temporary repair - overloading a star wired motor.

[Inteificio]

Hi,

I had to do a repair I was not certain on, so would like to run it past some experts here.

We have a large stirrer at work. Due to some incompetence the gearbox got damaged.
With a lot of work we have almost everything back together with 1 problem remaining.

There is a lot of wear on the main drive cog interface.

With the motor as is, the massive start up kick combined with the slip on the box is enough to shear the cotter pin.

So I changed the wiring from delta to star to knock the torque down.

It now spins up slowly and well.

This leads to the problem of setting the overload.

There is a 9-13A overload on there, it was 13, I moved down to 9. A little high, but it got me thinking.

If the motor can run fine at 13A star, why can it not run fine at 13A delta.

I have a few ideas why, but I wonder if anyone knows?

 

[Pete999]

In star you are putting 230v across each winding Delta 400v across each winding think about it!!!!

 

[Inteificio]

In star you are putting 230v across each winding Delta 400v across each winding think about it!!!!

I have been, I was wanting other people's' opinions though.


What are you leading towards?

 

[Pete999]

Ohms Law

 

[Inteificio]

From one Pete to another.

I have done all my maths, but want to check if I have overlooked something.

Therefore I am asking others for their opinion.

If I give my explanation I may lead others to view the problem the same way as me, and I learn nothing.

Is Ohms law that accurate taking in to account the large amount of induction?

 

[Pete999]

Just ask yourself, why is any motor started using the star delta method? to reduce current draw on start up, started in star, up to speed switched over to delta

 

[Darkwood]

You need to be careful here but i assume you already know the common voltages to star/delta of this motor - in assumption its a 3ph supply 400v.

Have you got a 400v Delta / 690v Star
or is it a 230v Delta / 400v Star

Just to confirm we are all singing from the same hymn sheet?

A star/delta starter works because you under voltage the field windings which will be 690v rated thus its current limiting on start up once speed is attained you swap to Delta that is more efficient for the motor to run at when the supply is 400v.

 

[Inteificio]

Hi Dark,
Glad you turned up.

It is 400v supply, and therefore undervolted wired in star.

The intent is to weaken the motor for a few weeks until it gets replaced.

 

[Inteificio]

Ok my concern is this.

I have deliberately undervolted the motor to reduce the power.
If the motor is underpowered, the slip may increase, it start overloading and drawing more current.

Does the fact that it is star or delta reduce the maximum current it can handle?

I am worried that as the motor has a higher slip, the efficiency may drop and there be more heat generated in the motor for a similar current.

So is that right, or am i overlooking something

Can I set the overload at 13A or does it need to be reduced and why?

 

[Lucien Nunes]

The supply current is equal to the winding current in star but sqrt(3) times the winding current in delta. So if the overload is measuring the supply it must be set lower for star. The starting current will have been reduced by changing to star but as you say the running current won't. You would do well to measure it under load to confirm.

 

[Inteificio]

That's what I was overlooking, thanks!

 

[Inteificio]

I should point out there is no star delta starter here. Just DOL.

All I did was change a motor from delta to star to reduce starting torque.

 

[Darkwood]

The worry here would be available torque as you will have reduced it and it may be this thats slowing the motor down as it struggles to drive the gearbox.

You say it runs slower now? - speed is associated frequency and the strength of field is associated with voltage so your speed should be the same as the 50hz hasn't changed but as you have under voltaged the windings by connecting in star the the field strength will be a lot lower consequently less torque available if is running slow and tripping O/L then the torque might not be strong enough.

 

[Inteificio]

Sorry, bad choice of words by me.

I should have said it takes longer to get up to speed.

 

[Darkwood]

The solution here would be a VSD so you can set a ramp up speed in that the damaged sprockets etc don't see the jolt ... although its clear these need replacing ASAP and TBH you should be able to take the old in and pick up a replacement of the shelf same day.