Four-quadrant controller

Four-quadrant controller controls DC motor

A four-quadrant controller consists of an electronic H - bridge circuit made up of four semiconductor switches , mostly transistors , which can convert a direct voltage into an alternating voltage of variable frequency and variable pulse width. Four-quadrant controllers in energy technology can also convert alternating voltages of different frequencies into one another in both directions.

Four-quadrant controller for DC motors

Integrated H-bridge

The task of a four-quadrant controller is clearly explained using the control of a DC motor for acceleration and braking in both directions of rotation. The basic structure of a four-quadrant controller consists of two transistors connected in series, each with a free-wheeling diode in reverse polarity. The DC motor to be controlled is located in the middle between the two halves. Its equivalent circuit consists of the inductance of the motor winding in series with its ohmic losses and the voltage source U _M , which is induced due to the rotor rotation.

For a better understanding, basic formulas are listed in advance:

If the motor delivers mechanical power, the product of U _M and I is positive. In the opposite case, the motor works as a generator and consumes mechanical power.
P _motor = U _M * I
The torque delivered by the motor is approximately proportional to the current flowing.
M _M ~ I
The excitation voltage U _M due to the rotor rotation is approximately proportional to the speed
U _M ~ n _red
The energy stored in the magnetic field is related to the square of the current.
${\ displaystyle W = {\ tfrac {1} {2}} \ cdot L \ cdot I ^ {2}}$

Depending on the operating mode, the four-quadrant converter works as a step -down converter for driving or as a step-up converter for braking and also enables the polarity to be changed to change the direction of rotation.

Operating modes

Animation for illustration

Buck converter

The buck converter operation is used to drive, the motor consumes power. In the circuit shown, T4 is switched through and a PWM signal is applied to T1 . If T1 conducts, a positive voltage is applied to the motor, the inductance is magnetized, a positive current flows and the motor generates an accelerating torque. If T1 switches off, the motor winding induces voltage and the current continues to flow via D2, whereby the magnetization of the motor decreases again slightly. The longer the lead phase lasts in relation to the blocking phase, the more current flows and the stronger the acceleration.

For opposite polarity, T3 is switched through and T2 is supplied with a PWM signal.

Boost converter

The step-up converter mode is used for braking and regenerative power, the motor delivers power. For this purpose, T4 is switched through and a PWM signal is applied to T2. If T2 conducts, the motor inductance is magnetized via U _M , a negative current I flows. The polarity of the current is opposite to that of U _M and the motor delivers power which is stored in the magnetic field. If T2 then blocks, the motor winding induces voltage and the current continues to flow via D1, the magnetization slightly decreasing again and the energy from the magnetic field being transferred to the supply voltage. The motor converts mechanical power into electrical power and therefore brakes.

It should be noted that the step-up converter U _M serves as the voltage supply and U _B as the load.

For opposite polarity, T3 is switched through and T1 is supplied with a PWM signal.

Neutral

No mention has been made of idle mode, in which at most one transistor conducts. After a residual magnetic field has dissipated, no more current flows through the motor. It is neither accelerated nor braked.

"Emergency brake"

An operating mode that is only conditionally recommendable is the emergency brake, in which T2 and T4 conduct and thus short-circuit the motor. The current generated by the motor is only limited by the ohmic losses and power is converted into heat. It is important that all components withstand the values that occur.

Overview

The quadrant alludes to the four areas in a coordinate system, where the current (≘ torque) is on the x-axis and the voltage (≘ speed) on the y-axis. The operating modes are clearly listed below according to their position in the coordinate system.

${\ displaystyle U_ {M} \ uparrow \ downarrow I \ Rightarrow {\ text {Generator}}}$ Brake quadrant 2 forward running	${\ displaystyle U_ {M} \ uparrow \ uparrow I \ Rightarrow {\ text {Motor}}}$ Accelerate quadrant 1 forward run
Accelerate quadrant 3 reverse ${\ displaystyle U_ {M} \ downarrow \ downarrow I \ Rightarrow {\ text {Motor}}}$	Brake quadrant 4 reverse ${\ displaystyle U_ {M} \ downarrow \ uparrow I \ Rightarrow {\ text {Generator}}}$

In the graphics, green indicates the alternating switch and purple indicates the permanently conducting switch.

Control

Control logic for MOSFET H-bridge

A control logic with resilient driver stages, called MOSFET drivers or H-bridge drivers, helps to safely control MOSFET H-bridges . The logic ensures that both transistors (T1 and T2, or T3 and T4) cannot be switched on at the same time. Furthermore, a "turn-on delay" is integrated (not to be confused with the switch-on delay), which only delays the switching on of the MOSFETs, but not the switching off. This bridges the delay time until a transistor blocks and prevents the switch-on phases of the transistors from overlapping and forming a short circuit (cross conduction or shoot-through) when switching. Even with the shortest overlaps in the µs range there are high current peaks in the supply lines. B. can lead to the permissible ripple current load of smoothing electrolytic capacitors being exceeded.

In order to switch the upper transistors (T1 and T3) through, a voltage above the supply voltage U _B must be present at their input . In the case of drivers in the low voltage range , this is usually done using bootstrapping .

Further considerations

The disadvantage of the four-quadrant controller is the low braking torque at standstill, since U _M assumes a small value. When viewed ideally, the current remains constant and thus the torque, i.e. braking force, remains constant. The ohmic losses are problematic, because if U _M remains small, then this also applies to the current (I = U / R). Correspondingly, a low speed results in a low possible braking force.

The motor must be controlled in the correct quadrant for proper operation. If this does not happen, two possible errors are possible:

Braking with the wrong direction of rotation: The motor inductance is no longer demagnetized and the motor behaves like a short circuit. The braking current is only limited by the ohmic losses of the winding. The engine brakes very hard.
Accelerating with the wrong direction of rotation: The motor inductance is no longer demagnetized. If both transistors turn on, the current limited by the ohmic losses flows, and U _M and U _B add up . The motor brakes heavily, depending on the pulse width ratio .

H-bridges in switched-mode power supplies

If a transformer is used in the circuit instead of a motor , an alternating current can be generated through the transformer by periodic switching. This principle is used in switched-mode power supplies with higher power and in welding inverters, but also in inverters and frequency converters.

In the case of switched-mode power supplies, the variable effective alternating voltage in the transformer is often generated by the fact that both half bridges work with a constant frequency and symmetrical pulses ( duty cycle 50%), but with a variable phase relationship to each other ( phase shifting converter ). This has advantages in terms of control and reduces switching losses .

Four-quadrant controller in power engineering

Triple half bridge for controlling a three-phase motor

Four-quadrant controllers in electrical power engineering are characterized by the fact that they can transport electrical power in both directions with changing polarities. In this way, a drive with an asynchronous or synchronous motor can be implemented in the form of a three-phase system, which feeds energy back into the network when braking. The description of these triple half bridges, as shown in the adjacent sketch, takes place with the help of space vector modulation .

High-performance converters are also used to couple energy networks with non-synchronous or deviating frequencies in the form of HVDC short coupling . They also allow the flow of energy in both directions. Instead of a three-phase motor, three-phase transformers , referred to as converter transformers , are provided.

Web links

Geared motors control with RN knowledge. Detailed application examples for H-bridges (discrete as well as integrated) in the robot network WIKI