220 VAC motor-Only One Lead Direct To Motor

[stripedbass]

This is a two-partr':

1-We have several well pumps that are 220 VAC, single phase. The person who wired them (several years ago) ran one lead from the breaker direct to the motor. The other lead is interrupted by the starter contactor. Isn't this dangerous? I installed new contactors and broke both leads.

2-How is a single phase 220 VAC motor constructed. What is the advantage over 110 VAC other than lower amp rating?

 

[jbartos]

Comment: If a 220 VAC single phase has one conductor grounded neutral, then the wiring was reasonably safe. In some countries, the safety codes require to break both hot and grounded neutral (which is in lieu of the equipment ground wire). Then, another conductor, namely "equipment ground conductor" is needed to make sure that the pump and associated conductive surfaces are grounded in case that phase (hot) wire leaks, or impresses on the conductive (metal) parts, which would pose a dangerous hazard since there would not be return to the panelboard ground to trip breaker/overload or blow the fuse, and the potential (hot) against the ground would stay posing the hazard.

 

[tabler]

Hi,
Comment:One phase motors usually are coonected with three wires-first is line (phase),second-neutral, third-ground.
It's enough to montage a protection device (from short-circuit connection ,overload, maybe also with earth-leakage relay) on the line wire.

tabler, Lithuania

 

[ControlMan]

Wow.

Unfortunately, this happens a lot. Single phase 220v can be switched through only one leg, but it is not advisable; and in motor applications like you describe it violates NEC. The reason you see this so much I suspect is that single-pole relays are cheaper than double-pole. The problem with switching only one leg is that while current flow is interrupted through the 220v device, the unswitched leg still carries 110v potential to ground - not a good thing. You were wise to install the new contactors as you did.

The advantage of using higher voltage motors for the same application is lower amperage draw, as you mentioned, but you also can yield a greater amount of torque, especially at startup if needed, depending upon motor (and starter) design.

 

[Chris33]

Controlman is right. It do happen alot but it do violate the code. you did do the right thing by putting in new contactors and breaking both legs.

Not to be negative here but jbartos, and tabler had the wrong idea about 220V single phase motors. Both the leads are hot and you do not have a neutral, but you do have a ground. If yo have a hot, neutral, and a ground you have a 120V motor.

Like Controlman said, you yeild a greater starting totque. This is why you use a Cap. to start a 120V motor.

 

[stripedbass]

Hi all,

Is the 220 VAC single phase motor wound the same as the 120 VAC?

 

[tabler]

Hi,
I bag Your pardon.I forgot, that we are talking about US. For American application it is necessary to montage protection on two wires.I must suggest with Controlman and Chris33.

tabler

 

[jbartos]

Comment: A distinction must be made between the official public power distribution system and the private power distribution. The 220 V single phase is not a standard power distribution system in USA; however, it is the standard power distribution system in some other Countries, e.g. some European Countries have it or had it in the past. They have certain rules for it, namely, one leg is always grounded since it is derived from 380Vdelta/220Vstar 3phase 4wire system with the neutral being solidly grounded. They may connect the grounded neutral to the grounded pole/pin in the 220V single phase receptacle. It is somewhat similar to 208Vdelta/120Vstar 3phase 4wire system with the neutral being solidly grounded in USA, except that the ground wire is mandatory according to NEC and the grounded neutral serves as a system ground rather than the equipment ground. Therefore, it is not being used for connection to the metallic parts of loads in USA.
If there is an ungrounded 220 V single phase being privately used as an ungrounded system in USA or elsewhere, then the both leads shall be open by the contactor and two-pole circuit breaker or two-fuse fuseholder. This posting somewhat clarifies my previous posting that was meant for the public or official 220V grounded single phase power distribution system derived from 3x380Vdelta/220Vstar 4wire 3phase system with grounded neutral.

 

[rhatcher]

This applies for 220V single phase only, or any single phase (line-to-line) of a three phase wye system where both conductors are hot and the equipment safety ground is provided by a third conductor.

The problem with interrupting only one of the two power wires is the danger of unintentional operation. This applies to any device including motors, coils, relays, clutches, brakes, etc.. The danger is if a ground occurs between the device and the 1 pole contactor. In this case the circuit would be completed from the uninterrupted hot leg source, through the device (whatever it is), and then through the unintentional ground to return to the source again. In some cases the device may operate despite the reduced voltages. This may pose a safety hazard.

For the record...with respect to single phase voltages derived from the phase-neutral of a system, Jbartos is right in saying that it is sufficient to interrupt only the hot lead.