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Freewheeling or Generation(Motor)

J

Jon Slaughter

Jan 1, 1970
0
Is it common for a motor that is configured in an H-Bridge to be shorted out
when it is "freewheeling"(so it no longer is freewheeling)? Does this cause
the motor to loose speed slowing it down and waste power?

Basically I'm talking about what I should do with the low side mosfet when
turning off the high side. If I keep it off then the motor will freewheel if
I turn it on then it will I'll waste energy?

But I have to turn it off to get my mosfet drivers to work.

I guess I can flip the low side mosfet on the other side off too so the
wheel isn't shorted?
 
H

hrh1818

Jan 1, 1970
0
Is it common for a motor that is configured in an H-Bridge to be shorted out
when it is "freewheeling"(so it no longer is freewheeling)? Does this cause
the motor to loose speed slowing it down and waste power?

Basically I'm talking about what I should do with the low side mosfet when
turning off the high side. If I keep it off then the motor will freewheel if
I turn it on then it will I'll waste energy?

But I have to turn it off to get my mosfet drivers to work.

I guess I can flip the low side mosfet on the other side off too so the
wheel isn't shorted?

Are you supplying DC or AC power to your H bridge? Is your
controller designed for single quadrant or four quadrant operation.
Does your motor only rotate in one direction or is it a bidirectional
motor? Do you want your controller to act as brake when stopping
your motor?
 
J

Joerg

Jan 1, 1970
0
Tim said:
Yes, depending on the application.


Yes. It's quite useful for braking, if that's what you want.

"Waste". You're so judgemental.

Maybe John is thinking that the manufacturer will soon be slapped with a
carbon tax if he includes freewheel braking in the design ;-)

Like the pirate-copy tax the Europeans supposedly slap on things if
there is a CD burning capability included.

[...]
 
J

Joerg

Jan 1, 1970
0
Joel said:
They did it with DAT here in the U.S. as well:
http://en.wikipedia.org/wiki/Digital_Audio_Tape

Absurd, but then again... maybe an "we assume all of you are pirates" tax is
preferable to the heavy-duty DRM seen on Blu-Ray discs, Winhdows Vista, etc.?

I just resent it when a government body makes a blanket assumption that
the majority of users is engaging in criminal behavior. It is not right.
I've never pirated any music, software, movies or whatever and I do not
plan to. Heck, call me a luddite but I wouldn't even know how to
download music. But I also do not want to pay a tax for something I do
not commit.
 
I just resent it when a government body makes a blanket assumption that
the majority of users is engaging in criminal behavior. It is not right.
I've never pirated any music, software, movies or whatever and I do not
plan to. Heck, call me a luddite but I wouldn't even know how to
download music. But I also do not want to pay a tax for something I do
not commit.

Certainly they tax blank DVDs in France (and AFAIK most EU countries)
which makes them about 100 Euros for a spindle or about 10 times more
expensive than in the UK. For that reason, there are several online
shops in the UK that sell blank DVDs and their webpages are, for some
very strange reason, in French. Most French consumers that I have
talked to don't seem to realise that >90% of the price of blank DVDs
in their country is tax that AFAIK goes to some pretty arbitrary
companies, otherwise I doubt they would tolerate it. I think they
also tax USB hard drives now, and they re-adjust the tax per gigabyte
every so often, so that it stays just below the threshold where
consumers would start to ask where all that money is going. And then
even after you've paid the private tax, they still say it's illegal to
sell copied movies etc. There is some weak excuse like that in
exchange for the tax you are allowed to make copies of CDs for "close
family" or something like that but that is very discriminatory against
people who either make their own music and movies, or just feel like
recording some data for whatever reason that is nobody else's
business. They should not get to have it both ways, (either stop the
private taxation or declare any and all piracy legal when using highly
taxed media) but I guess these companies must have very good
lobbyists.

Chris
 
J

Jamie

Jan 1, 1970
0
Jon said:
Is it common for a motor that is configured in an H-Bridge to be shorted out
when it is "freewheeling"(so it no longer is freewheeling)? Does this cause
the motor to loose speed slowing it down and waste power?

Basically I'm talking about what I should do with the low side mosfet when
turning off the high side. If I keep it off then the motor will freewheel if
I turn it on then it will I'll waste energy?

But I have to turn it off to get my mosfet drivers to work.

I guess I can flip the low side mosfet on the other side off too so the
wheel isn't shorted?
normally, Hbridges have both High and Low side off during the low cycle.
And in your case, you need to have the other low side on to
keep the boost cap in check in the low cycle.

You can turn on both low sides to do regenerative braking, in this
case it wouldn't matter if the cap got charge or not because you're
not requested a drive current how ever, in both cases the low side
fets should come on with a low enough Ron to provide a low side path
for the boost cap. If this isn't the case then it's possible the motor
you're using is generating more REGEN energy than the low side fets can
short out.
But like I said, you should always have the driving FETS off during
the low cycle to not brake the motor and only the opposite low side to
keep the boost happy for the next on cycle..

During a slow down braking, you don't need the high side any ways so
there really isn't a problem here that I can see and you'll only miss
one on cycle on restart of driving state.

http://webpages.charter.net/jamie_5"
 
J

JSprocket

Jan 1, 1970
0
Jon said:
Is it common for a motor that is configured in an H-Bridge to be shorted out
when it is "freewheeling"(so it no longer is freewheeling)? Does this cause
the motor to loose speed slowing it down and waste power?

You can short the motor, freewheel it, regenerate, or divert the current
via a load resistor. The options have different advantages and
disadvantages. If it's freewheeling, it's unbraked, so it rotates until
the friction uses up the inertia. If it regenerates the energy can be
recovered, useful in battery applications, however keeping the voltage
under control can be a problem and the circuitry is more complex.
Shorting the motor brakes it, but the inertial energy is converted to
heat in the windings, so you have to be careful about the motor rating
and cooling arrangements. The load resistor dissipates the heat
elsewhere, and you can achieve different levels of braking if you need
to, but again the circuit is more complex. And don't forget about the
possibility of an overhauling load.

JS
 
C

Clifford Heath

Jan 1, 1970
0
Jon said:
Is it common for a motor that is configured in an H-Bridge to be shorted out
when it is "freewheeling"(so it no longer is freewheeling)?

You have some good answers. It seems some facts were missed though,
especially if you're driving a big motor.

If you short your motor, you can get almost as much current as when
you short your power supply with a DC resistance the same size as
the load's. The L/R tc means it takes time to build up, but if there
are enough joules freewheeling it will still hurt. If you have enough
resistance in the loop to stop it hurting, that resistance dissipates
the braking energy.

If you don't have enough resistance in the loop, you have to PWM the
short-circuit based on the L/R time constant, alternately routing the
developed current back into the battery, which stores the energy
instead of dissipating it as heat. *That" is how regenerative braking
is done.

If you don't want to regenerate and can't or don't want to short the
motor (i.e. you want true freewheeling) you may need to PWM until
the motor current reaches a low enough value that you can safely open
the circuit without the inductive spike causing pain.

I.e. a short may not be feasible but instantly opening the circuit
can cause pain also.

Clifford Heath.
 
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