What's new
What's new

For VFD academics and circuit level fans. VFD internal operation question?

Theoretically, yes.

In reality, not so much.

It leaves the one unsensed output vulnerable to currents which do not return on one of the other wires. For instance, line to ground faults, which can blow the IGBTs with no sensing and so no drive cutoff.
The single CT with all three input wires going through it (picture 5 in the previous post) looks at the sum of all three phase currents which should always be zero unless there is a ground fault - if that CT sees any meaningful current it will trip the drive on ground fault. Since that protection is in place, when the drive is running normally you can apply the sum of currents rule with two phase current sensors to get the current in the third phase. Although there are still three current sensing devices, the ground fault CT is a much cheaper device than the output sensors so they save money.
Formula is OK.

3% and 5% have differing impedance, and also have differing protection from spikes as a result of that. They also have different voltage drops, which may be a consideration regarding which to use, vs protection.

A 3% reactor at 60 Hz, will be a higher percent reactor at 70, or 90 Hz, etc. The relation to frequency is linear, so the 3% will be about 6% at 120Hz.

But that is not all. The filtering of higher frequencies will round the square waves, and the lost area under the curve is lost rms voltage. So the voltage drop may actually be higher than expected.

That may not matter too much, since the inductance will add with motor inductance (if on the output side of VFD) and will still integrate the "volt-seconds" into a current. The power input to the motor will be slightly lower with than without.

As to the current sense, yes, with the GF sensing, you protect against the ground faults. But you still have 3 sensors, there is no savings that way.

The ground fault sensing may be more sensitive with one GF sensor than with a "calculated" GF sensing system and 3 output sensors.
The reactors all look the same with coils on a common frame.
Magnetic interactions between the coils?
Last edited:
Not very likely with properly wound toroid coils. If the windings are not symmetrical, then it can happen, yes.

Had that problem when some common-mode toroid line filter coils were picking up noise from an inductor in a piece of gear. They were coupling it to the line instead of blocking it.

Common mode coils have two separate windings, each of which typically covers about 1/4 of the core.