Re: 4Q Motor control paradox
- From: Spehro Pefhany <speffSNIP@xxxxxxxxxxxxxxxxxxxxxxx>
- Date: Tue, 06 Jun 2006 12:27:40 -0400
On 6 Jun 2006 08:40:09 -0700, the renowned "Roger" <irwin@xxxxxx>
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?
Other than losses in the supply and motor, the energy will go back
into the power supply. With a battery, it can recharge the battery,
which is generally good. With simple rectifier/filter capacitor supply
it will charge the capacitors, quite likely to an unhealthy voltage
for the capacitors.
Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@xxxxxxxxxxxx Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
.
- References:
- 4Q Motor control paradox
- From: Roger
- 4Q Motor control paradox
- Prev by Date: Re: RoHS => tin whiskers?
- Next by Date: Re: TO-220 Failures due to lead bending?
- Previous by thread: Re: 4Q Motor control paradox
- Next by thread: Simple discrete single cell converter with small standby current?
- Index(es):
Relevant Pages
|
