TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for driving a power transistor for performing power conversion of a servo control device for driving an AC motor. When a current command or a current feedback changes, the current is switched for a time considered necessary as a base point, and a current flows. The present invention relates to a method for controlling a motor.
Conventionally, the switching of a power transistor that performs power conversion of a servo control device that drives an AC motor compares a triangular wave with a current command as shown in FIG. 5, converts it into a carrier frequency PWM, and converts it into upper and lower transistors. The current was controlled by dividing and switching.
In a three-phase AC motor, the outputs (commands) of the three-phase current amplifiers are converted into respective PWM voltage commands for switching.
More specifically, as shown in FIG. 6, one cycle of the carrier frequency changes by switching the current from U, V to W, and so on. FIG. 7 is a waveform of a speed and a current when a constant torque command is applied. It can be seen that when the torque is small, the current is distorted and the excess current is large.
[Problems to be solved by the invention]
However, in the conventional technology, since the current is synchronized with the carrier frequency, an on-delay to prevent a short circuit prevents the necessary current from flowing when necessary, the current is distorted, a torque ripple occurs, a loss occurs, and the number of switching times increases. Since there is a large amount of current flowing in a place not directly related to the original, there is a problem that harmonics and distortion due to the carrier frequency are generated.
Therefore, the present invention turns off all the transistors when it is not necessary to supply current, puts them in a standby state, and when the current command or current feedback changes, supplies current for the time necessary to eliminate the relationship with the carrier frequency. The purpose of the present invention is to improve response and eliminate current distortion.
[Means for Solving the Problems]
In order to solve the above-mentioned problem, normally, all the power transistors are turned off to be in a standby state, and when a current command or a current feedback changes, switching is performed for a time considered necessary as a base point, and a current is supplied. The transistor is turned off to enter a standby state. Alternatively, when the power transistors are turned on for a certain time and then turned off to enter a standby state, all the power transistors are forcibly turned off for a certain time to provide a standby state. When the three-phase AC motor is driven, the two phases in which the current flows in the same direction overlap with the time in which the power transistor is turned on.
By means of the above means, an appropriate current can flow immediately by switching the transistor when necessary, so that the current response is fast, and since there is no carrier frequency, the current distortion is eliminated. Since the number of switching equivalent to PWM is small and there is no distortion. There is little power loss, and there is no torque ripple at low current, and the current becomes clean.
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, a specific embodiment of the present invention will be described with reference to FIG.
In FIG. 1, 1 is a current amplifier, 2 is a switching conversion circuit, 3 is a base drive circuit, 4 is a power transistor module, 5 is a motor, and 6 is a current observer.
The operation of the circuit configured as described above will be described with reference to the timing chart of FIG. The current amplifier 1 receives a current command and performs current control by current feedback. The output of the current amplifier 1 is converted into a switching signal by a switching conversion circuit 2. The switching signal drives the power transistor 4 via the base drive circuit 3 of the upper and lower transistors to control the motor 5.
FIG. 2 is an example of a switching conversion circuit. A counter is prepared for each transistor, and the transistor is turned on only during the counter operation and turned off when the operation is completed. Then, when necessary, the transistor can be turned on only at a necessary place to allow a current to flow. The upper and lower sides are determined based on the direction of the output of the current amplifier. When positive, the upper side is selected, and when negative, the lower side is selected. The switching time is calculated as follows.
Switching time = current amplifier output / maximum output × maximum switching time Here, the maximum output is uniquely determined by the current amplifier gain. The maximum switching time is a switching time possible due to a switching loss or the like. In other words, it is a switching time during which a loss generated when the maximum current flows can be radiated by a heat sink or the like. If the time is short, the number of switching increases and the loss increases. However, when driving the three-phase AC motor, the two phases in which the current flows in the same direction overlap the time for turning on the power transistor, so that the current is reduced by half for that time.
Therefore, it is necessary to correct the two phases having the same current flowing direction by doubling the overlapping time. Then, the remaining one phase adds the overlapping time. The three-phase switching times are summarized as follows.
Switching time = current amplifier output / maximum output × maximum switching time + overlapping time of two phases in the same current flowing direction First, the current observer 6 observes a current command and feedback, and notifies the current amplifier 1 when a change occurs. Then, the current amplifier 1 immediately calculates the switching time corresponding to the required command, and switches the power transistor 4 with the phase and torque (current) at the timing shown in FIG. For example, when the current flows from the U phase to the V and W phases, the upper transistor of the U phase is turned on, and the lower transistor of the V and W phases is turned on. The switching time is calculated by the method described above, and is set in the switching conversion 2. Then, when a current for a switching time flows in the switching 2, the switching is stopped until the next change to prepare for the switching. In other words, all the transistors are turned off and wait and wait. Then, in the case where the next change occurs and the switching is started, the setting is made as it is if a certain time or more has elapsed, and the setting is made after waiting if the time has not elapsed. This time is the minimum time during which the command changes and the upper side of the transistor turns on and the lower side turns on. If the change is rapid, it changes sequentially, and if there is not much change, such as during stoppage, the standby time is long and the current does not flow, so the operation becomes quiet. In addition, it can be used as a countermeasure for harmonics during stoppage. The current change may be observed by the current amplifier 1 in a short cycle.
FIG. 4 shows the current waveform in the simulation according to the present invention, which shows that there is no influence of on-delay and there is little distortion due to the carrier frequency. Therefore, it can be seen that the excess current is small. If there is an on-delay, the current does not flow due to the dead zone, so that the current waveform is distorted.
Here, the generation of torque ripple from a current that does not flow due to the on-delay will be described theoretically.
First, the current that becomes a dead zone due to on-delay is 
ΔIu, ΔIv, ΔIw: currents in the dead zone Δeu, Δev, Δew: uncommanded voltage R: motor resistance, L: motor inductance The resulting torque Δτ is:
Kt: Torque constant Id: Dead zone current for on-delay time From equation (2), it can be seen that DC ripple and AC ripple are generated.
Among them, the DC component has a constant torque and can be canceled to some extent by integration of the current amplifier, but the AC component has an effect as ripple. Also, a distortion of three times the current frequency occurs as a harmonic component.
Next, considering the influence of the carrier frequency, the harmonic current is generated mainly by the reactor component of the motor when switching is turned on / off.
The current dI due to switching is as follows.
dI = 1 / LΔedt (3)
dt: Switching time In conventional switching, the main circuit may switch from the maximum of + to the maximum of-, so that a large harmonic current is instantaneously generated. It can be seen that the switching is performed so as to affect three phases almost six times in one cycle. If voltage is not applied only in the direction of current flow as in the present invention, harmonics due to switching will increase.
Next, the loss will be described theoretically.
For example, assuming that torque ripple of KtΔIcos3θ occurs at the speed N in the on-delay, the power loss becomes | KtΔINcos3θ |. If the number of rotations of the motor is high, averaging is performed, so that 2KtΔIN / π becomes a power loss. The larger the dead zone due to the on-delay, the greater the loss.
Next, the loss due to the carrier frequency in the switching loss will be considered.
Loss in switching = edIdtN × f = e / LΔeddtdtN × f (4)
N: the total number of switching times in one cycle of the carrier frequency (assuming that two currents flow for two phases)
f: Carrier frequency As the carrier frequency increases and the number of times of switching increases, the loss increases.
【The invention's effect】
As described above, according to the present invention, an appropriate current can be supplied immediately when needed, and there is no carrier frequency, so that the response is quick and the current is not distorted. Since the number of PWM switching is small and there is no distortion, the loss is small. There is an effect that the current becomes clean without the low current torque ripple.
[Brief description of the drawings]
FIG. 1 is a configuration diagram of a specific example of a control device according to the present invention.
FIG. 2 shows details of switching conversion in the present invention.
FIG. 3 is a timing chart of switching in the present invention.
FIG. 4 is a simulation of current and speed when a torque command is applied in steps according to the present invention.
FIG. 5 is a configuration diagram of an embodiment using a conventional control device.
FIG. 6 is a configuration diagram of a conventional example.
FIG. 7 is a switching waveform of a conventional PWM conversion.
FIG. 8 is a simulation of current and speed when a torque command is applied in steps in the related art.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Current amplifier 2 Switching conversion circuit 3 Base drive circuit 4 Power transistor module 5 Motor 6 Current observer 8 PWM conversion circuit 9 Comparator