Line Reactors, transient voltages, VFDs...... oh my 1

[Sam654]

Hope you can help out an old gear head asking a questin that has probably been asked numerous times.

I'm installing VFDs on a boiler Over Fire Air fan (60 HP) and a Forced Draft fan (125 HP). If line reactors are installed on the VFD and using VFD cable, do I still need gounding rings for the motor bearings? Do I need VFD rated motors with a LR and VFD cable?

 

[djs]

A line reactor does nothing to limit the dv/dt at the motor. the high dv/dt does two things: produces reflected pulses and induces ground currents via capactive coupling. The reflected pulses place a higher than normal voltage across the windings. Motors rated for VFD use have high insulation values to cope with this. The grounding rings provide a bypass around the bearings for the coupled ground currents.

 

[LiteYear]

djs is right, but it might be worth noting that problem bearing currents are still the exception - in most cases they aren't an issue. Since your loads are both fans, you're probably at an advantage since the motor shaft wont be grounded outside the motor. Therefore there will be no shaft current except within the motor itself. The physical characteristics of the motor then become the determining factor - if the motor is long, or the windings are not symmetrical, bearing currents can still be a problem.

Unfortunately bearing currents are very hard to detect until they ruin a bearing. To be sure they're not a problem, you'd have to insulate one of the bearings. But that may turn out to be unnecessary!

 

[mikekilroy]

If you can afford 'vfd' cables (shield 4 twisted conductor) then by all means you should use it. Especially if these same wires have to run within say 1 foot of other wires, especially ow voltage control wires.

Others say what I would have: bearing currents are usually not an issue and I would not recommend messing with those rings unless they proved to be later.

Input line reactors are nice things to add if you have the space and money, to help protect the drive output power devices by slowing down current when/if a short circuit happens, as well as reducing some of the horrendous noise generated in the vfd from going back into your power system and effecting other stuff.

Output line reactors are nice things to add if you have space and money AND a specific reason. It will reduce the dv/dt voltage spikes approx. 30-50% to help your motor insulation MAYBE last longer. Maybe since if it has 'inverter duty' wire already this step is not required. It will make your motor run about 3% cooler by smoothing the current a tad. So again not normally recommended.

Last question you posed was about inverter duty motor. I touched on it above: good plan if you can afford it or if buying new, for sure buy it this way. Few name brand motors today are not 'inverter duty' anymore.

 

[LionelHutz]

Using a load reactor or output filter mostly depends on the cable length and motor voltage, which you didn't provide. However, they're not the guaranteed universal fix they're made out to be so I wouldn't just install one "because". I've scoped a motor with a short cable that had a large over-voltage ringing with a reactor and almost no ringing without a reactor. So, best bet would be to scope the voltage applied to the motor once installed.

As for the motor, it depends on how lucky you get. In North America anyways, buying a VFD rated motor really isn't a guarantee of much except for receiving a motor with a tag that says "VFD Rated" on it. Most if not all motors are built using the same insulation wire and I've seen certain VFD rated motors fail quicker than non-VFD rated motors.

 

[SceneryDriver]

I can't find the post on another forum I took this from any more, but it provides some good info:

Plain line reactors are simply series inductors placed in all three three-phase legs. They can be placed in the supply leads to the drive or in the motor leads coming out of the drive. Their effect on drive-motor system performance varies significantly depending on which place you put them. In every case, however, being simply inductors, they display a low impedance to low frequencies and higher impedance to higher frequencies, just as you would expect. Since they are inductors, you can expect them also to drop a small amount of AC voltage even at 60Hz.

Reactors are occasionally spec'ed in microhenries but more often are spec'ed in percent. This is highly confusing and leads to no end of trouble and, of course, more snake oil and false claims by the manufacturers. Just for the record, when a reactor is rated in percent, it means that, at its rated voltage and maximum continuous current at its rated frequency (60Hz, of course, for US reactors), it will drop its rated percent of the supply voltage across its terminals. So, by example, if you have a 5% reactor rated for 460V and 5 continuous amps, it will drop 460V times 5% or 23V when it is conducting 5 amps.

When reactors are placed in the supply leads to an inverter, the usual purpose is to prevent high frequency harmonics from the drive getting back into the AC supply grid. This is usually a no-problem deal unless you've got low voltage to start with. In that case, the extra voltage dropped across the reactor at heavy loads can get you into trouble since the drive cannot output more voltage than it gets in. Some other harmonic reduction scheme may be needed for this situation.

When reactors are placed in the motor leads, the usual purpose is to block high frequency components in the drive output pulses from traveling down the motor leads and causing trouble in the motor insulation. This is not usually a problem unless the motor leads get long enough to cause the high frequency components to reflect back from the motor towards the drive where they encounter new pulses coming from the drive. When that happens, the pulses can become additive and the motor insulation sees much more voltage than it should. Of course, the reactors also drop voltage as current passes thru them and you can again get low voltage conditions at the motor although this is not usually a big deal. The bigger problem is on precision speed and torque regulating sensorless vector drives where the drive cannot properly identify motor operating parameters because it can't see the motor accurately thru the reactors. The drive is left to include the reactors in the motor model even tho they don't belong in the model. On open loop or closed loop (shaft encoder feedback) systems this is not a problem.

There are additional complexities with motor lead reactors when you are dealing with motor overspeeds and output frequencies over 60Hz but I think the above will suffice for most users."​

Also attached is another white paper that provides more good info regarding line reactors, and dispels much of the myth surrounding them.


-SceneryDriver

 

[SceneryDriver]

There appears to be something seriously broken with linking to uploaded files with filenames beyond a certain length. Hopefully, these PDFs will actually post this time:

http://files.engineering.com/getfil...-4d4c-b014-3866519ccbea&file=LineReactors.pdf

http://files.engineering.com/getfil...a81-96b0-fcb1e1ac8b5f&file=ShaftGrounding.pdf

SceneryDriver

 

[LiteYear]

Great info SceneryDriver, but one important point has been left out - line and load reactors are "coupled" inductors. The coupling introduces a beneficial effect to everything already described.

To explain, suppose the reactors were ideal and therefore perfectly coupled. That means that the field induced by the current flowing in one phase sums entirely with the fields from the other two phases. In a balanced situation, the vector sum of the three fields is actually zero! So for balanced currents, the impedance of an ideal coupled reactor is also zero. Only unbalanced currents (such as those associated with harmonics) see an impedance. This is quite favourable, since it means that the impedance associated with real power is low while the impedance associated with harmonics is high.

In practice the coupling is somewhere around 96%. So when you use a reactor rated at 5%, it is actually the 4% of "leakage" inductance (ie. the proportion of inductance that is not perfectly coupled to the other phases) that creates the 5% drop. Using your example to illustrate:

If you have a 5% reactor rated for 460V and 5 continuous amps, it will drop 460V times 5% or 23V when it is conducting 5 amps. Since the voltage drop occurs for balanced currents, the drop is due to the leakage inductance (ignoring the small contribution due to the series resistance). If the coupling factor is 96%, then 4% of the reactor's "self inductance" is leakage inductance. Therefore leakage inductance is:

L_l = v / (di/dt) = 23*cos(2*pi*60*t) / ((d/dt)5*sin(2*pi*60*t)) = 23 / (5*2*pi*60) = 12.2mH

And so self inductance is L_s = 0.122 / 4% = 305mH.

It is this significantly larger self inductance that unbalanced currents will be impeded by.

 

[SceneryDriver]

Thanks, LiteYear. I've copied your post and put in my notes file.

I didn't see VFD cable talked about too much in the above posts, so I'll throw in my $0.02:
Regular wire is fine for most installations in metallic conduit, with the caveat that THHN wiring will actually have its insulation rating exceeded with a 480v drive. The wire, just like the motor, can actually see spikes as high as 1200v. 240v drives have fewer issues with this due to the lower voltages, provided good wire pulling practices are followed. Something to consider when choosing wire.

Much of what I do in scenery automation is "temporary" in nature (set up for a few days to a year) so I use shielded VFD cable between my Parker servos and my motors. The sound guys are much happier when I don't induce terrible hums and sequels in their low-level mic lines



SceneryDriver

 

[Sam654]

Thanks everyone for their input. Special thanks to SceneryDriver for posting those links.... very helpful.