A nice solution to this issue is to look around on ebay & find a deal on a KBCC-125R drive.  They're old school (compared to servo drives, vfd's, brushless, yada-yada) and are very reliable, being a industrial product.  They come with built-in circuitry & relay to allow instant reversing & dynamic braking plus easily configurable for different HP motors, PM magnet or wound field types.  I have one on my lathe & really like it.  I didn't have the patience or knowledge to do a bullet-proof relays circuit like shown above & kept looking 'til I found one for $65 NIB!

The on-board reversing function is also available in other later KB drives (PWM & others) but I like the older SCR types 'cuz they're usually cheap but good.

Here's the manual: http://www.kbelectronics.com/manuals/kbcc_manual.pdf

Those old-school drives are OK if the motor is rated for an SCR drive, but they can make the motor hot if the motor is not designed for them. You get a LOT of 100 Hz or 120 Hz ripple on the power, and some of this comes through.

On the other hand, the more modern MOSFET drives like the KBWT-26 leave the motor a whole lot cooler and quieter - but I have to admit they can radiate interference half-way to the moon. KB should have put a LARGE dual-winding L-filter on the output.

Both designs seem to have somewhat arcane interfaces, which may be fine for manual controls but are a pain for CNC interfacing. The KBWT unit analog input controls are at -100 VDC!

Cheers
Roger

Both designs seem to have somewhat arcane interfaces, which may be fine for manual controls but are a pain for CNC interfacing. The KBWT unit analog input controls are at -100 VDC!

Hi Roger, I was going to defer to your superior electronics training & keep my mouth (keyboard) shut but I have to voice my opinion.  I know my electronics knowledge is sadly lacking but I do read about this stuff & have been able to make my hacked-together stuff work pretty well.  I just don’t want someone down the road to stumble onto this thread searching for spindle drive info, take your statement as gospel & go away misinformed.

I have never owned or used a KBWT-26 but just couldn’t believe that KB would make a drive has a “somewhat arcane interface” that is “a pain for CNC interfacing.”  I took the trouble of Googling the drive’s manual & found the following:

“Signal Input: The drive can be operated with a 5 kΩ Main Speed Potentiometer (supplied), an isolated 0 – 5 Volt DC analog signal, or an isolated Pulse Width Modulated (PWM) signal from a microprocessor.”  To me that doesn’t sound like a pain.  Granted, one does have to observe the “isolated” instruction without fail.  Don’t ask me how I know!  “Note: If an isolated signal is not available, an optional signal isolator must be installed (KBSI-240D or equivalent).”  I picked up 1 of these on ebay for $30 and solved my isolation problem. 

As to the old school KB SCR drive’s propensity to “make the motor hot if the motor is not designed for them” both the minilathe’s drive & the treadmill motor’s MC-68 drive are both SCR types so the motors will be fine as far as heating is concerned.

I hope I don’t come across as argumentative; just trying to be factual.   I was wrestling with the exact issues as the OP with my ORAC conversion a few years ago & the KBCC-125R was suggested to me and was an immediate revelation.  When necessary, the lathe can be spinning a 6” 4-jaw at full chat in one direction, receive an M5 command & the KB quickly & smoothly brakes the motor to a stop, receives M4 command, relay clicks, motor reverses & smoothly accelerates to the set speed, all in a couple seconds.  I had asked around and was struggling with relays & a big resistor trying to do it myself.  When I stumbled onto the KB, the problem was solved & I was able to get onto the next thing on the list.

Granted, you fellows with solid electronics knowledge can design the circuitry needed to do the same thing using the OP’s MC-68 but I was just trying to provide an easier alternative.

Hi Dickybird

Sounds as though the KBCC is a good match for those motors. Useful information to spread around. And my experience has been that the KB drives have reasonable reliability too - maybe better than some Chinese VFDs. (However, one KB drive has failed on me.)

The arcane interface on the KBWT ... IS arcane imho. Yes, I know what the manual says, and I know what the front end circuitry actually looks like (because I managed to get part of the circuit diagram from KB). The problem is getting enough power out of the KB front end to power an optically-isolated interface **while keeping the response reasonably linear**. If you don't care about a somewhat non-linear behaviour it is much easier. OK, I was being fussy.

I must admit I had not searched on ebay for the KB SI interface. That might be easier, although my interface is reasonably simple NOW. It just took a bit of experiment on the bench - which was part of the fun after all.  :-)

Cheers
Roger

Thanks Roger, as usual all that linear/non-linear stuff zings right over my noggin.

Fortunately I chose commercial equipment & got real lucky on the price.  The KBSI 240D has adjustment pots, the KBCC-125R has adjustment pots and the CNC4PC C-11 has an adjustment pot.  When I finished twiddling them, I have solid Mach spindle speed control in both directions within a couple 10's of rpm from 50 to 1700 rpm.  Way better than I need for 99% of my lathe work.

It does have a teeny cyclic variation that affects threading at times.  With that in mind I picked up an AMC BE25A20AC analog servo drive a while back but haven't  yet taken the time to install it.  Too busy making parts to undo all that hard-earned success and start over!

> It does have a teeny cyclic variation that affects threading at times.
Could you somehow just add mass to the spindle drive? That can be really effective.

Cheers
Roger

Guys, still  traveling but wanted to say thanks for all of the great information. I'll be tackling this later next week. Probably have more questions then.

RT

Could you somehow just add mass to the spindle drive? That can be really effective.

You would think so but it didn't.  It exhibits the same symptom with the same work held in a light ER32 chuck & in a heavy 4 jaw as well.

Besides, I just wanna play with a servo drive when I get time.  I had already mounted a Servo-Tek 7V/1000 rpm tach generator on the motor & wired it into the feedback circuit of the KB.  The variation problem got better but was still there.  That's when I hunted down the AMC servo drive.  The existing tach/gen will be wired into the new drive & it will be configured in the velocity mode.  An encoder might be better but since the tach is already there I'll try it 1st.

(Sorry for the hijack RT, we're just keeping your thread warm 'til you get back. )

Hi DB

That cyclic variation - is there a feedback loop around the system? If it is not properly tuned it can sometimes cause some oscillation. The simplest solution in that case is to back off the P or gain term a whisker.

Cheers
Roger

Okay, back to the original subject  ,

When I started shopping for the relays I got confused by the nomenclature.  My motor at 70% power says 7.5 amps, 95vdc.  The product descriptions like the one below have ratings like the 4th line.  Will this one work? (DC 28V)

Product Name   Electromagnetic Relay
Model   JQX-13F
Coil Voltage   DC 12V
Load   10A AC 240V/ DC 28V
Type   DPDT
Pin Number   8
Size (Approx.)   40 x 27 x 20mm / 1.6" x 1" x 0.8" (L * W * T)
Weight   33g
Package Content   1 x Electromagnetic Relay