Chopper control

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The chopper control , and impulse control , is a low-loss control method for electric motors and frequency converters such as solar inverters . Their operating voltage is chopped up by electronic switches ( thyristor pulse controllers, direct current controllers, transistor or IGBT switches) and the mean value of the voltage is determined using the duty cycle , as with pulse width modulationvaries. In order to achieve the desired values ​​(duty cycle, voltage, speed or power), either the clock frequency or the duty cycle is changed.

The inductance of the windings plays an important role in motors , which - with a built-in freewheeling diode - means that the current flows continuously despite the choppers. The controlled mean current can be a direct current, a trapezoidal course or a (synthesized) sinusoidal oscillation.

Applications

Series motors

In electric railway locomotives and other electrical traces with series-wound motors were the introduction of the chopper control, the starting resistors omitted, a higher efficiency than is stage circuit obtained and are avoided by the continuous control of the pulling force jumps which would otherwise occur at the stage circuit. The same applies to motorized braking.

Furthermore, if the contact wire is able to absorb or an energy storage device is carried, energy can be recovered by means of recuperation (regenerative braking). The polarity of the field winding must be reversed for braking (generator). There must be residual magnetism to start generator operation.

The chopper frequency on light rail vehicles often varies in the low audible range and is therefore often easy to hear. The torque is proportional to the square of the electric current regulated by means of choppers.

When regulating the power of vacuum cleaners and drills, a triac controller is used for chopping; the frequency is determined by the mains frequency. However, these phase angle controls are usually not referred to as choppers.

Asynchronous motors / frequency converters

Frequency converters (FU) synthesize a three-phase sinusoidal signal using a three-phase chopper control in order to be able to operate asynchronous motors effectively in a wide speed range. Electric traction vehicles can therefore also have asynchronous machines in some cases.

The speed is proportional to the frequency of the synthesized current curve, the chopper frequency is much higher and can often be selected. In the past it was partly in the sensitive audible range of a few kilohertz, but today the chopper frequencies are beyond 20 kHz for powers of up to many kilowatts and are therefore no longer disturbing.

Synchronous and stepper motors

Two- and multi-phase electronically commutated synchronous motors ( BLDC motors) as well as stepper motors now have a chopper control, even with lower powers, which generates phase-shifted trapezoidal or sinusoidal current signals according to the number of phases or strings.

As a result, the motors achieve greater efficiency and increased starting and braking dynamics. The operating (direct) voltage, especially with stepper motors, is significantly higher than the nominal voltage of the winding phases, which means that magnetization and demagnetization can take place much faster - maximum speed and torque increase.

The speed is proportional to the frequency of the synthesized current curve, the chopper frequency used for this is much higher than that. The phase current is often measured back using current sensors. The torque is proportional to the current (BLDC motor) or to its square (reluctance and stepper motor).

Mechanically commutated DC motors

DC motors working with mechanical commutators with constant excitation (for example by permanent magnets) can be controlled very well in their speed by means of chopper control - if the ohmic armature resistance is neglected, it is proportional to the mean armature voltage and therefore - with a constant supply voltage - proportional to the duty cycle .

The torque is proportional to the current. For single-quadrant operation (only forward, only positive torque) only an electronic switch and a freewheeling diode are required.

Excitation and electromagnets

The excitation field winding of separately excited DC machines or alternating and three-phase generators as well as electric and pulling magnets can be controlled by means of a chopper. All that is needed for this is a single electronic (previously also mechanical) switch controlled by the current. For this purpose , the magnetic coil has a free-wheeling diode that ensures the continuous flow of current. One example is the charge regulator of an automotive alternator .

See also

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