Heat build up???? Need suggestion.

[Neil Jackson]

Hello All,
Newbie here.
I just finished my first CNC table/ Engraver carver. After burning up
several old style AT power supplies, and shutting a few down.
I broke down and purchased a 13.8v/ 10.3a Power supply from Radio Shack. I
was able to drop the power down 8.8v to power my controller board. And I
left the 13.8v legs to power my three (6vdc/unipolar motor). I'm told that
with a 6 volt motor I would be better off with 12 volts because at any given
time two phases of my motors could run at one time, thus needing at least 12
volts.

I'm getting an awful lot of heat from both my motors and the power supply.
Should I be running my motors at 13.8v as is or should I set up a current
limiter?
or should I step down the voltage from the line to only give the motors the
indicated 6 volts.
The machine seems to work fine, although I can stall one of the motors by
hand after some running time.

TIA
Neil

 

[Harvey White]

Hello All,
Newbie here.
I just finished my first CNC table/ Engraver carver. After burning up
several old style AT power supplies, and shutting a few down.
I broke down and purchased a 13.8v/ 10.3a Power supply from Radio Shack. I
was able to drop the power down 8.8v to power my controller board.

with a 5 volt regulator to power the ttl logic, I'd suspect.

And I
left the 13.8v legs to power my three (6vdc/unipolar motor). I'm told that
with a 6 volt motor I would be better off with 12 volts because at any given
time two phases of my motors could run at one time, thus needing at least 12
volts.

ah.. no, that's not how it works.

there are two kinds of drive, chopper and direct. Chopper drives
measure the current in the motor coils, then turn off the driver so
that the current stays within limits. The higher voltage (than the
motor) recommended for these supplies decreases the rise time for the
current in the motors, and increases the high speed torque. For these
kinds of drivers, you do want a higher voltage.


If you have a simple saturating transistor driver, you have at most
1.0 volts in the transistor in saturation (probably less than 0.5
volts), the motor voltage itself, and that's it. for a 6 volt motor,
you would run at no more than 7 volts. 13.8 volts is way too much, if
you have this kind of driver.

I'm getting an awful lot of heat from both my motors and the power supply.
Should I be running my motors at 13.8v as is or should I set up a current
limiter?

current limiter, but that depends on the kind of driver that's running
the motors.

or should I step down the voltage from the line to only give the motors the
indicated 6 volts.

6.5 to 7, roughly, but not much more.

The machine seems to work fine, although I can stall one of the motors by
hand after some running time.

Possibly when everything is hot.

Harvey

 

[Neil Jackson]

The below item is what I'm using to driver my motors, the specs say from 0-
to 24 vdc.
That was the reason for the middle of the road use of 12 vdc.
Also would a resistor setup work or should I use a series of diodes to step
down my voltage say to 7 volts from 13.8?


http://www.stepper3.com/
(S3ROC) Stepper3 Rev OC - 3 Axis 3 Amp Unipolar Stepper Motor Driver

Really
TIA
Neil

 

[Harvey White]

The below item is what I'm using to driver my motors, the specs say from 0-
to 24 vdc.

This turns out to be the maximum voltage that the board will tolerate.

That was the reason for the middle of the road use of 12 vdc.
Also would a resistor setup work or should I use a series of diodes to step
down my voltage say to 7 volts from 13.8?

Ok, reading page 9 of the manual:

1) the power supply used for the stepper motors shall be selected at
approximately one volt higher than the rating shown on the stepper
motor.

I'd use a power supply rated at 7 volts. You'd do well to find a
power supply rated at that voltage, or step the voltage down with
diodes. Figure about 0.6 volts/diode and experiment with them, and
they either have to be rated for the full current of the motors
combined, or you need a string for each motor rated at the full
current of what the motor will draw. Figure either the phase current
for full step, or 2x that for the half step mode. I'd go with at
least 2 1/2 to 3 times the maximum current just to be conservative.

http://www.stepper3.com/
(S3ROC) Stepper3 Rev OC - 3 Axis 3 Amp Unipolar Stepper Motor Driver

Really
TIA
Neil

NOTE: you do have this kind of driver.. so 13.8 volts is too much.

This is a very simple kind of driver. In hindsight, a xylotex board
would have worked better, giving you more torque, if the phases of
your motor were not common. (6 wire unipolar rather than 5 wire
unipolar).


I am not sure what the overcurrent will do to the steppers.

Harvey

 

[Neil Jackson]

Thanks Harvey,
Now for a new question, I have a couple of AT power supplies. Could I put in
series a resistor as a current limiter?
And achieve the same results. The below link has the same motors I'm using.
The SP3/HT is the complete setup. It shows an AT ppower supply being used.??
http://www.stepperworld.com/
The mechanics I've gotten worked out, but as you can tell know very little
about the electronics end of things. Your help is greatly appreciated.

Neil

 

[John Jardine]

Hello All,
Newbie here.
I just finished my first CNC table/ Engraver carver. After burning up
several old style AT power supplies, and shutting a few down.
I broke down and purchased a 13.8v/ 10.3a Power supply from Radio Shack. I
was able to drop the power down 8.8v to power my controller board. And I
left the 13.8v legs to power my three (6vdc/unipolar motor). I'm told that
with a 6 volt motor I would be better off with 12 volts because at any given
time two phases of my motors could run at one time, thus needing at least 12
volts.

I'm getting an awful lot of heat from both my motors and the power supply.
Should I be running my motors at 13.8v as is or should I set up a current
limiter?
or should I step down the voltage from the line to only give the motors the
indicated 6 volts.
The machine seems to work fine, although I can stall one of the motors by
hand after some running time.

TIA
Neil

For unipolar driving A 6V motor needs a 6V power supply (say 7V for cable
volt drops).
Keep the 13.8V supply and feed the motor coils via resistors mounted on
heatsinks (maybe a 20W 2.2ohms resistor per each coil pair). The motors get
their 6V when they are on, so don't burn out but you will now have some
extra performance out of them due to the initially higher supply voltage
being able to get the motor coils moving faster.
regards
john

 

[CWatters]

Thanks Harvey,
Now for a new question, I have a couple of AT power supplies. Could I put in
series a resistor as a current limiter?

I believe the standard way for home builders to drive stepper motors is to
use a high voltage supply and dropping resistors. I'm not sure this is any
better than using the correct voltage but that seems to be what people do.

If your motors are rated 6 V @ 3Amp and you have a 12 V supply the
calculations you need are:

Resistor value = (12-6)/3
= 2 Ohms

Power rating = (12-6)*3
= 18W

It might be cheaper to use multiple resistors in parallel if 18 W isn't
available or is too expensive.

Colin

 

[Harvey White]

I believe the standard way for home builders to drive stepper motors is to
use a high voltage supply and dropping resistors. I'm not sure this is any
better than using the correct voltage but that seems to be what people do.

The problem with a stepper is that you are switching a voltage through
the inductive coils on and off. Current lags and builds up slowly.
If the pulse is fast enough (rapid stepping, high RPM), then current
does not approach the nominal drive current... Result: low torque at
high RPM.

By running the motors at a higher voltage, but limiting the current
with an external resistor, you get the advantage that the current will
rise faster in the coils, thus giving better performance at higher
RPMS.

The real solution, if possible, is to use a chopper driver with
current limiting, which switches the motor coil to full supply voltage
(often 10 times the rated voltage) yielding a rapid rise time
(switches have a very low impedance), but turning off the switch when
the current has reached the nominal rating in the coil.

Gives you better torque at higher rpms as well.. this can be done
with two ICs for motor supply voltages to 35 volts and currents less
than 2 amps.

Harvey

 

[CWatters]

I didn't realise they were inductive enough and/or stepped fast enough for
this to be a problem. Thanks for putting me straight.