VFD suggestions for compressor

[not done it yet]

That is a very comprehensive assessment for VFD use and applicability to older motors. Thanks.

The BSI kitemark may well have a number to find an approx age of the motor. A brass(?) motor plate likely dates it, too. Using ‘mesh’ instead of Delta is yet another indicator? Most likely for UK/Europe - it is rated as 50Hz.

There will/should be a date of manufacture on the receiver, which may be relevant?

 

[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.

 

[minimutly]

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.

Now you're talking - proper forum engineering - and probably much more likely to succeed than trying to run that on single phase. Promise one thing - you'll start, or continue this post to let everyone know how you get on?
Re the plc control - Nudge and wait might be just as suitable as a PID block for loading the engine - maybe based on an rpm set point for pumping, with another for idling? Simpler is better sometimes.

 

[julianf]

My friend acquired a dive compressor with a 3ph motor.

As I've not completed my wadkin rebuild, I lent him my 7.5kw boost vfd (less than £100 on one of @Kram s Amazon specials! ; ) and it's doing him fine.

I forget the details on his motor but it's not pocket size...

 

[not done it yet]

and whether the motor can be configured for 230V.

No chance whatsoever. It’s already stated that it is a delta-wound (mesh) motor.

Running at ~57%of rated power is not really an option for a compressor, unless the gearing is substantially changed to reduce the pump speed and, therefore, output.

 

[Seadog]

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'm running a motor of 1953 vintage on my lathe, from a VFD. It's survived since 2013. The only thing that I think that I did was to replace the tails inside.

Visit Brocott's site. They do a red lacquer for this purpose.

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?

The unloader should be an integral part of the compressor pressure switch. When the pressure causes the switch to cut power it should open the unloader automatically. The VFD will directly feed the motor and you then use one of the pairs of power contacts in the switch to operate the stop function on the VFD.

 

[graffian]

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

My compressor came with a 5hp motor[three phase] and I replaced it with a 5hp single phase motor.
I was told you shoudl put aprox a third more horses single phase on it, but I had the motor and it worked.
All the lights dimmed and fluorescent lights restarted, when it started. When it had some pressure the lights flikered in time with the pump.
When more hp for single phase motor was suggested doubing the hp if using a petrol engine was mentioned!
I made a rotary phase converter, after 20 years of flickering all the light on the same phase as this house. The dimming and
flickering stopped. The idler motor rocks around in time with the pump.
I tried slowing the pump and it didn't make a huge difference.