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dave_ritt
April 9th, 2004, 04:52 PM
Can someone explain the correct wiring for a pressure switch
to a magnetic contact and relay. The power is 220v three phase.

Thanks

Dave

mdshunk
April 9th, 2004, 05:28 PM
Yessir....

You have the three phase conductors coming in the "top" terminals of the contactor. The 3 motor wires connect to the "bottom" terminals of the contactor.

The coil of the contator is where the slightly complicated connections are. Normally, the coil of a 220 volt air compressor is also a 220 volt coil. Double check this (written on the coil somewhere). Run a jumper wire from of the hot wires from the top terminal of the contactor (any one) goes to ONE terminal of the magnetic coil. Run another jumper from another hot wire from the top of the contactor (not the same one you used before) to one terminal of the pressure switch. A wire from the other terminal of the pressure switch goes to the other terminal of the magnetic coil. Basically, you're using the pressure switch to "make and break" one of the wires leading to the magnetic coil.

Best of luck... repost if you get stuck or find your magnetic coil voltage is something other than 208 or 220 or 240 (any of these will be okay). Remember, if this unit is 3 phase, you need 3 phase power to hook it up. You can't run this in the garage of your house, unless you have 3 phase at your house (not normal at all).

dave_ritt
April 9th, 2004, 08:34 PM
Thanks for the reply. I understand what to do, but I'm not sure
what terminals to use. I also have to run two wires to the
side of the relay that is attached to the contactor?? This
is what the terminals on the contactor are.


A1 , A2


3 , 1-L1 , L2 , L3


2, T1 , T2, T3

The coil is which terminals? Which terminals go to the side
of the relay? Let me know if you need more info.

Thanks

Dave

Homer
April 9th, 2004, 08:51 PM
The coil terminals are A1 & A2.

The 3-phase supply connects to the contactor at terminals L1, L2, & L3 that should be in a row across the top.

The connection to the motor overload relay is made at contactor terminals T1, T2, & T3 that should be in a row across the bottom.

Just follow MD's instructions for wiring the pressure switch and verify that the contactor's coil voltage is between 208V and 240V by reading the printing on it (don't connect it if it's only 120V).

Use any two of L1, L2, or L3 as per MD's instructions to wire the switch to the coil.

Homer

Homer
April 10th, 2004, 07:01 AM
Looking back at your original post I will add some comments just to be complete.

Do you have an overload relay (heater) for this motor or is the motor itself a thermally protected variety?

Typically the overload snaps on the bottom of the contactor and attaches to the "T" terminals. The motor is then wired to the overload relay terminals.

Also, have you sized your overcurrent device (fuse/breaker) for this motor?

Homer

dave_ritt
April 10th, 2004, 08:26 AM
You are correct. The overload relay attaches directly to the bottom
of the contactor.I have the motor wired from the "T" terminals on the relay. There are two terminals on the side of the relay also. Do any wires go to these?
The coil is 220 and wired like MD described.

Thanks

Dave

Homer
April 10th, 2004, 08:40 AM
The two terminals you see on the overload relay are an auxilliary contact. A normally closed contact (N.C.) will be provided so you can use the overload status for anything you wish (usually 120V control circuit). You don't need it for a simple application.

Also, auxilliary contacts on the contactor (N.O. and/or N.C.) can be used in a start/stop control circuit. You don't need these either since you're using a pressure switch. The N.O. is usually used to "seal in" the start button contact until the stop button contact breaks the control circuit.

Homer

dave_ritt
April 10th, 2004, 09:16 AM
Thanks for the help.

Dave

dave_ritt
April 10th, 2004, 10:08 AM
I thought of another question. Do you always run the one
hot wire directly to the coil? Since the pressure switch
has two poles, can you run both of the hot wires
through the pressure switch?

Thanks

Dave

Homer
April 10th, 2004, 10:23 AM
You are correct. If your pressure switch has two poles, you can wire it as you suggest.

When using a contactor for a motor you only need a single pole switch in the control circuit. This is the wiring configuration that MD gave you.

If you were using a small 240V single phase motor without a contactor, and were using the pressure switch directly, then the two poles would be necessary.

Homer

mdshunk
April 10th, 2004, 02:53 PM
This is a funny pic that goes well with this thread:

(alternate simple contactor control method :D )

Wgoodrich
April 11th, 2004, 06:29 AM
That picture is a backwoods engineering WOW, now that is a holding circuit that will never wear out. May burn down the house but won't wear out the mechinism. Ha Ha.

Seems to me I got an email with this picture from Mike Holt. Is that where you got it?

Wg

TheAccidentalWelder
April 14th, 2004, 01:52 PM
The control circuit also needs to pass through the Overload Relay's NC contacts, otherwise the motor has no overload protection.

Homer
April 14th, 2004, 02:42 PM
The control circuit also needs to pass through the Overload Relay's NC contacts, otherwise the motor has no overload protection.
That statement that this leaves the motor with no overload protection is incorrect. AccidentalWelder, you need to consider the operation of an overload relay a little more closely before making such a statement.

While it is typical to wire the NC overload contact in series with the control circuit this is to make sure that the contactor "drops out" and is not still energized after an overload condition. Yes, I should have instructed the OP to wire the NC contact in series with the control circuit. That's what I get for not sketching it out on paper before typing.

Thanks for pointing out my omission. I'm actually surprised that nobody else commented on this.

The overload relay not only actuates the NC auxilliary contact during an overload condition but it breaks the 3 high current contacts between the the contactor and the motor. After the overload heaters come up to temperature the relay will remain in this state until a manual reset is done. If the contactor is still "sealed in" when the overload is manually reset, then the motor will start immediately without the need for a start request. While that's not the best of situations, it hardly leaves the motor with no overload protection as you stated.

Homer

mdshunk
April 14th, 2004, 06:37 PM
I generally wire in the overload's aux contacts when there would be a special hazard created when the overload automatically reset and the motor restarted. I don't think an air compressor restarting without warning (which it does anyhow) creates any special hazard. I continue to believe that leaving the overload aux contacts out of the control circuit is completely proper for this application. A conveyor belt.... no way! An air compressor... okay!

TheAccidentalWelder
April 16th, 2004, 01:22 PM
The overload relay not only actuates the NC auxilliary contact during an overload condition but it breaks the 3 high current contacts between the the contactor and the motor.

Could you please provide me with a link or manufacturer/model info on these magnetic motor starters that break the 3 high current contacts between the contactor and the motor. Nothing short of the heaters melting into oblivion will deenergize the motor in any I have ever dealt with, without the control circuit being dropped. At that point the motor is likely cooked. The high current contacts are in the contactor not the overload section.

Homer
April 16th, 2004, 04:21 PM
Could you please provide me with a link or manufacturer/model info on these magnetic motor starters that break the 3 high current contacts between the contactor and the motor.

Certainly, AccidentalWelder,

Here is how an overload functions.

Overload Info (http://electronic-components.globalspec.com/LearnMore/Electrical_Electronic_Components/Relays_Timers/Thermal_Overload_Relays)

Quoting from the document, "If heat begins to rise, the strip bends and the spring pulls the contacts apart, breaking the circuit."

Take a look at the typical schematics for both the control circuit and the motor circuit attached below. You will see the three phase contactor's 3 contacts labelled "M". The overload relay's three high current heaters are labelled OL and are drawn with a symbol that even shows that they separate and break the circuit when they come up to temperature.

The NC contact from an overload that's wired in series with the control circuit is called an auxilliary contact. An auxillary contact has the same meaning for an overload relay as for a contactor. It's not the main contact, it just provides an indication of the main contacts' status.

You can even prove this to yourself. The next time you have a motor stop on overload, before you reset the overload, jumper the NC overload contact and try to start the motor. You will find that the contactor will pull in but the motor won't start. Then, immediately upon pressing the overload reset the motor will start.

Homer

mdshunk
April 17th, 2004, 08:06 PM
Could you please provide me with a link or manufacturer/model info on these magnetic motor starters that break the 3 high current contacts between the contactor and the motor. Nothing short of the heaters melting into oblivion will deenergize the motor in any I have ever dealt with, without the control circuit being dropped. At that point the motor is likely cooked. The high current contacts are in the contactor not the overload section.


You may be somewhat confused, since there are two types of contacts involved in a thermal overload. The first type are the 3, normally closed overload relay contacts. These are normally buried inside the overload unit, and are not visible by other than a hammer and chisel. The overload heaters that you suspected would melt are on the load side of the overload's overload relay contacts so they have no current flow after they trip the overload. The second type are the normally closed auxillary or "holding" contacts. They are generally used to drop out the contactor coil, if you wish to do so.

There is also sometimes a third set of contacts (generally on IEC style overloads)... normally open auxillary contacts or "alarm" contacts, used generally for setting an error light or setting a PLC input.

TheAccidentalWelder
April 19th, 2004, 01:25 PM
A typical magnetic motor starter does not have any contacts in the overload relay section in series with the phase conductors. T1,2,&3 from the contactor section have a continuous current carrying path to T1,2,&3 on the overload relay section provided that the heaters are installed.

Wgoodrich
April 19th, 2004, 06:21 PM
They are called heaters because when overloaded they heat. Then they trip a circuit dropping out the current going to the motor. Only way to get it restarted is when the heater cools off and resets if auto reset or push reset button if manual reset. The motor will not start or run again until the overloads are reset. The side contact on some motor control centers is required for the overloads to work some models of motor control centers do not rely on the side contact. Either way the coil drops out with current path broken by hte heaters heating the posts tripping an open circuit in the starter coil relay. Some need the side bar some do not. Then you may build a motor controller from components again using heaters. This way the run section of the control circuit is ran through each heater post so that when one heater post heats up it opens then shuts off current flow to the holding circuit when using component parts buildng your own motor controllers.

Depends on the type motor control center or component design you have but all do not melt and require replaced cutting power only to that one load line to motor. The heaters control shut down by breaking the holding circuit of the control circuits with or without the side bar contacts depending on make and design of your motor control center or component designs.

A heater does not just cut current to the motor leads it cuts current to the holding circuit holding in the motor starter magnetic coil stopping the motor by the heaters heating the posts that then break the control circuit feeding power to that magnetic coil.

Hope this helps

Wg

Pat the RiverRat
March 23rd, 2005, 04:48 PM
I'm was wondering if someone could tackle the issue I'm having wiring the starter on my compressor. Its a AB 509-BOD, with overloads and 1 set of aux contacts and a 120V coil. I'm using a start/stop setup as well as a pressure switch. Everything works fine except after the compressor get below the the startup pressure, the motor doesnt restart. You have to press the start button and it shuts off like its supposed to. I know the pressure switch is good because I can watch the contacts engage at the lower pressure setting. Any suggestions?

Thanks

Roger
March 24th, 2005, 09:26 PM
Pat are you saying you have to press the start button to get it to restart after it reachs low pressure limit? You say shut down? What kind of pressure switch (diaphram etc)are you using and how have you wired it to the starter and control circuit. Is this your starter.....
http://epub1.rockwellautomation.com/images/gl2/1268226.jpg

Pat the RiverRat
March 25th, 2005, 07:17 AM
Sorry, I should have said after repressing the start button, the compressor cylces like normal and the motor shuts off once the pressure switch contacts separate. I'm not sure what style of pressure switch I have, but its new out of the box. That is exactly what my starter looks like. I'll give the wire routing detail a shot, so bare with me.
The start/stop button I have are industrial models that have 4 terminals on the backside and are lighted. I have them wired just like the diagram posted earlier, using only 2 terminals on the same side..
Okay. From L1 there is a wire going to one stop button terminal. From the other stop terminal, I have 1 wire going to the #2 position on the aux. contact and 1 wire going to a terminal on the start button. From the start button terminal, I have 1 wire going to the #3 aux. contact and one wire going to the pressure switch. From the other side of the pressure switch I have 1 wire running to the coil. From there 1 wire goes back to the start switches 120v contacts on the bottom for the light. 2 wires go from the N.C. to each of the other terminal contacts for the button lights.
Actions are as follows for an empty tank:
Press start button. Starter activates compressor motor and fills tank like normal. When the pressure switch kicks out, the motor stops. Once the pressure is below 90PSI the pressure switch contacts close, but the starter doesn't restart the motor. Repress the start button to restart the compressor.
If you press the stop button at any give point, it will shut everything off like its supposed to.

I'm sure I confused you somewhere. I'll see is there is some sort if ID on the start/stop buttons so you have a better idea of how they are set up.

Pat the RiverRat
March 25th, 2005, 10:17 AM
The stop button is AB 800T-PT16 and start button is 800T-PA16.

Homer
March 25th, 2005, 01:49 PM
The typical control circuit for a compressor with a pressure switch is to use a switch that opens at one pressure and closes at another pressure to allow the tank to pressurize/depressurize before cycling the motor. With this setup, there is no latching of the motor. The pressure switch basically serves in place of a snap switch that is ON at low pressure and OFF at high pressure. See the sketch below.

If you are trying to combine a latched start/stop pushbutton control circuit with a pressure switch (PS) then you need two pressure switches. One pressure switch would turn ON the motor just like the start push button while the other would turn OFF the motor just like the stop PB. The motor would stay latched on after starting by a momentary contact (Start PB or PS Low) until unlatched by a different momentary contact (Stop PB or PS High). See the sketch below.

Based on your description, it sounds like you have wired up the second sketch without Pressure Switch #1. As you have found, Pressure Switch #2 can only STOP the motor in this configuration.

Homer

Pat the RiverRat
March 25th, 2005, 06:36 PM
So my only option is another pressure switch? That doesnt sound too bad. Two questions though:
1. What if I took the stop button out of the equation? The pressure switch has a separate lever for "auto" and "off". Not to mention once the start button has been pushed, the PS takes over shutting off the motor.The only reason I added the stop button was I bought them both w/ enclosure from ebay. Not to mention it gave me the option of putting the start/stop on the other side of the shop by the door. That way I could just hit the stop on the way out the door if the motor was running.
2. The pressure switch has 2 sets of contacts (its a 230V switch and I only used one set) could I incorporate the other set to make it work?
Thanks so much for your help guys!

Homer
March 25th, 2005, 07:21 PM
The pressure switch that you have now will be turning ON at a low pressure and turning OFF at a high pressure (hysteresis) just like the first sketch. That's the way your switch was meant to be used.

To use the setup in the second sketch you would need 2 pressure switches (one switch for low and the other for high) that only switched at a single pressure (no hysteresis). After seeing what you really want to accomplish I think that you should approach it differently.

Why don't you remove the stop push button. You can replace the start push button with a simple switch or even an Allen-Bradley 800T illuminated toggle switch. That way the compressor will cycle automatically and you can always turn the switch OFF. You would use the first sketch and place the switch in series.

Another alternative would be to wire the start and stop push buttons to a relay and wire a N/O contact from that relay in series with the pressure switch. This is by far the best way to achieve exactly what you wish. See the sketch below. The M in all the sketches is the contactor coil.


Homer

Pat the RiverRat
March 26th, 2005, 12:05 PM
Thanks Homer! I think I'll go with your first option. Sounds like it would be the easiest.
Thanks again!

Homer
March 26th, 2005, 01:19 PM
If you're going to remove the stop PB and replace the start PB with a toggle or snap switch be sure to also remove the latching aux contact from the circuit. If you leave your aux in place the toggle switch won't be able to turn off the motor until high pressure is reached.

The pressure switch will control the cycle and the toggle switch will be an ON/OFF.

Homer