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Old 09-21-2008, 08:56 PM   #1
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Default Do 3-phase motors require a neutral?

Let's take a 480/277v system for example. Why would the neutral not be required? Does it have to do with the load being balanced? I need someone to clear this up for me.

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Old 09-21-2008, 08:59 PM   #2
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A 3-phase motor by definition would never require a neutral conductor.

Neutral connections are only needed on single phase circuits.

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Old 09-21-2008, 09:24 PM   #3
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"Why is a neutral not required?" Well, because in three phase, the phases conduct through one another and a dedicated load like a motor uses all three phases and ONLY all three phases.

That said, three phase services can and do have neutrals to furnish single phase loads.

It should be remembered that single phase is really "split phase" with the center tap of the transformer grounded and serving as the neutral except in some cases where "isolated grounds" are wanted.
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Old 09-21-2008, 10:33 PM   #4
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Quote:
Originally Posted by waco View Post
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It should be remembered that single phase is really "split phase" with the center tap of the transformer grounded and serving as the neutral except in some cases where "isolated grounds" are wanted.
Not neccessarily so. If it is a wye service, each single phase is actually one compete winding of a 3 winding set.

You lost me on that "isolated grounds" statement. The neut is still derived at the center connection of the 3 windings of a wye or from the center tap of one of the windings of a single winding or. at the corner that is grounded in a corner grounded delta.

The only thing that makes a isolated ground isolated is it is isolated from other EGC's from service to point of final connection to the intended device.

Last edited by nap; 09-21-2008 at 10:36 PM.
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Old 09-21-2008, 10:37 PM   #5
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Quote:
Originally Posted by kbsparky View Post
A 3-phase motor by definition would never require a neutral conductor.

Neutral connections are only needed on single phase circuits.

A very straight forward answer to the original question, end of story.
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Old 09-22-2008, 01:52 AM   #6
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Quote:
Do 3-phase motors require a neutral?
Let's take a 480/277v system for example. Why would the neutral not be required? Does it have to do with the load being balanced? I need someone to clear this up for me.


Constantly one hot leg carries the difference of the unbalanced current between the other two hot conductors. This alternates between the three phases.

Once again A three phase motor would never require a neutral.
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Old 09-22-2008, 08:58 AM   #7
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"End of story?" Not really, the person wondered why, not just because.

In my part about "single phase," I was referring to a "single phase" service, not a three phase with a single phase used for single phase loads. Sorry about the confusion.

Your statement about what makes an "isolated ground" isolated is exactly right, but I wanted to stress that the neutral is always grounded at the service, but EGCs might be grounded at other places and at the place where the neutral is grounded.

Sorry, a pet peeve of mine are technically inaccurate explanations although I'm sure mine are as apt to be technically inaccurate, which is why I used the chalkboard sooooo much when I was teaching.
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Old 09-22-2008, 07:49 PM   #8
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Quote:
Originally Posted by waco View Post
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In my part about "single phase," I was referring to a "single phase" service, not a three phase with a single phase used for single phase loads. Sorry about the confusion.

.
I know what you meant but since this is an electricians board, a good many do deal with 3 phase services all the time so referrencing a single phase to mean only a center tapped single winding transformer supply is not the best way explain things.


I hope you were referring to your own explanations as being techinically inaccurate because mine weren't

The fact you tossed in the isolated ground was simply inappropriate for this thread. It has no bearing on anything discussed and your inclusion only lead to confusion.. As well, it inferred something quite incorrect.

and since you appear to relish in bringing in confusion, this statement:

Quote:
That said, three phase services can and do have neutrals to furnish single phase loads.

It should be remembered that single phase is really "split phase" with the center tap of the transformer grounded and serving as the neutral except in some cases where "isolated grounds" are wanted.
really did.

In the first part you speak of single phase loads in a 3 phase system and then you go to explain that a single phase is a split phase. Then you go and try to explain to me that you were not talking about a single phase load from a 3 phase system but right there, in the noted statement, you are the one that did include single phase load and 3 phase in the same post.

BTW: not all single phase loads use a neutral; 240, 208, 480 can all be supplied as single phase power and do not use a neutral.

I think you need to break out that chalk board.
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Old 09-23-2008, 11:11 PM   #9
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Default 3 phase neutral 120 volt motor

ok now here it gos guys , yes there is a motor that does have a neutral and its 3 phase also ? i just put this in to add to the confusion and to make more trouble , and i can not wait for the comments ? any takers come on ill bet theres someone out there ok just kiddin, its a synchronous[ 3] phase [120 ]volt ac 60 hz motor 4 lead special application low speed can go down to 24 rpm . its a wye connection motor only common neutral center tapped then each leg or winding a b c basic induction type style . but we as electricians will never use it or see it ever . ill get the model and post the type and who makes it ,but going to wait to see how many say iam nuts . and i didnt even go to the bar after work today .take care best to ya

Last edited by nick; 09-23-2008 at 11:23 PM.
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Old 09-24-2008, 01:43 AM   #10
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Quote:
ok now here it gos guys , yes there is a motor that does have a neutral and its 3 phase also ? i just put this in to add to the confusion and to make more trouble , and i can not wait for the comments ? any takers come on ill bet theres someone out there ok just kiddin, its a synchronous[ 3] phase [120 ]volt ac 60 hz motor 4 lead special application low speed can go down to 24 rpm . its a wye connection motor only common neutral center tapped then each leg or winding a b c basic induction type style . but we as electricians will never use it or see it ever . ill get the model and post the type and who makes it ,but going to wait to see how many say iam nuts . and i didnt even go to the bar after work today .take care best to ya
Yeah, post this part # i'd like to see it

A synchronous motor induction type? LOL Sounds like double talk

A synchronous motor doesn't depend on a induced current in the rotor to produce a torque? The only similarity it has to a induction motor is a synchronuous motor starts like a induction motor, the purpose being for starting torque and low current. It uses DC excitation to operate.

Some times ELECTRICIANS are called out to hook up synchronous motors, to correct the power factor on devices connected to the same line.

Once again,

A THREE PHASE MOTOR DOES NOT REQUIRE A NEUTRAL TO RUN

Last edited by 5volts; 09-24-2008 at 02:00 AM.
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Old 09-24-2008, 02:53 AM   #11
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So let me get this straight...... a 3 phase motor doesnt need a neutral.... EVER?



























































Sorry... I had to

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Old 09-24-2008, 06:09 AM   #12
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But wait, there is another type of three phase motor that does require a neutral
and it's not a synchronous motor either.

It's an AC Forklift motor.

Still has three phases, usually operates on 24vdc to 50vdc battery pack.

Confused yet?

The controller in the Forklift converts the voltage to AC through a CANBUS motor controller, inverter arrangement.

You can read more about it on the Crown Materials Handling website.
Where I work - we now have about 30 of these new machines added to the fleet.
I've recently just finished a 4 day training program on the new motors and CANBUS Controller.

Last edited by simmo; 09-24-2008 at 06:12 AM.
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Old 09-24-2008, 05:56 PM   #13
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well its a rare motor ,used in reelers cutting machines nuclear reactors liquid valves stuff like that , the steromotor is a conventional ac motor it can be single phase or two phase or three phase , and pulsed dc , standard voltage is 110/115 volts to 208/230 volts ac polyphase , meaning three phase . it can be asychronous or sychronous versions are made , but mechanically it has a unusal design in that it has a permanent magnet rotor is free to roll round inside of the stator bore without the usual restraint of bearings no metal to metal contact of rotor to stator it tracks on the stator by resilient tyres at each end meaning it floats no bearings . stator windings are conventional wound induction type the permanent magnet rotor has no fixed axis . its allowed to roll free in the stator bore by magnetic flux and multi pole construction ,not your common everyday motor . or it can have a dc excited rotor for synchoronous different types of models . this one has a per/ magnet rotor !! meaning its a three phase motor at 115 /208 volts ac its 6 leads are terminated wye all common neutral taps are made then at1 bt2 ct3 = y/connection it can run by pwm controlled freq drive ,as mostly you can only operate this motor at low speed the standard speed is sychronous = 20 rev /min asynchronus types at =2 rev /min . and can max out at 200rev/min. 60 or 50 hz standard ac power if you just want to run at full speed cont .these motors can go into a reversal meaning a back and forth 30 reversals per second if the load inertia and circuit permits this , its a step by step motor if needed or a open closed type valve motor ,we used these on a power plant job i never forgot the day 115 volts 3 phase ? now next i will get the model number had to email a old buddie for that but just go to steromotor its on the big web they are rare motors . or its in most motor books like mcgraw hill stuff nothing hard to find go to table 430.250 full load current three phase ac motors thats the chart to use with these see 115 volt column . these little motors are bad lots of torque at various low speeds . best to ya take care

Last edited by nick; 09-25-2008 at 05:16 PM. Reason: i never said it required a neutral it just has to have it to work on three phase 115v
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Old 09-26-2008, 05:04 AM   #14
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Three phase is self starting , single phase needs help rule one ! yes there is a difference between a induction motor and a sychronous motor i agree with p logic on that , yes [ asynchronous ]power is appllied to rotor by induction from the stator windings standard 3 phase motors . synchoronus power current is supplied to the rotor which interacts with the field on the stator windings the streomotor uses both starts and both running apllications but it only gos at very slow speeds and is very accurate on rotation its as close to a stepper motor you can get on ac power but not a dc digtal input type , i hope i didnt offend anyone i was just trying to give some input on a different type of motor , and yes i agree that generally all 3 phase motors do not have a neutral in our work sorry i hope you understand ,i know p logic you are a very knowledgeable electrician and respect you input , and can understand your dispute in this matter , i was also in when we hooked these up ? and we all looked at these motors and said the same thing you said WHATS THE [ ====] IS THIS ? still trying to get the company so you can see these but waiting on others is a pain, and these are not your standard off the shelf motors there made special for most projects . take care best to ya


Last edited by nick; 09-26-2008 at 05:27 AM. Reason: cant spell or write
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