3 PHASE motor
- Thread starter domnic
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- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
I think it is okay unless the relay is part of the disconnecting means-- 430.84
Tony S
Senior Member
- Location
- Resting under the Major Oak UK
It?s a method frequently used in crane control. That said I wouldn?t recommend it.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
Let's put it this way, there is nothing prohibiting you from controlling a 3 phase motor with a 2 pole contactor. The only requirements are that you protect all three phases from short circuits and ground faults, and that the motor has running thermal overload protection. If the thermal protection is inside of the motor already, you see this done quite often on cheapskate HVAC equipment vendors. That extra $3 for the 3rd pole is too rich for them I guess.
But if you are using an external (to the motor) overload relay, there is a requirement that the OL relay protect all 3 phases. So for motor starters, which are a convenient way to accomplish both the control and protection, it's easier to just get your 3 pole OL relay attached to a 3 pole contactor. Nobody sells a 2 pole contractor with a 3 pole OL relay. You could, I suppose, but a 2P contactor and get a separate 3P OL relay, then mount and wire them together yourself, but the $3 you saved on the contactor cost you 10 or more minutes of your labor. You then worked for about 50 cents/hour...
Why is it not good? No good technical reason really, but it tells the user that you built something as cheaply as humany possible. In reality though, even though it is technically illegal and a REALLY bad practice to rely upon a contractor to isolate a load from the supply, it happens nonetheless in the hubbub of field repairs, often done in a hurry. Leaving one pole live when the machine appears to be off is just asking for trouble.
But if you are using an external (to the motor) overload relay, there is a requirement that the OL relay protect all 3 phases. So for motor starters, which are a convenient way to accomplish both the control and protection, it's easier to just get your 3 pole OL relay attached to a 3 pole contactor. Nobody sells a 2 pole contractor with a 3 pole OL relay. You could, I suppose, but a 2P contactor and get a separate 3P OL relay, then mount and wire them together yourself, but the $3 you saved on the contactor cost you 10 or more minutes of your labor. You then worked for about 50 cents/hour...
Why is it not good? No good technical reason really, but it tells the user that you built something as cheaply as humany possible. In reality though, even though it is technically illegal and a REALLY bad practice to rely upon a contractor to isolate a load from the supply, it happens nonetheless in the hubbub of field repairs, often done in a hurry. Leaving one pole live when the machine appears to be off is just asking for trouble.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
By my math, that works out to $18 per hour, not $.50....
But that still has to be compared to the fully burdened hourly rate including overhead, etc.
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
A motor controller only needs to interrupt enough supply conductors to stop current flow in the motor. Open only one line of a three phase motor and you "single phase" the motor. Open two supply leads and though one is still energized there is no place for current to flow in a motor in good condition.
Same for single phase motors, open one line and you interrupt all current flow so the motor doesn't run, but does have voltage present. They like to do this with HVAC equipment, maybe even more so then they use a two pole controller for three phase equipment.
Same for single phase motors, open one line and you interrupt all current flow so the motor doesn't run, but does have voltage present. They like to do this with HVAC equipment, maybe even more so then they use a two pole controller for three phase equipment.
Tony S
Senior Member
- Location
- Resting under the Major Oak UK
On a crane we had to have one main contactor that opened if any motor detected a fault situation. If say one of the X travel drives tripped for what ever reason it would shut the entire crane down.
In our line of work where the main load would be molten metal I?d rather not have the load suspended while the electrician attended the fault. Had each drive handled its own fault condition and left the rest able to function the load could have been set down on the deck safely.
If a system is operating normally then there?s no problem. Under fault conditions it?s a pain.
It?s an old system and has seen its day and to my mind has no part in safe a safe control system.
I don?t like the system but it?s what I inherited and had to work with.
This begs the question, why would anyone suggest such a system? There must be a reason for the question being asked.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
That does not seem to have anything to do with a reason for using a two pole versus a three pole contactor as a motor controller. Unless the only time a two pole controller was ever used in cranes was to shut down power to the entire system.
Maybe I am missing the point?
I do not have an answer for why the crane you worked with had an all or nothing control system, other than perhaps that it simplified the wiring, as all fault sensors connected to the same place. Can you think of a situation where stopping one motion but not the others could in itself create a safety hazard until the operator became aware of it and reacted?
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
No wonder my panel shop went under... :slaphead:
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
For the other side of that, there is the classic story of the guy who was voted least likely to succeed in his high school class. Academically he was a total loss.
But at the 10th reunion he was driven up in a stretch limo and got out wearing a classic italian suit.
When asked how he had managed to do so well he replied:
"I just found a product that I could make for $1.00 and sell for @3.00. It is amazing how fast that 2% profit adds up."
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