Design of a regenerative braking control circuit

Abstract: A design method for a pump-up voltage control circuit is presented to regenerate energy back to the grid. The main circuit, control circuit, calculation method of main parameters and related waveforms are also presented.

Keywords: energy feedback; regenerative braking; synchronous control

1 Introduction

In general, the main circuit structure of the servo system is shown in Figure 1. Energy is transmitted from the power grid to the motor through rectifiers, filters, inverters, etc. When the motor is working in the power generation state, that is, when the motor brakes quickly or carries a potential load, the energy transmission needs to be reversed, and the energy will accumulate on the filter capacitor to generate a pump-up voltage. If the pump-up voltage is too high, it will threaten the safety of the system. The simplest way to control the pump-up voltage is: after the pump-up voltage is generated, connect an energy-consuming resistor between the DC bus to release the energy. If the motor brakes frequently or runs with a potential load for a long time, energy is seriously wasted; at the same time, due to the heating of the resistor, the ambient temperature rises, which will affect the reliability of the system. The circuit designed in this article can solve this problem well. 2 Overview of system working principle

The three-phase uncontrolled rectifier in Figure 1 is replaced by a controllable converter, and three high-frequency choke reactors are connected in series at the three-phase power input end to suppress possible bidirectional (grid-servo system) electromagnetic interference, and to play the role of an equivalent DC reactor when the converter works in the inverter state, as shown in Figure 2.

When the motor is operating in the electric state, the high-power switching devices S1 to S6 of the controllable converter are all in the off state, and the six freewheeling diodes form a three-phase uncontrolled bridge rectifier, and the working condition is the same as Figure 1.

When the motor is working in the power generation state, the inverter works in the rectification state, and the controllable converter works in the inversion state, so that the motor works in the regenerative braking state. At this time, the filter capacitor stores energy, the DC bus voltage increases, and after exceeding the grid line voltage value, the diodes D1~D6 reverse block; when the DC bus voltage continues to increase and exceeds the set upper limit allowable value UdH, the converter starts to work and inverts the energy on the DC bus and feeds it back to the grid. At this time, the high-frequency choke reactor will balance the difference between the DC bus voltage and the grid line voltage to ensure the inverter state.

Figure 2 Feedback converter main circuit

Figure 1 General servo system main circuit structure

Figure 4 Current detection and control signal generation circuit

Normal operation. When the DC bus voltage drops back to the lower limit setting value UdL, the converter is turned off. The issues that need to be considered for energy feedback are:

1) The feedback current must meet the feedback power requirements and cannot be greater than the maximum current allowed by the inverter;

2) The inverter can only be started for energy feedback when the DC bus voltage is higher than the set value;

3) In order to increase the feedback power, try to perform feedback when the grid voltage is high, because if the feedback current is constant, the higher the grid voltage, the greater the feedback power.

Therefore, the system must have a voltage control circuit, a synchronous control circuit and a current limiting circuit. The current and voltage control is completed by two hysteresis comparators, and the synchronous control is completed by the synchronous detection and control circuit.

3. Control circuit design

3.1 Design of voltage detection and control circuit

The purpose of designing the voltage control circuit is: when the motor is working in the power generation state and the DC bus voltage Ud rises to a value exceeding the set value UdH, the switch tube in the converter is started to invert the energy on the DC bus and feed it back to the grid, forcing Ud to fall back; when Ud is less than another set value UdL, the switch tube is turned off. In order to avoid the inverter starting and shutting down too frequently, the voltage control is a hysteresis control mode UdL

Where: U is the effective value of the phase voltage.

In general, when the grid phase voltage is 220V, UdL can be set to 630V, the loop width of the voltage hysteresis control is 20V, UdH = 650V, and the linear optoelectronic isolator NEC200 is used to detect the DC bus voltage, and the DC bus voltage is linearly converted into a weak voltage signal as the inverting input of the voltage hysteresis controller. The voltage detection and control circuit is shown in Figure 3. Uv is one of the conditions for controlling the on and off of the main switch of the feedback inverter.

3.2 Current detection and generation of current control signals

Since the current on the DC bus, the current through the converter switch tube, and the line current fed back to the grid are equal, it is only necessary to install a LEM current sensor at the converter end of the DC bus to detect all line currents IL during the energy feedback process. When IL is lower than the hysteresis lower limit ILL, UI is high, allowing the inverter switch tube to turn on; when IL is higher than the hysteresis upper limit ILH, UI is low, and the converter switch tube is turned off. After turning off, under the freewheeling action of the choke reactor, the direction of the energy feedback line current remains unchanged, the corresponding diode in the converter continues to flow, and the current on the DC bus is reversed. Therefore, it is necessary to rectify the voltage signal output by the LEM to obtain the line current feedback signal Iv for energy feedback, which is used as the second condition for controlling the on and off of the main switch of the feedback inverter.

The current detection and current control circuit is composed of a current sensor circuit, a precision rectifier circuit and a hysteresis comparator, as shown in Figure 4.

3.3 Phase Synchronous Control Circuit

When the converter works in the inverter state, in order to obtain greater energy feedback, when the feedback line current is constant, energy feedback should be performed in the high voltage section of the power grid as much as possible. Therefore, the switching state of each power switch device of the converter and the phase of the power grid voltage should satisfy the synchronization relationship shown in Figure 6. For this purpose, the synchronization control circuit shown in Figure 5 is designed. In Figure 5, ug1~ug6 are the conduction permission synchronization control signals of the power switch devices S1~S6 respectively. Connect A, B, and C in Figure 2 and Figure 5 together accordingly, and connect the system to the power grid. No matter how the phase sequence changes, the synchronization relationship shown in Figure 6 remains unchanged. Under this synchronization relationship, the ideal phase voltage and phase current waveforms are shown in Figure 7. Let u1~u6 be the drive control signals of S1~S6 respectively, high level conduction, low level shutdown, and the drive control signals u1~u6 can be obtained by the protection signal, voltage control signal Uv, current control signal Iv and synchronization control signal UTi respectively.

Figure 3 Voltage detection and control signal generation circuit

Figure 5 Synchronous signal generation circuit

Figure 7 Ideal phase voltage and phase current waveforms

Figure 6 Synchronous output waveform

Design of a regenerative braking control circuit

4 Application Case Analysis

When used in a DSP-controlled synchronous motor servo system, the required energy feedback power P=6.5kW, phase voltage U=220V, the maximum switching frequency fmax=10kHz of the converter, the maximum allowable current is 25A, and ILH=20A, ILL=10A, UdL=630V, UdH=650V are set.

4.1 Calculation of choke reactor The choke reactor plays a very important role in this pump-up voltage control circuit. There are three main requirements for it, namely inductance, operating frequency and operating current. The requirement for inductance depends on the set value of the feedback current and the rated switching frequency of the converter. Assuming that during the energy feedback process, the forward voltage of the reactor is: ΔUon=, under the action of ΔUon, the time for the feedback current to rise from ILL to ILH is ton. The inductance value of each reactor is L. When IL=ILH, the converter switch device is turned off, then ΔUon=L; ton=L

During the freewheeling process after the converter switch device is turned off, the reverse voltage value borne by the reactor is ΔUoff. Under the action of ΔUoff, the time for the current to drop from ILH to ILL is toff, then ΔUoff=; ΔUoff=L; toff=Lf==; L=; It can be seen that after the inductance of the reactor is determined by UdH, ILH, ILL, and L, the smaller the UdL, the higher the switching frequency of the converter power switch. When UdL is the smallest and the switching frequency is the largest, the required inductance is the smallest, and at this time UdL=××220sin30°=270V is the minimum value. The minimum value of the inductance Lmin=1.34mH.

4.2 Estimation of feedback power P

When the grid voltage is constant, the feedback power is related to the feedback current, which is: =; Due to the symmetry of the three-phase voltage, the average voltage during energy feedback is:

U=(ua－ub)d(ωt)=uabd(ωt)=[×Usin(ωt＋)]d(ωt)

=2.34U

but

P=U=2.34U=7.7kW

Meets the requirement of 6.5kW feedback.

5 Conclusion

The voltage and current waveforms captured by the oscilloscope during braking of the synchronous motor servo system are shown in Figure 8, which is consistent with the theoretical analysis and meets the design requirements. The application in the above system shows that this method is feasible and has certain practical value.