I have a job to replace a motor starter that was bad, an old Allis Chalmers. I go to install it and there are two wires that are on a set of N/C contacts. When I go to put the new Sq. D starter up I'm thinking where should these wires go. The only N/C contacts on the new Sq.D are the overload contacts for the coil. So happens that there is another Sq.D starter that works in conjunction with this one. They operate 2 air compressors outside. On the existing Sq.D starter, (after a few mistakes on my own), I notice that they have taken the wires to the coil off the overload contacts, and used these contacts for the other two wires, which are coming off of the pressure switches. Isn't this defeating the purpose of the overload for the coil? I don't do a lot of starter wiring, but can't you buy an extra set of N/C contacts that can be added to this starter? It is a Size 1, 3 phase, with a 460 volt coil. I didn't see a place to install another set of contacts, but it seems that I have run into it in the pass somewhere. I'm thinking this is rather dangerous for the compressor. In the event that one of the heaters blow, won't the compressor keep on running on single phase and burn the compressor up possibly without the overload hooked up? I know I'm missing something here. Like I said, I don't do a lot of starter wiring. Thanks for the input. Steve
Steve, I don't think that you have correctly traced the circuit. If you connect the pressure switches to the overload relay contact, they would have no function. You have just connected a number of switches together without a load or source. Also the OL contacts only open when the motor has been overloaded not when the motor starts and stops. Are you sure these switches are not connected between the power source and the hot side of the coil(term 3)? On most starters you can add at least 4 auxiliary contacts in addition to the motor power contact and the NO seal in contact (terms 2 and 3). Don
It appears that the pair of wires that were connected to the N/C aux. contacts need to be reattached to a set of N/C aux. contacts on your new starter. A kit should be available from your vendor to add the necessary contacts.
Don, maybe I explained it wrong. The new starter I installed did have the B phase wire breaking through the N/O overload contact, and before I knew exactly where two wires that were breaking through the pressure switches, I had connected them to the overload contacts also. I did this because the other existing starter had it's two wires breaking through these contacts. What I didn't notice on the existing starter was that the B phase wire that was feeding 1 side of the coil had been removed from the overload contacts and had been wired direct to the coil. After blowing 2 fuses on the starter I was working on, I traced the two wires that were breaking through the pressure switches and found out they were attached to the neutral of a 110volt circuit that was feeding some more small relays. So since I had these wires under the same terminals on the overload contacts as the B phase going to the coil, it was shorting out immediately. I think I need to get another set of the auxillary contacts and install them just for the pressure switch wires, so I can hook the overload back up the way it was intended to be. To do it right I probably need to do the existing starter the same way since they bypassed the overload also. Hope this makes sense. At this point I'm not sure what the reasoning is behind putting the pressure switch wires under the N/0 contacts anyway. Why not just wirenut them together. The way it is now, if I'm understanding it right, if the overload trips out on one of the starters, the other compressor want run, because the circuit for the pressure switch has been broken. Actually I know this because when trying to find the short I had the wires off the terminals and separated, and the other compressor wouldn't come on until I connected the two wires. Hope this is not too confusing. Steve
Ok, let me try to explain my question again to simplify it. By disconnecting the coil wires from the overload contacts on the starter and hooking them direct to the terminals, what effect does this have on the air compressors they operate? Remember that the pressure switches have been broken through the overload contacts. These wires have no voltage or amperage; they are just breaking the Neutral of the 110 volt circuit through them. ( This is the way one of the existing starters was wired when I got there )
Question #2 I went to the supply house and was going to get a N/C aux. contact to put on the starter. In looking at another overload relay contact on a starter that was on his shelf, I notice that the contacts are open, with no power on them. (I'm not too much on motor wiring, as you can probably tell.) Does this mean I need a N/0 contact instead of a N/C contact? What I want to accomplish is that these contacts are closed all the time. Remember, I'm just trying to keep these compressors running like they originally was, I'm not doing the designing of these circuits. But if I need to add contacts to make it safer, I do want to let the customer know, and let him make the decision.
Question # 3. I guess my lack of knowledge on this situation is showing up, but in the way the starter is designed to do out of the box, if the overload contact trips, it is because it is sensing overheating on the phase wires going to the compressor, right? Not the wires that are going through the overload contact, which is the control circuit going to the coil? Then that means that when the overload contacts do trip, that they break the circuit ( whatever is hooked up to the overload contacts ) ??? In my case it is the pressure switch wires. So if in case of a overload, the pressure switch is disconnected, thus preventing the compressors to come on, until the overload is corrected. One more time, Am I thinking in the right channel??? Thanks again. If I am wrong, whoever wired these compressors to start with was wrong also. Thanks... Steve
In general the coil side of the overload relay contact should never leave the starter. If you take it out of the starter, then a ground fault on the control circuit could prevent the overload relay from doing its job . The overload relay's fuction is to open one side of the power circuit to the contactor coil when the "heaters" sense excessive current. This contact is closed under normal operating conditions and opens under overload conditions. The pressure switch contacts should not connect to a contact...they should be connected to the power side of the control circuit. We really need a complete control sequence of operation to have any idea how this should be wired. Do both compressors normally run at the same time? Are the pressure switches designed to be a start/stop circuit with the compressors starting when the pressure drops below a preset point, and stoping when the pressure rises to a higher preset point?
I think you should try to simplify the wiring in your own mind first.
You have your line voltage to the motor connected to L1, L2, L3. Your load out to the motor on T1, T2, T3. The load current goes through the 3 heaters and out on the T1, T2, T3 terminals.
Now your control voltage usually has one side of the control power (we'll call L2) connected to one terminal of the overload block. The other end of the overload block connects to one side of the coil. That way if the overloads are ok your coil sees L2 and if the overloads trip it will kill L2 to the coil.
Now you need to take your L1 control side through whatever controls the motor to start it. In this case it might be a pressure switch or an alternating relay. L1 will connect to the other side of the coil from L2 when the pressure switch or relay makes.
The N/O contacts that you describe are usually used when a starter needs a latch-in circuit. I don't believe that is the case here. They also can be used as auxilliary contacts to power other things like indicator lights or open dampers, etc.
Edit: Sorry Don, I didn't mean to jump in on your reply. I was typing (slowly) while you answered.
Don, the compressors are wired so that one compressor comes on first when the preset pressure switch calls for it. There is a timer that allows the other compressor to come on after the 1st compressor has started as to decrease the start up load of both compressors on the breaker feeding them. Why the original electrician wired the pressure switch wires through the overload relay contacts, I don't know. I don't see why they can't be just wire nutted together, since there is a timer that regulates when the 2nd compressor comes on. If that could be done, then the wires feeding one side of the coil could be put back on the overload relay like it is intended to be. Remember they have got the control wire from the pressure switches tied in someway to the control circuit. They seem to be breaking the neutral that goes to one of the relays through the pressure switches then through the overload contacts on the starter. I know this is confusing maybe without seeing the actual wiring that is there. There is no schematic of his wiring, except just a simple diagram. I'm mainly concerned of whether I can need to wirenut the wires that he has on the overload contact now, and put the wires that feed one side of the coil back where they were. No big deal here I don't think. I've got his compressors going, I just want to see if there is something that could possibly be wrong that could damage his compressors or any other danger. Thanks again.. Steve
Most compressor controls that I deal with use a timer for the unloader circuit. The motor runs for the time set on the TDR, without compressing any air. You can definately hear the difference when it times out. A pressure switch with a deadband or differential setting wouldn't require the use of an aux N.O. contact. Using 2, open on pressure rise, switches, would require use of one aux. N.O. contact to seal in across the lower setting switch until the higher pressure setting switch opens. Alternators usually use one aux N.O. contact from each contactor, in parallel, to alternate when the compressor stops. The minimal components that you describe don't seem to be enough for a dual compressor ladder. Joe
Why the original electrician wired the pressure switch wires through the overload relay contacts, I don't know. I don't see why they can't be just wire nutted together, since there is a timer that regulates when the 2nd compressor comes on.
If you have the pressure switch wires scotched together, wouldn't this bypass the OL circuitry for the second compressor? Seems to me using a wirenut in that starter cabinet would be a bad idea.
I may not have read this enough times yet to see exactly what you are describing though, just trying to get a better picture myself..