Re: Controllable torque electric motor questions

On Mon, 14 Aug 2006 07:51:03 -0700, Tim Wescott <tim@xxxxxxxxxxxxxxxx>
wrote:

Richard Ferguson 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


You are correct that a universal-wound motor would be a bad idea. You
probably want a DC servo motor that's sized for the service you intend.
As already pointed out there are DC motors that are designed for
continuous high torque slow duty.

I don't think that you'll be happy with trying to control torque through
a 100:1 gearbox. I could be wrong, and the first thing you may want to
do when you get hardware is to give it a whirl, but I think you'll find
that the gearbox will introduce too much uncertainty in your torque,
even if the motor is doing exactly what you want.

Were I doing this I would go with the suggestion for a load cell.
Better yet, you might want to consider a torsion spring on the output of
your gearbox. If you could stand the low speed, winding up the spring
through the gearbox should give your controller time to react
appropriately, and putting rotation sensors on each side of the spring
should give you a good indication of torque.

Good idea!
.

Relevant Pages

  • Re: Why do electric powered automobiles need to be so complicated?
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  • 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. ...
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  • Re: RPM of variable speed DC Motors
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  • 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 ...
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