Line reactor question

Hi All,

Please educate me on line reactors.

VFD manuals suggest installing a line reactor between the AC supply line and the VFD when the supply has high fault current capacity.

I've always __assumed__ that this was just an inductor (per phase) to limit input current. (I'm working with single-phase power here.)

The descriptions on the Grainger website indicate that these have two windings. (These products are for three-phase applications.)

Anyhow, is this just a simple per-phase inductor, or something more complex?

And how does the "3%" spec relate to real inductance, as in Henries?

Thanks much.

- Leigh

If you mean the 3ph reactors by Sola, these are three phase line reactors, which is an inductor placed in each phase, They offer a cushion to fast rising currents and reduce the harmonics getting back into the supply, I assume the 3%Z is the because the impedance (Z) offers only 3% effect at 60Hz, obviously if they had say 100%Z then they would not any current to flow at 60hz!
Impedance value Z is the result of R,C,H & Fr.
So the Z value would vary at any other frequency and hopefully have a larger effect.
If using 1 phase, one of the reactors would not be connected, BTW any transformer on the input to the VFD also offers natural impedance to reflected harmonics.
M.

Hi M,

Thanks for the reply.

I'm an electrical engineer, so I'm reasonably familiar with terminology in the electronics industry. But sometimes terminology in the power distribution industry doesn't mean exactly the same thing.

My problem with the impedance issue is that the impedance of the load, i.e. the VFD and its attached motor, varies all over the place. The value at 10% loading would differ greatly from the value at 100% loading.

What I'm trying to do is find a single-winding choke that I can use on a single-phase line to accomplish the same thing. I hate to buy one designed for three-phase use and leave part/most of it unconnected.

Thanks.

- Leigh

I am not sure if you will get precisely what you are looking for in a single phase choke, but you could improvise by using the low voltage secondary of large 1 ph transformer that was rated around the same current as you motor FLA.
Seal off the primary ends, as you could get high voltages appear on them.
As to the varying impedance of the motor circuit, the output of the VFD varies in frequency and has a great deal of switching component, the input to the VFD is basically a fixed frequency of 60Hz.
M.

Leigh:

If you reference p22 of Alternating Current Machinery by Bailey and Gault, McGraw-Hill, 1951, there is an explanation of Per-unit System. From this I quote

"The meaning of per unit is the same as per cent expressed in decimal form:" ---- "The base for resistance, reactance, or impedance is simply rated volts divided by rated amperes."

Thus, a percentage impedance has to be related to the rated voltage and current that have not yet been specified here.

If this is a 240 V system and 20 amperes, then a 100% impedance would be 12 ohms and 3% would be 0.6 ohms. In this case the inductance is about 0.0016 H. For an inductor to be defined as a percent impedance the nominal voltage and current must be known. Otherwise percent impedance is meaningless.

.

gar said:

If this is a 240 V system and 20 amperes, then a 100% impedance would be 12 ohms and 3% would be 0.6 ohms. In this case the inductance is about 0.0016 H. For an inductor to be defined as a percent impedance the nominal voltage and current must be known. Otherwise percent impedance is meaningless.

Click to expand...

Hi Gar,

That sounds reasonable. I could certainly wind a 1.6 mHy toroidal choke that would handle 40 amps, or more likely two 0.8 mHy chokes and put one on each side of the 240-volt line. There are hash suppression chokes available in this current and inductance range, but they're intended for automotive use. I'd have to provide additional insulation for mounting.

I was having difficulty figuring out what impedance they were talking about. I suppose it makes sense to base all the calculations on full load.

My other option would be to install a bucking transformer to drop the line to 208v, which is the actual nameplate rating of the motor. This would also serve as a line reactor, so I get two functions for the price of ten or twelve, considering how over-priced this stuff is in the first place.

Thanks very much. I appreciate it.

- Leigh

Homebrew on-the-cheap...

Hey Leigh-

I made a similar AC choke for a 500W TV transmitter by stripping the windings from a microwave oven transformer, and it was mebbie six or seven turns of #8AWG around a 2x2 square core.

I didn't use any stellar math with mine- too many unknowns, too many variables. Instead, I swept it using a somewhat crude, but very effective technique- ran an audio sig-gen into a big audio power amp (a 1500w QSC long since retired from stage service), fed it through the choke, then a 2-ohm dummy load, and observed output signal across the resistor on a rather old scope... you could also use a diode/cap/resistor level detector circuit and a DVOM... just add or remove windings 'till your FCO is say... 75hz or so... when you get closer to 60, the choke's skirt gets too close to normal op, and you'll get core heating and voltage sag under load.

Hi Dave,

Thanks for the input. Good idea about the uwave transformer. I hadn't thought of that. Good cheap source of iron.

You measurement technique will certainly work. I have appropriate equipment to measure inductance in this range at 60 Hz, which simplifies life. This reactor is on the supply side of the VFD, so it only sees 60 Hz (plus whatever trash the VFD spits out in that direction).

I'm definitely going to do a homebrew on this. There's absolutely no reason something this small and simple should cost over $200.

Thanks very much.

- Leigh