On Thu, 10 Oct 2019 14:15:20 -0700, Bob La Londe
wrote:


On 10/10/2019 1:48 PM, wrote:
On Thu, 10 Oct 2019 13:11:46 -0700, Bob La Londe
wrote:

On 10/10/2019 11:34 AM, Jon Elson wrote: On Thu, 10 Oct 2019 10:29:22
-0700, Bob La Londe wrote:

Some years (10+) back I partially unwound the power transformer for my
power supply on the Hurco mill so that I could stay within the safe
margins for the servo drivers I chose. Sadly the servo drivers
did not
hold up. I replaced them with same more than once before finally
upgrading to something that could handle a lot more.

I've got one servo motor that is getting pretty hot. Its also got the
most load on the machine. The Y-axis motor. Its a belt drive and it
has to sling around the weight of the table and the saddle. It
gets hot
enough after a few hours to give a 1st degree burn. Not quite
enough to
make instant skin death, but its hot. You would suffer injury if you
wrapped your hand around it and held on.

This is a horizontal mill? The Y axis is vertical? Does it have a
counterweight or air spring to coutner the weight of the table?
No, no, and no. I'd guess that table and saddle at around 1000 lbs.
Conventional XY movement.

That would be the first thing I'd consider.
I have considered a couple options including converting it from a belt
drive to a direct drive. A direct drive would have much less
parasitic
load.
I really doubt the belt makes any difference.


Any? mechanical power transmission always has some associated loss in
usable power. The timing belts on this machine are pretty darned tight.
I matched the deflection on the Y up with the deflection on the Z.
They are the same center to center distance, and the Z has no issues.


There may be an issue with the ballscrew itself, but I'm holding
out. I have loosened the gibbs slightly, and I have made sure the
ball
nut and the gibbs are getting plenty of oil from the automatic oiler.

I am wondering if I increase the voltage by rewinding a few wraps back
onto the transformer to bump the voltage back up to the original
90(+/-)
VDC instead of the current 78VDC. If the voltage is higher the
current
should be lower at the same load requirements. Will that result
(within
reasonable limits) in a slightly cooler motor?

No, it will make no difference. The drive gives the motor the current
required to deliver the torque needed to stay in position. Higher
voltage to the drive MIGHT actually make it worse.

I considered there might be an issue with back EMF at higher voltage,
but generically total power is a function of amps*volts. Its my
understanding that DC motors typically have a sweet spot, and it was
originally powered at 90VDC. My thought was in addition to requiring
maybe less current for operation it might also be its "sweet spot."

I'm also considering wrapping a heat sink around the body of the motor
with a generous slathering of heat sink compound. I believe I can
make
a bendable heat sink that could be warped to fit the motor.

If this is a DC brush motor, that will have little benefit. It is the
rotor (armature) that is producing all the heat. Possibly forced air
cooling to the motor will help. If it is a brushless motor, then heat
sinks of forced air cooling will help a lot, as all the hear is
generated
in the stator.


Do you think the heat might have an affect on the field of the permanent
magnets? Its not an open motor, so forced air through the motor is not
an option. Of course air could be blown over the motor, but that would
not directly cool the rotor.



In all the years I have owned this machine i have always had
issues with
the Y axis. Its burned up servo drives, and a while back the motor
cooked. I do mean cooked.

And, if you can't hold your hand on it, then you are COOKING it now!


I am aware.
Greetings Bob,
Your DC brushed motor consumes current based on the torque
requirement while the voltage is related to speed. So higher voltage
will help it go faster but will not lower the current required. If you
could change the torque requirement that would help. Can you change
the gearing so that the motor spins faster? Years ago I ran into this
exact problem on a home made 4th axis. And the solution was to speed
up the motor while lowering the torque.
You are correct that total power is voltage times current.
Unfortunately supplying more voltage to the motor will not cause it to
draw less current.
Eric



I have been looking for a band-aid (rather than surgery) for this
application. I think that might just be the band-aid I need. Right now
its a 1:1 belt drive. I will have to think about that some. It would be
nice to keep the motor inside the knee where it is now.


I hope it works. Where is the encoder for the Y axis? If it's on the
ballscrew then you can probably get away with changing the pulley
ratio. If it's on the motor, then you will need to tell the control
what the new ratio is. Or move the encoder.
Eric