REQ: VFD recommendation

I have not seen any VFD vendor that goes beyond 3HP on 240 single phase mains. I do have a 7HP lathe with a 10HP Misubishi VFD on single phase. But I know I never push that sort of horse power. I've had this since

2004, and have not had the VFD "give up" yet. But this is a geared head lathe, and the motor input had triple V-belt reduction to start with. I never spin much faster then 600 RPM. I believe that a 5HP compressor is pushing that much near the 175 PSI fill of the tank. Now you could reduce your max pressure to 120 PSI or so, and reduce the max HP to near 3 HP (you would need to determine what pressure limits the HP to 3). I don't know how much the rectifiers diodes in the front end of the VFD can handle for peak currents, but that would be one issue. Also ensuring the IR drop of the wiring from the utility, though your meter and main panel to the VFD-air compressor must be controlled to prevent higher line current, causing the VFD to trip on low voltage. I thought there was a thread on this a few weeks ago. ignator

There are a lot of very useful features in VFDs, but some of them aren't very practical for use on an air compressor.

You might also want to look for a good used or new 5 HP air compressor duty single phase motor, with the same RPM rating as the 3-phase motor.

This might end up being a simpler solution.

DrivesWarehouse has them. Look for Polyspede PC1-50. It will work on a compressor, assuming it has unloaders.

You need to call them and ask before making your purchase. I would hate to be wrong about it.

Or I can sell you a new 5 HP 1725 RPM 184T Leeson motor. $300. It is expensive because it is 1725 RPM. You can buy a 3450 RPM motor for a lot less on ebay.

A 3 phase VFD running on single phase will need to be upsized in by 1.732, you need an 8.66HP drive to run the compressor at the full 5HP. I would start by checking with Iggy to see if he has any more drives available in the 10HP range. I have a 7.5HP motor on my CNC lathe and I run it with a

10HP Hitachi drive I bought from drives warehouse for a little over $600. Hitachi has info on running their larger drives from single phase.

This didn't seem too expensive to give me 3 phase conversion and variable speed control on a CNC lathe but seems too expensive to use on an air compressor.

But you may be able to find a similar 10HP drive used or on eBay for much less $$$. I would recommend finding a used drive you're interested in and then checking the manufacturers website to see if you can use it with single phase at a reduced rating.

RogerN

I do not have suitable drives in that size. (I have a 7.5 HP drive, but it is for fans and other variable torque loads and is not suitable for the OP).

This is a hard one, as I and Todd Rich have discovered.

As I mentioned in another post, there is a suitable drive by Polyspede that should work, but it is pricey.

The OP can also buy a 5 HP single phase motor. Be careful with buying them on ebay, as some are not really 5 HP. Example of a fake 5 HP motor is ebay item 140304911247. I have a real 5 HP motor for sale,

1725 RPM single phase by Leeson.

I run my 10 HP motor on my compressor using a phase converter, and I am basically happy with this arrangement. I may later use a drive if I convert mine to single phase, but I no longer care.

This is a myth. You have to read the manufacturers specs for the VFD as a number of the smaller units are designed and spec'd for full capacity operation on single or three phase operation. There are even a few models out there that only accept single phase input.

I have a PC1-150 running the 15HP motor on my Quincy with no problems.

Do you have any examples of 5HP drives designed for single phase in? The one's I've seen are rated for single phase in up to 3HP and then you have to upsize the drive.

Some drives are designed for single phase input, some are not. Also most drives I have seen the specs on are rated for 150% for a limited time for starting currents or temporary overloads. The LINE IN section of a drive is usually a 3 phase bridge rectifier to convert the AC line to a DC bus voltage, the outputs to the motor have 3 half H bridges with semiconductor switches. There are also capacitors to smooth out the DC supply.

If a drive is designed for 3 phase input, it doesn't need as much current through the Line In rectifiers and doesn't need as much capacitance per amp as one that is designed for either single or 3 phase. So if you have a 5HP drive designed for 3 phase input it most likely won't perform well (overheating, faulting out, burning out) if the application actually uses

5HP from the motor.

RogerN

5 HP rated, 240V single phase only (R&S, no T terminal at all) input:

It looks like they have taken Line In components suitable for single phase but the Motor out section is only 5HP, but it costs more than an entire 10HP

3 phase drive that can run over 5HP from single phase or 10HP from 3 phase.

I looked at their brochure, they have 50HP drives designed for single phase input, Now I just gotta get a 330Amp service :-)

RogerN

Why do you want to do this? You will only run the motor at one single speed anyway, won't you? It has to be cheaper to simply get a 5HP single phase motor to replace the 3 phase device, if you shop a little.

Pete Stanaitis

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Dev Null wrote:

Not necessarily the best unit cost wise, just the first example I found.

I pasted the following information from a Hitachi application note from here:

Application Note:

Sizing Three-Phase Inverters for Single-Phase Power Applications

Please refer also to the Inverter Instruction Manual

AN032404-1

Rev. A

Hitachi America, Ltd. © 2007 Hitachi America, Ltd.

Sizing Three-Phase Inverters for Use with a Single-Phase Supply

Although Hitachi does not offer inverters above 3 hp specifically sized and rated for single-phase operation, single-phase power can be safely used with larger 3-phase rated inverters, provided that care is taken to properly upsize and apply the inverter.

As background, for a given power (kW/hp) and voltage, the ratio of current for a single-phase circuit will be3 (1.732) times that of a three-phase circuit. This means that the input rectifier will see 1.732 times the current of the output devices. When powered by three-phase, these currents are nearly the same. This higher current would destroy the input of the drive if an oversized inverter were not used. Furthermore, full-wave rectified single-phase power has a much higher harmonic content than full-wave rectified three-phase power. This would introduce large ripple into the DC bus of the inverter, potentially causing other malfunctions. Larger size inverters have larger bus capacitors, thus more inherent filtering. So upsizing the drive ameliorates the ripple problem as well.

The rule of thumb Hitachi recommends is to start with the 3-phase motor's nameplate full load amperage (FLA) rating and double it. Then select an inverter with this doubled continuous current rating. This will give adequate margin in the input rectifier bridge and bus capacitors to provide reliable performance. NOTE: Fusing or Circuit Breakers should be sized to match the INVERTER input current rating, NOT the motor current rating!

As shown in the figure below, single-phase power should be connected to the L1 (R) and L3 (T) terminals, and optionally, a jumper should be placed between terminals L2(S) and L3(T). This jumper prevents the inverter from detecting a loss-of-phase should that function be active. Otherwise, the L2 (S) terminal should remain unconnected.

Beyond the inverter considerations, be sure to size components upstream of the inverter to match the INVERTER'S current ratings, NOT the motor's. This would include, but not be limited to wiring, fusing, circuit breakers, contactors, etc.

Hitachi

Yep, for Hitachi drives. Like I said, you need to check the manufacturer's specs for a given drive to see if derating is applicable.

>

Hitachi had another interesting application note on their website. They had one on powering your drive from DC. A person could buy a 5HP 3 phase drive for $300, add their own single phase full wave bridge rectifier of appropriate current rating, and add some capacitors, could have a 5HP drive with single phase in for less than $400 or so.

RogerN

>>

Yes, I recall Iggy was fiddling with feeding a drive DC.

I did not get very far with it as I did not work with it that much lately. The compressor runs great from my phase converter, which damps my enthusiasm a little.

Yes, but with the VFD and a pressure sensor, you can have the VFD vary the compressor speed to hold a constant tank pressure...

Well, that would only work properly for a certain consumption rate that is within safe RPM for the motor and compressor. Ergo, my compressor can go from 400 to 900 RPM on the pump pulley. And my motor, can probably be somewhere between 1300 and up based on motor cooling requirements. Adjusting from compressor RPM to motor RPM, that gives you a range of safe RPMs. An interesting idea.