Why do soft starts tell you to set the overload at (delta FLC/1.73)

[Jako13!]

Hi I have a question on soft starts. Why do soft starts have motor FLC delta /1.73. I’m confused as I understand star delta it is set at 58% of the full load current as the overload is in line with the windings. Thanks for your help in advance.

 

[Pete999]

Hi I have a question on soft starts. Why do soft starts have motor FLC delta /1.73. I’m confused as I understand star delta it is set at 58% of the full load current as the overload is in line with the windings. Thanks for your help in advance.

Voltage and Currents in Star- and Delta-Connected Loads | Introduction to AC Circuits | InformIT - http://www.informit.com/articles/article.aspx?p=101617&seqNum=6

 

[Bobster]

Well it entirely depends on where abouts in the circuit and what type of circuit you are using the softstart in.

Inline, or inline delta etc, I have never seen them mention 1.73 before. Which manufacturer are you using? Care to share a link to the instructions.

 

[Jako13!]

This is the soft start

 

[Darkwood]

Without been funny here try dividing 100 by 1.73 and see what the answer is

 

[Pete999]

Without been funny here try dividing 100 by 1.73 and see what the answer is

Hence my post 2, but your post was more direct, note to oneself stop trying to be a smart ---.

 

[Darkwood]

Hence my post 2, but your post was more direct, note to oneself stop trying to be a smart ---.

Soz Pete, didn't click the link hadn't realised you gave a indepth explanation via the link..

I just thought it easier to skip all the seemingly complicated maths here and show that dividing by 1.73 give you close enough to 58% regardless of why you need that percentage.. sometimes the simplest explanation is the best however if he was interested as to why 58% then you have covered that

 

[ipf]

Without been funny here try dividing 100 by 1.73 and see what the answer is

There are some numpties around you know, D..

Friday teatime, maybe?

 

[Pete999]

Soz Pete, didn't click the link hadn't realised you gave a indepth explanation via the link..

I just thought it easier to skip all the seemingly complicated maths here and show that dividing by 1.73 give you close enough to 58% regardless of why you need that percentage.. sometimes the simplest explanation is the best however if he was interested as to why 58% then you have covered that

Apologies not required Mate, the shorter the answer some times are the easiest to understand.

 

[Darkwood]

There are some numpties around you know, D..

Friday teatime, maybe?

To be fair using a one step sum to get a percentage is not how we are usually taught as it relies on knowing a very specific figure for each percentage, this is why we are taught to divide by 100 then multiply by the % figure we require as this uses 1 fixed figure and a known variable.

I think this has no reflection on numeric literacy but more on lack of recognition of that particular value, it isn't often expressed like that as most instruction for overloads setting would just specify 58% of the FLC when required.

I think the OP just over thought it a little and my only critique is he didn't use the calculation which would have shown it was approx 58%...

 

[netblindpaul]

DW,
The selection of soft start current depends on the design of the circuit and the motor configuration, remember that some motors are 230/400 & others 400/750 these days.
Also, if the motor was in delta and had a FLC of x then you would enter the delta FLC in the soft start as long as that was the correct value, not any mathematically corrected value, because the scaling could well be done in the start.
It does depend on the firmware and design of the motor, circuit and soft start unit.
Modern motors should hve the Y & D current values on the rating plate, so the overloads/soft start/inverters should be programmed in accordance with the OEM requirements, taking the value from the rating plate, without doing any maths.

 

[Darkwood]

@netblindpaul
Not too sure why you have responded as you have, I was simply expressing the OP's question and set-up, I did make that clear in a long handed way when I basically said in cases where you are specified to set at 58% then dividing by 1.73 is a quick one calculation route to getting a value.

I'm sure you must realise I wasn't making a sweeping statement that this is how to set up the many various methods of motor control and motors out there or have I missed the point you are making?

 

[Jako13!]

DW,
The selection of soft start current depends on the design of the circuit and the motor configuration, remember that some motors are 230/400 & others 400/750 these days.
Also, if the motor was in delta and had a FLC of x then you would enter the delta FLC in the soft start as long as that was the correct value, not any mathematically corrected value, because the scaling could well be done in the start.
It does depend on the firmware and design of the motor, circuit and soft start unit.
Modern motors should hve the Y & D current values on the rating plate, so the overloads/soft start/inverters should be programmed in accordance with the OEM requirements, taking the value from the rating plate, without doing any maths.

Thank you for your reply what ever the maths is I’m just confused as to why it is there I would have thought if you have connected a motor in delta you just use the FLC on the name plate as the soft start will be measuring the line currents not the phase current.But with star delta the overload is set at 58% of FLC as we are then measuring the current of the coils and not the line current.

 

[ipf]

To be fair using a one step sum to get a percentage is not how we are usually taught as it relies on knowing a very specific figure for each percentage, this is why we are taught to divide by 100 then multiply by the % figure we require as this uses 1 fixed figure and a known variable.

I think this has no reflection on numeric literacy but more on lack of recognition of that particular value, it isn't often expressed like that as most instruction for overloads setting would just specify 58% of the FLC when required.

I think the OP just over thought it a little and my only critique is he didn't use the calculation which would have shown it was approx 58%...

It's just the lack of basic common sense that gets me.
I can see it roughly by looking at the numbers.
Why not do both calculations and compare...about 30 secs on a calculator......maybe a couple of minutes, on paper.
Divide by 1 and 3/4 and multiply by 58%

 

[Darkwood]

The OP has just replied, I thought he was confused with the figure used to achieve 58% but he wasn't so my post went off on a tangent, he was questioning the rating of the overload in relation to the connection of the motor which I now realise, I simply read his opening post differently.