Re: 10 amps to drive steppers

The kit contains some really nice but large heatsinks. I don't like to run
mosfets much over 130F with heatsinks on them. One of my customers has one
of the few units that I sold as complete units. It ran very well for about
11 months but a student diconnected the Z axis motor while running and blew
a couple mosfets and a sense resistor.. It is back up running again.
Yes you can increase the current by placing 2 .1 ohm sense resistors in
parallel for 10 amps but I saw ecessive ground noise at about 9 amps so I
deliberately detuned the specs to 5 amps where it runs very reliably. Yes
the mosfets could be replaced with higher rated mosfets but you may run into
new problems.

One note. I NEVER run my stepper motors at the rated current. Why not ?
Bacause a stepper motor on draws full current when it is stopped. Who cares
that the motor is getting full current when it is stopped. NOT ME. The
faster you run the motor the less current draw there is. The reason is that
the coils can't charge fast enought at high speed. Additionally ,if you are
using a 6 wire motor and wire the coils in series you now only electrically
require about 80% of the rated current because the series resistance of the
coils has reduced the current requirement.
I have tested many motors with many drivers and measure the running torque
with a special torque tester and have proven to myself that using 80% of the
rated current is better for the motor and the electronics with no
degradation in performance.
Dan
-----Original Message-----
From: Mo <mo@...>
To: CAD_CAM_EDM_DRO@onelist.com <CAD_CAM_EDM_DRO@onelist.com>
Date: Saturday, June 12, 1999 7:05 PM
Subject: Re: [CAD_CAM_EDM_DRO] 10 amps to drive steppers

>From: "Mo" <mo@...>
>
>> know how it goes. The only modification to the board needed to allow
>higher
>> current is to parallel the existing .1ohm sense resistors with another
>.1ohms
>> which will double the adjustment scale to 10 amps.
>>
>> Wish Me Luck!
>
>Matt,
>I wish you luck!
>It is far nicer to explore at the ends of the envelope when you have a
>safety chute.
>Getting the board to run at high Amps is one thing, getting it to run
>reliably for years is another.
>My advice is to analyse and improve your protection system to keep the
>experimenting fun and painless.
>
>Do the units have a temperature sensor on the heatsink for over temp
>protection?(always a good idea when you are relying on forced cooling and
>fan health) If not consider providing a thermistor or other type temp temp
>sensor on the heatsink near to the trannys/fets this should be used to

drive

>the enable/disable pin.
>You may have several of these in key positions. If you need simple

circuitry

>let me me know.
>
>Use good quality heatsink compound and really go over the top with the
>cooling design. The simple baffles in Tim Goldsteins setup caused a 40F
>drop.
>Many fans are better than one. If one goes then you still have forced
>cooling from the other fan(s) On the subject of fans get the airflow specs
>for the fans you intend to use and go for the highest you can find - I use
>the Minebea Smartfan 4715ML-012P542-P1 12V 0.5A (commonly found in old
>Compaq Servers this has an external thermistor which allows it to run as
>fast as it needs to saving on life, you can replace this with a 5k pot for
>manual setting or with a 1k resistor for an airflow that will have your
>Bridgeport hovering around the shop!
>
>
>Get the max operating temps for the mosfets from the manufacturers
>datasheets or Dan. Run the units for extended periods measuring their temps
>and gradually increase the current until you don't feel safe going

further -

>need more current then add more cooling and continue your tests.
>
>With good cooling you will keep it going well and the attention paid to
>sensing/shutdown areas will be a great security blanket. Dan may be able to
>shed some light on these areas.
>
>Mo
>
>
>
>
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