25kva inverter single phasing can effct the 3ph induction motor

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Siemens 25kva 415vac 3ph inverter when become 1phasing .for this incident can effect the 3ph induction motor or burn?

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Type: Question • Score: 3 • Views: 1,216 • Replies: 16

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@arup baruah,

My immediate guess, Baru, is that yes it'd burn. Doubtless we will hear from Eng to confirm or deny and I'd ordinarily wait but in this instance wouldn't want you to experiment

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Current drawn by the operating phase is greater than design conditions for the winding. You may not discover single-phasing until the winding is damaged. Under some conditions, you may not recognize it at all. Preventing trouble is simple: use overload protectors in all three phases.

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@Tes yeux noirs,

Quote:

Current drawn by the operating phase...

That would presumably be the induction motor you plan to use

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is greater than design conditions for the winding

...which I'd guess means one-third of 25 kva

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You may not discover single-phasing

,...may not learn you've overloaded the supply winding

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until the winding is damaged

...probably when it burns out. Of course it should first get hot but don't depend on that

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Under some conditions, you may not recognize it at all.

...., that is, if it doesn't give you a warning of some sort such as its heating

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Preventing trouble is simple: use overload protectors in all three phases.

To myself at first this wasn't simple but that's because Tes is doubtless much smarter than me; until I realized that this protection could be fuses

Tes you hafta forgive me

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@dalehileman,

Quote:

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Current drawn by the operating phase...

That would presumably be the induction motor you plan to use

The single operating phase winding of the 3 phase motor which is single phasing. The one that's gonna cook.

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@Tes yeux noirs,

Aha Tes of course, now it's much clearer as I had assumed a single-phase motor. Arup is apparently asking whether the very motor designed for the source would be damaged if for instance two of the phases went off for some reason

...And y'know what....I dunno

My guess is, it might depend upon whether the motor was under load

Tes whaddya think

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@dalehileman,

I was thinking of thermal protection. Item 49 in this diagram is a thermal relay operated by a bimetallic strip in contact with current transformers on 2 phases. Overload will make one or more of these hot and trip the relay. This de-energises the starter coil and stops the motor

http://electrical4u.com/electrical/wp-content/uploads/2013/04/motor-protection-3.gif

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@dalehileman,

The OP mentioned it was a three phase motor.

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@Tes yeux noirs,

Wow Tes that's 'way beyond my capabilities
Incidentally I ought to explain that at 85 I misread that "3ph" as "3hp"so, sorry....

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The OP mentioned it was a three phase motor

Good lord I feel like such an idiot but just wait Tes til you're my age

Still, I wonder if it wouldn't keep running if it weren't under full load

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@dalehileman,

Quote:

My guess is, it might depend upon whether the motor was under load

One-third load or less, not much will happen. Motor will appear to work normally.

Somewhat higher than one third load, it will continue to operate, but it will draw current more than its rated value. Could burn if no thermal overload protection.

Higher load (near to rated load), motor speed will drop gradually to zero over some seconds, depending upon amount of the overload and inertia of the load connected. Again, could burn if no thermal overload protection.

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@Tes yeux noirs,

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One-third load or less, not much will happen. Motor will appear to work normally

Hafta take your word for that Tes. However I'd think lacking the other two phases that it might stall anyhow

But why didn't we provide overload protection on all three phases

Yo Eng, where are ya

1 Reply

@dalehileman,

Quote:

However I'd think lacking the other two phases that it might stall

If two phases go down it won't rotate. Single-phasing in a three-phase motor is when one phase is missing from the supply. In that situation the missing voltage is created in the affected phase winding by induction and at light load no symptoms are seen, although voltage measurements at the terminals might look a bit odd.

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@Tes yeux noirs,

Quote:

In that situation the missing voltage is created in the affected phase winding by induction

That is, of course, if the winding isn't shorted

Perhaps we ought to explain for Arup's benefit at least in this case no current would be flowing and so the other two are doing all the work

1 Reply

@dalehileman,

Quote:

That is, of course, if the winding isn't shorted

That's a different situation. Single phasing in a three phase machine is usually caused by removal of one phase due to faulty contacts or blown fuse somewhere in the supply wiring.

1 Reply

@Tes yeux noirs,

Indubitably

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@arup baruah,

DO YOU HAVE ANY SCHEMATICS TO EXPLAIN THIS EFFECT ???

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@Tes yeux noirs,

Quote:

If two phases go down it won't rotate

Yea Tes, pretty obvious when ya think about it. The armature wouldn't know which way to go

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25kva inverter single phasing can effct the 3ph induction motor

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