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 as way to switch between them.
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.
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 get 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.
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) 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.
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.