Replacing a 3-phase speed-varying drum switch with a VFD?

Question

I am working on a stock feeder / power feeder (used with woodworking equipment). It has an 8-position drum switch to control both fwd/reverse direction as well as vary the speed. The ON positions of this switch are:

• Fast - forward
• Fast - reverse
• Slow - forward
• Slow - reverse

as well as 4 OFF positions.

The motor itself has 6 leads (as shown in the diagram) which are all switched. My understanding is that the speed control function of the switch works by changing the topology of the motor coils with respect to each other and to the 3 phase inputs.

I would like to replace this switch with a VFD, both to run the machine on single phase power and also to provide more granular speed control.

The essence of my question is: what's the best way to wire the motor-side of the VFD? Is the "slow" or "fast" wiring topology more appropriate? (Or, would either work?) Assuming that is determined, what is the correct wiring combination of motor leads that would achieve said topology? Just from studying the diagram below its not clear to me what that is.

I could perform resistance tests or other inspection of the motor or existing switch, if that information would be useful.

Here's the control switch:

Each of the non-animal positions are OFF.

The motor is integral to the machine... its not a separable standard mount motor. It has no nameplate. However the plastic switch housing (also integral to the machine) has this:

Manufacturer's wiring diagram:

I've compared the diagram to the actual machine and its pretty accurate. Annoyingly the leads on the motor are all labelled with the same number (all red leads are "4" and all yellows are "2"!) but I can distinguish them with the help of the diagram.

FYI I originally had it on a static phase converter but it ran quite hot, that's one reason for the VFD idea.

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It may help someone else trying to do the same thing to know that this was a Powermatic model PH-3 (aka PF-34). Maybe it will turn up in web searches.

Answer 1

The motor in question appears to be a two-speed, one winding, variable torque motor. When operating at the low speed, the two sections of the winding are essentially connected in series and form 4 poles. When operating at the high speed, the two sections of the winding are connected in parallel and form 2 poles. The parallel connection results in a higher voltage per winding section. As a consequence, the motor can provide more torque when operating at the high speed.

If the high speed connection is used for VFD operation, the motor will have the same capability as it has now for full-speed operation. At reduced speed, the motor will be capable of providing the rated full-speed torque at reduced speeds. Because the self-cooling capability of the motor is reduced at reduced speed. Continuous use of the full torque capability at reduced speed is likely to overheat the motor.

If the low-speed connection is used with a VFD, operating above the low speed will not be possible without operating above rated voltage. That would probably not be a feasible mode of opreation.

It would probably be possible to leave the switch in service for VFD operation, but it could only be operated when the VFD is shut off. There is probably not a good reason to use it.

Image from Smeaton, Motor Application and Maintenance Handbook McGraw Hill, 1960

Answer 2

The answer by Charles Cowie was absolutely correct. Having finished the project I thought I should separately add some details specific to the machine I was working on.

The 6 motor leads were divided into yellow & red triplets, marked 2 and 4 respectively (corresponding to the 2– & 4– pole configurations, I believe). I first labelled them individually based off the wiring diagram in the manual. e.g., yellow #s 9, 11, 24 and red #s 13, 15, 18.

I also marked the drum switch terminals for easy reference.

3-phase inputs were colored brown, blue and black.

I then disconnected all wires from the drum switch and successively tested each switch position to see exactly what wiring combination it produced. These positions are labelled based on the following diagram:

Switch Setting	Red 13	Red 15	Red 18	Yel 9	Yel 11	Yel 24
Turtle-Left	Brown	Blue	Black	Open	Open	Open
Turtle-Right	Blue	Brown	Black	Open	Open	Open
Rabbit-Left	Conn	Conn	Conn	Brown	Blue	Black
Rabbit-Right	Conn	Conn	Conn	Blue	Brown	Black

('Conn' meaning connected to all the other reds).

This examination of the switch confirmed the other answer. It also confirmed that the forward/reverse direction change is just a matter of swapping 2 of the input leads, as expected.

I completed the wiring of the motor using one of the "rabbit" configurations having entirely removed the drum switch. (So that "forward" direction on the VFD matched the sense of how the machine is supposed to operate normally).

Answer 3

The Powermatic PH-3 feeder is driven by a 2-speed, 1 HP motor.

The wiring chart on the switch housing shows that it's a constant-torque

Dahlander-connected motor.

Here's the schematic / wiring diagram.

Use of a VFD, to control the motor, should pose no problems.

The motor is to be configured for 2 poles and the VFD for constant V/f.