Could you run a big 7.5hp compressor with vfd on a 240v 16amp supply?

[Domeo_jones]

People... just searched for vfd info abd I can see a lot of lathes and milling machines.
Which of course is next on my shopping list.
(Btw any myford super 7s going?
But... just looking at a decent compressor and 3 phase ones are so much cheaper... so.....
Any help would be amazing.
Thank you all so much.

 

[skotl]

In a word.... no

7.5HP is 5.6KW, or 25A at 220V, not allowing for startup current or losses through the VFD.

 

[Kram]

Yes a VFD will run it but you'd have to set freqency low, between half -2/3rds speed to stay within 16A limits.

VFD's are listed by power, but you need to look at motor full current. 7.5kw VFD's are not cheap. It wont run from a 4kw vfd.

Startup current is no great issue with a vfd - big capcitors and soft start smooths it out.

 

[skotl]

Yes a VFD will run it but you'd have to set freqency low, between half -2/3rds speed to stay within 16A limits.

Startup current is no great issue with a vfd - big capcitors and soft start smooths it out.

Is that defeating the purpose, though? Would it not be more straightforward to get a single phase compressor that will run within 16A?

(I'm also considering the cost of the VFD presumably negating the cost saving of a 3 phase compressor)

 

[Kram]

I didnt say its good idea just that it'd work. One advantage is slow motor should be less noise, and 3ph are generally? belt drive and better quality. If its old, check the tank in case its a rusty bomb.

Another option is get it, sell the motor, put a 4kw 3ph and vfd. Still not cheap.
Or a 32A socket fitted.

 

[Domeo_jones]

I didnt say its good idea just that it'd work. One advantage is slow motor should be less noise, and 3ph are generally? belt drive and better quality. If its old, check the tank in case its a rusty bomb.

Another option is get it, sell the motor, put a 4kw 3ph and vfd. Still not cheap.
Or a 32A socket fitted.

So would a 4kw 3ph motor be able to run a 5.6kw pump?
I might have to pm you.

 

[Agroshield]

The 5.6kW power is the rate of doing work. So to lower the amount of power it demands, you have to do the work slower.

The original motor is likely to be sized to give a specific flow rate (cubic feet per minute). It is also sized for across-the-wires starting under considerable load (i.e zero to full speed in no time at all with maybe 80psi pushing on it). It will be sized for the working pressure of the tank.

If you can sacrifice a great deal of the plated performance of the compressor, you can make it work on a 16A supply.

You could lower the working pressure and you can change the pulleys so it fills slower. Starting from an empty tank, and with an unloader valve and slow ramp up on the VFD should be no issue.

However, it seems to me that the biggest requirement for it will be restarting on a part-full tank and going up to full pressure. You might be better to ask a general question in the compressor sub-forum concerning when the machine is at its most power hungry and what options are available to deal with this.

 

[Domeo_jones]

The 5.6kW power is the rate of doing work. So to lower the amount of power it demands, you have to do the work slower.

The original motor is likely to be sized to give a specific flow rate (cubic feet per minute). It is also sized for across-the-wires starting under considerable load (i.e zero to full speed in no time at all with maybe 80psi pushing on it). It will be sized for the working pressure of the tank.

If you can sacrifice a great deal of the plated performance of the compressor, you can make it work on a 16A supply.

You could lower the working pressure and you can change the pulleys so it fills slower. Starting from an empty tank, and with an unloader valve and slow ramp up on the VFD should be no issue.

However, it seems to me that the biggest requirement for it will be restarting on a part-full tank and going up to full pressure. You might be better to ask a general question in the compressor sub-forum concerning when the machine is at its most power hungry and what options are available to deal with this.

Thank you.

 

[brightspark]

its pointless buying a big comp and reducing the speed and pressure . just buy a smaller comp

 

[armalites]

Yes a VFD will run it but you'd have to set freqency low, between half -2/3rds speed to stay within 16A limits.

VFD's are listed by power, but you need to look at motor full current. 7.5kw VFD's are not cheap. It wont run from a 4kw vfd.

Startup current is no great issue with a vfd - big capcitors and soft start smooths it out.

I suspect a motor on a compressor would get pretty hot if you were running it like that. I run the pump on my 4 post ramp slightly slower but it runs for less than a minute a time.

 

[Domeo_jones]

So next question.... could I possibly build a 2x4kw tandem compessor using vfds to delay start ups but also get some pretty impressive cfm from 240v 16amp ?
Just wondering if/How you would stagger the start ups?

 

[skotl]

So next question.... could I possibly build a 2x4kw tandem compessor using vfds to delay start ups but also get some pretty impressive cfm from 240v 16amp ?
Just wondering if/How you would stagger the start ups?

I thought you were looking at this cos 3 phase compressors were cheaper? Sounds like it's getting expensive now

Anyways, you could rig up a delayed starting circuit relatively easily enough, if you're somewhat comfortable with a little electronics & soldering. I'd have a power switch that trigger the low-voltage start signal on the first VFD, and then triggered a timer circuit that slept for 30-60 seconds, then initiated the start on the second VFD.

So staggering the startup is easy enough, but your problem is going to be when they go to recharge their cylinders as the pressure drops. There will always be variations that you can't account for, so there's always a chance that both will fire at the same time.

But.... why? If you just want more air capacity, get a single compressor (single phase seems most appropriate) and add a second tank.

 

[Kram]

Compressors have an unloader valve, so they are not starting against tank pressure.
There is no way any of this will work out cheaper. What compressors andvfds areyou looking at?

Are you intending to run two 16A compressors together, or, staggered such as to maximise duty cycle?

If your going to be spending decent money, the best result will be, upgrade your socket to 32A and a large VFD to run it.

Get a proper lathe, the money you've saved from not getting a Myford, will pay for everything And if your not using compressor and lathe at same time, it could run both, ideally same spec motor so mimimal fuss with settings.

 

[Hopefuldave]

Compressors have an unloader valve, so they are not starting against tank pressure.
There is no way any of this will work out cheaper. What compressors andvfds areyou looking at?

Are you intending to run two 16A compressors together, or, staggered such as to maximise duty cycle?

If your going to be spending decent money, the best result will be, upgrade your socket to 32A and a large VFD to run it.

Get a proper lathe, the money you've saved from not getting a Myford, will pay for everything And if your not using compressor and lathe at same time, it could run both, ideally same spec motor so mimimal fuss with settings.

Right about getting a decent lathe not a Myford, just the name more than doubles the price compared to something with the same (or much greater) capabilities! You could have niceties like a quick-change gearbox for threading (Imperial and metric without juggling gears if you're luckly and patient!), power feeds, plenty of HP and multiple speeds without faffing around with belts, a taper attachment...

[EDIT] - a decent lathe would justify messing about with a VFD thereby opening up the world of lathes from the Golden Age of British Engineering... \[EDIT]

Compressors... the work done by the air tool is always going to be less than the work put into compressing the air, if it's not for a big spray setup or grit blasting (and you're not fretting about explosive atmospheres) electrical tools are a lot more efficient and take up a lot less space (says he with a big-ish twin-cylinder compressor and a load of air tools because... I can).

Dave H. (the other one)