460/575 connection 1

[petronila]

Dear All,

Sometimes we find some motors rated 460/575 v connected in Delta for 460 V and connected with a combined wye-delta connection for 575 V rated for the same hp at the same speed at both voltages(i.e. 100 hp 4 poles 460/575 v 60 Hz). See attached.

Thoughts why the manufacturer rated the motor with the same HP for both voltages?

Thanks

Petronila

 

[waross]

That connection looks a little scary, but I may be about to learn something new.
For the sake of example, lets assume a rated FLA of 10 Amps on the delta connection.
Each phase winding will pass 10A /1.73 = 5.8 Amps.
On the wye-delta connection, the wye windings may now be presented with 10 Amps to supply 10 Amps to the delta.
That raises a couple of interesting issues:
What are the actual voltages across the wye and the delta windings?
What is the effect on motor torque and line current when the phase angle of half the windings is shifted?
Solving the wye-delta voltages and currents as a simple resistor circuit will ignore transformer action between the phase shift of the physically adjacent windings.

The original question:

Thoughts why the manufacturer rated the motor with the same HP for both voltages?

If you will accept a WAG as thoughts as to why;
1. Frame size?
2. On one voltage or the other, the safe allowable HP may be greater than the rated HP.
3.It may be easier to market the motor as a single HP motor.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[jraef]

That connection looks a little scary, but I may be about to learn something new.

You and me both...

I have never seen that before, but it kind of makes sense in that by combining the Delta on half of each windings with the Wye on the other half for the 575V connection, you would be dropping the PHASE voltage seen by each set of windings by 1/2 of 1.732 (.866), so the NET voltage to the motor becomes 575V x .866 = 498V, which is within the +-10% tolerance of a 460V motor. That would explain why the HP remains the same; you have a 460V motor that is connected to 575V, but is not USING all of the available voltage due to the connection pattern, so the torque it produces is (nominally) the same as it would in the 460V pattern and since it is still 60Hz, the RPM is the same. HP = Tq x RPM / 5250, so the HP is the same.

The problem I see with this though is that 575V is the utilization voltage for a 600V system, so if you really have 600V at the motor terminals, the voltage across the windings will be .866 x 600 = 519V which is OUSIDE of the tolerance of a 460V motor (506V). So I would be careful using this as a 575V replacement.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[waross]

What concerns me more. not to dismiss your concerns, is the phase shift of half of the windings from star to delta.
Your comments, Jeff?
Petronila,is there any chance that you may give us a picture of a nameplate, or a link to a spec sheet? Thanks.
ps.LPS for showing me something new. grin

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[petronila]

Dear All

No nameplate picture

Petronila

 

[Gr8blu]

petronilla: The reason the machine has a dual voltage rating (in this case, 460 and 575) is that it is intended for multiple NATIONAL markets. 460 is a pretty common "load side" voltage rating for the US, which has a 480 V nominal distribution voltage. Canada, on the other hand, uses 575 V at the load for a nominal 600 V distribution system. To look at it another way - it would be the same as having the machine rated for both 50 and 60 Hz operation for use in either North America or Europe.

Some manufacturers get cute and try to put the same HP into both voltage conditions by messing with the winding (as you've seen in your particular case). Others simply dropout a section of the winding entirely for one of the two voltage levels (think about it like a variable resistor with a tap). And finally, there are those who actually rate the machine based on the applied voltage and connection: which is going to be slightly different at 575 than at 460.

Converting energy to motion for more than half a century

 

[waross]

voltage rating for the US, which has a 480 V nominal distribution voltage. Canada, on the other hand, uses 575 V

Maybe not oranges and apples but certainly different varieties of apples.
US nominal voltage: 480 Volts
Canada nominal voltage: 600 Volts
US motor voltage 460 Volts
Canada motor voltage 575 Volts.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[petronila]

Thanks for the inputs.

Yes, I agree with Bill and Jraef that this Y-D combined connection is scary, and accordingly, with an EASA´s technical article named A closer look at winding conversions by reconnection, it is. The article states the combined Y-D connection is intended for 628 volts rather than 575 V and because the torque varies with the square of the voltage the wye-delta combined connection @ 575 v will result in [575/628]2 x 100 or 84% rated torque so for the same speed, the hp for 575 v should be 0.84 (hp @ 460 v) or hp @ 460 v is derated about 15% for 575 v.

My 5 cents on this is I believe the manufacturer is rating the motors 460/575 v for the same hp because its original design was done 15 % stronger for 460 V (i.e. 115 hp rather than 100 hp) that will produce 100 hp @ 575 v also I believe too the manufacturer is misleading the nameplate data rating a 115 hp motor as a 100 hp.

Tip: If someone requests to redesign a 460 v (delta connected) motor to dual voltage (460/575 v) and if the 460 v starting method is not wye-delta, a better approach will be to redesign the winding 2 Y @ 460 V and 1 D @ 575 using the same 12 leads.

Petronila

 

[waross]

That will drop the 575 voltage at the same Volts per Hz to 531 Volts. That is a lot closer, and no odd phase angle of the windings.
I am still wondering what effect the phase shift from delta to star of half of the windings will have on the numbers.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[zlatkodo]

This is nothing new.
This is just one of the applications (not the main) of so-called “combined YD windings,” the connections described in books more than 100 years ago.
In this particular case, the reason is to obtain a larger range of rated voltages.
Such a connection was later developed by some Russian scientists and recently is in use especially by Chinese manufacturers.
More at:

[URL unfurl="true"]https://winding.wixsite.com/design/post/2016-1-21-combined-yd-windings?hc_location=ufi[/url]

or try google "combined YD windings".