Single phase motor wired to 3 phase overload

stasis said:
breaking the grounded conductor isn't just a no-no...it's against the NEC.

just my 2 pesos

I don't think this would apply to the grounded Neutral. You are breaking your current carrying conductor, not your ground. You'd still leave your Ground wire intact running out to the motor.

I might be wrong, but I don't think the NEC says you cannot open your circuit if it may have 0 potential to ground. Not all grounded conductors are grounds.
 
Gil47, I have to disagree with you on a three phase fan that looses a phase. When it looses a phase it will slow down and as the load for a fan works to the cube of the speed even a small drop in speed will cause a large drop in current. The motor will not stall it has no reason to stall.

Bryan
 
Ahhh the terminology gets us again
GROUNDED CONDUCTOR is current carrying is BONDED to ground and in USA is white or gray, can be derived from a "common point" of transformer winding(s) and is often called neutral erroneously (24 or 120 control power derived from a transformer and one side of line BONDED to ground)

GROUNDING CONDUCTOR is not a normal current carrying conductor is bonded to ground at the service entrance is a green or bared conductor and serves to assure all equipment stays at ground potential and also serves as a second current path to trip a breaker

In USA the NEC does allow breaking the GROUNDED CONDUCTOR ie neutral PROVIDED all other current carrying conductors are simultaneously broken.

I agree that a single phased fan motor will slow down. Yes in accordance with affinity laws the power delivered to fan will drop.
HOWEVER you are now suppling the lowered power with only two of three phases. Phase current will rise by a minimum of 1.73 (you are now single phase not 3) and if you are lucky trip the overload.
The gamble here is
will decreased power demanded by fan result in more current decrease than the current increase caused by going from 3 to single phase (ie throwing 1.73 out of the power calculation).

Dan Bentler
 
Last edited:
BryanG said:
Gil47, I have to disagree with you on a three phase fan that looses a phase. When it looses a phase it will slow down and as the load for a fan works to the cube of the speed even a small drop in speed will cause a large drop in current. The motor will not stall it has no reason to stall.

Bryan


When a standard 3 phase motor is using Direct on Line starting running at full load there is a slip of about 5% , now if you lose a phase then the motor slows down,
Then the slip gets greater say 10% , the motors impedence changes dramatically causing a higher current probably double full load and so it goes on, creating a cascading slower speed reducing the load but a lower impedence giving a higher current situation.
The lower speed of the fan applying the cube rule will not compensate enough to stop a correctly set motor overload tripping the motor, if the overload is set to motor full load current.


Now if that one phase was to fail to earth and ended up supplying 2 phases and an earth then you do have a situation that may keep running but not well.
 
OK, practical experience beats theory. I went to the font of all knowledge, my Dad, who used to sell motors and motor control gear. Hi reply was less polite that mine and started with a 'bull' and ended in 'it'. He used to work out in Brazil and because of the dodgy supply had to be able to run a flour mill with only two phases available. So a fan will slow down but will continue to run, a machine may continue to run depending on the load it is driving. Would you like me to put any parts of that statement in big letters to emphasize it.

Oh, and my Dad is bigger than your Dad :)

Bryan
 
BryanG said:
OK, practical experience beats theory. I went to the font of all knowledge, my Dad, who used to sell motors and motor control gear. Hi reply was less polite that mine and started with a 'bull' and ended in 'it'. He used to work out in Brazil and because of the dodgy supply had to be able to run a flour mill with only two phases available. So a fan will slow down but will continue to run, a machine may continue to run depending on the load it is driving. Would you like me to put any parts of that statement in big letters to emphasize it.

Oh, and my Dad is bigger than your Dad :)

Bryan

Motors I've seen with only two phases sometimes start backwards, but I imagine if a phase drops out while running then it would probably keep going.
 
To add: It is the excessive current drawn on the remaining 2 phases that cause motor winding failure.

Is it against NEC to break the neutral if you break the line also?

Never heard that before. I know I've broken that law. Oops.
 
boardmaker said:
To add: It is the excessive current drawn on the remaining 2 phases that cause motor winding failure.

Is it against NEC to break the neutral if you break the line also?

Never heard that before. I know I've broken that law. Oops.

I previously posted and copied here again.

Ahhh the terminology gets us again
GROUNDED CONDUCTOR is current carrying is BONDED to ground and in USA is white or gray, can be derived from a "common point" of transformer winding(s) and is often called neutral erroneously (24 or 120 control power derived from a transformer and one side of line BONDED to ground)

GROUNDING CONDUCTOR is not a normal current carrying conductor is bonded to ground at the service entrance is a green or bared conductor and serves to assure all equipment stays at ground potential and also serves as a second current path to trip a breaker

In USA the NEC does allow breaking the GROUNDED CONDUCTOR ie neutral PROVIDED all other current carrying conductors are simultaneously broken.

I agree that a single phased fan motor will slow down. Yes in accordance with affinity laws the power delivered to fan will drop.
HOWEVER you are now suppling the lowered power with only two of three phases. Phase current will rise by a minimum of 1.73 (you are now single phase not 3) and if you are lucky trip the overload.
The gamble here is
will decreased power demanded by fan result in more current decrease than the current increase caused by going from 3 to single phase (ie throwing 1.73 out of the power calculation).

Dan Bentler
 
BryanG said:
OK, practical experience beats theory. I went to the font of all knowledge, my Dad,

Oh, and my Dad is bigger than your Dad :)

Bryan

Seems like our Dads would have got along real well
I also am standing right behind my Dad
He is 101
Would you really hit an old man :)
 
Leitmotif, I agree 100%.

Stasis had me thinking there for a minute, and I thought it may be a terminology issue, but I have been wrong before.

Ungrounded conductor-current carrying conductor (hot)
Grounded conductor-current carrying conductor (neutral)
Grounding conductor-Grounding conductor (for lack of better terminlogy)

One to many code classes is hard on a guy.
 
leitmotif is correct about breaking all conductors simultaniously...and about the terminology being erronous.

a simple way to remember this is... Ed White.
if the term ends with 'ed' (grounded) it's white (or gray), and if it ends with 'ing', it's green (or bare). Far too often, I have come across residential electricians who use the terms neutral & common to refer to the grounded conductor. I don't like either term, they are misleading.

a 'common' is NOT white, expecially in industrial controls. A common is a wire that shares potential between two or more points. This is usually the 120VAC (or whatever your power scheme may be) after the E-stop that supplies power to pushbuttons, limits, output cards, etc.

Neutral implies that the wire is unbiased, but the reality is that a grounded conductor carries current. It just carries it TO ground, i.e, as a dump of excess current away from the load.

Try removing the grounded conductor off a grounded bus in a panel, and if the circuit is under a load (or under tension for the European guys) the wire will arc.
 
Many ways to skin this cat IMHO. First check on code compliance...particularily NEC 2008 430.225 (B) and 110.3(B). As far as Neutral disconnection to the motor, it seems to be not required 430.225 (B) (3). Haven't seen neutral switching commonly used for a motor load circuit. But have seen it commonly used in motor control overload circuits.
 
HI all,

Can anyone explain to me the correct way to wire a single phase 120 volt AC motor to a three phase solid state overload/contactor?

I have seen L1 through contact 1 to the motor with neutral to contact 2 looped back throught contact 3 out to the motor.

I have also seen L1 through contact 1 looped through contact 2 out to the motor and neutral through contact 3 out to the motor.

Also I have seen L1 looped from contact 1 through contact 2 and contact 3 out to the motor with a direct connection to neutral (not through contactor/overload).

Having seen all of these in the field, I am not sure which is correct and why.

Any help would be appreciated.

THX

Marc

I have several controls (for 1 phase sump pumps) looped from L3 to T3.
 
Sorry for reviving an old thread but when connecting a single phase into a 3phases device, today we were debating about the direction of running the current thru the device and i would like to know what you guys think.

If we imagine L1-L2-L3 as input of the overload device and T1-T2-T3 as output:
Would it make a différence if we reverse polarity of the middle leg for a 120v circuit:

option1
-enter line into L1
-jump T1 back to L2
-jump T2 back to L3
-Use T3 to the motor

Or option2 (easier to wire)
-Enter line into L1
-Jump T1 direct to T2
-Jump L2 direct to L3
-Use T3 to the motor

We were doing it like option 1 for years and someone new with us want to use option 2 and i'm wondering if it make a difference either on a thermomagnetic overload or with an electronic one.
 

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