starting a 7.5 hp single phase compressor

[boyle78]

I was looking for some options that some of you might have for my problem. Have a 7.5hp 240v single phase compressor in a detatched dwelling garage. No surprise that when that pig starts the lights flicker. I'm worried about reduced life of the electronic ballasts, and if the compressor is being used alot the alarm clocks in the house (off the same transformer) have to be reset. I know Baldor has a soft start...but way too pricey. A vfd isn't going to work since it's single phase, so I was wondering about adding an accessory starter capacitor or a deep well controler/starter. I wasn't sure if this was the way to go. Other than that, I'm looking at swapping out the 7.5 for a 5 hp single phase. I would think this option would reduce startup current, but it would just take longer to fill the tank right? Any help or ideas would be awesome.

 

[iwire]

As far as swapping to a 5 hp motor you would likely have to reduce the motor sheave size to reduce the load on the motor which as you guessed would mean it would fill the tank slower.


However you might get away with keeping the 7.5 hp motor but reduce the sheave size on it making it start up easier ..... Again the tank would fill slower.

 

[hillbilly1]

Make the power company put in the right size transformer. I don't know your location, but up north they are notorious at putting in a small transformer, then running 5-6 houses off of it. My uncle in Ohio had an automotive repair shop that had a 7.5 HP single phase compressor, the ups units on his computers would beep while the compressor was running. They had a 10 kva transformer running his shop, his house and the farm across the road. I told him to get the poco out there to upsize it, and his problems went away. They do that all over the place In rural areas, which was probably fine 60 years ago.

 

[ptonsparky]

A first for me in this area. The POCO to the NW informed me that service upgrades now costs the customer $350. The meter is moved to a pole in the alley and the customer is responsible from there on. They have been moving the meters but no charge. Not sure what will happen when we request a transformer chang.

 

[templdl]

I was looking for some options that some of you might have for my problem. Have a 7.5hp 240v single phase compressor in a detatched dwelling garage. No surprise that when that pig starts the lights flicker. I'm worried about reduced life of the electronic ballasts, and if the compressor is being used alot the alarm clocks in the house (off the same transformer) have to be reset. I know Baldor has a soft start...but way too pricey. A vfd isn't going to work since it's single phase, so I was wondering about adding an accessory starter capacitor or a deep well controler/starter. I wasn't sure if this was the way to go. Other than that, I'm looking at swapping out the 7.5 for a 5 hp single phase. I would think this option would reduce startup current, but it would just take longer to fill the tank right? Any help or ideas would be awesome.

Mmmmmmm, detached garage, where is the service entrance and if it is the house have you considered the voltage drop that could be occurring because of the feeder to the garage. That's where I would start first.
I would measure the voltage at the main CB while you have somebody start the compressor. If you have a voltage drop you may consider the service drop itself may not be sufficient.
Also, if the feeder to the garage is causing a voltage drop to the compressor it may be causing the compressor to take to long to start which may be reflected in the service entrance by a lengthy voltage sag causing the lights to dim noticeably for a long time. Should the feeder to the garage be changed to a larger size the compressor may take a shorter length of time to start causing only a momentary dimming of lights in the house which I don?t think would be objectionable.
You will notice the a compressor with a motor of that size is optimized with (2) capacitors, one to start and the other for running which improves the power factor of the motor allowing for the HP.
I fail to see what a well pump deep well capacitor would bring to the party. The well pump capacitor is located above ground on the motor controller because it can't be locate on the well pump itself. Besides that it would be a real pain to have to replace one if it was a part of the well pump.
Clocks in the house on the other hand would not be affected as they are base on the 60Hz frequency and as such is a non issue.

 

[Rick Christopherson]

Instead of looking at the electrical end, consider looking at the mechanical end. For a compressor of that size, find out if a head-unloader kit can be added (assuming it does not have one already). Your symptoms tend to indicate that the motor is starting under load, especially when you say that with frequent re-starts it kills the clocks in the house (The cylinder is still pressurized on the next restart).

Not all compressors are set up to accept a head unloader, but I'm guessing that one in this size range will. It opens compression in the cylinder so the motor is not starting under such a heavy load.

 

[Hv&Lv]

Clocks in the house on the other hand would not be affected as they are base on the 60Hz frequency and as such is a non issue.

What he means is that the voltage goes so low that the clocks (digital) will start blinking 12:00 and will restart from there. He should install a battery in them anyway.

 

[GeorgeB]

this is kinda DIYish here (but I'm a fluid power professional and do the real stuff too), but the addition of a check valve, single normally passing solenoid valve, and perhaps timer depending on compressed volume make the unloaded startup an easily implemented improvement. And plumbing DIY is ok, right? <g>

To add to Rick's comment, bleed the compressor (with the pressure switch OFF!) to atmospheric pressure and start from there. If the inrush, dimming, clocks resetting, etc still occur, unloading won't help.

 

[iwire]

Instead of looking at the electrical end, consider looking at the mechanical end. For a compressor of that size, find out if a head-unloader kit can be added (assuming it does not have one already). Your symptoms tend to indicate that the motor is starting under load, especially when you say that with frequent re-starts it kills the clocks in the house (The cylinder is still pressurized on the next restart).

Not all compressors are set up to accept a head unloader, but I'm guessing that one in this size range will. It opens compression in the cylinder so the motor is not starting under such a heavy load.

If this is a factory built assembly of compressor, motor, receiver and controls I would be very surprised if it did not have an unloader, that is very basic equipment often part of the pressure switch on smaller units. Even unloaded compressors are often hard starts.

Of course it is worth checking for it's presence and operation.

 

[boyle78]

Make the power company put in the right size transformer. I don't know your location, but up north they are notorious at putting in a small transformer, then running 5-6 houses off of it. My uncle in Ohio had an automotive repair shop that had a 7.5 HP single phase compressor, the ups units on his computers would beep while the compressor was running. They had a 10 kva transformer running his shop, his house and the farm across the road. I told him to get the poco out there to upsize it, and his problems went away. They do that all over the place In rural areas, which was probably fine 60 years ago.

I spoke to PSNH and they said that their 25kva is more than enough....just the house and the garage are tapped off it.

 

[boyle78]

What he means is that the voltage goes so low that the clocks (digital) will start blinking 12:00 and will restart from there. He should install a battery in them anyway.

that's exactly what's happening......but only on heavy usage days. (building race cars uses alot of air tools)

 

[ptonsparky]

Time to get the meters out and narrow it down. May be less expensive in the long run to replace those digital clocks with ones that have battery backups.

Check the unloader valve as suggested and make sure the start capacitors are functioning properly

I see HvLv already suggested batteries as well.

 

[boyle78]

THANK YOU for all the responses and ideas so far....some I've tackeld already, some I will very soon. I am going to revisit this from scratch, recheck my calcs, and take more measurments. Depending on what I find, changing the pully size on the motor may be my least expensive option....I'll keep ya posted!

 

[hillbilly1]

I spoke to PSNH and they said that their 25kva is more than enough....just the house and the garage are tapped off it.

They already have a 25 kva? That's unusual for a poco to use on a single residence without being prodded, must have been a relatively new install. If that's true, then it probably is a wire size issue then, but it also can be overloaded poco primary too, If it is a rural area with a long run back to the sub station, the voltage drop on the lines can also cause trouble.

 

[kwired]

Instead of looking at the electrical end, consider looking at the mechanical end. For a compressor of that size, find out if a head-unloader kit can be added (assuming it does not have one already). Your symptoms tend to indicate that the motor is starting under load, especially when you say that with frequent re-starts it kills the clocks in the house (The cylinder is still pressurized on the next restart).

Not all compressors are set up to accept a head unloader, but I'm guessing that one in this size range will. It opens compression in the cylinder so the motor is not starting under such a heavy load.

I can't ever recall seeing one that don't have unloader - especially if single phase motor. The problem is not enough capacity to start without suffering voltage drop - very common at typical dwelling services - and complicated even more by voltage drop on the feeder to the garage. Voltage drop is probably fine while running - it is the drop that occurs when starting that nothing was designed to handle.

If this is a factory built assembly of compressor, motor, receiver and controls I would be very surprised if it did not have an unloader, that is very basic equipment often part of the pressure switch on smaller units. Even unloaded compressors are often hard starts.

Of course it is worth checking for it's presence and operation.

:thumbsup:

I spoke to PSNH and they said that their 25kva is more than enough....just the house and the garage are tapped off it.

THANK YOU for all the responses and ideas so far....some I've tackeld already, some I will very soon. I am going to revisit this from scratch, recheck my calcs, and take more measurments. Depending on what I find, changing the pully size on the motor may be my least expensive option....I'll keep ya posted!

Changing the pulley size on your compressor effectively makes your 7.5 Hp compressor a smaller compressor. If you need the capacity of what you have you may find it will not keep up after doing this. It will definately run longer cycles as it is trying to do same amount of work at a slower rate = less horsepower = a lesser expensive compressor likely would have done the same job.

If you want soft starting you about need to use a VFD (oversized because you don't have a three phase source) and a three phase motor. This might cost as much as the original compressor when all is done.

Another possibility is starting motor unloaded and engaging a clutch to load it after it has started - but that involves redesigning the whole machine to some degree.

 

[templdl]

What he means is that the voltage goes so low that the clocks (digital) will start blinking 12:00 and will restart from there. He should install a battery in them anyway.

Good point but at what voltage does that happen? The voltage drop must be pretty serious then.

 

[Hv&Lv]

Good point but at what voltage does that happen? The voltage drop must be pretty serious then.

I think it is somewhere around 90 volts. The DC voltage will be too low to keep the devices on.

 

[tom baker]

A vfd isn't going to work since it's single phase, .

Its very common to use an oversized VFD for single phase in, three phase out, with this you only need to change the motor. We used to do this with a rotary phase convertor.
First I would try a softstart, it will eliminate any inrush. Also make sure the AC has an unloader on the compressor - it opens so starts with no load and then closes after up to speed.

 

[kwired]

Its very common to use an oversized VFD for single phase in, three phase out, with this you only need to change the motor. We used to do this with a rotary phase convertor.
First I would try a softstart, it will eliminate any inrush. Also make sure the AC has an unloader on the compressor - it opens so starts with no load and then closes after up to speed.

Soft start on a single phase capacitor start motor?

I have asked if this was even possible to make work before, so far no one has said it is possible without specialty built equipment that will cost more than alternate common methods.

 

[augie47]

I have seen them installed on single-phase air conditioner compressors numerous times so I would think it would be applicable for an air compressor but it's a good engineer or manufacturer question.
With that large of a single phase motor I think you will experience some "blink" regardless of what you do short of installing a soft-start.

This might be interesting:

http://www.baldor.com/pdf/manuals/834-602.pdf