Info from Ah-ha! on BOSS's/Motors/HVdrivers

howdy guys,

Sorry for the delay with this 'tech post'.
Business has been real hectic this week.

Lots of interest in BOSS's here lately, so
I'll start with that.


BOSS Speeds
------------

I've seen posts here recently talking about 30ipm
rapids; and one about 50 ipm even on higher-voltage
drivers....75v I think it was.

I can only guess that they're doing something wrong;
or have bad components; or have clogged way-lube
orifices.

Ah-ha! has done around 500 BOSS conversions over 10
years, and I don't know of a single customer not getting
100 ipm or more; assuming the machine and size-42 motors
are in decent shape. And remember that that's with the
regular Ah-ha! 48v drivers. With old/weak/damaged motors;
even then they're getting 60+ ipm; again on 48v.

One does -not- need 160v drivers to get decent speed.
But more about that below.

The vast majority of BOSS motors were Sigma or Superior
size-42 double-stack; rated for 8-9 amps, 1,100 oz-in.
Most of the BOSS manuals tell you to set the current to
8.3 amps or something like that.

But remember that these were all FULL step drives. In
other words, both coils were full on, all the time; at
8.3 amps each.


Microstep Driver Current Ratings;
----------------------------------

Now, fast-forward 25 years...to today's microstepping driver
There isn't any simple 'on' or 'off' with these. It's a
-sine- wave of current; i.e. it's only at -full- current a
tiny fraction of the time.

Virtually all microstep drivers are rated in terms of
PEAK current. In other words, a 10-amp driver only puts
10 amps into the winding at the very peak of the sine wave.
At that time, the -other- phase is at -zero- current. I.e.
the phases in a stepper are in quadrature; 90 deg apart.

The -RMS- current with a "10 amp" microstepper is never
more than 7 amps; usually less. In measurments we've made,
most of the popular '7 amp' drivers average around 3.5 to
4.5 amps RMS into a stock BOSS motor; depending on how
sensitive they are to low-inductance motors.

Our own Ah-ha! PowerDrivers are rated likewise; i.e. the
12-amp is the peak of the sine wave. One small difference
is that ours does include a real full-step mode, and it will
in fact pump 12 amps/phase into both phases, simultaneously,
all day long. But ya didn't buy microsteppers to run
full-step, right? <g>

Microstepping in general produces only 65% of the motor's
rated torque; bearing in mind that the motors all have
torque -ratings- based on "full stepping, both phases on".

So, take your 1,100 oz-in BP motor, and multiply by .65.

Now it's a 700 oz-in motor. OK, and you're only giving
it half the 8-9 amps required for full rated torque
in the first place. hmmm...now it's only about 350 oz-in !

Imagine someone handed you a couple of little size 34
double-stacks (about 350 oz-in) and said "run your BOSS
with these". You'd say they're crazy, you'll never get
any performance with only 350 oz-in on that big a machine.

And that's why you're not getting the rapids you want.

Want a BOSS to go fast? Give it a good high-CURRENT
driver, and it will. Those boss motors never needed
160v drivers to go 120 ipm in the first place, from the
factory. Most original BOSS controls run 60 to 90v.
As I said, all our customers get 100 ipm or more, reliably,
on safe, cool-running, 48v drivers.

How do we do that?


Motor Speed:
-------------

Motors are inductors. Inductors take time to 'charge';
i.e. when the voltage is first applied, no current at
all flows. Over time, the current flow rises. The
rate of rise of current is determined by only two things
(basically):

- Voltage applied
- Inductance of winding

Further, you can only step the motor as fast as you can
charge, and discharge, those coils. That's what a 'step'
is all about; turning one coil off, and the next one on.

You can immediately see that fast operation requires
either high applied voltages =OR= a low-inductance
motor. BP motors are pretty low-inductance motors
to begin with. Most any motor rated at 8 amps or
more has reasonably low inductance.

They don't need HV drivers in order to perform.

If you're trying to get a 3-amp high-inductance
motor to perform, then yes, you'll have to whack
it with a lot of voltage. But that's not a BOSS
motor.

With modern 12 amps motors, we've got Boss's running
in excess of -200- ipm rapids on a daily basis.

Again, on regular ol' 48 volts.


High Voltage...a good idea?
---------------------------

In addition to our own 48v Ah-ha drivers, we do sell 160v,
and even 320v (!), drives from PacSci. Real nice stuff.

But you know something? They're not usually the best
solution. Here's why; and with special regard to the
home-shop situation. I really wish these issues occurred
to more people during their system-design phase:


- Safety

A 160V 10 amp PWM circuit acts a whole lot like an HF TIG
welder. It doesn't just make a spark...it will -destroy-
an entire connector instantly. I mean turn it into a ball of
expanding plasma!

The high-frequency keeps the air around the arc ionized,
which keeps the arc going, which keeps the air ionized....
and I've seen it eat several square-inches right out of
a glass epoxy circuit board, including the IC's, before
anyone could hit the power switch. Nothing but a gaping
hole left.

Now, if your 6-yr old is dinking around behind your control
cabinet while you're out taking a leak, and yanks a cable
loose; you might just end up attending a really sad funeral.
At the least, the kid could lose some fingers in a real serious
hand-injury; and be pretty damn traumatized too.

I've -seen- kids wandering around behind controls...playing
with the pretty color connectors. I've handled support
calls where the guy calls me back to say he found the problem
...his kid had yanked one of the cables loose; no big deal he
says laughingly...cute, isn't it? yah buddy...real cute.

I think we -all- lose track of our kids once in a while.

Think about it.


Legal:
------

In the National Electrical Code, anything 60v and under is
called "low voltage wiring"; and you can pretty much run
it like you want. However, in most parts of the US, it
is illegal to run any 'machine' or 'industrial' wiring
over 60v outside of metal conduit. And that nice flexible
metal conduit costs several bucks a foot!

It turns out that if you run it outside of conduit, your
homeowner's insurance is voided. If you happen to have
a fire; even a little one, and any fireman or insurance
adjuster sees that wiring; you might be in for a serious
legal and financial problem. Again, something to think about.


Now, given the above two issues, plus the fact that reasonably
high speeds -are- easily acheivable with reasonably safe 50-60v
drivers; then does it make sense to run 160v drivers in a home
shop? For me and my family: no. You gotta decide for your
family.

I expect Ah-ha! will bump our drivers up to 60v one of these
days; but I doubt we'll do any 160v step drivers of our own.
There just isn't the -need- for it. Above 200 ipm, it
starts making more sense to go servo anyway.

Motor Heating:
---------------

One final issue with HV drivers. They really make the
motors run -hot-. The internal hysteresis and eddy
current losses go up as the -square- of the applied
voltage. i.e. from 50v to 100v, the temp-rise goes
up 400%.

The fact that you're using PWM drives already puts you
a step behind on the heating issue. Back then, they weren't
expecting chopper-drives. The older motors didn't use
laminations as thin as today's motors. The thicker the lams,
the worse the eddy-current losses/heating. It's exactly
the same as older spindle motors running hotter when
driven by variable-freq pwm drives.

Put HV on top of PWM, and you're really cooking those poor
motors. Again, it's not -necessary-, so why do it. Instead
of getting high-voltage drivers with insufficient current
capability; just get a reasonable voltage driver who's
-amperage- is properly sized for the motor in the first place;
and you'll be quite happy with the performance.

note: a 'low inductance' motor would typically be:

Size42, 8-15 amps; size34, 5-8a; size23, 4-6 amp

As another point of reference, at home here I get 400 ipm
on my Techno gantry table used for PCB drilling. That's
with 6-amp 48v drivers on 4.7a 23-doublestack PacSci's.


Motor Wiring;
--------------

I saw a couple of posts about running motors 'half coil'.

Perhaps, but it's not a foregone conclusion. Half-coil gives
one quarter the inductance of using both halves in series.
Good for max top speed with -light- loads. However, what
kind of work are you doing? Point-to-point relatively long
rapids, like drilling? Yup, max rapids is probably your need.

But what if you're doing lots of contour-cutting? I.e.
the machine is spending 90% of it's time around 5-15
inches per minute, and having to constantly make sudden
directional changes (i.e. big accel/decel) on every little
segment ?

Well, in that case you might be better off running the
motors full-coil; because the same current will give you
40% more torque in the low-mid speed range. That allows
you to set your -accel- rates much higher, and quite possibly
reduce the overall job time. Worth considering anyway.


- Rewiring Motors, 6w to 8w

The best of all solutions is to pop the back of the motor
off and turn that 6-wire into an 8-wire; then wire the
coils in parallel for each phase. Now you have the low
inductance of half-coil with the full torque of full-coil.
You then give the motor 40% more current to get full torque.

Actually, back on Boss's for a moment, I've yet to see an
original BOSS motor that didn't -already- have 8 wires
brought out. Someone mentioned a unipolar Boss drive,
and 6w boss motors, but I've never seen one. I'll bet
they're 8w inside tho...

And virtually every 6-wire stepper of -any- size I've seen
over the years was in fact a bifilar-wound 8-wire motor
internally. With a little time, and care, they can be
converted to 8-wire for optimum operation.


- Motor Failures, and Troubleshooting

Someone posted recently about steppers being indestructible;
and a stepper almost certainly not being the cause of
weak performance.

Steppers, when not abused, -are- in fact extremely long-lived.
However, the majority of BOSS machines have had at least one
motor replaced by now. Why? It was very common for BOSS drivers
to pop an output transistor, sending a big current pulse of
about 50 amps through the winding!

Most steppers use Alnico magnets, and will be permanently
(and instantly) damaged (partially demagnetized) with any
current over (roughly) 200% of rating. A common occurence
on Boss controls, sad to say.

Dropping a motor will do the same thing. Shock demagnetizes
magnets. bye bye expensive motor.

Further, some of those Boss puppies ran -hot-; and that can
reduce the torque and/or cause a rubbing rotor. It's common
to pop the back off a Boss stepper and smell that unique
aroma of 'overheated transformer'. Any ham will know what
I mean. It's a really unique fragrance...usually an expensive
one <g>.

In any case, bad/damaged motors are quite common on Bosses.

Common Symptoms: Weak axis. I.e., stalls frequently even
with reasonable accel/speed settings. Can't reach good
rapid rate. Shaft (of demounted motor) seems to spin very
freely. (note; some brands/models -do- seem to have very
little detent-torque even when in OK shape; but this is rare).
Microstepping highly non-linear. I.e. will take several
microsteps without really moving; then next step it moves
a lot.

Troubleshooting: Just swap it with the motor on your other
axis. If the problem moves with the motor....QED. If problem
stays with the axis, then suspect dry ways, bad driver, wrong
driver settings etc.. Usually only one axis is weak. If both
are, borrow a known good motor and try it on one axis.


Final note, about the series-II spec: I think it must
be a misprint. The BOSS manuals I've seen, both
series-I and series-II, said either 90 ipm or 120 ipm
rapids, depending on the control, machine, and year.
I can't quite imagine BP releasing a machine as recently
as 1979 with only 24ipm rapids...I can't see how it would've
sold. Fanuc was already shipping the model 5 by then, with
300 ipm.

Assuming this info is useful to somebody; I'll get to the
other tech-topics I mentioned in a few days.


Richard, KC0FVV