Re: unipolar or bipolar?

Thanks Mariss.

You might be disgusted to learn that someone can read that several
times and still not be enlightened. I'm sure at some point soon
I'll wake up in the middle of the night screaming "YES!!", but for
now, maybe you could spoon feed me some advice...

Let me very plainly describe my situation.

First, I've never retrofitted a machine before. I obtained a
Bridgeport BOSS with smashed control to use as my first attempt.
This is basically your older 3 axis CNC knee mill (2 hp,
2800lbs..etc)

What I know about the axis drive system is tons less that I know
about every other part of the machine, but here is what I have
learned...

First, the axis motors are 8 wire NEMA 42 steppers.

Second, an offlist email informed me that they are rated at 7-
12amps, but nobody seems to know the voltage ratings.

Next, the original power supply gives 3 phases of 60-70-80vac, is
rectifed to dc later and this votage travels through (2 each) rather
large capacitors to the amps and also through about 4 in-series
resistors (Dale ph-50 50W 10 3% 8025 if I am reading them correcly.)
per phase (or 12 of them total) on the back side of the amps. These
are the gold heatsink type at about 1.5" long each and were mounted
to the main heatsink.

There was no way for me to test anything prior to ripping it all out
as it looked as if someone got mad and used a boot in a rather
aggressive fashion on the internal electrics and I could not risk
applying power to the system in this state.

I have no idea (nor would I) if these motors were unipolar, bipolar,
full, or half wound, nor the type of original drivers or their
ratings. I simply have 3 motors with 8 wires coming out of them, a
source of DC power, 3 210(42)'s, and a boatload of electronics
removed from the original.
I have no idea if the originals were microstepped or not, but I'm
assuming they were since the lowest jog increment on the BOSS system
is .0001.

My goal is this....
Get the machine running in the "correct" way under PC control. It
can run the same, better, but no worse as far as speed, power, and
efficiency are concerned (regarding the axis drives and motors) than
the original system.

My concerns are these...
First, the 210's are rated at 7amps and according to rumor that is
where the motors start.
Not knowing the original voltage of the motors (it doesn't say
anywhere...even internally), I cannot do the 4-25X calculation, nor
would I know which is best if I had a choice (would 25X be too close
to the max and would I need to be constantly checking the motors for
overheating...etc).
Being that the original system had all these resistors in the line
would I be better off with a higher amprege, lower voltage driver
system like some of your competitors make (I only ask because they
have nearly the same resistors pictured in on of their setups), or
am I still doing the best with the 210's?

Although I know a fair amount about machines as a whole, I am quite
ignorant regarding motor windings, microstepping, and all the
formulas that are entailed in motion control of the axes, and I must
admit that I am at the point where I am just tempted to hook it up
and if it sorta works, say "well, I guess that's right".
To make matters even worse, I've never done much more on the BOSS
mills than jog the axes, so I don't have any basis to compare the
original performance with the retrofit performance to allow me to
tune the new system. I have more experience with machines with
cutting speeds that more than likely far surpass even the rapid's of
this BOSS. In other words, I don't know what I should expect out of
it and when to be disappointed or delighted.

Maybe there's something in all of that which you could answer?


Many Thanks
Sean

--- In CAD_CAM_EDM_DRO@y..., "mariss92705" <mariss92705@y...> wrote:
> UNIPOLAR, BIPOLAR, SERIES, PARALLEL, FULL-WINDING, HALF-WINDING
>
> These terms can be confusing but they don't have to be. Unipolar
and
> bipolar properly refer to motor drives, the other terms refer to
> motors.
>
> In the end, 4 wires from the motor must be connected to the drive;
> it's all a matter of which ones go where. The following
definitions
> will clear things up.
>
> DRIVES FIRST:
>
> UNIPOLAR: An outdated motor connection requiring only 4 power
> transistors. It also refers to 6-wire motors, (not outdated).
Motor
> current is applied to the center tap of a winding. One end of the
> winding or the other is switched to ground by the power
transistors
> but never at the same time.
>
> ADVANTAGES: Simple
>
> DISAVANTAGES: Many. The connection acts a 2:1 step-up transformer,
so
> the transistors "see" twice the power supply voltage. Inductive
> energy is not inheirently clamped, so complex and inefficient
> clamping circuits must be used to prevent damage to the
transistors.
>
> BIPOLAR: Almost universally used in modern, high-performance
drives.
> Bipolar refers to how the winding is driven; when one end of the
> winding is grounded, the other end is connected to the power
supply
> voltage and visa versa. This method requires a full-bridge driver
per
> winding for a total of 8 transistors.
>
> ADVANTAGES: Many. The transistor voltages never exceed the power
> supply voltage. Inductive energy is clamped to the power supply
> voltage rails and is efficiently recirculated back to the power
> supply. No circuit voltage is above or below the power supply
voltage
> rails at any time.
>
> DISADVANTAGES: More complex than unipolar.
>
>
> NOW, THE MOTORS:
>
> SERIES: A method of connecting a split winding (8-wire motor) end
to
> end so that the current flows through one winding and then through
> the other one. Mind the phasing of the windings; they must be
> connected so that they "add" (boost). If they are connected
> to "subtract" (buck), the result will be no torque and possible
> damage to a switching drive since the inductance will be zero as
well.
>
> ADVANTAGES: Smaller power supply because only half of the rated
> current is necessary for the same holding torque. Less motor
heating
> at a given voltage.
>
> DISADVANTAGES: The motor will develop only half the power at
higher
> speeds because this connection has 4 times more inductance.
>
> PARALLEL: A method of connecting a split winding (8-wire motor)
side
> by side so that current flows through both windings
simultaneously.
> Mind the phasing again (see SERIES: above).
>
> ADVANTAGES: Develops maximum power at higher speeds.
>
> DISAVANTAGES: Greater motor heating at a given power supply
voltage
> when compared to the series connection.
>
> FULL-WINDING: A method of connecting the end wires of a 6-wire
motor
> leaving the center tap unused. It is exactly identical to a series
> connected 8-wire motor since the winding is internally connected
in
> series at the center tap. The advantages and disadvantages are
> identical to the series connection as well (see SERIES: above).
>
> HALF-WINDING: A method of connecting the center tap and one end
wire
> of a 6-wire motor leaving the other end wire unused. It makes no
> difference which end wire is used. It is almost identical to a
> parallel-connected 8-wire motor.
> The single difference is only half the copper (hence half-winding)
is
> used, so the winding resistance is twice what it would be for an
> identical 8-wire motor. At higher speeds this higher resistance
> develops a slightly greater voltage drop, leaving a little less
for
> the motor to work with, and so the motor develops a little less
power
> compared to an 8-wire motor.
>
> The difference is almost insignificant; dynamometer tests show
only a
> 3% difference in power output between a half-winding 6-wire motor
and
> a parallel-connected 8-wire motor. It only stands to reason; at
> higher speeds phase currents decrease and therefore the I squared R
> loss differences decreases as well.
>
> The advantages and disadvantages are the same as a parallel
> connection (see PARALLEL: above).
>
> CURRENT SETTINGS FOR WINDING CONNECTIONS:
>
> The proper phase current is very important for best low speed
> performance of a microstep drive. It relates to how smoothly and
low
> speed resonance free a good microstepping motor will be.
>
> There is a common misperception some users make regarding current
set
> with microstep drives. It has to do with the fact microstepping
> drives the motor with sine and cosine weighed currents. When the
> current in one winding reaches its maximum the other winding
current
> is at zero. From this some surmise that it should be possible to
> increase the set current to 141% of rated since the motor power
> dissipation would then be identical to a full step drive. That is
> correct as far as power dissipation goes but it neglects a very
> important point most are not aware of.
>
> The magnetic flux path is not shared by the windings. This means
the
> current in one winding does not contribute by adding or
subtracting
> to the to the magnetic flux of the other winding. Though the
> dissipation may equal a full step drive by running the current at
> 141% of rated, the iron magnetic saturation is dependent on the
> current of one winding entirely.
>
> The result is the iron in the motor saturates at the higher
current
> and degrades the motor's linearity. Consequently the microstep
> placement is not optimal anymore and the motor will exhibit
greater
> low speed resonance than it otherwise would have.
>
> A microstepping drive may not have as much holding torque as a
full
> step drive, but this is meaningless in practical terms. A full
step
> drive invests about 35 to 40 % of its available torque in
vibrating
> the motor and mechanism once it begins to turn. A microstepping
drive
> wastes less than 1% of available torque this way.
>
> Though it may have less torque to work with, a microstepper
applies
> nearly all of it to the load, so in the end it is at no
disadvantage
> compared to a full step drive. There is no point to trying to
bring
> up a microstepper drive torque to the same level as a full step
drive.
>
> The conclusion is: set the current to the motor's rated current.
> If it is a unipolar (6-wire motor) use the nameplate rating. If it
is
> a parallel wired 8-wire motor, use the parallel rating.
>
> UNIPOLAR: Use the motor's name plate rating
>
> SERIES: Use one-half the parallel rating
>
> PARALLEL: Use the motor's name plate rating (usually the parallel
> rating)
>
> FULL-WINDING: Use one-half the name plate rating
>
> HALF-WINDING: Use the motor's name plate rating
>
> Mariss
>
> --- In CAD_CAM_EDM_DRO@y..., "kevinagilent" <scoob22@h...> wrote:
> > what is the difference? what are the benefits
> > and, or drawbacks of either
> > thanks kevin