How can 400V & 690V on e-motors result in the same power

Forgive my ignorance, but how can two 3-phase ratings on the same motor result in the same output power?

The one I have is rated for 0.7kW in triangle for both 400V and 690V.

• delta/star or triangle/star configuration. You can't connect this motor in triangle on 690V, but star. On 400V you connect it in triangle, power is the same. – Marko Buršič Nov 17 '18 at 9:21
• @MarkoBuršič I think it's the other way around; star/wye is 400V (phase-ground), and triangle/delta is 690V (phase-phase). – marcelm Nov 17 '18 at 9:53
• @marcelm: Marko is correct. Have a look at my answer. – Transistor Nov 17 '18 at 20:52

simulate this circuit – Schematic created using CircuitLab

Figure 1. Connection possibilities.

• On a 400 V phase-to-phase supply the motor windings are connected in delta (from the Greek 'D' or Δ).
• On a 690 V phase-to-phase supply the motor windings are connected in star or wye (from the American letter 'Y').
• From trigonometry you can show that the orange lines in Figure 1b are $$\ \sqrt 3 \$$ times the length of the black lines. $$\ 400 \times \sqrt 3 = 692.8 \$$.

simulate this circuit

Figure 2. Connection options.

How can two 3-phase ratings on the same motor result in the same output power?

In each case the voltage across each winding is 400 V so the same current will flow in each winding and the same power will be produced.

• The 'American' letter? Well okay then transistor. – sidA30 Nov 17 '18 at 11:37
• Ha! It's "star" on this side of the Atlantic. I've never seen "wye" as a word for 'Y' except in American literature. – Transistor Nov 17 '18 at 11:39
• Ah of course. I misread the name plate on the motor. Indeed it says 400/690 Δ/Y. I confused it with being rated for both 50/60Hz. Additional question: Since it is 400/690 for 50Hz and 480V/60Hz in Δ - what would the Y voltage @60Hz be? It isn't mentioned on the name plate. – Claws Nov 18 '18 at 4:24
• 480*sqrt(3)~831V? – Claws Nov 18 '18 at 4:30
• I've never seen 830 V mentioned anywhere. Maybe it can only be run in delta on a 60 Hz, 480 V supply. Don't forget that insulation breakdown voltage becomes a problem at some point. – Transistor Nov 18 '18 at 7:59