would a 4-wire star connected induction motor start with one phase open?

[Power0020]

I wonder if a wye connected 4-wire motor had one phase open, would it start? a rotating magnentic field will be there but skipping one phase.

if the motor is delta or 3-wire wye connected with one phase open, it will be simply a single phase one and a pulsating magnetic field will not allow it to self-start.

any clues?

 

[waross]

Never seen a four wire induction motor. If such a motor existed it would probably start.
A rotary phase converter starts with one true phase and one phase offset by capacitors. Once running the back EMF from all three phase windings produces fairly good three phase power.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ashtree]

Power0020
Motors will normally run in the circumstances you describe as long they have little or no load. Performance will be well down obviously.
This often confuses troubleshooters, because the motor will run seemingly as normal with no load but wont run properly if at all under load. A check on phase current normally tells the story however.

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[waross]

You can get a rotating magnetic field with two phases and a neutral. That's how an wyepen delta works. You may also produce three phase 120:208 Volts from two phases and a neutral. I have heard this connection called a "Winnipeg" connection.
The problem is finding a three phase, four wire induction motor. Not a standard motor.
If you use instead an open delta transformer bank, you will get full power out of the motor and will be able to use any standard motor as a spare.
If you have a special application and only two phases are available, you may be able to get a motor shop to break out the wye point so you have a wye point connection to work with. The motor will have to be oversized by at least 50%.If your load is 10 HP the motor will have to be at least 15 HP.
With a couple of 2:1 autotransformers connected in open delta you will be able to run a 230 Volt motor from two phases at 120 Volts, and the motor will not need to be oversized.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Power0020]

Makes sense.

A loading test will reveal this issue.

when a phase fails open in a three phase IM with start or delta connection the motor continue running with a higher current on live phases. I wonder how it does without a rotating magnetic field? or it uses the pulsating field component in its own direction (similar to single phase motors)?

 

[waross]

As an example, if you lose "A" phase on a three phase motor, You lose "A-B" and "C-A". You have only "B-C" left.
Yes the motor becomes a single phase motor.
Now two things can happen depending on where the phase has failed.
If the phase has failed just at the motor the motor will probably start to overheat with anything above about 1/3 rated load.
If a phase has failed on a feeder to a panel and there are non-motor loads on the panel, the motor acts as an induction generator and tries to power the non-motor loads connected to the panel. The motor may overheat even with no load.
Back to your original question, Power;

I wonder if a wye connected 4-wire motor had one phase open, would it start?

Do you have such a motor or is this a question to help understand motors? No problem either way. Just wondering.
My problem is that I have seen a lot of three phase induction motors but I have never seen a four wire, three phase induction motor.
There are a couple of good reasons to NOT use a four wire connection for a three phase motor.
With the hypothetical four wire motor, if you lose "A" phase you will still have "B-N" and "C-N".
The motor will start but a little slower and possibly a little hotter.
If the phase has failed just at the motor the motor will probably start to overheat with anything above about 2/3 rated load.
If a phase has failed on a feeder to a panel and there are non-motor loads on the panel, the motor acts as an induction generator and tries to power the non-motor loads connected to the panel. The motor may overheat even with no load.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[iop95]

A 3phase IM don't start-up with 2 phase connected only.
It's like a single phase motor, that have two magnetic fields with equal values and rotating in opposite directions, so no torque.
Need an external torque or an auxiliar winding to "help" one field to "become" as main and predominantly.
If 3ph IM motor is working and lose a phase it transform in a single phase IM and keep to run in same direction. Torque decrease and current increase, being an abnormal operation mode.

 

[waross]

I guess I didn't make it plain enough. There is no such a thing as a three phase, four wire induction motor.
If you take an existing motor and break out the star point to make a home brew three phase, four wire motor, then yes, the motor will start and run on two phases and a neutral.
This is similar to developing three phases with two phases and a neutral and an open delta transformer connection. (I believe this is called a "V" connection in IEC land.
I once visited a shipyard in the Third World that had taken delivery of some three phase welding machines. The boys connected them the same way as the existing single phase machines. Two phases and a neutral. The cooling fans rated three phase, 208/230 Volts were running on 120V, 120V and 208V. They must have been oversized for the load because they continued to run, day after day. The welding machines worked for most work but if heavier current was needed the boys would use one of the older single phase machines. The new machines were not that good at higher amperage settings.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[roydm]

Two phase motors were quite common in NZ when I did my apprenticeship, you would see them mainly out in farming areas driving low torque loads like milking and shearing machines.
A 3 phase 4 wire motor with one phase missing is a 2 phase motor isn't it?
As others indicated it's essential to have the neutral to get a rotating field.

 

[davidbeach]

Two phase is not three phases with a phase missing. In a two phase system the two phases are in quadrature to each other while in a three phase system there is 120 degrees between phases. Two phase is now extremely rare and most people haven't ever seen it (I know I haven't).

 

[EngRepair]

I wonder if a wye connected 4-wire motor had one phase open, would it start?

Try it and you'll see.

 

[waross]

I haven't seen it either David. The Scott and Fortescue connections were included in the older textbooks but there never was an exam question.
Interestingly, the two transformer Tee connection that was popular for awhile for three phase power was very similar to the connections used to transformer three phase to two phase and back.
A lot of the TEE connected transformers that I saw were about 25 KVA or 37.5 KVA, 480:120/208 Volts. They were used to power lighting panels in industrial plants.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[iop95]

I think term "2 phases" must be used with reference to:
a) 2 phases - a voltage distribution system with 2 voltage 90° phase shift. Was used before 3-phase system but still exist today.
b) 2 phase - a situation that may occur in a 3 phase system (one phase lost); it's a particular situation (failure) and result a single voltage (line voltage in no neutral used distribution) or 2 voltages 120° phase shift (2 phase voltages in neutral used distribution).
A 3 phase motor with neutral conection (I don't know to exist... no sense), with one phase lost, will start, being supplied by a 2 phase voltages 120° phase shift.
If no neutral connection, motor don't start, being a supplied by a single voltage which don't have "ability" to rotate.

 

[waross]

Two phase power was an old system with a 90 degree shift between phases.
Not to be confused with using two of three phases. Using two of three phases is very common but the users are very seldom aware that they are using two phases and not single phase. There are probably 10 million apartments in North America supplied with two phases out of three and nobody ever noticed.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[stevenal]

faq238-777

 

[ashtree]

Despite what everyone is saying there are such beasts in service. It is quite a number of years ago but i worked on a small 3 phase star connected motor that had four wires. From memory it was either on some sort of machine tool or some specialist piece of equipment and it was not an off the shelf standard motor.

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[waross]

Was that a motor or a rotary phase convertor?
A three phase induction motor may be modified by bringing out a connection to the internal wye point. A capacitor(s) is used to provide a phase displacement to facilitate starting. When the motor is up to speed, the back EMF will produce tree phase power with fairly good phase angles.
As far as a four wire motor:
We are familiar with the overheating and over-current issues associated with unequal voltages and/or phase angle errors.
These issues can be expected to become worse with a four wire connection. Hence, no one makes a four wire three phase motor.
Yes. You can start a rotary phase converter with two phases and a neutral. It will start as well or better than it will with the normal capacitor starting system.
In practice, with two phases and a neutral available, it is cheaper and more efficient to use an open delta transformer connection to develop three phases.
However, if a suitable rotary phase converter is available and transformers would have to be purchased, then by all means start and use the rotary phase converter with the available two phases and the neutral.

Bill
--------------------
"Why not the best?"
Jimmy Carter