VFD suggestions for compressor

[Gas mark 5]

Hi all,

I finally have a garage / workshop of my own, and after struggling with tiny compressors for so long, I decided to get something big enough:

​

It's a Broom & Wade that should be ample for everything I need it for, I just have to work out how to power it. It has a 10hp 3 phase motor, but I don't have 3 phase at this property. The options I've considered so far:

• Get 3 phase. Haven't actually looked into prices, I just assume it would be hideously expensive.
• I have a 10hp Briggs & Stratton petrol engine that would probably power it fine, but noisy, and a bit of a faff working out mounting, ratios, control systems etc.
• Use a VFD.

I've never used a VFD before, is there any reason not to go for the absolute cheapest 10hp unit I can find? That one reckons it needs 35A on the input, but I only have a 32A supply to the garage. Can I get away with it, perhaps if I limit the max rpm to ensure It's not drawing too much current? Apparently there's an issue with the star/delta starter on this compressor. When using a VFD, is that still used, or is it wired up solely in delta and the VFD handles soft starting?

Any insights appreciated!

George

 

[Stupoty]

You can get motors rewound for different voltages , this is what I did for my lathe to run it on 240v

Stu

 

[Neilj]

The specs on that Vevor VFD look a little dubious; 35A input, 50A output? That makes it 142% efficient
If you're using a VFD, all the existing starter gear gets binned, the motor connects directly to the VFD and that controls everything.

 

[dave h 2]

Depends if you can get at the Star point to run 230V

 

[Arc Tourist]

Correct me if I'm wrong, but what you may need is a phase converter rather than a (VFD) variable frequency drive, as the compressor motor only needs to run at one speed ?
For my 3-phase machines which do not need a variable speed control, I had fitted simple digital phase converter's.

 

[brightspark]

Correct me if I'm wrong, but what you may need is a phase converter rather than a (VFD) variable frequency drive, as the compressor motor only needs to run at one speed ?
For my 3-phase machines which do not need a variable speed control, I had fitted simple digital phase converter's.

View attachment 347430

what sort of price are they

 

[mtt.tr]

Don't ask me I'll suggest you run it PID as a long winded project

 

[Arc Tourist]

what sort of price are they

I paid £88.oo for the " MY-PS-1 " model, and don't recall what I paid for the other I bought. They are made in Korea, and prices vary according to requirement. These can be found on Ebay. The seller is KIDD 2127 (90) . You can send a message with your motor's specs and they will tell you which phase converter is best for your application.

I mounted them in a box so it could be plug in and play for different machines I might acquire of comparable horse power.

 

[Plum]

Hi all,

I finally have a garage / workshop of my own, and after struggling with tiny compressors for so long, I decided to get something big enough:

View attachment 347389​

View attachment 347390
It's a Broom & Wade that should be ample for everything I need it for, I just have to work out how to power it. It has a 10hp 3 phase motor, but I don't have 3 phase at this property. The options I've considered so far:

• Get 3 phase. Haven't actually looked into prices, I just assume it would be hideously expensive.
• I have a 10hp Briggs & Stratton petrol engine that would probably power it fine, but noisy, and a bit of a faff working out mounting, ratios, control systems etc.
• Use a VFD.

I've never used a VFD before, is there any reason not to go for the absolute cheapest 10hp unit I can find? That one reckons it needs 35A on the input, but I only have a 32A supply to the garage. Can I get away with it, perhaps if I limit the max rpm to ensure It's not drawing too much current? Apparently there's an issue with the star/delta starter on this compressor. When using a VFD, is that still used, or is it wired up solely in delta and the VFD handles soft starting?

Any insights appreciated!

George

Be very careful of motor inrush current if you’re sailing close to the wind on supply amperage

 

[Gas mark 5]

I paid £88.oo for the " MY-PS-1 " model, and don't recall what I paid for the other I bought. They are made in Korea, and prices vary according to requirement. These can be found on Ebay. The seller is KIDD 2127 (90) . You can send a message with your motor's specs and they will tell you which phase converter is best for your application.

I mounted them in a box so it could be plug in and play for different machines I might acquire of comparable horse power.

View attachment 347471

Thank you for this, I shall look into it!
I was initially thinking a VFD would make the most sense, because I could share it between lathe / mill if I end up with more three phase kit further down the line. But it would be easier just to have something dedicated to this compressor. My only other concern would be the current requirements - I don't imagine it will need to run flat out to keep up with my air delivery requirements, so having the ability to limit the maximum torque / rpm might be useful if I find it tripping. Or is that only likely to be a concern during startup, perhaps use soft start or just fit a slower breaker? To be honest I'm not really sure what all these current ratings refer to, is the 16A on the motor plate the typical draw while running? The peak at start up? The maximum draw under load before it'll start damaging the windings?
Edit: and what does 16A.MESH mean, is mesh just another word for delta?

 

[Guzrik]

A smaller motorpulley will get the drawn current down as well.

 

[Dcal]

Does that compressor need a 10HP motor?
If it does you are not going to run it from a 32amp 240v supply.
The starting current could be 6 times the rated current. It will (should) be a short spike but it will probably overload your supply.

Do you know anyone with 3 phase?
I would put it on a 3 phase supply and measure the starting and running current with a clamp meter.
A lot of things will affect the current required but if the compressor pump is worn you might get away with a good bit less and live with the lower air output.
The way the unloader valve works will have a big effect on the starting current.
As @Guzrik just said above, gearing it down a bit will reduce the current required at the expense of air output, but I would try and run it on a three phase supply to see where you are before spending any more cash.

If you are stuck with three phase 380 / 410 volt you can get inverters that have 240v single phase input and 3 phase 380v output.
The Chinese ones are allegedly rated in Chinese amps and you need to buy a size or two up so you’re looking at a 15 or 20HP version.
I can't see you running that on a domestic supply.
I needed one for a Hydrovane that was rated at 4kW, I bought the 7.5kW version.
When I set it up it was only drawing 3 amps a phase but that was because the compressor pump was scrap.

Also, some old motors are supposed to not be compatible with inverters.
I haven’t had an issue with motors of a similar vintage but you never know.

 

[minimutly]

You are going to really struggle with that on single phase and also with a vfd Or converter. I had a very twin on various types of vfd, they didn’t like it. Slowing it down won’t help - the current will just pulse less often.
My advice is to sell it and get a hydrovane. That’s what I did in the end.
Sorry

 

[Burdekin]

I have three 3hp comps so I can keep start up amps lower and also have one on a different ring main. Maybe worth considering if that one is going to have too much start up current.

 

[Arc Tourist]

..........................................................having the ability to limit the maximum torque / rpm might be useful if I find it tripping. Or is that only likely to be a concern during startup, perhaps use soft start or just fit a slower breaker? To be honest I'm not really sure what all these current ratings refer to, is the 16A on the motor plate the typical draw while running? The peak at start up? The maximum draw under load before it'll start damaging the windings?
Edit: and what does 16A.MESH mean, is mesh just another word for delta?

Sorry, I'm not an electrician by trade, but I'm confident there are some on the forum qualified to answer this.

 

[mike os]

You will probably need a converter rather than vfd as it’s 389/400v not 220/400v

 

[m_c]

For a 10HP motor, you're looking at a 7.5KW inverter minimum.
Depending on whether the motor is currently delta or star, or if indeed the ends of the windings are accessible to convert, will dictate if you need a 240V or 380V out inverter.

You'd then need to add an unloader solenoid, so you can spin the motor/compressor up to speed off load, then have the VFD activate the solenoid once at speed.
Normally this is part of the pressure switch, and the unloader gets closed as soon as the pressure switch closes, as the motor will accelerate fast enough on mains power to avoid the compressor stalling, but a VFD would need programmed to take several seconds to avoid overload/putting too much load on the incoming power, so you'd need to avoid loading the compressor too soon.

The big issue though, is if your 32A supply is powering anything else, as 7.5KW is right on the limit of a 32A supply.
Then there is the risk that due to the age of the motor, I suspect the winding insulation won't be VFD compatible, so there is a risk they'll breakdown over time when run from a VFD, unless you add a sine wave filter.

Running that compressor from a VFD on a 32A supply would likely be possible, but I'd say you'll be looking at a cost of at least £600 using cheap parts.
Plus on a compressor that age, I'd be wanting to get the tank inspected. There's no saying what condition it will be in internally.

 

[eddie49]

I would be taking advice from one (or more) reliable VFD, or other phase converters, supplier. They should know/advise if your requirements can be fulfilled. They can also provide accurate costings, too, as appropriate of course.

One example of a UK-based company that know about VFDs is: https://www.drivesdirect.co.uk

 

[timgunn1962]

As some of the answers above may have made apparent, there can be a pretty steep learning curve with VFDs. There can also be a pretty steep learning curve with compressors.

Running it from a single-phase 230V supply is likely to need a fair bit of thought and some money.

The motor rating plate shows 380/420V 50 Hz Mesh (aka Delta). It's not obvious how old it is and it would be wise to find out. The motor frame size looks to be an old Imperial one and there is still a NEMA 254 motor frame size listed. NEMA is the American system. Here in the civilized world we use IEC Metric frame sizes.

The 400V Delta configuration means it probably can't be run from a 230V single-phase in, 230V 3-phase out VFD. It will either need a step-up transformer to 400V (ish) for the input and use a VFD that doesn't mind if there is only single-phase power coming in, instead of the normal 3-phase, or it'll need a single-phase 230V in, 400V 3-phase out "boost" VFD. These are harder to find and usually carry a substantial price premium.

As a 3-phase 10 HP (7.5 kW) motor, the "normal" starting system will have been Direct-On-Line starting, which will usually pull 6-7 times the nominal running current during starting. Effectively, that is likely to mean you'll be pulling around 50 kW on startup, even if you use a perfectly lossless phase converter (no such thing exists AFAIK). A VFD can ramp up the Voltage nice and slowly, reducing the starting surge or even limiting it to no more than the normal running current. The downside of this is that it takes a lot longer to reach full speed and it shouldn't try to do any work until it up to speed. With a decent-sized receiver, this need not be a problem: a solenoid valve that unloads the line from the compressor head to the Non-Return Valve on the receiver, closed by an "at speed" signal from the VFD would almost certainly allow you to overcome any starting surge issues.

7.5 kW tends to be about the point at which "Star-Delta" (aka Wye-Mesh in North America) starting becomes normal. This was/is a way of reducing the starting current by initially connecting the motor in Star to reduce the Voltage across each of the windings and running it up to speed before changing over to Delta and giving the windings their normal rated Voltage. The starting current reduction from Star/Delta is much less than that available with a VFD and VFDs offer other advantages as well. Star/Delta starting is now pretty unusual on new installations as VFDs have taken over. Star/Delta starting typically draws around 2 1/2 times the Full-Load Current. Motors for Star-Delta starting on 400V 3-phase are typically wound for 690V in Star, 400V in Delta and run in Delta on 400V. Motors able to run (in Delta) from a single-phase-input VFD (230V single-phase input, 230V 3-phase output) are wound for 400V in Star, 400V in Delta and run in Star on 400V supplies and Delta on 230V supplies. 400V/230V is the "normal" configuration for modern motors up to 2.2 or perhaps 3 kW (3 or 4 HP), but above this, they tend to be wound for 690V/400V and can't be run to full speed from a 230V VFD. The output from a VFD varies both the effective Voltage and the Frequency. Being able to output 230V 3-phase would allow a nominal 400V motor to run to about 57% of its rated speed, where it would produce 57% of its rated power. In some applications, that is fine. Whether or not this is one of those applications is not clear.

When it comes to 400V input VFDs, most of the big-name VFDs will simply refuse to run without 3-phase input. Others need derating. Some of the cheaper Far-Eastern-made units will happily run on single-phase 400V. This is not something any of us normally have available, but step-up transformers can be bought. Oil-filled welders that accept both 230V and 400V power actually include an autotransformer that can provide 400V when fed with 230V. Such welders are often used in homebuilt phase converters, but can also be used to supply suitable 400V VFDs.

The age of the motor matters. A lot. "Old" motors tend to have relatively weak, brittle impregnation of the windings. Some may even have used shellac.

On normal mains power, interactions between the coils of the windings as their electric fields varying as a result of the AC waveform cause the windings to bunch tightly together at peak Voltage/Current and to relax slightly at zero Voltage/Current. It is this that causes the characteristic "transformer hum".

On 50 Hz mains, there are 100 peaks and 100 zero-crossings per second (there is a positive- and a negative-peak in each cycle and 50 cycles/sec).

A VFD uses Pulse-Width Modulation, typically switching at between 4 kHz and 12 kHz. Each pulse involves an on- and an off-switching event. At 12 kHz switching frequency, there are 12000 peaks per second. Also, because the switching is effectively instantaneous, the rate of change of Voltage (dV/dt) is much higher and, as a result, the forces on the winding impregnation during each event are many times greater than with "real" mains power.

If you try to run an "old" motor from a VFD, the insulation is very likely to fail, often within a second or so, but I'm told that sometimes it may take hours. There doesn't seem to be any hard-and-fast rule for the transition from "old" winding impregnation to newer "plastic" resins, so you take your chances somewhat. Most of us try to avoid problems and I have very little experience of killing old motors on VFDs: once was enough and mine was instant. It took out the VFD along with the motor.

I tend to regard anything before about 1960 as almost certain to fail and anything after about 1980 as almost certain to survive. I have nothing whatsoever to support this though. It is this uncertain age range that I suspect your compressor falls into.

If you really need to run an old motor from a VFD, you can use a sinusoidal filter between the VFD and the motor. Expect to pay more for such a filter than for the VFD or a modern motor, maybe both: it ain't usually the cheapest option, unless it's a particularly unusual motor that can't be changed for something standard without expensive mods to the machine.

Rewinding of a motor will be done using the winding impregnation current for the rewind date, so if you know it was rewound in, say, 2005, you can treat it as a 2005 motor from a VFD perspective. Rewinding is not cheap. Decades ago, it used to always be cheaper than a new motor. Now a standard motor is cheaper than a rewind. Rewinding is only normally viable on motors that are unusual enough for replacement to be very expensive, or where the lead-time is enough to justify the rewind cost by the reduction in downtime.

It seems likely that the most-realistic option would be to fit a modern motor and drive pulley, cannibalise an oil-filled welder for use as a 230V-400V step-up transformer so that you can use a cheap Chinese VFD to run the motor, set up an "at speed" output relay on the VFD, and add a solenoid valve to unload the air pressure while the motor is running up to speed. It's a lot of of faffing about.

 

[Gas mark 5]

As some of the answers above may have made apparent, there can be a pretty steep learning curve with VFDs. There can also be a pretty steep learning curve with compressors.

Running it from a single-phase 230V supply is likely to need a fair bit of thought and some money.

The motor rating plate shows 380/420V 50 Hz Mesh (aka Delta). It's not obvious how old it is and it would be wise to find out. The motor frame size looks to be an old Imperial one and there is still a NEMA 254 motor frame size listed. NEMA is the American system. Here in the civilized world we use IEC Metric frame sizes.

The 400V Delta configuration means it probably can't be run from a 230V single-phase in, 230V 3-phase out VFD. It will either need a step-up transformer to 400V (ish) for the input and use a VFD that doesn't mind if there is only single-phase power coming in, instead of the normal 3-phase, or it'll need a single-phase 230V in, 400V 3-phase out "boost" VFD. These are harder to find and usually carry a substantial price premium.

As a 3-phase 10 HP (7.5 kW) motor, the "normal" starting system will have been Direct-On-Line starting, which will usually pull 6-7 times the nominal running current during starting. Effectively, that is likely to mean you'll be pulling around 50 kW on startup, even if you use a perfectly lossless phase converter (no such thing exists AFAIK). A VFD can ramp up the Voltage nice and slowly, reducing the starting surge or even limiting it to no more than the normal running current. The downside of this is that it takes a lot longer to reach full speed and it shouldn't try to do any work until it up to speed. With a decent-sized receiver, this need not be a problem: a solenoid valve that unloads the line from the compressor head to the Non-Return Valve on the receiver, closed by an "at speed" signal from the VFD would almost certainly allow you to overcome any starting surge issues.

7.5 kW tends to be about the point at which "Star-Delta" (aka Wye-Mesh in North America) starting becomes normal. This was/is a way of reducing the starting current by initially connecting the motor in Star to reduce the Voltage across each of the windings and running it up to speed before changing over to Delta and giving the windings their normal rated Voltage. The starting current reduction from Star/Delta is much less than that available with a VFD and VFDs offer other advantages as well. Star/Delta starting is now pretty unusual on new installations as VFDs have taken over. Star/Delta starting typically draws around 2 1/2 times the Full-Load Current. Motors for Star-Delta starting on 400V 3-phase are typically wound for 690V in Star, 400V in Delta and run in Delta on 400V. Motors able to run (in Delta) from a single-phase-input VFD (230V single-phase input, 230V 3-phase output) are wound for 400V in Star, 400V in Delta and run in Star on 400V supplies and Delta on 230V supplies. 400V/230V is the "normal" configuration for modern motors up to 2.2 or perhaps 3 kW (3 or 4 HP), but above this, they tend to be wound for 690V/400V and can't be run to full speed from a 230V VFD. The output from a VFD varies both the effective Voltage and the Frequency. Being able to output 230V 3-phase would allow a nominal 400V motor to run to about 57% of its rated speed, where it would produce 57% of its rated power. In some applications, that is fine. Whether or not this is one of those applications is not clear.

When it comes to 400V input VFDs, most of the big-name VFDs will simply refuse to run without 3-phase input. Others need derating. Some of the cheaper Far-Eastern-made units will happily run on single-phase 400V. This is not something any of us normally have available, but step-up transformers can be bought. Oil-filled welders that accept both 230V and 400V power actually include an autotransformer that can provide 400V when fed with 230V. Such welders are often used in homebuilt phase converters, but can also be used to supply suitable 400V VFDs.

The age of the motor matters. A lot. "Old" motors tend to have relatively weak, brittle impregnation of the windings. Some may even have used shellac.

On normal mains power, interactions between the coils of the windings as their electric fields varying as a result of the AC waveform cause the windings to bunch tightly together at peak Voltage/Current and to relax slightly at zero Voltage/Current. It is this that causes the characteristic "transformer hum".

On 50 Hz mains, there are 100 peaks and 100 zero-crossings per second (there is a positive- and a negative-peak in each cycle and 50 cycles/sec).

A VFD uses Pulse-Width Modulation, typically switching at between 4 kHz and 12 kHz. Each pulse involves an on- and an off-switching event. At 12 kHz switching frequency, there are 12000 peaks per second. Also, because the switching is effectively instantaneous, the rate of change of Voltage (dV/dt) is much higher and, as a result, the forces on the winding impregnation during each event are many times greater than with "real" mains power.

If you try to run an "old" motor from a VFD, the insulation is very likely to fail, often within a second or so, but I'm told that sometimes it may take hours. There doesn't seem to be any hard-and-fast rule for the transition from "old" winding impregnation to newer "plastic" resins, so you take your chances somewhat. Most of us try to avoid problems and I have very little experience of killing old motors on VFDs: once was enough and mine was instant. It took out the VFD along with the motor.

I tend to regard anything before about 1960 as almost certain to fail and anything after about 1980 as almost certain to survive. I have nothing whatsoever to support this though. It is this uncertain age range that I suspect your compressor falls into.

If you really need to run an old motor from a VFD, you can use a sinusoidal filter between the VFD and the motor. Expect to pay more for such a filter than for the VFD or a modern motor, maybe both: it ain't usually the cheapest option, unless it's a particularly unusual motor that can't be changed for something standard without expensive mods to the machine.

Rewinding of a motor will be done using the winding impregnation current for the rewind date, so if you know it was rewound in, say, 2005, you can treat it as a 2005 motor from a VFD perspective. Rewinding is not cheap. Decades ago, it used to always be cheaper than a new motor. Now a standard motor is cheaper than a rewind. Rewinding is only normally viable on motors that are unusual enough for replacement to be very expensive, or where the lead-time is enough to justify the rewind cost by the reduction in downtime.

It seems likely that the most-realistic option would be to fit a modern motor and drive pulley, cannibalise an oil-filled welder for use as a 230V-400V step-up transformer so that you can use a cheap Chinese VFD to run the motor, set up an "at speed" output relay on the VFD, and add a solenoid valve to unload the air pressure while the motor is running up to speed. It's a lot of of faffing about.

Wow, this is really useful information, thank you for taking the time to write it all out. The plate on the receiver dates from 1965, so towards the earlier end of the range you mentioned, if the motor is of a similar vintage. Is there any way to determine for sure the type of insulation used on these particular windings? Would a plastic burn test reveal any obvious differences between old and new style insulation? Any way to 're-impregnate' or overcoat the windings in situ?

I'll look into it a bit more, see if I can work out what the insulation is made of, and whether the motor can be configured for 230V. But it's not sounding hugely promising for the original motor. I'll probably just use the spare petrol engine I have, it's a bit of a faff to set up, but no more so than getting electric running would be by the sounds of it. I only spent £100 on the whole compressor, so don't really want spend many multiples of that on VFDs and inverters with marginal chances of success.

The day job is embedded engineering (more familiar with milli and micro amps) and I have a spare PLC lying around ear-marked for the job, so shouldn't be too difficult to set something up. I'm imagining it just needs to actuate an unloader valve, power the starter until the engine gets up to idle speed, maybe run a PID loop with a servo on the throttle control to maintain a particular RPM (or just leave it at a constant throttle position if it doesn't vary that much). Close the unloader valve until it gets up to pressure, then kill the engine, and wait for the pressure to drop below a threshold before repeating the cycle. Am I missing anything there?

It was in use until recently, so I'm hoping the receiver is usable. It has what looks like inspection covers, so I'll see if I can have a look inside, and hopefully borrow a hydraulic testing kit.