses fixed time triggers the way.
In the traditional intermediate frequency induction heating power source, fixed time triggers the way generally is realizes by the circuit of the cells voltage and the electric current composite signal. This kind of electric circuit fixed time is approximate. But in the ultrasonic frequency induction heating, as a result of control circuit’s inherent delay’s existence, causes this kind approximate no longer to establish. Therefore, uses the voltage and the electric current synthesis triggering way fixed time, the leading time along with the circuit of the cells resonance frequency, the output voltage peak-to-peak value change will change.
Use in phase-locked loop feedback circuit insertion delay link, on the one hand has compensated control system’s inherent delay, on the other hand may obtain the precise triggering time in advance. Obviously, in control circuit’s this detention link’s time-constant and the circuit of the cells resonance frequency and the voltage peak-to-peak value are relatively independent.
5 conclusions
Through the experimental verification and afterward scene ultrasonic frequency induction heating power source’s actual movement effect, this kind of control circuit has the strong antijamming ability and steady transformation ability, constant triggering time in advance, with big start scope.
