H bridge

A four-quadrant controller is composed of an electronic H- bridge circuit of four semiconductor switches, usually made of transistors which can convert a DC voltage into an AC voltage of variable frequency and a variable pulse width. Four-quadrant power engineering and alternating voltages of different frequencies in both directions can be converted into each other.

Four-quadrant controller for DC motors

Clearly, the task of a four-quadrant controller based on the control of a DC motor for acceleration and braking in both directions explained. The basic framework of a four-quadrant controller consists of two times two transistors connected in series, each with a freewheeling diode in blocking polarization. In the middle between the two halves to be controlled is a DC motor. The equivalent circuit composed of the inductance of the motor winding in series with the ohmic losses and the voltage source is induced due to the rotor rotation.

For a better understanding here basic formulas are listed first:

• Is the engine from mechanical power, the product of UM and I is positive. In the opposite case, the motor functions as a generator and receives mechanical power. Pmotor = UM · I
• The torque output from the engine is approximately proportional to the current flowing. MM ~ I
• The excitation voltage UM due to the rotor rotation is approximately proportional to the speed UM ~ NROT
• The stored energy in the magnetic field is related to the square of the current.

Depending on the operating mode of the four-quadrant controller works as a buck converter for driving or as a boost to the brakes and also allows a change of polarity of rotation direction change.

Modes of operation

Of the step-down converter mode, is used to drive the motor receives power. In the illustrated circuit is switched to set and T4, a PWM signal on T1. Passes T1, the motor is a positive voltage, the inductor is magnetized to be a positive current flows, and the motor generates an accelerating torque. T1 switches off, the induced voltage of the motor winding and the current flows via D2 further, the magnetization of the engine slightly decreases again. The longer the conduction time in relation to the take -off phase, the more current will flow and the greater the acceleration.

Of opposite polarity with T2 and T3 is connected is supplied with a PWM signal.

The boost converter operation is used for braking and feeding back, the engine generates power. T4 is turned on and to set a PWM signal at T2. Directs T2, the motor inductance on UM magnetized, a negative current I flows. The current has an opposite polarity to UM and the engine emits power that is stored in the magnetic field. Then disables T2, induces the motor winding voltage and the current flows through D1, further, the magnetization decreases slightly again and the energy is released from the magnetic field in the supply voltage. The motor converts mechanical power into electrical power and brakes so.

Note that the boost converter UM is used as a power supply as a load and UB.

Of opposite polarity T2 is turned on and T4 is supplied with a PWM signal.

No mention was made of the idle operation, passed in at most one transistor. After a residual magnetic field has degraded, no current flows through the motor. It is neither accelerated nor decelerated.

A limited recommended mode is the emergency brake, guided, in the T2 and T4, and thus short-circuit the motor. The power generated by the motor is limited by the ohmic losses and power is converted into heat. It is important that all components withstand the occurring values ​​.

Survey

Word part quadrant passes on the four areas in a coordinate system, wherein the current ( ≘ torque) is on the x-axis and the voltage ( ≘ speed ) on the y -axis. The following operating modes are shown graphically according to their position in the coordinate system.

Quadrant 2 Slow forward running

Quadrant 1 Speed ​​forward running

Quadrant 3 Accelerate reverse run

Quadrant 4 Slow reverse motion

Green features in the graphs correspond to the alternate switch and purple to permanently conducting switch.

Control

In the safe control of MOSFET H-bridge control logic with a reliable driver stages, MOSFET driver or H- bridge driver provides assistance. The logic ensures that not both of the transistors (T1 and T2, and T3 and T4) are switched on simultaneously. Furthermore, a "turn- on delay" integrated ( not to be confused with the delay), the delay only to turn on the MOSFETs, but not shutdown. This is the delay time before a transistor blocks, bridges and prevents the switching overlap the switch-on of the transistors and a short form (English cross conduction or shoot- through). Even with the shortest possible overlaps in the ms range arise in the supply lines, high current peaks, which may lead, for example, means that the allowable ripple current load of smoothing electrolytic capacitors is exceeded.

To cycle through the upper transistors (T1 and T3), must be present at the input of a voltage higher than the supply voltage UB. For drivers in the low voltage range, this is usually done by means of bootstrapping.

Other considerations

Disadvantage of the four-quadrant is the low brake torque at standstill, as UM takes a small value. In an ideal observation of the current remains constant and thus the torque, that is, braking force constant. Problematic are the ohmic losses, because if UM is small, then the same is true for the current ( I = V / R). Accordingly, results in a low speed low potential braking force.

For proper operation of the engine in the correct quadrant must be controlled. If not, two error cases are possible:

• Braking in the wrong direction of rotation: The motor inductance is not demagnetized and the engine behaves like a short circuit. The braking current is only limited by the ohmic losses in the winding. The motor brakes very strong.
• Accelerating in the wrong direction of rotation: The motor inductance is no longer demagnetized. Now switch both transistors through which limited by the ohmic losses current flows and thereby to UM and UB add. The motor brakes hard, depending on the pulse width ratio.

H- bridges in switching power supplies

, Instead of a motor, a transformer in the circuit used, an alternating current can be generated by the transformer by periodically switching. This principle is used in switching power supplies greater power and welding inverters, but also in inverters and frequency converters.

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

Four-quadrant power engineering

Four-quadrant controller in electrical power engineering are characterized by the fact that they can transport electrical power with alternating polarities in both directions. Thus, in the form of a three-phase system, for example, a drive to be implemented with an asynchronous or synchronous motor, the energy fed back into the mains during braking. The description of this triple half-bridge, as shown in the adjacent sketch, done with the space vector modulation.

Large power inverter also serve the coupling of energy systems with non- synchronous or different frequencies in the form of HVDC short coupling. Also, they allow the flow of energy in both directions. Instead of a three-phase motor are three phase transformers, referred to as the converter transformer, are provided.