Application Guide

ManualsBrandsHoneywell ManualsHVAC SuppliesSmart VFD HVAC Variable Frequency Drive, 208/230V, 3 HP, NEMA 1, Drive Alone

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Line Reactors

Line reactors can be used on both VFD inputs and outputs.

Input Line Reactors

The installation of a large number of VFDs or other similar equipment can distort the current and voltage waveforms of the

power supply, to the point which exceeds the power supply company guidelines or to the a level that other equipment served by

this power supply is prevented from operating correctly. In such a case, including input line reactors provides additional

smoothing to that provided by the DC choke. This reduces the overall VFD disturbance. Input line reactors can reduce the

amount of radio interference produced by the VFD on the power line back into the distribution system. Tests have shown that

typically 10 dB reductions in noise levels can be achieved, however the optimum situation is to install both input line reactors

and RF filters.

Output Line Reactors

Reactors may need to be fitted on the VFD output for several reasons:

— If the VFD to motor cables are long, 320 feet or more, then capacitive leakage from the cables to earth can be so high that

the VFD sees this as a fault and can trip on earth fault or over-current. It is difficult to be specific on this point as the

capacitance varies according to: type of cable, proximity of individual conductors relative to each other, and the conductor

shielding.

— With one large VFD driving multiple small motors, it is often specified that individual motors can be switched on and off the

VFD without stopping the whole system. When the motor is disconnected from the VFD, a line reactor in the supply to

each motor: acts as a current limiter, and helps absorb voltage spikes.

— Lightly loaded motors, when driven from a commercial power supply, can often mislead the surveying engineer into

assuming that a VFD several frame sizes smaller than the motor can be selected for the application. Problems can occur

when the VFD starts the motor. Even though the load is small, the motor current draw at low speed can be much higher

than at full speed. This is due to the fact that the motor winding at low speed is a more resistive than inductive load.

Typically, the VFD is unable to accelerate the motor due to over-current and it finally trips. Adding a line reactor to the VFD

output reduces current magnitude and can

rescue

the situation.

— When sudden shock loads, contactors, or switches on the VFD output are encountered, an output line reactor installed can

reduce the impact of these occurrences that cause trips or damage to the VFD.

— Mineral insulated cables from the VFD to motor have often been a problem. The conductors in these cables are so close to

each other that higher than normal leakage losses occur. Also, there have been occurrences of gland failure at cable ends

for no apparent reason. On installations where output line reactors have been installed together with mineral insulated

cables, no problems have been reported. This may be a coincidence, as there is insufficient data to form a positive

conclusion. It appears that the

softening

of the output pulses by the line reactor has a beneficial effect where mineral

insulated cables are used.

RFI

Sources Of Emissions

VFD emissions fall into 2 categories: conducted and radiated (see Fig. 28). It is important to recognize that the VFD itself does

not radiate much RF energy. Within 8 inches of the VFD, the field strength can be high and can interfere with the correct

operation of sensitive equipment. Beyond the region shown in Fig. 28, the field strength quickly diminishes by the inverse cube

law. Beyond 300 mm the field strength is generally insignificant.

Conducted Radiated

75cm

2.5m

1GHz30MHz

Emission

150 kHz

500m

Fig. 28. Conducted and radiated emissions.