# A challenge for the VFD experts



## Electrorecycler (Apr 3, 2013)

So here's the challenge I face. We were awarded a tender to take a community public works buildings, office and workshop, completely off-grid. This is nothing new for us, we specialize in off-grid power systems and install them all over Ontario. This particular system is a large, robust off-grid system capable of 18kw @ 240VAC and can handle surge loads of up to 36kw for 5 seconds. It's powered by 20.8kw of solar, a 12kw prime generator and a 2460ah battery bank. This system can handle anything except, it seems, their 5hp 240V single phase air compressor motor. It takes the system down within 1 to 2 seconds of trying to start. I have tried a soft-starter to no avail.

After considerable research into this motor, it's starting current is borderline too high for this system with other loads present. WEG DP005102. FLA = 21A, LRA = 136.5A (32.8kw!), Locked Rotor Time = 6s

My question is; does anyone feel that a single phase VFD would solve this problem? Would a VFD be capable of ramping this motor up to speed while remaining at a reasonable current or am I barking up the wrong tree and should just be telling the customer to buy a smaller compressor? Any feedback would be appreciated!


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## gpop (May 14, 2018)

look for the un-loader valve it may be in the pressure switch. Hold that down and try to start the compressor. Once you think the motor is up to full speed release the un-loader. (take amp readings while you are doing this test).

If that reduces the load to the point where the motor can get up to speed then a simple timing relay and solenoid would be your best bet.


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## wiz1997 (Mar 30, 2021)

Here is one of many articles about using a VFD as a soft start and how it differs from an actual soft starter.



VFDs for Soft-Starting: Not Glamorous, but Necessary - EMA Inc.


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## backstay (Feb 3, 2011)

Just in my limited experience, your two parallel 9kW inverters are only capable of starting a 2 hp motor. You have a long way to go to get to 5 hp.


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## 460 Delta (May 9, 2018)

This is a long shot indeed, but some more capacitance in the start circuit, [hard start kit] may help.
If you are going to go with a VFD, swap in a 3 phase motor also to take full advantage of the drives capabilities.


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## gpop (May 14, 2018)

Starting any 5hp will be a pain even with no load (worth testing to see if that's even possible). Add the worst load you can find which would be a compressor just makes it even more challenging.

Now as you already have a generator that means you probably have propane / gasoline. I would just buy a gas powered compressor (add a propane conversion kit if required)


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## SWDweller (Dec 9, 2020)

I do not understand how you think a VFD is going to help when a soft start failed. They are pretty much the same animal except for the variable speed in start mode.

It has been a while for me with a soft start, how much time does the soft start provide? Before latching in. Did you get a big enough soft start? The units I worked with had a lot of settings for "every situation".

Does the 12kw generator have the ability to parallel with the solar to increase capacity?
How does the gen start?

Using the numbers you provided extending the time for the softstart would seem to be the place to begin. The max of the system and the compressor seem to be pretty much the same value which is not good.


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## Wirenuting (Sep 12, 2010)

It’s possible that depending on their application, a scroll compressor might be a better choice.


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## just the cowboy (Sep 4, 2013)

5 hp *single phase* motor? 
What VFD would you use, most that take single phase in put 3 phase out.
I would like to see if anyone has one that works.

Same with a softstart, most single phase don't work well.
Hard start kit like @460 Delta said maybe.


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## Almost Retired (Sep 14, 2021)

can you change the motor to 230V ?
that would cut the amps in half


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## just the cowboy (Sep 4, 2013)

Almost Retired said:


> can you change the motor to 230V ?
> that would cut the amps in half


He is already running it at 240V


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## micromind (Aug 11, 2007)

460 Delta said:


> This is a long shot indeed, but some more capacitance in the start circuit, [hard start kit] may help.
> If you are going to go with a VFD, swap in a 3 phase motor also to take full advantage of the drives capabilities.


Yep, it'd need to be a 3Ø motor, I don't know of any VFD that will run a 5HP single phase motor. 

If you go the VFD route, you'll need more than a 5HP VFD since a compressor starts hard even if the unloader works. 

Speaking of which, I seriously doubt if any VFD will start a fully loaded compressor. You'll need to ensure that the unloader actually works. 

As far as I know, there are 3 types of unloaders for a compressor like this.......

1) Pressure switch valve. This one has a small tube from the check valve to a small valve on the side of the pressure switch. When the switch opens, this valve will open and release the pressure in the line from the cylinders to the check valve. When the switch closes, this valve closes and the pump starts unloaded but it builds pressure fast so it needs to accelerate quickly. A VFD may or may not be quick enough, especially with a weak supply.

2) A combination check valve and unloader, commonly called a 'load genie'. This valve will release the pressure in the lines from the pump to the check valve based on flow. When the flow stops, the check valve will close and at the same time release the pressure. There's a small hols in the side of the valve that air will come out of when the pump stops. This one also needs to start fairly quickly but the pump can accelerate a bit slower than the pressure switch valve type. There are no small lines with this type, it's self-contained. 

3) Centrifugal unloader. As the name implies, this one will release pressure based on the speed of the pump. There's a small line that looks like it goes into the bearing but it's actually the unloader. 

To test the unloader, allow the compressor to build pressure until the pressure switch turns it off. Move the pump by hand, at least one full revolution,it should move fairly easy. If it is easy then gets hard, the unloader needs to be fixed. 

One other idea is that if they don't need the full capacity of the compressor and it's belt-driven, use a smaller pulley on the existing motor. This will reduce the energy needed to start it but it will also reduce the airflow. 

One problem I see here is the motor is 2 pole (3500 RPM). These motors often have considerably less starting torque and take longer to come up to speed. If you go the VFD route, you'll need to change the motor anyway so use a 4 pole (1740 RPM or thereabouts) and use a pulley on the motor that's a bit less than double the diameter as the one you have. The existing motor is 184T frame, the majority of 5HP 4 pole motors are also 184T frame so the mounting holes and shaft will be the same.


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## 460 Delta (May 9, 2018)

To tag onto @micromind post, you could venture out into the weeds with old 460 and build an unloader with a ON delay TDR and a diaphragm valve (we call them ASCO valves). When the motor starts, power the TDR and send power through the N/C contacts, and in say 10 seconds or so the TDR times out and goes to the N/O contacts killing power to the valve. It shuts stopping the air release and into the tank.
Why buy for $50 when you can build it yourself for $100. Lol.


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## JRaef (Mar 23, 2009)

VFDs and reciprocating compressors are not a great fit. it can be done, but usually you have to over size the VFD to avoid having it trip on high DC bus. A Soft Starter should have worked, so if it didn't, I'd say you have an unloader issue to deal with first.

As others have said, make sure that it starts fully unloaded. That's usually the reason for high starting current. An unloaded compressor should start in under 1 second, but it will still demand that 600% current for that second.


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## paulengr (Oct 8, 2017)

SWDweller said:


> I do not understand how you think a VFD is going to help when a soft start failed. They are pretty much the same animal except for the variable speed in start mode.
> 
> It has been a while for me with a soft start, how much time does the soft start provide? Before latching in. Did you get a big enough soft start? The units I worked with had a lot of settings for "every situation".
> 
> ...


Way off base here.

A soft start reduces starting current but also reduces torque. All it can do is manipulate the voltage. That’s fine for some applications. In 3 phase you’d just do a quick 1-2 second ramp to by easy on the power train then start at 450% of FLA minimum. Some may need 500-550%. So it’s not very soft.

A VFD reduces the voltage too but it also reduces the frequency, keeping torque constant. The only “gotcha” is when sizing VFDs the price goes up a lot with the maximum current so the natural tendency is to undersize them. Since you get full torque at nearly any speed a VFD can “soft start” even loads that ATL doesn’t work on.

But this is single phase. At running you have a sizable run capacitor that cancels the bad power factor of single phase and boosts voltage from the motors point of view. But you also have s second starting winding 90 degrees apart with a big starting cap boosting voltage and giving it a roughly 25-30 degree phase angle advance. This is set up for a stuff voltage. If you are dipping a lot you need to increase the starting cap. In very bad conditions connect a second identical size one in parallel.

Soft starting single phase seems like an incredibly terrible idea. I can’t see any legitimate good reason to attempt it. Starting sucks enough as it is. We could just reduce the start cap.

Single phase VFDs only work on PSC motors which have lousy starting torque to start with though not as lousy as shaded pole. The exception is you can get three phase VFDs to run on single phase power. But you are going to be pushing close to 200-300% FLA on the starting. Plus you need to double the VFD because it has only one phase. So that’s 5 HP x 3 x 2 = 30 HP. I believe the biggest 240 V single phase drives are 20 HP. This just has bad idea written all over it. It also isn’t going to solve much if your starting issue. Plus the two compressor thing is a bad idea. Sell them both and buy a legitimate 10 HP. And if it’s a home workshop thing what are you using it for? Most “air” uses are not good painting works much better with HVLP which doesn’t need 90 PSI. There are better compressors. As far as air tools I went battery or corded long ago. Pneumatic controls are much better all electric. Plus think about it…what are the losses? Estimates are it takes 8 HP of electricity to drive a 1 HP pneumatic load.


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## FisherElectricAutomation (Jan 5, 2021)

I'm very late to this thread but happened upon it browsing and I'm curious what you came up with. My two cents are to skip the vfd and simply use a smaller compressor that may just run longer periods through the day. Although this would be entirely based on the air demand in question. Is it a remote water treatment facility or a maintenance shed? In my experience the maintenance depts just buy the biggest compressors they can get their hands on within budget. If the air is for a daf skimmer or some process the township will have the engineered requirements on hand, work shed a small portable will suffice. A 2hp unit can usually supply [email protected] Hope you got it squared away


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