Adding a VFD to an Odd-Duck Milling Machine (Enco 100-1525) Motor

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Fig 1.VFD box wiring

A while back, I got a nice deal on an early 90’s Enco 100-1525 milling machine. It’s a Chinese 2/3 size Bridgeport copy with an 8″x32″ table. This machine has a 3-phase motor, which I consider a plus because it means I can have knob-controlled speed by adding a VFD (Variable Frequency Drive).

Adding a VFD is usually not too complicated with typical two-speed three-phase motors, but this machine turned out to have a 4.3-to-1 speed ratio between the two ranges (unlike the typical 2-to-1 ratio). With a 2-to-1 ratio, it’s usually fine to run the motor only on the high speed range and then use the VFD for speed reduction. There is a small torque reduction at low speeds when doing this, but for home shop use, it’s fine.

But with the Enco mill motor’s 4.3-to-1 ratio, if I only drive the high speed motor coils and then try to dial down the speed to cover the slow range, the torque loss is too much. This meant I had to keep BOTH motor speed ranges and devise a way to switch between them.

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Oddball 3-phase motor with 3200 RPM on the high range and 740 RPM on the low range, a 4.3-to-1 speed ratio. Huh?

I started by tracing through the original drum switch on the mill. It had 5 positions: FORWARD HI, FORWARD LO, OFF, REVERSE LO, REVERSE HI.

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The motor is a “Dahlander” style:  https://en.wikipedia.org/wiki/Dahlander_pole_changing_motor

which requires some non-trivial connections for each of the speed ranges. In the low range, three nodes ( 1U, 1V and 1W in the above Wiki link) get powered in a “delta” connection. In high range, the 1U, 1V and 1W nodes get shorted to each other while the 2U, 2V and 2W nodes are powered in what is called a “double star”. Some head scratching was required to decode the old switch. The next step was to figure out how to duplicate the connections in an automated way.

It wouldn’t be silly to ask why I didn’t leave the drum switch as is and just power the mill from the VFD. The reason is that VFDs do not like to have switches between them and the motor. If I NEVER switched speed ranges while the mill was running, nothing should break, but I wanted a fail-safe idiot-proof design that did not depend on me always being on the ball. That leads to the following caveat.

I’D LIKE TO INTERJECT A WARNING HERE: THIS HOOKUP VIOLATES A BASIC RULE OF MOST (MAYBE ALL) VFD MANUFACTURERS. THAT IS NO SWITCHING DEVICES SHOULD BE CONNECTED BETWEEN THE VFD AND THE MOTOR. THE REASON FOR THIS IS THAT INTERRUPTING POWER TO A MOTOR WHEN A VFD IS POWERING IT CAN CAUSE A BIG INDUCTIVE SPIKE THAT CAN DAMAGE THE VFD.

I AM VIOLATING THE ABOVE RULE AND MOST LIKELY VOIDING MY VFD WARRANTEE. I’M INSERTING CONTACTORS BETWEEN THE VFD OUTPUT AND THE MOTOR, HOWEVER I HAVE THE VFD PROGRAMMED TO NEVER CHANGE THE CONTACTOR POSITION WHEN THE MOTOR IS MOVING. I AM CONFIDENT THIS PROVIDES SUFFICIENT PROTECTION, BUT IF YOU ARE UNCERTAIN, DO NOT DO THIS.

I did not violate the above rule lightly. I simply could not think of a cleaner more reliable way to accomplish the two speed ranges and prevent operator error. Note that this is only possible because the selected VFD (Hitachi WJ200) can be programmed and is able to store and select between two sets of motor parameters. I’m treating the two motor speed ranges like two separate motors. Not all VFDs are capable of this.

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Fig 1. Wiring diagram for 2-speed range control of 3-phase motor using a single VFD

Link to PDF of Fig. 1: vfd-2-speed-motor-wiring

The key to the motor connection is the use of three contactors. In Fig. 2, one (CONT_1) is activated for the low speed range, while the other 2 (CONT_2 and CONT_3) are activated for the high speed range. For added safety, the contactors are driven from opposite sides of the VFD’s internal relay (AL1 and AL2) so that it is not possible for high and low speed to be activated at the same time.

Internal programming of the VFD (Hitachi WJ200-015SF) prevents any range changes while the motor is spinning. If the HIGH/LOW speed range switch is flipped while the motor is running, the motor remains in its current speed range until it is turned off and spins down. When the motor stops, the speed range is then changed.

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VFD in electrical box
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Machine control box on mill head. Clockwise (sorry, no labels): tachometer RPM readout, speed knob, “HI SPEED RANGE ON” indicator light , toggle switch for HIGH/LOW speed range selection,  and FORWARD/OFF/REVERSE drum switch. The drum switch is surplus, hence the “HAND” label for the reverse position. Maybe I’ll make proper labels someday.

All the mill controls are housed in a small box attached to mill head. This box contains only low voltage controls that go to the VFD. It contains is no high voltage. High voltage is restricted to the VFD box.

This setup has been running flawlessly for about 5 years.

9 thoughts on “Adding a VFD to an Odd-Duck Milling Machine (Enco 100-1525) Motor

  1. Len,

    I’d love to do the same thing to my identical Enco mill. Can you share with me a parts list for the rpm display, the P/N for the enclosure, the power supply and the rest of the components.

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  2. I to would like to do the same thing on my enco mill. More information would be helpful. I have been trying to figure out the VFD programming but I am lost could you make your program available?

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    • Hi John – Unfortunately I didn’t write down what I did to program the VFD. I remember it *was* kind of painful to read to manual. Do you have the same model VFD that I used? Also, do you have the a weird motor like mine where high/low is not 2/1? If you have a normal 2/1 dual speed 3-phase motor, you don’t need to go through the hoops I did. If you have the same VFD as me, I think I remember finding a site where someone wrote a more readable set of instruction for programming. If you do have the same model, I could try to find that.

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      • I have the same Enco 100-1525 mill with the same motor. It isn’t running as I haven’t bought a VDF yet. I have been looking at different VDF’s for the two motor control but haven’t found one that I could get my head around. If you could find that site you mentioned I would appreciate it. I will look also and spend more time reading the manual for the Hitachi before I buy it.

        Thanks

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  3. Awesome write up. I have the same machine and motor. You just saved me a ton of head scratching. Curious, Did you go with the 1hp Hitachi or larger rated drive? Seems the amps are right on the cusp of 1hp.

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    • Thanks. I used the WJ200-015SF (rated for 2HP in CT continuous torque). It might not have been necessary to go that big, but I figured for the $50 price difference, I didn’t want to go through design and build to find out I needed to change the inverter later.

      The other thing I remember was that programming was a pain. I wish I had taken notes on the programming, but unfortunately I didn’t. If I ever need to go back in, I’ll have to relearn the programming from scratch. I do remember there were some online programming write ups that were much better than the manual, but I don’t have links anymore. Hopefully Google will get you there.

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  4. Thanks Lens, I went ahead and ordered the 2hp. Kinda weird, Marshall Wolf called Hitachi and their engineer said to go with the 3hp. Did not make much sense to me. Anyways, on your main box I see a toggle switch that looks to have the main power to it, is that just a 240 volt toggle so you can power down the whole unit? Not a toggle that operates a magnetic contact?

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