Motor locking

Hi,
I was working on a robot which has a crane like lifting assly. I
am using a motor to windup the wire which is attached to the weight.
Now I have noticed that if the motor terminals are shorted, the motor
cannot be rotated by its shaft as easily as it can be without its
terminals shorted. I was kinda hoping that I could use this property
of the motor for holding a load at some height. I have been suggested
many mechanical alternatives, but I find the motor soln a better one.
I guess that since the motor is acting like a generator (mechanical
input at shaft and terminals are shorted) the value of load that can
be held depends on the torque generated in opposition to the input.
Now how can I calculate this value of torque. Is it same as the
stall-torque or the starting torque (as specified in the motor's
datasheet) or is it something different. I am using a 12v DC motor
(2400rpm geared down to 120rpm) and a 1cm dia pulley to wind the
string on.
Thanks,
Abhijit Karnik

 

The motor must rotate to generate braking

 

The motor must rotate to generate braking

But it seems that there is a min. value of torque to be applied to the
input shaft, before the rotation can occur. I guess it could be
similar to the detent torque in a stepper motor, or was it holding
torque?

 

A PM motor can provide braking to slow the motor down or to slow the rate
at which it accelerates but cannot hold it stopped by itself. This
braking combined with the normal starting resistance MAY be enough to
hold it close to stopped under some load conditions but once you break
over the starting resistance you can expect creep.

On the other hand this regenerative braking can be used very effectively
to reduce the required size of the mechanical brake. You may only
require a small holding brake rather than a full size stopping brake.

Another alternative is to fully drive the motor so that you essentially
provide a reverse current to give you your holding torque at rest. Of
course this complicates the control, drains the power supply and requires
increased confidence in the controller's reliability.

Robert

 

The effect you noticed is called poling (google), there is no way AFAIK
to externally increase.