Compressor electrical question

[Stephen Newell]

I keep burning up pressure switches on my compressor. 5 hp single phase 220. It was recommended that I use a magnetic switch to keep from doing that. Should the power run through the magnetic switch, then the pressure switch to the motor or through the pressure switch, magnetic switch then the motor.
 

[Ken-Pa]

Are you using the correct magnetic safety switch ? Wiring goes thru the safety switch , then pressure switch to motor . Make sure you"re using the correct amperage wiring & breaker also . Its been several years since I"ve done any compressor installations , so I"ve forgotten a lot . You also need to take into consideration how far the compressor is from your main source of power due to voltage drop . Remember 8 ga wire is good for up to 40 amps & 6 ga is for 50 amps . I generally used 6 ga wire from breaker to compressor magnetic safety switch & then 10 ga from there thru pressure switch to motor . I"ve never had any of my installations fail to this day (PTL). I used to be a registered installer for a couple compressor companies . So if the purchaser wanted their compressor warranted by the manufacturer , it had to be installed by me . But of course we did have a couple purchasers try collecting on a warranty issue but lost out when they couldn"t produce my installation paper work . That also saved my butt & reputation back then . I have drawings somewhere of how I used to wire up large compressors if interested & I can locate them . But it"s been 20+ yrs so I can"t guarantee my locating them . Let me know if this helped you any . You might check with your local industrial electrical supplier for help & maybe a drawing of wiring . Just be safe . Make all connections tight & secure . Remember you will have vibrations to contend with . God bless , Ken
 

[buickanddeere]

Neither.
Line power from a local shut off switch or wall receptacle to the line side of the magnetic starter. Load side of be starter to the motor.
Control power from the line side of the starter to the pressure switch and back to the coil, through the overload contacts.
Coil voltage must match line voltage . Don't be one of those d!cks that use the 240V supply to the coil and wire to ground to obtain power for a 120V coil.
Time to hire a pro before you burn the place down, cripple or kill somebody. You hire a pro to do your dental work .
 

[John T]

If youre constantly burning up pressure switches ITS HP AND/OR CURRENT RATING MY WELL BE LESS THEN THE 5 HP YOU REQUIRE???????? The pressure switch may well be a low current rated device (designed to operate a seperate independant magnetic starter coil) and NOT switch and operate the 5 HP motor itself!!

Of course it takes much more current to START that compressor then to run it, and when those contacts just close they must handle that current ya know.

HERES THE DEAL: You could use a magnetic starter which has sufficient HP and Current rating to start and run the compressor motor, AND JUST USE THE PRESSURE SWITCH TO CONTROL THE STARTERS CONTACTOR which is wayyyyyyyyy less current then the actual motor requires. The pressure switch would ONLY activate the starters low current coil, which when activated and conducting VIA THE PRESSURE SWITCHES CLOSURE then closes the higher HP and higher current rated magnetic motor starter contacts...

A tyical farmer Billy Bob installation could be a wall mounted 2 pole Safety Switch,,,,,,,,,,which feeds the magnetic starter,,,,,,,,,which is controlled on and off via the closed contacts (and low current) inside the pressure switch.

ORRRRRRRRRR when I designed such at our industrial facility I USED A COMBINATION STARTER INSTEAD. It consisted of a combination single enclosure safety switch PLUS a magnetic starter all in a single enclosure and was cheaper then buying BOTH seperate safety switch and seperate magnetic starter

NOTE a magnetic motor starter will have whats called "heaters" or "thermals" sized for the motor to protect against overload and overheating. Size them to match your motor HP, Voltage, Amperage. ALSO the starters coil voltage (which pressure switch activates by its open and closing) can be same as your operating voltage or less (240 or 120 or even lower like 24 etc)

Buttttttttttt if the pressure switch is big enough and the above is NOT the problem, look for excess current especially excess start up current, caused by too long of wire runs or excess voltage drop or too small of wire or a faulty compresor head pressure unloader etc

John T
 

[Ken Macfarlane]

Those little step down transformers for 240-120 are common and handy as a lot of stuff uses 120 V control voltage. Like he said, ground wires are not there for you to wire into a circuit.
 

[David G]

I have a 5HP single phase that only has a pressure switch.

Are you sure it is unloading?

what upstream breakers or fuses do you have?
 

[Stephen Newell]

OK here is the full story. When I bought the compressor 20 years ago it came with a magnetic starter. It was too complicated for me so I hired an electrician to wire the compressor. When the guy got there he said I didn't need a magnetic starter so I let him wire the compressor without it. He just put in a sub-panel with a breaker box with 8 gauge wire about 40' from the main and a 40 amp breaker. The compressor is just wired from the pressure switch to the breaker. The motor is 5hp and 22amps. Ever since then I've been burning pressure switches about one a year until I found a place I could get a 25amp pressure switch. They are lasting me about two years. Now its burned out again and in searching for a heavier duty pressure switch I came across a place that said if you need larger than 25amps you should have a magnetic starter. That got me thinking about the magnetic starter that I got with the compressor which I still have and thought I would install it on the compressor like perhaps it should have to begin with.
 

[George Marsh]

Stephen,
If you plan to use a magnetic switch, a contactor, the pressure switch will be used to power up the coil, not the motor. So you will need to find a contactor with a 220v coil. IF you use a 110v coil, then make sure you only run 110v to the pressure switch and wire up the contactor with 220v.

That said, when the pressure switch on my 5 hp compressor went out I couldn't find a switch rated at 5 hp. 3 hp was the largest. So I connected leg 1 of the power in to both sides of the pressure switch and both sides of the out side of the pressure switch which got connected to leg one of the motor. Leg 2 of the power got connected directly to the other leg of the motor. Basically, the pressure switch only turned on and off one side of the 220v. That was 6 years ago. Switch is working just fine.

If anyone has a source of a 5 hp pressure switch, please post. I got my 3 hp switch from grangers.
George
 

[buickanddeere]

Not code legal as it leaves the motor always energized.
Why not just do the job right when at it?
 

[John T]

If you already have a magnetic starter assuming its rated for your HP and voltage etc I would definitely use it and just let the pressure switch handle the low current to operate the magnetic switches coil as I described for you below PIECE OF CAKE and your pressure switches will last a longgggggggg timeeeeee that way. Insure the "heaters" or "thermals" in the starter are sized for your motor and find out the voltage rating of the contactors coil and wire it properly

Post back any questions

John T
 

[John T]

Yeppers that will "work" (at least motor will stop) but if I had ever tried that when I was an industrial electrical design engineer my boss would have fired me GRRRRRR. We ALWAYS disconnected BOTH HOT LEGS, the maintenance electricians (not to mention NEC) sorta frowned on a hot leg (120 to neutral and case frame) still being present when the motor was supposedly OFF. Of course, if done properly (and as we trained our electricians) the safety switch (in sight and within 50 feet and lockable OFF) was off and locked off BEFORE maintenance...

Take care George, it never hurts to be over safe

John T
 

[George Marsh]

Breaking one leg of 220v is done all the time. I have a 12 year old central air conditioner, Rudd, the contactor only broke one side and the other leg was wired direct. When I replaced the contactor 2 years ago, I replaced it with a 2 pole.

Look at any 220v window air conditioner. They only break one power leg. Current can't flow until the other leg is picked up.

Better tell the people who make air conditioners they are breaking the law.
 

[Dusty MI]

Manufactures are allowed to switch only leg within an assembly. BUT it is NOT allowed in the field. Both legs MUST be switched per the National Electrical Code/NEC.

Dusty
 

[Stephen Newell]

Thats the reason why I posted the thread. I thought it was a mistake to not use the magnetic starter the compressor came with. I could have wired the compressor the way it is myself. Now I'm back to square one and theres no money for an electrician at present and I'm having trouble understanding the magnetic starter. The thing is so old now there is no schematic on it anymore. The only name on it is Telemecanique D32 but there is another part of it below LR2 D2353 . So far all I know is L1 and L2 are incoming power and T1 and T2 go the the motor but I lost on the rest of it.
 

[Dusty MI]

Think of it as 2 separate things in the starter.
You have figured out the motor part of it.
Now the other part is the coil. First you need to find out how many volts the coil needs. That is printed on the coil, you may need to take it apart some to see that label.
Think of the coil as just another motor but a very small one. That coil closes and opens the contacts of the starter, just like the air pressure closes and opens the contacts in the pressure switch.
Then have the pressure switch turn on and off that small motor/coil.
The heaters are just switches that are normally closed and open if the motor gets over loaded, and they are in series with your pressure switch, and they are already wired within the motor starter.
Most likely the coil is 220/240 volt and one side already fed from L-1 or L-2, you need to figure which.
Then take a wire from the other L- terminal to the pressure switch and another wire from the other side of the pressure switch back to feed the other side of the coil, there is a terminal for both of these wires, smaller than the L and T terminals.
If we knew where you are at someone close could help.

Good Luck,
Dusty