Re: Motor trials and tribulations (long)

Christopher Tidy wrote: 
Jon Elson wrote:

<snip>

I've never seen a motor of this size which uses a delta connection for 415 V before. In fact, I don't think I've seen a motor below 10 hp which uses a delta connection for 415 V. But I've never seen a motor of this size with a fan cowling which weighs 20 lbs, or a rotor which is fabricated from individual pieces of stock before, so I guess I shouldn't be surprised. I guess in 1931 there may have been less export, and it seems LSE specialised in large motors, so maybe a delta connection was their standard?

The data plate (earlier message) says 400 V, but seems to also say
"Star Delta" on the connection line.  If you have 6 wires, you should
be able to (re)connect it in the star configuration for 230 V.


That's what I thought, but I'm puzzled by the fact that the motor was delta connected in its original factory installation. I would have expected a motor of this size to be star connected for 415 V operation, but I think it's unlikely that a factory in the UK would have a supply with a 240 V line-to-line voltage. The only thing other thing I have to go on is the winding resistance (6.5 ohms) but I can't see how this is a help.

OOps, I think I got this backwards! I should have said :
If the STAR voltage is 400 V L-L(from the plate, not 415) then the actual voltage across the individual windings would be 400/1.732 = 231 V. (1.732 is the square root of 3.) So, if this motor only has 3
windings, your only connection choices are delta or star. (Most motors
in the US are Wye only, with nine wires. There are 2 windings for each
phase. One set has an inaccessible center point, the other is wired
totally separately, so it can be connected in series with the other set
for high voltage, or parallel for low voltage. This is generally for
240 or 480 V systems. In the special case of 230/400 or 240/415 V systems, you have the additional trick of the Delta/Wye conversion.

Jon

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