Using Two Pole Contactors for Three Phase Supply

When motor in off run by contactor one live wire is maintained. If motor is double insulated is allright. Security is good if contactor and motor resides in same unit. CE logo is as NEMA standard. Not need certification, is only an manufacturer categorization. Underwriter Laboratories may certificate if contactors and motors resides in same unit box (no external wiring). Sorry by my english, I'm from Argentina.

But this connection leads to one line is always live even if motors and compressors are at off mode; this is what i'm worried about specially in case of any leakage current or even during regular mentainance for the A/C unit.

British Standard 7671 would require all three poles to be locked off prior to maintenance.

The forum doesn't know the original poster's location, so BS7671 might be inapplicable.

You.ve got a run capacitor and a start capacitor. What is your problem?

I didn't mention any capacitor fuanction; Three phase motors and compressors don't need any capacitors !!! what do you mean ???

hi, i am looking for the manufracturing company in Argentina, which are in Swichgear and transformer making. I believe you can help me.

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I've never seen this problem.

Is it possible the manufacturer sent the wrong print or is expects the same print to work with single or three phase? Have they found a new way to save money? Check the control enclosure to see if you were sent a single phase unit or someone made an error and sent the wrong print. See if three contacts are installed on the Motor Starter. You are correct, three contacts are required to disconnect power from a three phase motor and each motor phase should be protected by a fuse. After all, the motor coils are all tied together internally in the motor, yielding a path for a live phase to energize the other two motor phases.

Lock Tag and Try, don't just trust an open motor starter as a disconnect switch. Test phase to phase and phase to ground on both sides of the motor starter to find a welded contact.

I have worked with refrigeration equipment for 16 years now. Usually the economy units only use a double pole contactor instead of a three pole to break the power to a three phase compressor. This does not harm anything, because remember, even though the compressor might be on vibration mounts isolating it from the grounded enclosure, the compressor ground bonding wire is always connected providing a low impedance path to earth ground. This being said, any leakage currents from phase to ground (should any occur) will take the path of least resistance through the bonding conductor to ground. Should the leakage currents be great enough it will eventually clear an over current safety device.

Typically units are never "leaky". You either fault to ground or not. 480 VAC is not forgiving and will break over and leave carbon trails. 230/240 VAC is the same way. You must remember that the dielectric strength of the oil and refrigerant rich mixture inside the compressor is very high under normal conditions and does not lend itself readily to leakage currents.

In single phase domestic units, almost always only one pole of the 230 VAC is only broken instead of both. This is done not only for economics, but by leaving one pole unswitched, the unswitched leg paired with the neutral can now provide 115 VAC to operate the oil heater in the compressor in the off cylce to stop liquid refrigerant migration to the compressor.

For servicing the refrigeration unit, there is always a service disconnect switch (usually fused) within five feet of the unit that breaks all three poles. This is NEC code for safety. When we service a unit, we pull all three fuses for additional safety while working on the machine.

post make and model # so i can find on web.

This set up sounds all wrong. You need to make/break all points. Is there an overload relay? How about an Motor Circuit Protector. If you only break two points, a short circuit and overload condition still exist in one phase. The machine will now b unbalanced. Unless this is a new device and someone has thought of a new way, then I would love to hear about it.

The setup you are refering to is common practice for all commercial refrigeration and HVAC. When controlling a three phase motor all you need to disconnect to stop the motor is two phases. Likewise with a single phase motor just disconnect one phase. As far as the safety aspect, its not important because if you working around the high voltage section your main disconnect would be shut off and locked out to protect you. You do know about "Lock out / Tag out" I hope. If not I would suggest checking it out as soon as possible.

When manufacturing the equipment the 2 pole contactors cost less than 3 pole, and single pole cost less than 2 pole. So the manufacturers save alot of money buy designing the equipment this way.

Had to figure this out, so I talked to an old timer in the HVAC and he stated he saw it all the time. I stand corrected. It still doesn't sound safe to me.

I can only see this type of application being installed on refrigeration compressor applications to keep one winding always hot so it may act as a compessor crankcase heater.

You are correct, it appears that something is wrong, and you should be talking directly with the equipment manufacturer/supplier to be sure there is not a simple mis-understanding. In the mean time I would like to make the following 2 points:

1. UL is not a safety certification, it is a standard of conformity. UL will certify a ladder if asked; yet if used wrong the ladder can still kill. My opinion CE is worth even less if viewed as a safety standard--Follow NEC and NFPA directions-Where these codes disagree look to local inspectors. (Assuming you are working in USA)

2. A motor contactor (regardless of #poles) is never to be used as a disconnect device for servicing. Hopefully your units have 3 phase input disconnects or circuit breakers.

'There are many old electricians, and many bold electricians; however there are not many old, bold electricians'

Guest #8 is exactly right. I also work in this field. It is not uncommon, and am suprised we don't see this more often. There would be very little negative effect of this wiring scheme, but the mfg. cost can be decreased greatly considering the volume of units produced.

If the manufacturer did this there will power on both the terminal and load side of the contactor due to the power returning from the motor. The motor will always be powered and could provide safety risk on service personel. The input to the system would have to turned off and trouble shooting time may be extended due to the topology of the system.

Is it possible the motor starter has three poles but only two Over Load heater elements? The unit would look like a 2 pole relay, but not really. The contacts are hidden by an ark suppression cover so cannot be seen unless the unit is taken apart. The manufacturer could save some money yet the motor would be completely disconnected from power when off, for safety. I wish I had one to test…

For guest #11. Yes you would need to disconnect the main power for safety purposes. If you need to take current readings or check to make sure you have full voltage on the load side of the contactor, you need to be careful. When I'm working on any unit I treat the unit as if it were powered up whether it is locked out or not. When tracking down problems in a control circuit, sometimes it has to be powered up to run the problems down. Control circuits in my plant can be 24vdc, 24vac, 110vac and 240vac. The higher voltage control circuits can kill you just as fast as the power circuits.

We have three weapons that protect us as we work on these systems: Intelligence, training, and common sense. If you are missing any of these three weapons then this type of circuit and any other type of electrical circuit is dangerous and you should leave it alone until you can obtain the training or whatever is missing.

I've been working on systems like this for over 25 years and I'm still alive and unharmed. The equipment if installed as per NEC is properly fused, so there won't be any problems.

Some manufacturers leave one phase to power all the time and another phase will be ran through a coil or resister to provide enough current to create heat, but not enough to run the motor so they don't have to use a crankcase heater on their compressors.

All this goes back to : Be careful and always treat your electrical circuits as if they are hot. If you try to save 60 seconds by not turning of the main power switch it won't be profitable if you are dead.

There is another inherent danger involved in this, aside from the personnel risk. In a 3 pole contactor situation, welding one contact will not necessarily result in the destruction of the motor because there would not be a return path for power flow through the motor winding. Using a 2 pole contactor means that the 3rd leg must be permanently connected to the motor. If either one of the 2 contactor poles welds, you WILL lose the motor.

I've had this happen many times and never lost a motor yet. When the motor gets single phased the line protection (fuse / Breaker) breaks the circuit.

Can you recomend a two pole switch, for remote control, to on/off a 3 phase unit, interupting the 24 volts side of the relay?