Re: Controllable torque electric motor questions

On Mon, 14 Aug 2006 04:17:09 GMT, Richard Ferguson
<fergusonOMITsculpture@xxxxxxxxxxxxxxx> wrote:

I was having a discussion with someone about a machine design, and I
suggested that a variable torque motor would be a good solution. I
further suggested that a DC motor might be a good choice, high torque at
low RPM. But neither of us are electrical engineers.

When I did a little internet research I concluded that there are
probably multiple solutions, given today's electronics, and that a DC
motor might not be the best choice. I noticed, for example, that most
universal motors are not reversible. Operating motors at zero rpm can
tend to overheat them. Operating universal motors at low load tends to
overspeed them.

This application calls for a controllable torque to be output from the
motor. This torque would be applied for several seconds at zero RPM,
until the load started to move, and then the motor would rotate for a
few seconds and stop. In a few seconds, the motor would reverse at
minimal load, stop, and the cycle would start again. The machine would
need to be able to apply the same torque in both directions. Some kind
of position sensor would be used to determine the motor starts, stops,
and reversals. A gear reduction would apply the motor torque to the
machine, with a total machine rotation of 1/4 or 1/2 turn. 1/2 turn in 3
seconds would be 10 RPM at the final gear reducer output. The amount of
power involved is not a great deal, since rapid operation is not
important, and a lot of gear reduction makes sense. My guess is that
the final torque, after gear reduction, would be in the range of 10 to
100 foot pounds. If we assume 100:1 gear reduction, the motor would
need to put out around 1 foot pound of torque at zero to 1000 RPM. A
small variable speed electric drill would have more than enough power,
but might not survive the tough duty cycle. Without finding the
formula, my guess is that the power required would be much less than 1
HP, given appropriate gear reduction, but perhaps good to oversize the
motor to prevent overheating it. Kind of an odd application. The RPM
is not a major concern, but being able to set the torque is key. Most
likely, the torque needs to be controlled by a PLC (showing my age) or
computer, but manually setting the torque by turning a dial or punching
in a number may be acceptable. It is not clear how precise the torque
control needs to be, probably +/- 10% would be more than adequate, given
that we are automating something that has been done by hand by a craftsman.

Oh, and the application is similar to twisting square bar for decorative
railings, something common for blacksmiths. Pretty easy to do if you
get the bar red hot. (Obligatory metal content).

I tried to describe the application thoroughly, but I am sure that I
left something out.

Anyone care to propose a motor/controller solution that would meet these
requirements?

Does anyone know of a good reference that would help me understand the
options?

Or am I asking at the wrong forum? Anyone care to suggest a better
place to ask this question?

Richard

The torque of a DCPM (permanent magnet) motor is linearly proportional
to its current almost regardless of speed. Some if not most
servomotors are designed to deliver a rated torque down to and
including zero speed, indefinitely.

If your accuracy requirement is only 10%, then straight current
control would probably work fine for you. If you want more accuracy,
then you'd have to measure torque somewhere in the system, perhaps
with a loadcell, and use that as a feedback signal in a closed-loop
control.

One possible error source is the stuff between the motor and the load,
e.g. gears and other power transmission components. Their drag torque
may not be constant with speed, temperature or over time as things
wear and/or the state of lubrication possibly changes. The most
accurate and reliable approach would be to control motor current using
a feedback signal derived from a torque sensor that meauses actual
torque delivered to the load.

One ft-lb of torque at 1000 RPM is about 142 watts or about 0.2 HP.
That'll give you a rough idea of what size motor you need -- but
you'll want to select a servmotor that can deliver 1 ft-lb
continuously at stall. Just one example: Baldor MT-3363-BLYCN can
deliver continuous stall torque 11.25 in-lb (almost a ft-lb) when
drawing 4.75 amperes. It's rated speed at 100 volts is 2400 RPM, so
with slightly more gear reduction than you contemplate this motor
would easily do your job. There are many other servomotors
available from a number of manufacturers. Google is your friend.


.

Relevant Pages

  • Re: Why do electric powered automobiles need to be so complicated?
    ... 10,000 RPM). ... Torque conversion of one sort or another is a must. ... very fast - whatever the standard 3-phase 60 Hz induction motor runs at). ... pole, 8 pole, and 12 pole motors, which run correspondingly slower but with ...
    (sci.electronics.design)
  • Controllable torque electric motor questions
    ... I was having a discussion with someone about a machine design, and I suggested that a variable torque motor would be a good solution. ... I further suggested that a DC motor might be a good choice, high torque at low RPM. ... The amount of power involved is not a great deal, since rapid operation is not important, and a lot of gear reduction makes sense. ...
    (rec.crafts.metalworking)
  • Re: RPM of variable speed DC Motors
    ... I've never heard of anyone calculating amps per RPM, ... then motor A is slightly more ... some manufacturers do not make finding their torque graphs readily ... you'll be able to get nearly that torque out of it at higher speeds (and ...
    (rec.crafts.metalworking)
  • Re: Controllable torque electric motor questions
    ... suggested that a variable torque motor would be a good solution. ... Operating motors at zero rpm can ... A gear reduction would apply the motor torque to the ...
    (rec.crafts.metalworking)
  • Re: how to calculate reqd motor strength ?
    ... Multiply the force by the wheel diameter to ... Torque is force times moment arm ... That's all accounted for in the motor output specs. ... the max RPM will always be lower. ...
    (comp.robotics.misc)
Loading