High power DC motor test power supply design source

Compared with AC motors, DC motors have advantages such as excellent speed regulation performance, convenient speed regulation, smoothness, and a wide speed regulation range, so they are still widely used in many industrial occasions. There are mainly the following ways of DC speed regulation: armature series resistance speed regulation, changing armature voltage speed regulation, PWM DC adjustment system, double closed-loop DC speed regulation system, digital DC speed regulation system and constant power speed regulation by changing excitation.

The DC motor test power supply should be tested according to the test requirements of the DC motor, including voltage regulation test, light load test, load test and overload test, as well as dynamic process test. This requires it to have continuous voltage regulation and overcurrent functions, and requires fast response speed. This article will introduce the design of high-power DC motor test power supply.

Circuit Design
1 Rectification Circuit Calculation
The rectifier circuit uses three-phase uncontrolled rectifier. The characteristics of this circuit are simple and fast, and the output waveform can meet the requirements of the inverter circuit. The main function of the input rectifier filter circuit is to convert AC voltage into DC voltage; in addition, it must have a certain output voltage holding ability, which can prevent interference from the power grid from entering the power supply and prevent interference generated by the power supply.

The diode should be selected according to the effective value. The calculation process is as follows.

The waveform of the rectifier output voltage Ud fluctuates 6 times in one cycle, and the pulsation waveform is the same, so:
(1)
U2 is the effective value of the phase voltage, and its value is 220V.
Ud=2.34×220=514V (2)
P0 is 50kW, and the efficiency η is 0.8, so P0=Ud×Id×0.8, Id=P0/Ud×0.8=122A.
The effective value of the current flowing through the rectifier diode is:
(3)
And the maximum reverse voltage that each rectifier diode can withstand is:
(4)
Considering the fluctuation of the grid voltage -10% to +10%, the minimum parameters of the diode should be:
IA(VT)=I(VT)×1.1=78A
UFM=URM×1.1=593V
Therefore, the rated parameters of the diode are: 200A, 1200V. Therefore, the selected diode model is ZP200-ZL20 bolt-type ordinary rectifier diode.

Figure 1 Main circuit diagram

2 Filter capacitor calculation
The power frequency filter in the power supply is connected between the power frequency rectifier and the inverter circuit. It can not only convert the pulsating current into a smooth DC, but also suppress high-frequency interference, especially the high-frequency interference generated during the conversion.

It is best to use an electrolytic capacitor with low equivalent series resistance and large capacitance as the filter capacitor, because the equivalent series resistance value has a direct impact on the output pulsating voltage value. Therefore, in order to reduce the equivalent series resistance, four capacitors are connected in series and parallel to obtain the required capacitance circuit.

The grid voltage is 380V, ignoring fluctuations, and the no-load DC voltage Ud0 is approximately the line voltage peak value of 540V. When loaded, the voltage drops, and the capacitor voltage is:
Ud=1.35×380=513V (5)
Input current Id, that is, the high-frequency transformer input current is 125A. The DC side voltage is pulsating when loaded, as shown in Figure 2. The maximum voltage drop △U is considered to be 10%. In the T1 interval, the capacitor C discharges to the load, and completes a charge and discharge cycle in the T/6 interval.

Figure 2 Analysis of filter capacitor filtering process

Assuming that the discharge current is constant during the capacitor discharge period, then:
Id=C△U/T1 (6)
(7)
From formula (7), we get
(8)
(9)
In the formula, Id is the input current 125A; △U is the voltage change (△U =Ud0×10%=54V); T1 is the capacitor discharge time; Ud0 is the no-load DC voltage 540V; T is the industrial frequency AC period 20ms; is the industrial frequency AC angular frequency), we can get:
(10)
The line voltage peak value 540V borne by the capacitor.
In practice, four 5600μF/400V electrolytic capacitors and two large resistors are connected in parallel as a group, and two groups are connected in series to form a filter capacitor group. Its capacitance is 5600μF. It is greater than the calculated value, and the filtering effect meets the requirement of less than 10%.

Therefore, a DCMCE 1669 electrolytic capacitor with a specification of 5600μF/400V is selected.

Since the electrolytic capacitor is not an ideal capacitor, its own impedance will affect the voltage across the capacitor, and the voltage is pulsating. Therefore, in order to stabilize the voltage across it and make the voltage across each group of capacitors equal, a voltage-equalizing resistor R2 and R3 are connected in parallel at both ends of each group of capacitors, and R2=R3=30kΩ is selected.

As for the capacitor C1 in front of the filter for removing high-frequency interference, its capacitance is difficult to determine, because high-frequency interference includes interference from the power grid and power supply. Therefore, you can try to select C1 = (2.5 ± 5%) μF or other capacitors of the same order of magnitude, as long as the peak withstand voltage of capacitor C1 meets the requirement, the peak withstand voltage Up = 600V.

When the capacitor input rectifier filter circuit is connected to the AC voltage, a large surge current is often caused due to the charging of the capacitor. Here, the current limiting resistor R1 is selected as 20Ω/20W. After starting, after a delay period, the switch S1 is closed to remove R1 from the main circuit.

3 Inverter circuit calculation
For the inverter circuit, a full-bridge inverter circuit is selected, and IGBT is used as the switching device.

IGBT combines the advantages of MOSFET and GTR. It has the characteristics of high input impedance, good thermal stability and low driving power of power MOSFET, and the advantages of low on-state voltage, small conduction loss and high withstand voltage of GTR.

Calculate the main circuit and select the IGBT according to the design objectives.
The function of IGBT is to convert DC voltage into square wave voltage through its periodic on and off action. It is the key core component of the inverter circuit. Because it is relatively fragile, its design and selection are directly related to the safety and reliability of the entire system. Therefore, the selected parameters must be within its forward bias safe area (FBSOA). A large margin is left when calculating the parameters. [page]

① Rated voltage
The maximum voltage of the DC output after the input grid voltage is rectified and filtered:
(11)
Where Ud is the maximum steady-state voltage that the IGBT can withstand, U is the effective value of the grid voltage, which is 380V; 1.1 is the fluctuation coefficient; α is the safety factor, which is 1.1.

The peak voltage Uce at shutdown is 988V, the rated voltage should be taken upwards, and the actual voltage level value is 1200V.

② Rated current Ic:
primary current of high-frequency transformer
I1=I2×N2/N1=125A,
where I1 is the primary current of high-frequency transformer, I2 is the output current of test power supply, N1 and N2 are the number of turns of primary and secondary sides of high-frequency transformer.
The average current I on each IGBT is 63A.

The rated current Ic is the rated value given in the IGBT manual at a junction temperature of 25°C, Ics=186A.

Among them, Ics is the calculated value of the IGBT rated current; I is the average current on each IGBT tube; 1.414 is the peak coefficient; 1.5 is the Imin overload capacity coefficient; 1.4 is the Ic reduction coefficient. The rated current Ic is taken as 200A according to the tube current level. In summary, the rated voltage of the IGBT tube is 1200V and the rated current is 200A. Therefore, the IGBT selects the Siemens high-power IGBT module BSM200GB120DLC.

4 Output rectifier circuit design
The switch rectifier diode should not only have a short reverse recovery time and a small reverse recovery current, but also a slow recovery speed of the reverse current to reduce noise. Commonly used types include gold-doped diffusion type, epitaxial type, Schottky type and fast recovery type (PIN). Among them, the fast recovery type is characterized by a low forward voltage drop, which is 0.85V at room temperature. As the junction temperature increases, the forward voltage drop will be lower, only 0.6V at 150℃, close to Schottky tube; short reverse time, no more than 200ns; reverse leakage current is only 1mA at 150℃ and rated voltage, close to ordinary rectifier diodes. Therefore, a fast recovery diode is selected.

For a single-phase full-wave rectifier circuit, the rated current of the rectifier diode is:
IN=0.5×I2=500A
, where IN is the rated current of the rectifier diode; I2 is the output current of the power supply of 1000A.
The maximum reverse voltage on the tube is:
Um=2×U2=136V
, where U2 is the output voltage amplitude of the test power supply of 68V. Considering that a certain margin is left, the diode is selected according to the voltage of 300V and the current of 2000A. The final choice is the ZK300-35ZT3 flat fast recovery diode.

Control circuit design
In this design, the control circuit uses phase shift control. The phase-shift PWM controller turns on the four switches of the full bridge in turn by phase shifting. In the process of the two switch tubes in the same bridge arm turning on in turn, the leakage inductance of the transformer and the output parasitic capacitance of the switch tube form a resonant cavity to discharge the voltage on the capacitor at the fastest speed, ensuring that the switch tube is in the zero voltage switching state (ZVS), thereby avoiding the overlap of voltage and current during the switch operation.

PWM phase shift control is achieved through the error amplifier of UC3875. PI regulators are connected to the four corners of UC3875, and the output voltage is compared with the given reference voltage using a voltage sensor to control the phase between A, B and C, D, and finally adjust the waveform duty cycle to stabilize the power supply at a predetermined value. The two half-bridges A/B and D/C can control the turn-on delay (i.e. dead time) separately, ensuring that the output capacitor of the next power switch device is discharged within the dead time, providing voltage turn-on conditions for the switch device to be turned on.

Figure 3 shows the control circuit diagram of this design.

Figure 3 Control circuit

Drive circuit design
With the widespread application of IGBT in various types of converters, the selection and performance of IGBT drive modules have attracted more and more attention from users. In addition to providing sufficient gate voltage for IGBT to turn on and off, the ideal drive module should also have rapid and reliable protection functions, while striving to make the circuit simple and stable. There are many common drive modules for IGBT, among which the EXB series is the most widely used. EXB841 drive module is widely used in switching power supplies, UPS, power transmission and power compensation.

FIG4 shows a driving circuit diagram of the IGBT of the present invention.

Figure 4 Driving circuit

IGBT requires a +15V voltage to obtain a low turn-on voltage during the switching process, and also requires a 5V gate voltage to prevent false operation during shutdown.

Both voltages can be generated by the driver's internal circuitry from a 20V supply.