Star Delta Theory

[collinsd70]

Hi All.

Firstly would like to say thanks to everyone who reads this and provides any useful information.

I have used the search button and have read through previous posts/ stickys before posting this.




Basically I would like to understand the voltages clearer in star and delta, I know that with a 400v three phase supply would give a phase voltage of 400v in delta (across any winding) and 230v in star (across any winding) but delta is termed to be lower voltage higher current than star and is recognisable like this from a motor plate but how does this work out from the information I have given?

I understand wiring the connections in a terminal box aswell as star delta control etc but I just dont seem to understand the theory so well.

Could anybody also explain how 720v can be achieved ( have seen this on a wiring plate and also confused me).

Thanks in advance.

Dan.

 

[silva.foxx]

View attachment Three-Phase_Circuits.pdf

This will give you some light reading for bedtime, but should explain it all. Hope it helps.

regs// s.f

 

[collinsd70]

Hi Silva.

Thanks for the pdf, unfortunatley I already have it and have read it a couple of times but still do not understand, the calculations for working out phase voltage etc all seem correct but does not show how voltage in star is aparently higher than in delta.

 

[silva.foxx]

Are you asking in relation to motor nameplates?

 

[collinsd70]

yes thats where i seem to be getting confused.

 

[silva.foxx]

I'm assuming you are relating to dual voltage motors where the voltage is shown as 230/400 or 400/690.

I have recently seen this confusion at the place I work.

With dual voltage motors the 230v relates to delta. The 400v relates to star.

If this motor is connected to a 1ph inverter which provides 3ph out then you would connect in delta as the inverter outputs 3ph 230v (You cannot put more out than you put in, effectively). If you connect this motor to a Direct on line (DoL) starter then you would connect in star as the higher voltage of 400v is applied.

With a 400/690 or 415/720 motor... using a DoL starter the motor would be connected in delta.

 

[silva.foxx]

See http://www.electriciansforums.net/c...l/43772-how-know-whether-wire-star-delta.html

Is this what you mean?

 

[collinsd70]

Silva.

Thanks for your help, the reason I have become confused is because of the various voltages displayed on the motor rating plates, does the voltage listed on the motor refer to line voltage or phase voltage?.

 

[silva.foxx]

People, including myself, often incorrectly use the term phase.
The 3 lines are L1, L2 and L3. These have to go somewhere for a potential difference to arise.
Therefore the 3 phases are L1-L2, L2-L3 and L3-L1. So in effect your line voltage is your phase voltage (until you introduce a neutral).

If you have two lines to a device, say L1 and L2 at 400v, then this is 1 phase 400v, not 2 phase 400v.

You will connect the motor depending on the potential difference between each pair of the three lines. Don't get too scientific. If you have 400v between L1 and L2 then you have 400v. You've got what you've got.

As said... if you have a 400v DoL... you will connect the motor in the connection relating to 400v on the nameplate.

 

[Knobhead]

Don’t know if this helps or confuses the situation.
View attachment 8951

 

[collinsd70]

I understand the potential difference between phases and phases to neutral (ground) but why is wiring in star considered as high voltage and delta considered as low voltage?

 

[silva.foxx]

I think most of the confusion is from comparing the star and delta connection with the star and delta of supply systems.

What must be understood is that the 3ph AC motor is a machine connected to a supply. The motor contains three windings that can be connected in two ways. We are talking about the way these windings are connected to accept the voltages supplied and carry the design current. The motor windings are consumers. Each winding is created for the lower voltage, the line voltage across one winding is the lower voltage (as connected in Delta). To use the higher voltage you must connect two of the lower voltage windings in series (as star). If you placed 400v across a lone 230v winding then you risk burning up the winding or tripping the supply.

View attachment AMP_REF_DualVoltMotorConn.pdf

I'm sure there is a better Layman's explanation as I know that I'm not answering your question efficiently.

 

[collinsd70]

thanks for the attachment, so by connecting 400v in star you are only putting 230v across each winding and in delta you would be putting 400v across each winding?

so you could wire it in either star or delta depending on the rating of the windings?

does the voltage on the nameplate mean the supply voltage (phase to phase) or the voltage each winding is designed to carry?

 

[silva.foxx]

... so by connecting 400v in star you are only putting 230v across each winding and in delta you would be putting 400v across each winding?

so you could wire it in either star or delta depending on the rating of the windings?

does the voltage on the nameplate mean the supply voltage (phase to phase) or the voltage each winding is designed to carry?

Yes to the top question. 400/1.732 = 230.9 ~ 230v

On dual-voltage motors yes, not too sure on single voltage motors... depends on voltage and connection... you don't really want to vere away from the nameplate. If you have a 400v Y connected motor with the plate stating this fact, then you don't really want to cause confusion by fitting it on the end of a 1ph-3ph inverter and putting the links in delta.

Voltage on nameplate relates to supply voltage.

 

[Knobhead]

I’ve only ever come across motors like these a few time in 35 years. I didn’t like working on a 660V system at all. Because everything looks like a bog standard 400V system you tend to forget it’s got that extra Umph behind it.
Normal star connected motors are the realm of HV motors 3.3KV and upwards. You tend to notice the difference then.

View attachment 8996

 

[laurence6202]

Most 3ph motors > 3kW are nameplated 400V delta/660V star in South Africa (and I think in the rest of the world). That means the windings can cope with and will deliver 3kW in 400V delta or 660V star. So using the arrangement below running it 660V star would give 17A winding current and 380V winding voltage the same as the delta connection, but with a reduced 17A line voltage as oppesed to the 30A line voltage in delta.

However the more common application for the star is to use it as a starting arrangement at 400V - with the figures in black on the left > therefore with only 10A line current as opposed to the 30A under delta. Of course the starting torque is also less, but this is OK for pumps where at lower speeds the torque required is less, so the star delta starters work well for loads that have low starting torque.

The nameplate is stating what the motor/windings are capable and designed to cope with, not necessarily how it is used.

If you had a 660V 3phase power supply you could use this same motor at the same power rating, but you would not have the option for lower starting current using the star delta arrangement, as the motor would have to run permanently in star.