temporary repair - overloading a star wired motor.

[Inteificio]

VSD would be the ideal solution, however this unit is being decommissioned/replaced in a couple of weeks. So the odds of me being able to order an expensive piece of kit that would take a week to arrive ~0.

Am risking making myself look more foolish here, but my maths/logic falls to **** when I work nights. So sorry to double check something so basic.

The previous poster said Ip and Il on star had a ratio of 1.7....

I thought star was ip=il and 1.7vp=Vl

If you keep the resistance of the windings the same, changing wiring from star to delta drops phase current by a factor of 3?

So for delta with a 1 ohm winding.
You get 400v across each winding, therefore 400Ap.
Al = 1.7Ap so there should be 690A at the overload.

With star you get 230v at each winding, therefore 230A
Ip = Il therefore only 230A at overload.

So ratio of 3:1.

So one of us is wrong (and as I was awake at 5am and woke up at 9, it's probably me =-) )

 

[Knobhead]

The O/L’s need to be set at √3 Delta FLC to compensate for the reduction in the current carrying capacity of a single winding.

Slip will increase, efficiency will fall and the temperature will rise.

I don’t think that motor is long for this life.

 

[Lucien Nunes]

The current does not depend on the winding resistance, it is a function of voltage and mechanical load. Changing Vp does not therefore produce a proportional change in Ip, because the effective load impedance changes.

 

[Inteificio]

lol, good point.

 

[Darkwood]

The motor will be effectively trying to do the same job but with only a 3rd torque so as soon as you load up you may find this is where you have your issues as it stands it may be fine driving the gears and stirring arm but if your not mixing clouds and candy floss with this stirrer then you may end up pushing the motor a tad too much.

Unless it was oversized for the job to start with or your work load is light.

 

[Inteificio]

Odds are this motor was WAY oversized, as long as it lasts two weeks it's fine. It's for the bin anyway.


So to confirm I got this.
When rewiring a 400vl delta motor to 400vl star you rate the overload down by root 3?

EDIT: just thinking about this more. you rate down by root 3 when switching delta to star 400vp, so in this circumstance is it rate down by factor of 3?

 

[Darkwood]

Its not quite a simple as a straight forward sum ... don't forget you are current limiting a starting motor so run up time will be extended - usually a Star/Delta starter is used and the O/L is set to 0.58% of FLC giving protection in Delta 6wire but when in star the current is usually high as you are starting the motor so to protect the motor connected in star like your situe then you may find issues with O/L tripping on start-up if the start-up is long.

For running current in star the O/L setting is simply subject to standard formula but you are both starting and running in star what is a 400v/690v motor you could find this causes problems even with the time/curve allowance of the O/L.

You are setting a motor up to do what its not really design to and this can be difficult to protect and as Tony says it may be a short lived experiment there again you may get away with it but many variable will play there part here so a straight forward answer is hard to give.

 

[Clive P]

Hi,

One of the best posts for a while, this is just what I like to see, a good old informative discussion.

Regards.

 

[SPARTYKUS]

Hi,

One of the best posts for a while, this is just what I like to see, a good old informative discussion.

Regards.

Bah! Clive P beat me to it!

 

[Thefailsafe2012]

Hi,

One of the best posts for a while, this is just what I like to see, a good old informative discussion.

Regards.

Yeah it is!

But as I haven't a clue what yer all on about I'm going to dive back into the mosh pit with all the Electrical Trainee's and DIYers and stir up some trouble

:rockon:

Honestly, an interesting thread though!

 

[Darkwood]

Use the RATE THIS THREAD option at the top of the page if you like it and give it a star rating.

 

[Marvo]

Slip will increase, efficiency will fall and the temperature will rise.

I don’t think that motor is long for this life.

My thoughts in a nutshell. Add to this if it's a TEFC motor the extra slip will reduce the air flow over the motor so it get nailed from both sides as it were. There's a good chance you'll get thermal runaway so if there's a thermistor on the windings make sure it's connected to the control circuit.

 

[Deleted member 72666]

Chances are it wont make it till the end of the shift let alone 2 or 3 weeks!

 

[Knobhead]

Add to this if it's a TEFC motor the extra slip will reduce the air flow over the motor so it get nailed from both sides as it were. There's a good chance you'll get thermal runaway so if there's a thermistor on the windings make sure it's connected to the control circuit.

Chances are it wont make it till the end of the shift let alone 2 or 3 weeks!

There you have it Inteificio, it doesn't look too good. It may last the fortnight but I don't think so.

 

[Darkwood]

Lets get a wager going... now this is kinda pot luck as we don't know the working duty expected of the motor or the duration .. but here's mine -

Friday at 2:30pm a strong fishy smell confirms the demise of the motor! :behead: