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is there any gain to switch an from single phase air comp motor to 3 phase with a VFD to draw less amps

everdingfp

Plastic
Joined
Nov 28, 2022
I have a hypothetical question, i don't think will work But i need to ask it- keep in mind i realize its a big investment in materials but its worth it to me if its possible? We have portable temporary 2 hp air compressors that are 120-240v 1 phase 15/7.5 FLA compressors - we install these in emergency repairs to keep dry sprinkler systems in service for a couple weeks at a time when there is a failure of the existing sprinkler air compressors until a proper replacement can be available- these temp air compressors draw to much current and trip the 120v 15 amp common receptacle circuit breakers- bringing in an electrician to hard wire them in temporarily is always a delay and takes time- sometimes a new circuit and wiring has to be installed - is there any gain to switch this 2hp air comp motor from single phase to 2hp 3 phase with a VFD to draw less amps and be able to use them as plug and go or is there another way I'm not contemplating. We have used smaller temporary air compressors but they just don't last and are just too small for the job.
 

markz528

Hot Rolled
Joined
Sep 25, 2012
Location
Cincinnati
Technically you will not be able to run a 2 hp motor on a 15 amp 120 vac circuit. 20 amp circuit - yes. Remember that per code, a breaker can only be run at 80% of its rating continuously, so 16 amps on a 20 amp circuit and 12 amps on a 15 amp circuit.

I think you would use a step up transformer and use a 240 volt drive. I believe the single phase 120 volt drives have a high input current rating. And even 240 volt micro VFDs have higher input amps. A good general purpose 240 volt VFD would have the lowest input amps.

The advantage of the drive is that you eliminate the inrush current and you can run it a little slower to reduce load - very beneficial if you don't need full flow.
 

JST

Diamond
Joined
Jun 16, 2001
Location
St Louis
Probably no gain.

Anything you add is going to use some power, and increase draw at full power.

The VFD "might" give an advantage if you can run the compressors at less than full power. That would be if you really do not need the full power of the compressor, if the airflow is less than the pump capability, and the compressors spend much of the time shut off as the tank drains air.

However, if so, it could then be just as good to replace the motors with motors of the lower power, less by roughly the proportion of time the motors are off now. If they are off 50% of the time, a 1 HP motor might be enough (with an adjustment to the motor pulley size). That would be simpler and likely cheaper and more durable.
 
Joined
Jan 15, 2005
Location
The Netherlands
Like JST says Also a VFD can reduce inrush current significantly
The pressure setting is also important The motor will draw only full current at max pressure A VDF you can configurate probably that it will work If that is enough flow is unsure
A step up transformer is not going to help BTW
 

markz528

Hot Rolled
Joined
Sep 25, 2012
Location
Cincinnati
A step up transformer is not going to help BTW
He wants to run it on a 120 volt input. Most logical is to use a 240 volt drive so the step up transformer works well. I have a portable 1 hp skid and that is what I do.

Would have to look at at the details from an engineering standpoint, but the increased losses from a transformer and drive may very well be offset plus some in amps by the improved power factor input of the drive. Depends on drive input power factor and motor power factor.
 

reggie_obe

Titanium
Joined
Jul 11, 2004
Location
Reddington, N.J., U.S.A.
These temporary compressors are for the dry side of the sprinkler system? Can the motor be downsized along with the pulleys to run the pump slower and still maintain system pressure? Smaller motor, less current draw.
 

BT Fabrication

Stainless
Joined
Nov 3, 2019
Location
Ontario Canada
I have a hypothetical question, i don't think will work But i need to ask it- keep in mind i realize its a big investment in materials but its worth it to me if its possible? We have portable temporary 2 hp air compressors that are 120-240v 1 phase 15/7.5 FLA compressors - we install these in emergency repairs to keep dry sprinkler systems in service for a couple weeks at a time when there is a failure of the existing sprinkler air compressors until a proper replacement can be available- these temp air compressors draw to much current and trip the 120v 15 amp common receptacle circuit breakers- bringing in an electrician to hard wire them in temporarily is always a delay and takes time- sometimes a new circuit and wiring has to be installed - is there any gain to switch this 2hp air comp motor from single phase to 2hp 3 phase with a VFD to draw less amps and be able to use them as plug and go or is there another way I'm not contemplating. We have used smaller temporary air compressors but they just don't last and are just too small for the job.
HP = Wattage which is also directly related to air CFM flow.. So changing and adding more devices is making more current used if all you ever have is 120V 15A.
2Hp in anything will still draw the same if not more if ran through a vfd as there are losses in the vfd.
if you had a single leg from a 3 phase service like 600V here goes down to 277V on one leg to neutral. 480V i think goes to 208V in the usa.
 

mksj

Cast Iron
Joined
Aug 10, 2010
Location
Tucson, AZ
There are some motors that are considered very high efficiency, a single phase may be around 60% vs. Striatech DVR motor may be around 90%. They have a 1.75Hp motor with controller which may be just be enough and not trip the breaker. Unfortunately they provide no specifications for their motors as to current draw, but you can contact them and request further information. I do not follow where hard wiring changes the situation other than the plug rating limitation, with a 2 Hp motor you are drawing more than the 120VAC circuit rated at 15A.

 

everdingfp

Plastic
Joined
Nov 28, 2022
Thanks for the reply's it has to be portable and small enough to get into a apt building parking garage sprinkler/ water service room, usually at night mostly on weekends these never fail when its convenient, I like the idea of maybe a smaller motor 120v 1 hp is13.1 amps, trying to find a motor that only draw 12 amps to replace it maybe too small to be efficient- is there a calc to size a smaller diameter pully
 

johansen

Stainless
Joined
Aug 16, 2014
Location
silverdale wa
Friend of mine uses Schwartz oil less compressors and has in general found them to be fairly reliable with the exception of the connecting rods fatiguing, I have repaired them several times, and found some interesting failures particularly with the crap we bought on ebay that were "rebuilt" by someone who didn't know a gasket is needed to set top dead cylinder clearance.
The motor pulls 13 amps no load, increasing to 15 at 80-90psi on the tank. (1hp, 120v connection) -this is utterly terrible.

I would speculate the power factor is pretty low. You can certainly go the vfd route with a voltage doubling 120v vfd driving a 1hp 3 phase motor. It will work, but your actual gains are nearly zero. the reason why is because the vfd draws a power factor of about 0.6, maximum of about 0.7 for a voltage doubler.. the problem being it won't last long. the capacitors blow up. (and unlike a motor, you can't add capacitors to get the power factor back up, because its not a phase shift, but rather the vfd draws short spikes of current well in excess of the rms amps)

The motor increases from as low as 70% power factor 60% efficiency.. to 80% efficiency for a 1hp 3 phase motor, but the vfd can only be guaranteed to be 0.6 power factor.

But adding the right amount of capacitance to the motor, will decrease both no load and full load amps significantly, for nearly free and will last 100,000 hours where as a cheap vfd might last a few years.

there is another solution: call up your local hvac companies and ask them for these types of motors:
more often than not, you can take a few broken ones and get one that works. the magnets fly off and crash for example, leaving the inverter good.
https://www.supplyhouse.com/Carrier-S14S0016N01-120-240v-1-ph-1-HP-1250-rpm-Motor -there are several similar on ebay right now for $150 to 250.
The problem is.. it will be 1hp actual. no 50% overload available.

You may find your "1" hp air compressor is drawing 1.5hp shaft power just before it shuts off. that's why the shwartz oil free compressor i mentioned is pulling 15 amps. If the motor was producing only 1hp (assuming efficiency and power factor are the same) it would be: 64% power factor and 64% efficient and would probably burn up. In reality the motor is probably .7 power factor and .7 efficient and it is developing 1.2hp. Or 80% power factor 70% efficient and its developing 1.3hp.

anyhow if you want to get your line amps down below 10 at shut off and develop 1 shaft hp.. you need to get a 2hp motor, wire it for 120v, add some power factor correcting capacitors, and reduce the line voltage to the motor by 10-20% and set the pullies so the compressor only draws 1shaft hp actual. not "nominal".

Ihe inverter ECM motors can deliver 1hp at 10 amps on a 120 circuit because the motor is 90% efficient and the rectifier 70% power factor. 1200 x .9 x .7 = 746. --- you ain't going to get a 1hp 3 phase induction motor to operate at 90% efficiency. maybe 80%.

sorry for the long post...
 

everdingfp

Plastic
Joined
Nov 28, 2022
Friend of mine uses Schwartz oil less compressors and has in general found them to be fairly reliable with the exception of the connecting rods fatiguing, I have repaired them several times, and found some interesting failures particularly with the crap we bought on ebay that were "rebuilt" by someone who didn't know a gasket is needed to set top dead cylinder clearance.
The motor pulls 13 amps no load, increasing to 15 at 80-90psi on the tank. (1hp, 120v connection) -this is utterly terrible.

I would speculate the power factor is pretty low. You can certainly go the vfd route with a voltage doubling 120v vfd driving a 1hp 3 phase motor. It will work, but your actual gains are nearly zero. the reason why is because the vfd draws a power factor of about 0.6, maximum of about 0.7 for a voltage doubler.. the problem being it won't last long. the capacitors blow up. (and unlike a motor, you can't add capacitors to get the power factor back up, because its not a phase shift, but rather the vfd draws short spikes of current well in excess of the rms amps)

The motor increases from as low as 70% power factor 60% efficiency.. to 80% efficiency for a 1hp 3 phase motor, but the vfd can only be guaranteed to be 0.6 power factor.

But adding the right amount of capacitance to the motor, will decrease both no load and full load amps significantly, for nearly free and will last 100,000 hours where as a cheap vfd might last a few years.

there is another solution: call up your local hvac companies and ask them for these types of motors:
more often than not, you can take a few broken ones and get one that works. the magnets fly off and crash for example, leaving the inverter good.
https://www.supplyhouse.com/Carrier-S14S0016N01-120-240v-1-ph-1-HP-1250-rpm-Motor -there are several similar on ebay right now for $150 to 250.
The problem is.. it will be 1hp actual. no 50% overload available.

You may find your "1" hp air compressor is drawing 1.5hp shaft power just before it shuts off. that's why the shwartz oil free compressor i mentioned is pulling 15 amps. If the motor was producing only 1hp (assuming efficiency and power factor are the same) it would be: 64% power factor and 64% efficient and would probably burn up. In reality the motor is probably .7 power factor and .7 efficient and it is developing 1.2hp. Or 80% power factor 70% efficient and its developing 1.3hp.

anyhow if you want to get your line amps down below 10 at shut off and develop 1 shaft hp.. you need to get a 2hp motor, wire it for 120v, add some power factor correcting capacitors, and reduce the line voltage to the motor by 10-20% and set the pullies so the compressor only draws 1shaft hp actual. not "nominal".

Ihe inverter ECM motors can deliver 1hp at 10 amps on a 120 circuit because the motor is 90% efficient and the rectifier 70% power factor. 1200 x .9 x .7 = 746. --- you ain't going to get a 1hp 3 phase induction motor to operate at 90% efficiency. maybe 80%.

sorry for the long post...

But adding the right amount of capacitance to the motor, will decrease both no load and full load amps significantly, for nearly free and will last 100,000 hours where as a cheap vfd might last a few years.

can you add capacitance to an existing 120v single phase 1 hp motor? how would you do that ?
 

JST

Diamond
Joined
Jun 16, 2001
Location
St Louis
can you add capacitance to an existing 120v single phase 1 hp motor? how would you do that ?
You are increasing the power factor, so the motor plus capacitors draws less current by canceling the magnetizing current, and drawing net only the "power" current. Magnetizing current is roughly constant at all power levels.

The capacitors simply go across the input line. You can calculate the required capacitance for a 0.9 power factor, if you know the starting power factor (you probably do not have a solid number for that).

You can also just assume that the current draw of the motor when running completely unloaded (not even a belt connected) is about 75% to 80% magnetizing current. (it's probably higher, but you do not want to over-correct).

Put in a capacitor that will draw about 90% of the current you assumed as magnetizing current, and you should significantly reduce the total input current.

The capacitors draw "leading" current, while the inductance of the motor draws "lagging" current.* The two cancel out, leaving only the "real power current", which will be less than the sum of the "power current" plus magnetizing current.

You do not want to go above a correction to around 0.8 to 0.9 power factor.

* "Leading" refers to the current being maximum before the voltage is maximum, while "lagging" means the current reaches a maximum after the voltage does. An ideal load (a resistor) has voltage and current reaching a maximum at exactly the same time, and has a power factor of 1.0.

Te "power factor" is the proportion of total current that is "power" current. So a power factor of 0.6 means that 60% of the total current is producing actual power, and the rest is "reactive" current, out of phase with the voltage. An ideal resistor has a power factor of 1.0. An ideal inductor or capacitor has a power factor of 0.0 (zero), and draws only reactive current.
 

Almost Retired

Aluminum
Joined
Mar 23, 2019
Location
north central louisiana
Than You very much very helpful
short answer no
but as mentioned above there are things you can do to decrease your amp draw
if you can reduce required watts for the end device
then you wont require as many watts at the supply
it will all depend on how much money you want to spend for a small reduction that will actually do the required work


a transformer will actually increase your amp draw due to its own losses
a transformer works similar to a vehicle transmission
low speed = high torque
hi speed = low torque
it takes the same hp to make the vehicle run either way

where volts is equated to speed
and torque is equated to amps
and HP is equated to watts
higher voltage uses less amps
lower voltage uses more amps
for the same watts (watts = volts x amps)

so a 230 motor uses less amps at 230V 3 phase
but the transformer uses more amps on the input at single phase
again volts x amps = watts
there is no free lunch
what ever HP you want to run at whatever phase and voltage draws X amount of amps
now change the formula to solve for amps
amps = watts divided by volts

you cant cheat the utility or the circuit breaker feeding what ever combination you try
it still boils down to required watts at supplied voltage must equal required watts at user voltage

however if you can reduce required watts for the end device
then you wont require as many watts at the supply
it will all depend on how much money you want to spend for a small reduction that will actually do the required work
 

Almost Retired

Aluminum
Joined
Mar 23, 2019
Location
north central louisiana
a better fix would be using the supply for the original compressor which is now out of service
even it it was hardwired, a 240V 20A plug can be installed beside the existing supply at much less than the eqpt you are talking about buying for every compressor you put in service
 








 
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