I am trying to wire a new motor to a variable frequency drive (VFD) and having some trouble figuring out how to wire the motor to the VFD. The motor is a 1 HP, 3-phase, 208-230/460 volts, and I'm wiring a 120 VAC supply voltage into the VFD, which steps that up to 3-phase 230 V. I have included a picture of the wiring diagram attached to the motor below.

Motor Wiring Diagram from motor.

Based on that diagram and the fact that the motor will be operated at 230 V, I gather that wires 9 and 3, 8 and 2, 7 and 1, and 4 and 5 and 6 all need to be wired together - giving me 4 separate wire bundles. The VFD has wire terminals for U, V, and W, so it looks like I have 3 terminals and 4 wires, so I'm not sure which wires go to the U, V, and W terminals on the VFD or what to do with the extra wire bundle. I have included an image of the motor wiring diagram from the VFD manual below.

Motor wiring diagram from VFD manual

I was thinking that the extra wire bundle would be a ground, but there is also a green screw inside the wiring box on the motor that I'm thinking is where I connect the ground wire (and connect the other end to the protective earth PE terminal on the VFD), but please correct me if I'm wrong. With the extra wire, I'm also left wondering how to differentiate between U, V, W and whatever the 4th wire is. In the VFD motor wiring diagram, I also can't figure out what the Physical Earth Shielding (PES) is - is it just saying to use shielded wires and attach the shield to ground? The model numbers for the items are: Baldor CEM3546 Motor, and Lenze SMVector ACtech ESV751N01SXB. So my specific questions are:

  1. What is the 4th wire?
  2. Where do I connect the 4th wire?
  3. To ground the motor, do I attach a wire between the green nut and the PE (protective earth) terminal on the VFD?
  4. By connecting the ground wire on the incoming 120 V single phase to that same PE terminal, is the motor grounded?
  5. Is the PES just another way of saying to use shielded wires, with the shielding being attached to ground? Is this really necessary?
  • \$\begingroup\$ If at all possible (and it usually is, unless you need portability to a high degree) feed the VFD 240V (if you have 120V, you have 240V available in almost all cases.) Metal conduit is one handy form of shielding. \$\endgroup\$
    – Ecnerwal
    Dec 20, 2015 at 10:34
  • \$\begingroup\$ Where have you got the information that VFD steps up the voltage to 230V. I never seen such VFD. \$\endgroup\$ Dec 20, 2015 at 10:44
  • 2
    \$\begingroup\$ There are VFDs (typically 1 HP Max for practical reasons) that will do that. You'd be unlikely to see them outside of countries with 120VAC as the normal supply. All or nearly all of them will also take 230VAC input. e.g. automationdirect.com/static/specs/gs2drive.pdf \$\endgroup\$
    – Ecnerwal
    Dec 20, 2015 at 17:50

2 Answers 2

  1. and 2.: The terminals 4, 5 and 6 are not to be connected to your VFD. This is indicated by the drawing, as there is no wire "leaving" the interconnected terminals. i.e. those are bridging terminals to build the correct wiring of all windings of the motor.

  2. probably yes. Normally PE connectors should be designated by green/yellow, but I've seen it often as only green.

  3. Depends from your wiring scheme on your premises. If you have combined neutral/protective-earth connect that to your VFD and your motors PE. If you have separate neutral / protective earth you have to connect the PE to your motor's PE terminal.

  4. The PES is for EMC-purposes. It starts at your motor's housing and is connected back to the shielding cabling between the motor and the VFD. It will reduce electromagnetic interference. For this purpose you have to use shielded cables. Is it necessary? Probably you won't get any problems with not shielding it. But others may. So in my eyes not applying the appropriate measures for EMC is like letting my dog shit onto the green of my neighbours. So: just do it and use shielded wires. According to the datasheet your motor cable should be low capacitance (<75 pF/m wire-wire, <150 pF/m wire-shield).


This doesn't answer your question which has been well answered by @Ariser, but is relevant to the subject.

Be careful with long cable runs between your VFD (variable frequency drive) and the motor. My (recent) understanding is that it exacerbates problems with bearing erosion caused by axial shaft voltages. The problem can be reduced by using insulated bearings and the correct cable type.

SKF have a good article - Prevention of electric erosion in bearings which goes into some depth.

The asymmetric flux distribution inside the motor induces an axial shaft voltage, which further leads to a low-frequency circulating current flowing through the bearings. Asymmetric, non-shielded motor cabling also generates bearing current. These “classic” bearing currents are a problem especially for large motors with low numbers of pole pairs (e.g., two-pole motors). They have larger flux asymmetries than small motors or motors with many poles. Some countermeasures have been developed – mainly different means of insulation. But better design of motors and cables has also helped to minimise these problems.

In the 1990s, the increasing use of variable speed drives (VSD) has had a dramatic effect on the number of identified bearing failures. The reason is that the power-switching semiconductor devices used in frequency converters have changed from thyristors to gate turn-off transistors (GTOs) and further to the insulated gate bi-polar transistors (IGBTs) that dominate the VSD market today. These IGBTs are used to create the pulse width modulated (PWM) output voltage waveform and thereby improve efficiency and dynamic performance of the drive. Sadly there is no advantage without compromise. So apart from the classic voltages and currents generated by the motor itself, new effects have been observed when the motor is supplied from a PWM converter (frequencies of 3 to 12 kHz, depending on the power range). It has been discovered that the bearing damages are now caused by a high frequency (5 kHz – 10 MHz) current flow that is induced by these fast-switching (100 ns) IGBT semiconductor devices.


simulate this circuit – Schematic created using CircuitLab

Low and high voltage motor connections.


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