Wednesday, April 12, 2017

Mill: Speed Control

NOTE: If you're not comfortable working with electrical component and AC power, then do NOT attempt this modification. Touching the wrong thing can potentially kill you.

My mill is a Grizzly G8689 with the standard brushed motor. All its wires are numbered, I assume for ease of assembly, so I'll refer to the wires by their numbers. I assume the wiring is the same for all brushed motor mills, but your mileage may vary.

The stock controller. You can see the numbered tags on the wires and the terminals.

The X2 mill's DC motor speed controller is the weak link and not the motor itself. Short of doing a treadmill motor/controller upgrade, you can upgrade just the speed controller and see good gains in both torque and speed. Although the stock controller is a PWM unit, it's not a very good one and is quite limited. It's also a much easier and potentially cheaper upgrade than a treadmill motor.

The common DC controller you'll see in searches if the KBLC-19PM which is just a custom version of the KBIC-120 rated for 125VDC out instead of the standard 90VDC. The KBIC-120 is only rated up to 0.5 HP without a heatsink and the X2's motor is actually 0.5 HP or less, regardless of what the sticker on it says. so you can safely use the KBLC-19PM or the KBIC-120 using the 0.5 HP resistor (0.25 ohm, PN#9841). You can also use the KBMM series of controllers, but the cheaper KBIC works fine. Since I was able to get the best deal on it, I went with the KBIC-125B, which is a KBIC-125 uprated from 90VDC to 125VDC. Aside from the KBIC itself, I also needed a 5k potentiometer. Forunately my KBIC come with a motor fuse, otherwise I would have needed to add one (the mill should use a 7.5A fuse).
KBIC-125

The stock controller is screwed to the back of the mill's electrical box, with a small accessory board next to it. The accessory board converts AC to 20VDC to power the fan. To remove the controller loosen all the terminal screws, remove the wires, remove the four screws and remove the board. To make room for the KBIC I also removed the accessory board, turned it, and mounted it to the side of the box using a couple newly drilled holes, some plastic washer, and a pair of pop rivets.

To mount the KBIC I reused one of the vent holes on the side of the box, then measured and drilled a second hole, and using two screws and nuts secured the KBIC's side to the side of the box.

KBIC mounted in box with accessory board mounted on the side.
In this picture it's already been wired.

Inside the small controller box I disconnected the motor leads from the switch and connected them to wires 1 and 2 running back to the electrical box. The 5k potentiometer was wiring in using the old potentiometer's wires with a direct swap over. The old potentiometer also incorporates a switch with three wires going to it (AC neutral), I took those three wires and soldered them all together and heat shrinked them.

The three wires on the old potentiometer which I wired together.

Back in the electrical box I soldered spade connectors onto the wires to make installation and troubleshooting easier. I then wired it up per this diagram:


On the 20VDC accessory board I connected wire #3 to the AC neutral terminal and connected #4 wire with a short jumper to the AC hot terminal. I removed the yellow LED which was wired in series with the accessory board since it was disrupting its operation. Now the fan runs whenever there's power. I don't think the added cooling is needed, but I didn't see a reason not to.

The emergency cutoff switch is still used and turns AC on and off before it connects to any switch or board. The AC fuse in the diagram isn't used as the mill's existing fuse is used instead.

The "Inhibit" function on the KDIC allows the motor to be turned off electronically. I decided to take advantage of it, as it allows me to set a RPM, and then use the switch to turn the motor on and off without having to reset the speed every time like on the stock controller (note: the switch's direction is now reversed so you might want to relabel it). The KBIC also gave me much finer control over the motor, which was a pleasant surprise. The stock board was frustrating to try and adjust, since the adjustments frequently overlapped. However, on the KBIC they're all nicely separated and discrete.

Once the KBIC is connected, I needed to adjust it for my motor specifically. The first thing I did was adjust Max Speed so the output was 110VDC at full speed. I then adjust the Min Speed so the motor stopped just before the speed knob hit zero. The factory Accel setting was good, but could be adjusted if the motor either spun up to speed too fast or too slow. I left the Current Limit at it's factory value of 1.5x the HP resistor. I also left the IR Compensation as is, but might look at it later if I find the motor RPMs dropping under heavier load; it allows the KBIC to compensate (to a degree) for load to keep the RPMs steady.


However, a couple weeks after upgrading to the KBIC I again upgraded to a KBWS which uses PWM to control the DC voltage.There are two commonly used ways to power a DC motor from AC voltage: SCR and PWM.In short, an SCR will essentially chop off half the AC input and feed that to the motor, with the speed being controlled by where in the AC waveform it starts to feed it to the motor. When the motor is set to full speed this is fine, but at anything less than full speed the motor won't be getting the full voltage. While with PWM the AC is rectified to DC, and then the DC is very rapidly switched on and off to control the speed, so the motor sees rapid, brief inputs of full voltage.

The KB PWM control I use produces a peak voltage of 160VDC which is very rapidly switched on and off to produce a lower average voltage. I was worried about the 110VDC motor being hit with 160VDC, but after research I found if the frequency is high enough, then it doesn't really matter (assuming the peak voltage isn't absurdly high). In fact, a high frequency PWM drive will actually allow the motor to run cooler while producing more torque.

In real life this means SCR controllers are cheaper, more robust, and able to handle more current, but are louder (they produce an AC buzz), the motor runs hotter, and the brushes and commutator have a reduced lifespan. PWM controllers are more expensive, have lower current limits, are somewhat easier to break, but they run very silently and the motor runs cooler with a longer brush and commutator life.

Since both the KBIC and KBWS use the same chassis, it was very simple to swap them out. Please note newer KBWS controls come with a shorter capacitor which doesn't require modification of the control box, while the older KBWS would need modification to accommodate their height. 

KBWS installed in the mill.



The quieter operation was immediately noticeable and very welcome. After an extended run the motor was still cool to the touch, unlike with the KBIC. Ultimately, I think it was a worthwhile upgrade.

Both the mini mill and mini lathe mostly use PWM controllers stock. However, the stock controllers are very limited and prone to die. If my choice was the stock controller or a KBIC/KBLC I would go with the KBIC/KBLC, even though they're SCR simply for the significantly greater performance and adjustability they offer. However, if you can get a KBWS for a good price, that's a better the way to go.

A quite note, KB Electronics makes a cheaper version of the KBWS called the KBWD. However, while the KBWD will work, it doesn't have an electronic stop like the KBWS has. On a mill/lathe the electronic stop is very handy, since you can set a speed and then use it to turn the motor one and off without disturbing the speed setting. That feature is useful enough to me to specifically seek out the more expensive KBWS.

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