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Larry T · Jan 31, 2009, 05:49 AM

I recently bought an air compressor with three phase power. I do not have three phase in this older property so I knew I would need to change the motor. I found a single phase 7.5 hp. Motor with the same shaft, etc. dimensions as the old three phase that I traded to a guy with the opposite problem.
He told me that it didn't have a mag starter so I ordered one. Now that I have the motor picked up I found that it is a capacitor start. So I guess no need for the starter.

Now the big question. So now I have a rebuilt motor on my compressor with none of the knock outs removed. I have a pressure switch with two wires in a casing hanging. I have a diagram that shows proper connection for direction so I am good there, and also on the ground placement.

How does this pressure switch wire into the motor ? I assume it runs inline with one of the power wires to cut it out at set pressure. Also, since a 3 phase motor came out of this with a magnetic start switch which has been removed and sent with the old motor I now have no switch for this new motor.

I have a service disconnect ( non-fused ) that will be within 5 feet of the compressor. Would this be my switch ? Or is there something I am really missing here. Do I need to get some type of actual switch for this motor which is a continuous duty, or is it wired direct with the service disconnect.

Basically I was good using the old configuration with the mag starter, but without the need for the starter I am now confused as to how the pressure switch wires in, and also if I need a switch.

Can anyone offer some suggestions ? Thanks !

Stratmando · Jan 31, 2009, 06:42 AM

Since this is likely 240 volts, have the pressure switch break both poles if available, if not, break 1 side of power with the pressure switch.
Disconnect should break both hots.

Larry T · Jan 31, 2009, 06:57 AM

Stratmando, it has a pressure switch that has 2 line terminals at the bottom, and 2 motor terminals at the top. I assume you mean to run a line in each and then out to the motor. Is this correct ?
Yes, the disconnect I have is a pull type that disconnects both hots. IDK but the motor says 230 V and says it requires a ground. Reversing schematic is on the box cover, so no prob. There. Wires marked well.
Do I need another switch though ? Or is the service disconnect and pressure switch sufficient ?

Thanks !

Stratmando · Jan 31, 2009, 07:54 AM

I think I would add an on/off switch, Personally, I don't like using a Disconnect or a breaker for the switch. Will work fine either way.

Larry T · Jan 31, 2009, 04:27 PM

Well Stratmando, I got it going. I picked up new wiring and a new disconnect box. First time it ran I blew a fuse, but now it starts up great. I think it needed the capacitors charged or something.
I am also going to change out the pressure cut out switch and relief valve. The switch failed the first time it ran and kept climbing so I opened the air powered it down and gave it a little light oil and worked it manually. Works now, but I am still going to replace it. Better safe than sorry.
Thank you for your help !

Larry

KISS · Jan 31, 2009, 04:31 PM

Are you sure the pressure switch is rated for 7.5 HP? That may be the reason why it fused closed.

Larry T · Feb 1, 2009, 02:51 PM

Well, things were working OK until today. When I tried to power it up today it blew a fuse again. We have an old farm house with an older service panel so yes fuses, not a breaker panel.
I replaced the fuses again. I tried to run it and it began to run somewhat slowly and then started to smoke from the pressure switch and the capacitor box as well as the side plate where the connections are made.
I immediately cut the power and let it cool down. The two hot leads from the pressure switch to the motor have melted. The ground is fine. The two hot leads were a little smaller, I believe they were 12 and the rest of the wiring was 10. I reused these two from the old three phase that was on it.
I also see that the wires from the main to the sub panel are 12 gauge as well.

So what do you think ? Is the motor cooked or is it possible that it was not hurt. The wires going to the motor all look OK, but the leads coming in from the switch are fried.
I looked at the capacitor wiring and it looks Ok, but I do not know where the smoke came from unless it is smoke that came through from the wires melting.
At this point I have removed the wiring from the pressure switch, and also disconnected the power from the switch box.

Do I need to run bigger wire all the way from the main panel ? AM I drawing too many amps or do you think the smaller power leads from the switch to the motor could be the problem. The motor is a 7.5 hp. Single phase, and the switch was from a 10 hp. Three phase motor with two lines in and two out to the motor. I ran both lines through the switch and then out again to the motor. The ground connected inside the switch box and then again in the motor cover. How can I tell if the motor is still good ? Is there a way to test it with a meter ?

Thanks

Tev · Feb 1, 2009, 10:36 PM

Is there a nameplate on this motor? What numbers does it give for amperage?

I'm guessing that you are trying to run the motor on a circuit that is too small. 7.5HP is fairly big for a 240v single phase motor. I'd be surprised if the 20 amp circuit you are using is big enough.

KISS · Feb 2, 2009, 01:05 AM

Yo. The pressure switch is toast.

~17A for 3 phase and 29 amps for single phase. (THESE #'s ARE NOT FOR SIZING THE FUSE OR WIRE SIZE)

P is greater than 746*240/1000; 100% efficient which it isn't.

I used the default values for efficiency and PF here: Veris Industries - Motor Calculators

Converting Ampere between Single Phase and 3 Phase

This table suggests #6 up to 100 feet

Circuit Wiring - Single Phase (230V) Electrical Motors

The fuse has to be a time delay type fuse.

For proper sizing of the circuit we need distance and the nameplate data from the motor and how often it's expected to run. We can guess that it's not continuous.

The pressure switch is toast and needs a contactor.

For the same motor, 3 phase current per phase is shared somewhat by a factor of sqrt(3)

KISS · Feb 2, 2009, 01:53 AM

Here is an example of a single phase starter 7.6 hP:

230VAC, Single - Phase Motor, 7-1/2 Horsepower - FarmTek

The starter doesn't use overloads because the current (Full load Amps) is electrically adjustable. You need the starter because the contacts of the pressure switch cannot handle the motor. Note FLA is adjustable from 37 to 50 amps.

This starter does not contain an enclosure, disconnect or fuses. The coil can be 240 V. If the compressor is going to be used intermittantly you can place the pressure switch in series with an ON/OFF rotary switch.

It's best to find a combination fused/starter/disconnect for a motor of this size.

Larry T · Feb 7, 2009, 03:49 PM

Well, I am back again. The weather got nice and I have the new panel in and working well. I have the new 6/2 running to the disconnect, the starter and then the motor.
Unfortunately when I push the button on the starter nothing happens. If I manually push in the contactors the compressor is powered, starts up and runs great. I cannot cet the coil to draw in at all. Can someone please help as I am having issues with where the wires from the pressure switch should go.
The pressure switch has 4 connector screws with 4 points contacts. The starter has the 2 lines coming in at the top and the two out at the bottom. There are two red wires at the bottom of the starter. One goes to to the top at each line in. The one on the right is way up on the right by the line. There is another spade connector beside it and another behind the two of them. The other red wire goes to the back side of where the line in is. The two spade connectors in the front are empty.
Like I said, I can power the compressor up great by just pushing in on the contactors at the top where the lines connect, but no voltage or no " draw in " at the coil.

Any thoughts??

Thanks !

KISS · Feb 7, 2009, 05:10 PM

Time to put your thinking cap on or take some pictures.

I'm going to suggest the latter. Use "Go Advanced/Manage attachments". To re-size pics, use the free software IrfanView - Official Homepage - one of the most popular viewers worldwide.

Lots of goofy things can happen. You could have a DPST switch on your pressure switch (4 terminals) or an SPDT or a DPDT. Who knows.

I did suggest a small fuse on these lines, so you don't let out the magic smoke so easy.

Starters can be intimidating. First, the overloads have to be installed.

Second, they are normally used as "start/stop" stations where "start" pulls in the contacts and "stop" or a power failure drops them out.

You cannot use this method.

That's were I said you need: line1----fuse---switch(rotary)---starter----line2

The (starter) portion of the wiring has a small switch in it that's operated by the overloads. This drops out the contacts.

The starter may have an external reset button on it as well.

I'll look at the starter data that you provided when I get a chance. Meanwhile take a few pics.

KISS · Feb 7, 2009, 06:01 PM

The motor data, name plate data, starter data all are missing from this thread. I guess time to go to sleep until the starter model re-appears.

Missouri Bound · Feb 7, 2009, 06:21 PM

While covering all bases, make sure the unloader is working properly. This would cause severe overloading trying to start the compressor motor with a load. I suspect that this motor needs at least a 30 amp circuit, with 10 ga. Wire. Your disconnect will work, but if you can get a pressure switch with an off position, that will take care of things.
Read the nameplate.