1
\$\begingroup\$

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.

\$\endgroup\$
  • \$\begingroup\$ 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. \$\endgroup\$ – Marko Buršič Nov 17 '18 at 9:21
  • \$\begingroup\$ @MarkoBuršič I think it's the other way around; star/wye is 400V (phase-ground), and triangle/delta is 690V (phase-phase). \$\endgroup\$ – marcelm Nov 17 '18 at 9:53
  • \$\begingroup\$ @marcelm: Marko is correct. Have a look at my answer. \$\endgroup\$ – Transistor Nov 17 '18 at 20:52
1
\$\begingroup\$

schematic

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 \$.

schematic

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.

\$\endgroup\$
  • 1
    \$\begingroup\$ The 'American' letter? Well okay then transistor. \$\endgroup\$ – sidA30 Nov 17 '18 at 11:37
  • \$\begingroup\$ Ha! It's "star" on this side of the Atlantic. I've never seen "wye" as a word for 'Y' except in American literature. \$\endgroup\$ – Transistor Nov 17 '18 at 11:39
  • \$\begingroup\$ 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. \$\endgroup\$ – Claws Nov 18 '18 at 4:24
  • \$\begingroup\$ 480*sqrt(3)~831V? \$\endgroup\$ – Claws Nov 18 '18 at 4:30
  • \$\begingroup\$ 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. \$\endgroup\$ – Transistor Nov 18 '18 at 7:59

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.