Laminated reactors are used in a great many application, particularly in industrial automation, where their use is becoming indispensable, explains ANDREW KOTAS of Schaffner.

Laminated reactors can perform many roles in motor drive applications. In pulse-width-modulated (PWM) drive systems, harmonics - sinusoidal oscillations with a frequency n times the fundamental mode) occur as an undesirable secondary effect.

Harmonics do not arise in the case of purely resistive loads, which have sinusoidal currents. However, non-linear components, where currents and voltages are not sinusoidal, can give rise to extremely high harmonic emissions, depending on the application. Motor drives with frequency converters and switched-mode power supplies are classic sources of harmonic emissions.

Harmonics deform the supply voltage, which can cause faults or even system crashes in systems synchronised to the zero crossing of the supply voltage. In addition, harmonic emissions cause serious losses in RC combinations of relays and contactors, which can often not be calculated beforehand.

Not least, harmonics are a nightmare for energy suppliers, as parasitic energy consumption above 50/60Hz cannot be measured, and therefore cannot be billed.

The new harmonics standard EN 61000-3-2 stipulates appropriate measures, and has been in force since 1st January 2001. Appropriately dimensioned reactors installed on the line side of the load can considerably reduce these harmonic emissions, depending on the size of the inductor and the resultant voltage drop.

At the moment of switch-on of an installation, a current rise exceeding the effective value during operation can occur for a short time. An appropriate inductor is usually provided at the mains input to protect the electronics of the frequency converter. However, under certain circumstances, peak currents can occur now and again in normal operation. These have to be restricted by a line reactor.

The reduction of the effective current has a fortunate side effect, allowing the specification of a lower current (and hence cheaper) mains input filter. The overall current consumption of an installation is considerably reduced and the efficiency increased, thereby improving the power factor.

In the case of frequency converters with a DC link, the rectified energy is temporarily stored in the DC link capacitors. Regularly occurring peak currents considerably reduce the service life of these electrolytic capacitors and, as a result, the service life of the inverter.

A line reactor restricts these peaks and prolongs the conductive interval, thereby reducing stresses on the capacitors.

Commutation Reactors:
When using power converters with natural commutation, a current is transferred from one branch circuit to the other branch circuit. A total short circuit arises for a short time at the moment of transfer. The short-circuit current which arises as a result must be restricted by means of a commutation reactor, in order to protect sensitive electronic circuitry.
The total nominal current flows in each branch circuit (phase) for 120°. As a result, the thermal current is reduced to approximately 82% of the nominal current. This in turn means that the copper winding, which is dimensioned in accordance with the thermal current, can be reduced. As a result, a commutation reactor is considerably less expensive than a line reactor.

Line reactors:
When using power converters, the line reactor has the task of protecting the supply from voltage dips during commutation and from thereby arising harmonics. The supply system voltage dip during commutation amounts to a maximum of 20% of the peak value. The harmonic emissions entering the supply are thereby reduced. The prime function of a line reactor is to protect the supply. Compared with a commutation reactor, the inductance values of the line reactor are usually higher, and the nominal current is the same as the thermal current. As a result, a line reactor is always larger than a commutation reactor, which is also reflected in the price.

Spike Killers:
As a result of reciprocal switching of power semiconductors in an inverter, time-shifted voltage blocks arise, which generate a rotating field. The voltage and consequently the speed of the motor can be regulated by means of PWM with a high pulse frequency. The extremely short make and break times of the IGBTs result in exceptionally steep rising voltage edges (spikes), and have the voltage overshoot characteristic for this application. Both phenomena exert heavy stress on the motor windings and can considerably shorten their service life.
A dV/dt reactor (spike killer) not only reduces this overshoot many times over, but restores the rise time of the voltage to a tolerable level. Capacitance free dV/dt reactors are used (e.g. in servos) because time-delaying components cannot be utilised for that application.

Sine-wave reactors:
It is an unfortunate side effect of modern power electronics that disturbing pulses can be transmitted to downstream motors. Parasitic capacitances (such as the screened motor supply cables) can cause high oscillating circuit voltages that may damage or destroy motor windings. Parasitic earth currents can run through motor bearings, vaporising lubricant and causing damage to the bearings. In addition to this, the motor frequently generates an unpleasant noise.
In order to reduce or eliminate such interference and Other secondary effects, a sine-wave reactor, or a sine-wave filter is connected between the frequency converter and the motor. The eddy current losses in the motor can also be minimised in this manner. In addition to protecting the motor, the sine-wave filter also provides protection for the inverter, because the lower pulse load is reflected in lower semiconductor losses.

Energy recuperation reactors:
Many of the latest frequency converters incorporate energy recuperation modules. When braking a motor running at a very high speed or with a heavy load, energy is generated for a short time in accordance with the motor/generator principle. In the past, this energy was usually converted into heat via a power resistor. In order to be able to make better use of the energy generated, the ER (energy recuperation) module, which is nothing more than an inverter in the direction of the supply, is activated for energy feedback.
This application not only requires special input filters (possibly also harmonics / pulse frequency filters), but also the use of a special line-impedance in the form of a reactor. The ER reactor restricts the feedback current and converts it into a form similar to a sine wave, so that the supply quality is not adversely affected. Operation of the drive is not guaranteed without this reactor.