Mail Archives: geda-user/2015/04/23/17:45:11
On Thu, Apr 23, 2015 at 11:57:52PM +0300, Sabin Iacob wrote:
> On 04/23/2015 11:30 PM, Rob Butts wrote:
> > One last question regarding steppers.
> >
> > I am looking at the Pololu DRV8825 stepper motor drive rated for 1.5A
> > without a heatsink or 2.2A with cooling. The nema 23 steppers that
> > I've found are mostly 2.8A. Is that the coil max amp rating or what
> > it draws when driving it (clearly I know little about steppers)? I'm
> > guessing this will overload the DRV8825.
> > ???
>
>
> that would be the max amp rating, I guess (that or the current which
> guarantees the nominal holding torque, but I know from experience that a
> stepper rated for 350 mA will get pretty hot and jumpy and noisy when
> driven with 600mA - found this out when testing some drivers I made a
> while ago, also DRV8825 (it's a nice little chip); unlike my board, the
> Pololu drivers are made with thicker copper and probably have the back
> of the board turned into a heatsink, as the datasheet recommends
I can easily drive 1A with an A4988 on my own boards.
I do have MEGA1284 boards with RS485 and A4988 driver.
Not bumpy at all, but board layout and supply at critical factors.
I would be surprised if the DRV8825 has problems with it - it is said
that the DRV8825 is the better one.
ANyway - no personal expirience with the DRV8825.
Not too familar with NEMA to size imaginations and didn't expect it to
be such a hight current with 4 ohm, usually those have lower resistense.
But it isn't easy to measure low resistence, because your measureleads
also sum up.
Some DMM can zero out, but for good results you want 4-wire measuring.
Anyay - the motor can handle up to 2,8A - I assume that's in fact 2A only
and the 2,8A are 2A / sin(45°).
The 2A is average current and if you rotate you apply a sine patern
giving 2A effective at 2,8A, but if you run at full 2,8A you need to be
carefull if you stop the motor, since it may stop at highest current
point.
The usually way to go is reduce holding current.
The current is what makes the torque.
The higher the current the higher the torque.
The driver will apply as much current as configured and if you underrate
a motor then you will have less tourque than the motor would be able
to deliver.
At stopp the driver with apply the amount voltage given by the coil
resistence, so for 4 Ohm (if that really isn't less) with 2A it is 8V.
The motor is heating 8Watt in it's coil resistence, but full 2A will
mean high torque.
However when you move the applied current _moves_ as well.
To change the coil current you also have to overcome it's inductance,
which means that the intended current is less because it needs to
recharge the inductence.
The offset is higher the higher the speed is and with higher speed the
torque reduces because of this.
You want a higher supply voltage to recharge the coil faster, which
results in higher torqueue at higher speeds.
Unless you want to move slow or use a high supply voltage you won't
see the maximum current anyway.
For example for CNC milling the milling process often requires high
torque, but you may process at low speed.
You want a motor with high current and high voltage only if you want
speed too.
For pick and place you often don't need high torque, but fast movement,
so you want high voltage, but may use low current, since at high speed
you may have troubles to apply a high current.
First thing you need to know what speed and torque you need to have
for your application.
If you really need high current I would suggest going for a bigger
driver.
Leadshine makes rather inexpensible ones and is often used for
hobby class milling machines.
--
B.Walter <bernd AT bwct DOT de> http://www.bwct.de
Modbus/TCP Ethernet I/O Baugruppen, ARM basierte FreeBSD Rechner uvm.
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