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Magnetic contactor for 20 hp VFD


May 10, 2017
I have a 10hp 3 phase motor that I'm driving with a 20hp VFD using plain ol' single phase 240 volt. The VFD manual states that the VFD can be used with single or 3 phase input.

The manual specs 4 awg wire from the panel to the VFD. The manual also specs a 65 amp magnetic contactor between the panel and the VFD.

Is a 65 amp magnetic contactor robust enough if I'm supplying the VFD with single phase and not 3 phase? I was thinking an 85 amp contactor would be more in line if using single phase to power the VFD, which would also match the ampacity of the 4 awg wire.

For reference the 10hp motor is on an air compressor, running at 208 volts. The motor tends to draw roughly 30 amps, and the VFD draws about 45-55 amps (depending on the air pressure), but can spike to 70 amps when the motor is just starting.

Thanks for the help.
This is one of those "where are you located" deals.... In the EU, contactors are rated one way, in the US, for motors, it has been different.

In your case, the VFD will handle the motor inrush, the contactor won't see that. The single phase current conversion at FLA is 52A, the UL current for 10 HP single phase is 50A, and the VFD specifies a 65A contactor, with 4 AWG wire.

The 4 AWG wire is rated at 85A for 75C wire, but 70A at 60 C wire. The terminals on the contactor may only allow rating at 60C. So you probably have to use the 70A limit and an appropriate overcurrent device in the supplying breaker box, which would be no greater than a 70A rating.

You should be able to use a 65A contactor, especially if you use a supply breaker rated below that.

The VFD almost certainly specifies an overcurrent protective device (fuse or breaker), so what is that rating, if given?
I'm located in the US.

The VFD is a Mitsubishi FR-A220-15K. The manual states a 125 amp breaker.

Link to the manual is here: https://vfds.com/content/manuals/mitsubishi/mitsubishi-a200-manual.pdf

See page 5 for the wiring setup and page 113 for the breaker/wire size recommendations in that pdf.

So what I have planned currently is this: 125 amp breaker from the 240 volt single phase panel to a 65 amp magnetic contactor via 4 awg wire. From the magnetic contactor to the VFD again using 4 awg wire. From the VFD to the motor using 8 awg wire.

The manual says, " The breaker should be selected with care since a large inrush current flows in the inverter at power on."

You are suggesting a 70 amp breaker? 125 amp seems very high to me as well, but I was trusting the manual on that point.
Well, those specs are a mess of inconsistency, aren't they? You actually cannot do what they suggest, mostly per section 240 of the NEC.

I also do not find any reference other than a note on one wiring diagram, referencing single phase input. So, if there is a change in requirements for single phase, which I would expect, I cannot find it. Current input will be considerably higher for single phase.

If you put a 125A overcurrent, then you cannot, per the NEC, use #4 wire, UNLESS you fall under one of the "tap rules", which are actually for taps taken off of higher rated feeders, but could potentially apply here. Those specify certain conditions, and maximum length of wire between the feeder and a lower rated overcurrent protective device at the load end of the "tap". See section 240-21 (B) (2) of the NEC. (the following is per the copy of the NEC I can easily access with this computer, which is a couple versions old)

One case is taps not over 10 feet (240-21 (B) (1). I assume your plan would have the wires longer than that, so it would not apply.

The other is taps not over 25 feet long. There are three conditions.
1) the wire is rated at least 40% of the breaker rating supplying it. Yes, satisfied.
2) The tap is terminated in a single set of breakers or fuses appropriate to the conductor size. NOT satisfied.
3) the wire is protected from physical damage by being enclosed in a "approved raceway" or "other approved way" . You can do that.

So, using the "tap rule", you need a breaker or fuses at the end which are rated for the wire ampacity. That would be less than the 125A breaker specified, so what's the point of that breaker?

Given that, it is simpler to use larger wire, if that is accommodated by the contactor you plan to use. And, the contactor should be rated for the current it can see, which is the 125A, so that all your connected equipment is "listed" for the use you intend.

Since they specify the setup in the manual, one is obliged to assume that the UL recognition of the VFD was arrived at with the specified stuff in use, so the VFD can deal with the 125A in fault conditions. We do not know this, but it is really a requirement that the manual requirements are used in the testing.

I don't know if the 65A contactor and the #4 wire would actually change anything enough to invalidate the recognition if larger capacity wire and contactor were used with the same breaker.

I also don't know if the 125A rated wire (# 2/0) would even fit on the terminals of the VFD.

It's a bit of a "cluster****".

The other option is to use a breaker of the wire rating. That is simple and direct. It's not in excess of the manual specs, so probably would not affect UL. What it would do for performance I cannot judge from the manual.

This is all assuming you are intending to conform to the NEC.

There is an obvious conflict between what is specified and what is allowed.

Any more details of the installation you expect to use?
Yeah, the manual is a cluster****. It only mentions single-phase in one place, and like you said, it is uncertain if the specs they list are for single phase or three phase or both.

I think the most sensible setup would be something like this:

70 amp breaker in the 240 volt panel, as that is the highest amperage 4 awg wire is supposed to be paired with. Then an 85 amp contactor, which would equal the 4 awg wire ampacity, then 4 awg wire from the contactor to the vfd, and finally 8 awg wire from the vfd to the motor.

In testing the vfd and motor, I didn't see any current draw above 70 amps, so hopefully this covers me with a margin of safety. I did read some other (different brand) 20 hp VFD installation manuals and they spec 2 awg wire, so that makes me a little nervous. (I already bought the 4 awg wire)

Thanks for the help.
There is no reason for a higher rated contactor than the overcurrent protection, unless you are intending to somewhat "futureproof" the installation. That seems a reasonable thing to do.

Otherwise, plan seems pretty decent to me. I see nothing that actually says it won;t work, but of course the proof is in the operation once you go "off the data sheet".

I assume you are doing as the manual suggests, and making the setup drop out and stay "out" if power is cut, by means of the contactor.

Specs would not be for both, since the single phase starts out at 1.73x current, and the actual RMS current is higher due to being drawn at a bad power factor, in short bursts of high current. That is characteristic of rectified AC into a capacitor, as with the bus capacitor of a VFD.
You may consider adding a DC choke, some VFD manufactures require it when running a 3 phase input VFD in single phase mode to decrease the current peaks. It also changes the current rating/fusing or breaker size. Example in the Fuji manual for the same rated VFD size, would be 50.2A input current with a DCR and a 75A MCCB breaker, w/o DCR it is 72.4A and 125A breaker. They recommend #4 copper input wiring. It also depends on the duty mode.
I was oversizing the contactor simply to give a little headroom above the breaker capacity. The price difference is negligible so I figured why not.

The contactor is normally open and non-latching, so if power is cut it will drop out and stay out, if that's what you mean?

Once I get it all wired up I'll put the air compressor through some tests and watch the amperage draw of the VFD and make sure I'm not making anything hot.

Thanks for walking me through this.
You may consider adding a DC choke, some VFD manufactures require it when running a 3 phase input VFD in single phase mode to decrease the current peaks. It also changes the current rating/fusing or breaker size. Example in the Fuji manual for the same rated VFD size, would be 50.2A input current with a DCR and a 75A MCCB breaker, w/o DCR it is 72.4A and 125A breaker. They recommend #4 copper input wiring. It also depends on the duty mode.
I just read through the manual regarding the DC choke, and they list it as an OPTION. If added then the current draw and breaker size is slightly less. The breaker size and wire size I decided on was based on not using a choke.

Thanks for the heads up.
Yes, you wire the contactor so that when you push the "on" button (a momentary, normally open switch), the contactor is closed, and then is held closed by power through a contact on the contactor, either a regular power contact, if the coil is rated the same as the power supply, or an aux contact, if coil power is different.

That way, if power drops, the contactor opens, and the coil gets no power until you hit the button again.

The choke reduces the high pulses of current, and smooths them out to a somewhat ore continuous current. That reduces the input current somewhat, but does not change the 1.73 to 1 current relationship of single to three phase.
You do not need to use the factory DC buss chokes, the MTE are more reasonably priced and what I typically use.
The contactor would de-power the VFD, where the internal "no restart" still has the VFD powered-on and entirely ready, just not driving the motor.

There is a potential difference between those, as viewed by the safety agencies, particularly since that specific VFD is apparently not recognized as a "motor control". Some newer ones are.
So the issue is not just a positive disconnect for the drive, which could be done at the panelboard breaker, but rather a no re-start in the event of a momentary power interrupt?
Well, the no-restart fulfills the requirement to not re-start the motor, which is the primary issue. The machine, or at least the VFD, would still be "on", just not driving the motor. If there are any other portions of the machine that may be running, then those might be an issue as well.

It really depends on the need, the type of outage, etc. We are not given information on what else may be associated with the VFD/motor in whatever machine thins is.

In a long power outage, you may not want to wait for power to return, yet you may not be able to determine what the status of a machine will be when power comes on. The only alternative is to pull the disconnect, but then you may not know how it will react when you restore power to it after the outage is resolved.

That's a good reason for a "dropout contactor", as it restores the machine to a known status, which is "disconnected, not running".

Also, the disconnect may not be intended or rated as a "power switch", although it carries power. As a "disconnect", it's primary purpose is to allow a safe positive removal of power, normally when the machine is already not running. So, when you restore power to the machine by closing the disconnect, you do not want it to have power surges from charging bus capacitors, motors starting, etc.

A contactor that breaks power when power goes out will eliminate these concerns/problems. You don't need to use the disconnect, so it is not an issue.

If the possible problems above are not an issue, then the contactor may be un-needed. The manual for the drive seems to have it as an "option", so it is not required to maintain the UL recognition of the drive.
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I can see the need for no-restart on machine tools, but in my experience compressors are generally set up so that on restoration of power they restart automatically. They're on automatic pressure controls and fall into the 'could start moving at any time unless locked out' category. Pushing the start button on the contactor won't be notably different to switching the disconnect back on.

A contactor simply seems like another thing to fail, and more inrush cycles on the drive rectifiers/capacitors.

I would not expect a breaker or switch-disconnector to be any worse at making the inrush current, but then IEC rules generally require all switches/breakers to be capable of making/breaking the loads they're controlling unless you want to mark them as not to be used under load, and put them under lock and key. I certainly wouldn't trust one of those 'removable link' disconnects.

An advantage could be to avoid the VFD equivalent of 'chatter' during a brownout - voltage rises, VFD boots up and applies load, voltage collapses due to the load, VFD dies, voltage rises...

That's what a lockout-on-x-faults is for.