Thread: 5.5HP 3Phase motor on Single Phase with a VFD ????

Im still on the lookout for a compressor...

Peerless P17 with larger tank is probably my first choice if I have to buy a new single phase unit.

Im also looking for second hand ones, what Ive noticed is that (generally) 3phase compressors are cheaper and provide more air that the single phase units.

Ive seen a Pilot K25 (three phase) and a Peerless PV25 for very good money - both provide significantly more air than the Peerless P17 but both use a 5.5HP 3 phase motor.

Is a 5.5HP 3-phase motor too big to run off a domestic supply with a suitably sized VFD? (I know Id need a dedicated 15A or 20A supply from the distribution board).

If its doable, any suggestions on which VFD to use? Any anything I should be aware of?

Thanks
Jon

I have a Clisby compressor running a 5HP motor with a single phase 5HP VFD.

5HP = 3750 W = ~15.6A @ 240V

Technically a 20A SP line and breaker are required because of high start up currents and for a continuous use, but a VFD can be programmed for a slow start so the current never goes above the max draw.

I have an ammeter permanently wired into my SP line so that I know what the draw is on the breaker. Most VFDs will display the current output but that is the current on one of the 3Phase lines and requires a conversion to determine the SP current being draw and because of AC power factors its is not always directly reflecting what the Breaker is delivering.

Mine is set up with a 20s slow start and peaks at 15.5A (4.96HP) at 135 PSI which is where I have the compressor triggered to switch off the VFD.
BTW By changing the max PSI you can change the max current draw by the motor.

The motor must be switched on and off by the VFD and not directly by the compressor pressure switch. This means hardwiring the motor to the VFD and using one of the 3 contacts available in the pressure switch to switch the VFD on and off. This works really well.

I have my compressor outside in a semi sound proof enclosure and I use one of the remaining 2 comp press switch contacts to switch 240V to a fan which vents the compressor enclosure while it is running and for an adjustable period thereafter. Some details of my circuits are here http://www.woodworkforums.com/showth...ght=compressor
More details and final setup here http://www.woodworkforums.com/showthread.php?t=184473
As usual it is more complicated than necessary but most of this is because I built most of it from bits and pieces I had on hand.

Bob has covered it pretty well, you can get 4KW VFD's for a few hundred dollars, it will need to be a permanent connection ( like an airconditioner or a stove ) and wiring rated for 20A. You might get away with a 15A circuit, but it's pushing the limit.

This is the cheapest 4KW drive I could find on ebay http://www.ebay.com.au/itm/4000W-5Hp...item51cc8fb410

A bit more searching might come up with a better deal.

Ray

Originally Posted by BobL

I have a Clisby compressor running a 5HP motor with a single phase 5HP VFD.

5HP = 3750 W = ~15.6A @ 240V

Thanks Bob, I need to re-read your posts again...and again... At least I now know that a 5HP will run on a 240V supply.

At the risk of asking a stupid question though, how do you determine a 3 phase hp current draw at 240V?? In short how do I work out how many amps at 240V from a 5.5HP 3 phase 415V?

Also, the link that RayG posted to a 4KW VFD specifies 5HP. For a 5.5HP motor would I need to go to the next size up (appears to be 5.5kW) or would a 5HP VFD drive a 5.5HP motor? Im assuming with losses in the electrical system a 5HP motor would be pushing the limits for a 5HP VFD and so little chance of it running a 5.5HP motor.


Thx
J

Originally Posted by ventureoverland

Thanks Bob, I need to re-read your posts again...and again...

Sorry I try to be clear as possible - maybe too technical?

At the risk of asking a stupid question though, how do you determine a 3 phase hp current draw at 240V?? In short how do I work out how many amps at 240V from a 5.5HP 3 phase 415V?

Not quite sure what you are asking here.

I prefer to work on the 240V side of this because a) I can measure it easily, and b) because this is what the breaker is seeing
Irrespective of what side is being used.
1 HP = 0.745 kW so a 5.5Hp = 5.5 0.745 = 4100 W or 4.1 kw

On the Single phase side
Divide the Power/Watts by Voltage will give you ~Current or Amps
ie 4100 W/ 240V = 17A.
The 4 kW VFDs are good for about 18A single phase.
Aside from when it starts when high currents are drawn but the VFD can be programmed to control) usually the motor will not draw 17A until the compressor is approaching its max pressure.
As I said in my previous post the max current drawn can also be controlled by adjusting the pressure at which the comp switches off.

Working on the 3 phase side of things, determining the total current drawn using the current shown by the VFD is more complicated because a square root of 3 is needed in there, and something else called a power factor is involved. This makes it trickier to use the VFD current read outs.
Some limited teasing shows that on my setup the Single phase side is over estimating the current and the 3P side is possibly under estimating, but I would rather stay with the over estimate.

Also, the link that RayG posted to a 4KW VFD specifies 5HP. For a 5.5HP motor would I need to go to the next size up (appears to be 5.5kW and twice the price of the 4kW!!) or would a 5HP VFD drive a 5.5HP motor? Im assuming with losses in the system a 5HP motor would be pushing the limits for a 5HP VFD and so little chance of it running a 5.5HP motor.

again 1 HP = 0.745 kW so 5.5Hp = 5.5 0.745 = 4100 W or 4.1 kw
A 4 KW VFD should safely drive that especially as the motor will only be delivering max power for a few seconds at the end of its recharge cycle.

When the compressor is in constant use the pressure will usually be below the max power draw and depending on the draw will even be below the switch on pressure i.e. 80 or 90 PSI. At this pressure the current drawn is about 2/3rd of the max current. The VFD and compressor should be able to run continuously at these currents - subject to compressor duty cycle of course.

Without testing I cannot guarantee any of the above, compressors are strange things.

One thing you have to be sure off is that the compressor motor is wired as either
240V ∆
or
415V Y (which can be converted to 240 ∆)

A 415V ∆ motor cannot be converted to 240 V ∆.

Originally Posted by BobL

One thing you have to be sure off is that the compressor motor is wired as either
240V ∆
or
415V Y (which can be converted to 240 ∆)

A 415V ∆ motor cannot be converted to 240 V ∆.

One of the compressors Im looking at has this photo of the motor...
Screen Shot 2015-02-27 at 10.51.34 pm.jpg

Does this give the information needed to know if it can be driven by a VFD??

Thx
J

Originally Posted by BobL

Sorry I try to be clear as possible - maybe too technical?



Not quite sure what you are asking here.

I prefer to work on the 240V side of this because a) I can measure it easily, and b) because this is what the breaker is seeing
Irrespective of what side is being used.
1 HP = 0.745 kW so a 5.5Hp = 5.5 0.745 = 4100 W or 4.1 kw

On the Single phase side
Divide the Power/Watts by Voltage will give you ~Current or Amps
ie 4100 W/ 240V = 17A.
The 4 kW VFDs are good for about 18A single phase.
Aside from when it starts when high currents are drawn but the VFD can be programmed to control) usually the motor will not draw 17A until the compressor is approaching its max pressure.
As I said in my previous post the max current drawn can also be controlled by adjusting the pressure at which the comp switches off.

Working on the 3 phase side of things, determining the total current drawn using the current shown by the VFD is more complicated because a square root of 3 is needed in there, and something else called a power factor is involved. This makes it trickier to use the VFD current read outs.
Some limited teasing shows that on my setup the Single phase side is over estimating the current and the 3P side is possibly under estimating, but I would rather stay with the over estimate.



again 1 HP = 0.745 kW so 5.5Hp = 5.5 0.745 = 4100 W or 4.1 kw
A 4 KW VFD should safely drive that especially as the motor will only be delivering max power for a few seconds at the end of its recharge cycle.

When the compressor is in constant use the pressure will usually be below the max power draw and depending on the draw will even be below the switch on pressure i.e. 80 or 90 PSI. At this pressure the current drawn is about 2/3rd of the max current. The VFD and compressor should be able to run continuously at these currents - subject to compressor duty cycle of course.

Without testing I cannot guarantee any of the above, compressors are strange things.

One thing you have to be sure off is that the compressor motor is wired as either
240V
or
415V Y (which can be converted to 240 )

A 415V motor cannot be converted to 240 V .

Thanks Bob,
Excellent reply.



Thx
Jon

Originally Posted by ventureoverland

One of the compressors Im looking at has this photo of the motor...
Screen Shot 2015-02-27 at 10.51.34 pm.jpg

Does this give the information needed to know if it can be driven by a VFD??

Thx
J

You're out of luck with that one. Clearly marked as Delta (Δ) at 415V.... bad luck. You are looking for a motor marked Y at 415V

Originally Posted by jhovel

You're out of luck with that one. Clearly marked as Delta (Δ) at 415V.... bad luck. You are looking for a motor marked Y at 415V

Thanks. I was hoping that someone would be able to say that the W U V symbols next to the 415Δ symbol suggests that the windings maybe changeable to 240Y.

I think from reading about changing from 415Δ to 240Y, when in 415Δ the bridging straps should be vertical (W1-W2, U1-U2 and V1-V2, as per picture) and when in 240Y they should be horizontal (W1-U1-V1).

R
Jon

Originally Posted by ventureoverland

Thanks. I was hoping that someone would be able to say that the W U V symbols next to the 415Δ symbol suggests that the windings maybe changeable to 240Y.

I think from reading about changing from 415Δ to 240Y, when in 415Δ the bridging straps should be vertical (W1-W2, U1-U2 and V1-V2, as per picture) and when in 240Y they should be horizontal (W1-U1-V1).

R
Jon

Other way around.. if it's wired 415 Δ then when changed to Y it will be 720 V.. what you are looking for is a motor that's 415 Y, and that then becomes 240 when wired in Δ.

Ray

PS.. But you can run a 415 Δ motor off a 240V 3 phase supply, but the power will be way down at 50 Hz... but strangely it will be the same at 25 Hz.. if you program the 240V VFD to have 25Hz as the rated motor frequency.

Thanks RayG - need a 415Y / 240Δ.

Im not sure if I should start another thread or continue this one as Im now starting to wonder from the original question, but its still all related to a compressor...

After not much look googling around for ideas of prices and availability of an appropriate used motor I called Teco in Sydney yesterday to try and get a bit more background info. In the process of discussing my needs they offered:

a) a new Monarch 4kW Single Phase 2 pole (Teco) motor with the same foot dimensions, shaft & spin speed for $561.
b) Teco VFD - $1300 !!!!!!
c) a new Teco 415Y / 240Δ 3 Phase motor with the same foot dimensions, shaft & spin speed for $390


Which got me thinking...
I understand the 4KW single phase to be a 3 phase motor wired up with 2 BIG capacitors (to run on single phase). I have heard over the years that one of the most common failures on larger single phase motors are the capacitors.

Now, discounting the Teco VFD in favour of a 4kW eBay cheapie ($350 ish)... If I were to proceed down the "new bits" route, with an eye on reliability and the "best" solution, would I be well advised to get the 3 phase plus VFD or just get the single phase motor and do without the VFD?


Next question...
The Teco man suggested that a (Teco) 4kW VFD would be more than happy driving a 4kW motor on a compressor. A chap on eBay (in Melbourne) that sells the $350 - $700 VFD's suggested that I should go for a 5.5kW vFD to drive a 4kW motor on a compressor. I wonder if there is any sense in this, or if he was just trying to up-sell?


Whilst Id love to be able to afford a VFD plus new motor to utilise a brilliantly sized (for me) compressor, by the time Ive finished its not going to be cheap. Indeed I suspect it will be similar money to a new Peerless P17... but on the other hand I would have an extra 150l/m more air.


Looking at the used motor market - where do you guys pick up these cheap motor bargains?


Decisions decisions.

Thx
J

My 5HP 3P WEA motor cost $35 and was obtained via Gumtree.
The motor was in very good nick plus it came with about 4m of quality 3P cable and a new Clipsal plug - I could not by the plug for that price .
The motor was from a near new large panel saw. The previous owner had to move house and had no 3P at his new place so he was replacing it with a single phase motor.

I bought a 5HP HY VFD from Aliexpress for $169 with free express shipping.
With the drop in the AUS$ and the added cost of shipping I see the latest price from the same seller is now $344! but I see others for sale at $260 including shipping.
The same VFD is available on ebay. The best price I could find from an AUS ebay supplier is $311 including shipping - still a fair deal IMHO.
It's interesting to note the seller does not recommend running a compressor motor with then, but mine has been running happily for almost a year now.
I'm not a heavy compressor user and I only use my compressor every other day and apart from when I use the plasma cutter (perhaps every other week), it rarely runs continuously.

This "cheapest available" option wouldn't be my preferred pathway if I was running a business, but I'm just a small time back yarder on a tight budget that likes to tinker so it suits me.

Originally Posted by ventureoverland

Looking at the used motor market - where do you guys pick up these cheap motor bargains?

There was a pallet load of 3 phase motors that no-one wanted at a place I was at on Monday this week. They were part of an auction lot, that the purchaser didn't want, so he left them there. Free to anyone who wanted.

These things last forever, the worst you might need is new bearings, and they mostly use commonly available cheap bearings.

Once you start looking, they are easy enough to find.

Ray

Yep, keep your eyes/ears open and check out ebay, gumtree, local garage sales, etc. I recently picked up a BRAND NEW Teco 5HP motor for $50 from a garage sale. Also recently I got a really old 3HP motor (a very, very solid "made in England" old thing) that had been reconditioned and then never put back into service (butchers meat bandsaw) for a carton of cheap beer.

Well, by way of update... I bought the compressor that started this whole thread off.

I sent the bloke a bit of a cheeky offer, fully expecting him to say no... but instead he said "yes, if you can pick it up in the morning". So $250 and a few hours drive later I have this:

New (to me) Peerless PV25 Compressor by Jon_Kelly, on Flickr

Now, when I got there I couldn't see it running so I was in two minds to walk away. The chap had all the paperwork that the compressor came with, plus a new set of reed valves for it (he had apparently just changed one cylinder), had owned it since new and he seemed genuine. In the back of my mind I knew I would need a new motor for it anyway and spares are available for the pump so I parted with the cash.

I did note that it had a broken drain valve, the compressor was full of oil and the air cleaners were pretty clean.

First thing I did when I got home was make the new drain valve insert (utilised the original rubber O'ring):
New (to me) Peerless PV25 Compressor by Jon_Kelly, on Flickr

Second thing was to pressurise the tank and see if the pump was leaking into either cylinder. It isn't - so far so good .

Now the not so good...
Pic 1 - (Apologies for being upside down - looks OK in Flickr!!)
This was taken at about 80psi. Its not a massive leak by any means, but a leak none-the-less:
New (to me) Peerless PV25 Compressor by Jon_Kelly, on Flickr

Pic 2:
(Apologies for being upside down - looks OK in Flickr!!)
New (to me) Peerless PV25 Compressor by Jon_Kelly, on Flickr

I think a rusted out tank is the most serious problem I could have had.

Is it dangerous / Is it safe to pressurise upto 145psi (normal shut-off pressure)? I guess I don't mind the leak, but can't run the risk of a sudden de-pressure.

Im assuming the fix is to grind off the foot, clean up the tank and re-weld it (taking care obviously to remove the 2" drain plug on the end of the tank to make sure no gas build up inside during welding).


Any thoughts? Have I bought a lemon or a peach ?

R
Jon