Motor Wiring

[accord]

I have been asked to connect a new 3 phase motor to existing wiring.
Motor is 3.8Kw dual voltage. Reason its being changed is because the old motor had been rewound too many times and couldnt be done again. The motors run 24/7 driving pumps. Have to be rewound roughly 12 monthly.
The existing wiring is DOL and the motors are in Star . This apparently is how its always been (premesis open over 10 years)
For a motor of this size would it not be better to fit a Star Delta Starter to both motors so they run at 230.
The cost of removing , rewinding, re-fitting and eventually re-placing. Against initial cost of fitting Star Delta.
A

[spinlondon]

As far as I remember, the motors are right on the cusp between requiring soft start and DOL.
If the lack of soft start, is causing the re-wind each year, then yes fitting a soft start would be a good idea.
However, you state that the motors are run 24/7, which would suggest that starting is not the cause of the motor windings failing.
You may want to investigate other possible causes, such as phase imbalance, or under voltage?

[accord]

Thanks for that , Sorry but i didnt mean to say that I thought the starting method was to blame for the constant re-winding.
In my opinion the fact of 24/7 running is the cause of eventual breakdown of windings.
What I would like a view on is do you think that if they were run in Delta the windings would be under less stress (230v).
If thats true then a Star Delta Starter could be fitted.

[spinlondon]

It would depend how the motor was chosen for it's intended task.
The output torque of the motor will depend on whether it's wired in star or delta.
If the motor was chosen for it's output in delta, and the motor is run in star, yes the motor will be placed under stress.
If however the motor is chosen for it's output in star, then it shouldn't make any difference, except that during changeover, the motor could be stressed by the extra current required to run it up from the star speed to the delta speed.
The RPM in star needs to be around 75-80%, of the RPM in delta when a star/delta starter is used, and the change over time has to be set to allow the motor to achieve this speed to avoid stress caused by the change over.
You need to determine what output torque is required from the motor, whether this will vary, etc. etc.
To be honest, it sounds as though the wrong type of motor has been chosen for the job.

[accord]

Thanks for your reply.
The load on the motor remains constant. They are pulling water from a swimming pool its passed through the filters and back to the pool. The club was built 17 years ago and I have to assume that the motors were chosen correctly.
At present they are in star with no other option other than changing the the link plates to delta at the motor.
That option is no good as the starting current would take out the breaker immediatly.
As there is a history of windings burning out approx every 12 months , I was looking for a reason for this and thought possibly that wiring a star delta starter so the motors would run in delta would reduce stress and heat.
The only other reason for this premature burning out is a poor rewind , or bearing failure causing heat .
Phase balance is ok.
Probably should just connect it up as before and not worry, but I have an enquiring mind and just thought I would see if anyone has any ideas.