I’m rewiring a Desktop CNC controller and came across the attached info on one of the internal modules (variable frequency drive).

The two 220v mains power inputs are labelled R / S / E where I was expecting L / N / E or similar for live (active), neutral, and earth.

What does R / S stand for and is this a standard notation that I haven’t seen before, or something else (perhaps a translation issue)?

VFD schematic

  • \$\begingroup\$ Links to the actual documentation would be helpful. \$\endgroup\$ Apr 15 '18 at 18:47

"R,S,T" are arbitrary sequential identifiers with no direct meaning (as opposed to "E" meaning "Earth," for example.)

They correspond to 3-phase power inputs:

  • R = L1
  • S = L2
  • T = L3

Additional info and color conventions can be found in this wiki.

Edit per request:

enter image description here

  • \$\begingroup\$ If you could copy and paste that color info to this answer you may get some up votes. Takes a while to find it. Do a screen shot and crop it. \$\endgroup\$
    – user105652
    Apr 15 '18 at 22:48
  • \$\begingroup\$ @Sparky256 There ya go. Just did a quick copy/paste and adjusted the formatting a bit. Hope it's helpful. \$\endgroup\$
    – Phil C
    Apr 15 '18 at 23:19

The documentation clearly shows 2 phase and 3 phase inputs being R,S and R,S,T but clearly different model numbers.

...so I'd say the controller is probably using US 120/240V consumer supply (and here using 240V as R,S) split phase.
Note here that the two active wires are not 120deg separation but are 180deg separation. So it is NOT 2 phases of a 3 phase supply.

The first diagram shows the input as single 220V input, so this should work using either split phase (240V) in the US or as UK/Aus/etc style 230/240V single phase line if you have an E100S.
It should work for 120/208V 3 phase only if you have an E100T.


When you are using only single phase AC, connect the line and neutral to R and S, in either order.

R, S, and T are referring to three phase AC input. There is no neutral or ground. All three lines are "hot", just 120° out of phase with each other.

  • \$\begingroup\$ Are you saying that the OP should use just two phases (R, S) of a three-phase supply? Does that mean that a single-phase supply would be unsuitable? \$\endgroup\$ Apr 15 '18 at 18:47
  • \$\begingroup\$ @AndrewMorton. Any two wires of a 3-phase source is considered single phase. The voltage depends on if the source was 3-phase or split-phase or L-N. Only the voltage between R and S counts. \$\endgroup\$
    – user105652
    Apr 15 '18 at 18:56
  • \$\begingroup\$ @Sparky256 I think I have found the documentation at www.automationtechnologiesinc.com/wp-content/uploads/downloads/2017/02/NOWFOREVER-MANUAL-E-100.pdf which says the S in E100S means single-phase, so the (R, S) would be left over from the (R, S, T) of the E100T three-phase series. ICBW. \$\endgroup\$ Apr 15 '18 at 18:58

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