Can anyone point me to a web resource that lays out the basics of DC
motor controlling or possibly put down the bullet points of same?
I would like to convert my drill press to DC and after reviewing the
myriad of options on ePay, decided I need experienced help.
I would like to buy surplus/used parts if at all possible and need some
guildlines as to what to look for such as DC field voltage of motor,
typical controller types used, pitfalls, etc.
If anyone could comment on what they did when they converted their
machine(s) it would be greatly appreciated.
DC motors work very well on DP's. The size of the DP, whether is has single
or double pulley reduction and the material you want to drill normally are
all considerations. If it's a large DP, then I'd go with something like
Surplus Centers GE 1.25hp 104VDC motor. I have one on an old Hercules DP
that was used to drill 1.5" holes in tool steel. With a small DP, a .25 or
.50 hp motor might be plenty. With double belt reduction (center step
down/up pulley) Smaller hp will do more. If you're old school, you can use
a Variac and a bridge rectifier to run it. You will be adjusting while
running when drilling because you're varying current and voltage at the same
time. For better results, go to a solid state controller (KB, Minaric,
etc.) and your spindle will maintain speed under load as current is
maintained and voltage (pulse width) is varied (simplistic description).
Some controllers have integrated reverse switching either by switches,
relays or regenerative circuits. This is helpful/required if tapping. With
the Variac supply, a reversing switch can easily be installed. The same
with the solid state type, but they are a bit more sensitive to reverse
surges (reversing motor while at speed). In most cases, one should stop the
spindle before reversing direction. The regenerative type is the exception.
A resistor can be used to apply braking for more effecient control. BTW,
beware of small motors with high rated hp as this sometimes refers to
"developed hp" due to high speed. Mass is usually a good thing.
Basically, the above refers to PM (permanent magnet) motors. Shunt wound
motors are effectively similar, but with a required field power supply
needed. Most solid state controllers have this available at full or half
supply voltage. A Variac can be tapped at the correct point for this. With
two Variac supplies, interesting results can be had by varying the field
supply. This method was used on old DC systems, ie. Monarch EE. The higher
field voltage suppresses motor rpm but can add torque at low speeds. Very
low field voltage can/will result in VERY high motor speeds and possible
Thanks Ron. I will see deeper into this, it is on my list of nice
things to do when I get done with other stuff. Sounds very
interesting. The motor on my Chinese drill press does not have any
nameplate on it at all, so it is hard to match NEMA frame. Not an
insurmountable difficulty, though.
First you need to understand the motors themselves.
There is a ton of motor control stuff at
This is primarily aimed at use of Microcontrollers for motor control,
but theres some good basic stuff, like drive cctry, there as well.
Just look for motors that have the speed, torque and/or power you
think you need. Matters of field voltage (if not PM motors) can be
dealt with. Understand that DC motors do have a maximum torque
limited by max current. Reducing speed with pulleys or gears also
increases delivered torque, but if speed control is done electrically
then the motor must deliver the full load torque even at reduced.
Best speed control is obtained with tachometer feedback. Some motors
have built-in tachs. An external tach or encoder can also be used.
I installed a surplus center DC treadmill motor and controler on my drill
press about 5 years ago and it is a wonderful thing :) Thanks to Don and
others for help with modifying the controler to work better as a tool motor.
The treadmill controller had a slow ramp up and would only start at zero and
you had to bring the speed up with a pot. This motor is no longer available
from surplus center but it is rated at 2HP. The controler limits the
current to 10 Amps so the actual HP is much less ~3/4hp. It is a series
wound motor with a tach feedback to the control to maintain the set speed
under variable loads. It has a max of 6KRPM and I set it on the slowest
belt speed on the drillpress. I get very useful power from about 100 to
1000 spindle RPM without moving belts. The motor is biased (brushes offset)
to run in one direction but is reversable and seems to do ok with that at
slightly reduced power if you only use the reverse occasionally. You should
be able to source a motor and controler from a used treadmill. I put one on
the drill press, one on a 6"Atlas and one on a wood lathe. All are still
"Buyer beware". Not all treadmill motors are equal. I have scavenged a
few from the dump and one in particular was very surprising in its
performance: with it running at full speed (6K rpm), I was able to bring
it to a stop by putting my bare hand on the 10" flywheel!! This was
supposedly a 1 1/2 hp (+-) motor. Yeah, right. I was very glad that I
hadn't paid anything for it, or had invested any time installing it on a
machine. POS for sure.
--If you're lucky enough to live in the SF Bay Area the Crucible has
just released the schedule of Spring classes. Included is one on how to
build a DC power supply. I'll bet they could tell you a bunch about the
right stuff you'll need to do the job right..
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