Re: 4Q Motor control paradox

Roger wrote:
Consider a 4Q motor control with a H bridge driver. The bridge is being
driven in PWM by means of a current control loop inside a speed control
loop. The motor is required to ramp up, maintain a velocity, and then
ramp down (braking).

There is a lot of conventional wisdom out there which states that
whilst braking you are using the motor as a generator (regenerative)
and therefore a load resistor must be switched into the DC link to
dissipate the kinetic energy from the load. Attempting to brake the
motor simply by taking the PWM to the opposite extreme (presumably
respecting the max current of the motor) will cause the energy to be
dissipated in the armature resistance.

An alternative view is the physics one. If we apply a force to a load
it starts moving, and will carry on moving until we apply a force in
the opposite direction. If this problem were in space, we would fire a
rocket to accelerate, and then turn the ship round to fire in the other
direction to ramp down again (hey, who remembers Defender :-) . Now, in
our 4Q bridge all we are doing is forcing a current to flow against the
generated emf, that is we are creating a force in the opposite
direction. So where does the kinetic energy go?

And which is the correct way of looking at the problem?

Most physicists these days would want to talk about energy balance, and would only resort to the details of the forces, currents, whatever, when they had to.

If the kinetic energy goes somewhere and it doesn't dissipate as heat in the motor and it doesn't dissipate as heat in the amplifier then it must come out of the amplifiers power terminals -- which it will if the amplifier has synchronous rectification and is sufficiently efficient and if the motor is actually pumping energy into it as it slows down.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google? See http://cfaj.freeshell.org/google/

"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html
.

Relevant Pages

  • Re: 4Q Motor control paradox
    ... driven in PWM by means of a current control loop inside a speed control ... The motor is required to ramp up, maintain a velocity, and then ... dissipate the kinetic energy from the load. ...
    (sci.electronics.design)
  • Re: dc motor control for a balancing platform (segway)
    ... that lets me control two motors. ... to stter the motor torque to get it back in balance. ... usually either a permanent magnet or a classic squirel cage induction ...
    (sci.engr.control)
  • Re: AC amplitude modulation for inductive loads
    ... Mosfet bridge. ... I probably should mention that the pump wants to see a sinus like (or ... You want to pulse the bridge to get the sine wave and rely on the motor ... 24VDC to 230VAC) lay out and to add a control input for the output ...
    (sci.electronics.design)
  • Re: AC amplitude modulation for inductive loads
    ... My present design is powered from 24V DC and utilises a full Mosfet bridge. ... The bridge is controlled by a MC 33035 which is a PWM motor controller with an analogue input and a direction signal. ... I probably should mention that the pump wants to see a sinus like excitation which limits the use of a triac. ... It is done by a converter up front and that gets the sine-wave control signal. ...
    (sci.electronics.design)
  • Re: AC amplitude modulation for inductive loads
    ... So far I have used a 24VAC pump, which is no longer available, the ... Mosfet bridge. ... The bridge is controlled by a MC 33035 which is a PWM motor ... Documentation and quality control is up to about 40% ...
    (sci.electronics.design)