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Home > Archive > Electrical Engineering > August 2005 > Braking Resistor
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| Hello, can someone give me a explanation on how a braking resistor works on
a VFD for a 480 V 3 ph motor?
I know that it is used to dissipate the energy, but how is the resistor used
in the motor cct?
Thanks, Dave
| |
| Ignacio Simón Yarza 2005-08-22, 8:21 am |
| Hello,
Hope that these might clarify the facts:
1) VFD FOC technology gets equivalent behaviour to inverse braking but in
such a way that differential speed between stator and rotor magnetic fields
is completely controlled. A lagged magnetic field is produced on stator
windings applying voltage by means of PWM technique. As this magnetic field
is lagged with respect to the magnetic field driven by the rotor, a force
appears that makes the rotor speed to slow down .
2) In terms of voltages, these rotor and stator magnetic fields result in
two differential voltages. The difference between them yields a voltage
resultant which generates a current flowing back into the VFD. This energy
returning path accounts for the dissipation of kinetic energy on the rotor
axle. But, Where does the kinetic energy goes?
3) Braking resistors are just a junk box for energy dissipation. It is
useful as long as it has got energy dissipation capability. They are passive
elements, rough and basic elements. While the VFD is only an energy
transducer, braking resistors are electric loads which must evacuate the
mechanical energy produced on the motor axle (resistor powerdissipation
ability>braking torque*angular speed)
4) This energy is evacuated by the operation of a transistor placed at the
DC bus bar of the VFD, which switch on the path between bus bar and
resistors when it is detected that energy is being sent back from the motor
to the VFD.
3) The additional point to be clarified is why using this braking technology
instead of using a magnetic field acting proportionally to axle speed or
phase changing?? just for the same reason for which VFD are used instead of
direct feeding of motor or static voltage-starters. As FOC theories allow
treating AC motors as if they were DC control-prone motors, so does VFD
braking. As long as theThyristor bridge is able to handle the currents, any
braking curve may be obtained. Other technlogies as DC braking are cheaper
but on its basic development it only allows pre-defined braking profiles
like damped braking, with a counter-force proportional to speed, and it is
usual that the mechanical brake have always to work at the end of the
braking cycle.
Best Regards.
Ignacio Simón Yarza.
Mech eng
Electronic and automatism eng
"Bob" <licku45@yqahoo.com> escribió en el mensaje
news:od84d1lfflftmkebostclpm3v7q5fguqds@4ax.com...
Quote:
Actually, most VFDs do both. Dynamic Braking, i.e. using the braking
resistors, is a good way to bring the motor down CLOSE to a stop, but
as the motor slows, the energy in it becomes less andless, so the
braking energy that can go into the resistors becomes less as well. So
at some point, the law of diminishing return takes over, and you have
to finish the job using DC injection braking.
On Wed, 6 Jul 2005 18:47:57 +0100, "Ted Rubberford"
rubberford@hotmail.com> wrote:
"Fred" <KirkJohnson@shaw.ca> wrote in message
news:syNye.1875618$6l.883601@pd7tw2no...
Correct me if I'm wrong but I believe that AC motors are braked by
introducing a DC current across the stator windings??
Fred
Read the post again boy, "motors controlled by a VFD"
Teddy Rubberford.
Bizarre Bugger...
me@home.com> wrote in message
news:dhbmc1hp68hq14o2a7m7gsvqlet9lmt358@4ax.com...
The only place for motor energy to go is into the load or into the
resistor. To slow the motor the drive the energy has to go somwhere,
its only path is back through the IGBT or similar device back to the
DC bus, where a resistor bleeds off the excess energy to keep the DC
voltage under control.
The AC/DC converter on the front end is different for a regenerative
AC drive. SCRs vs. diodes. It lets you shut off or otherwise control
the incoming power to control the braking.
On Tue, 05 Jul 2005 23:58:49 GMT, "Dave" <feltcher@telus.net> wrote:
Hello, can someone give me a explanation on how braking resistors work
on
AC
motors controlled by a VFD?
Thanks, Dave
"Dave" <davco1@telus.net> escribió en el mensaje
news:gJQNe.151327$wr.27141@clgrps12...
> Hello, can someone give me a explanation on how a braking resistor works
> on a VFD for a 480 V 3 ph motor?
>
> I know that it is used to dissipate the energy, but how is the resistor
> used in the motor cct?
>
> Thanks, Dave
>
| |
|
|
"Ignacio Simón Yarza" <ignaciosimon@ya.com> wrote in message
news:dec8oa$3ni$1@news.ya.com...
> Hello,
> Hope that these might clarify the facts:
>
> 1) VFD FOC technology gets equivalent behaviour to inverse braking but in
> such a way that differential speed between stator and rotor magnetic
> fields is completely controlled. A lagged magnetic field is produced on
> stator windings applying voltage by means of PWM technique. As this
> magnetic field is lagged with respect to the magnetic field driven by the
> rotor, a force appears that makes the rotor speed to slow down .
>
> 2) In terms of voltages, these rotor and stator magnetic fields result in
> two differential voltages. The difference between them yields a voltage
> resultant which generates a current flowing back into the VFD. This energy
> returning path accounts for the dissipation of kinetic energy on the rotor
> axle. But, Where does the kinetic energy goes?
>
> 3) Braking resistors are just a junk box for energy dissipation. It is
> useful as long as it has got energy dissipation capability. They are
> passive elements, rough and basic elements. While the VFD is only an
> energy transducer, braking resistors are electric loads which must
> evacuate the mechanical energy produced on the motor axle (resistor
> powerdissipation ability>braking torque*angular speed)
>
> 4) This energy is evacuated by the operation of a transistor placed at the
> DC bus bar of the VFD, which switch on the path between bus bar and
> resistors when it is detected that energy is being sent back from the
> motor to the VFD.
>
> 3) The additional point to be clarified is why using this braking
> technology instead of using a magnetic field acting proportionally to axle
> speed or phase changing?? just for the same reason for which VFD are used
> instead of direct feeding of motor or static voltage-starters. As FOC
> theories allow treating AC motors as if they were DC control-prone motors,
> so does VFD braking. As long as theThyristor bridge is able to handle the
> currents, any braking curve may be obtained. Other technlogies as DC
> braking are cheaper but on its basic development it only allows
> pre-defined braking profiles like damped braking, with a counter-force
> proportional to speed, and it is usual that the mechanical brake have
> always to work at the end of the braking cycle.
> Best Regards.
> Ignacio Simón Yarza.
> Mech eng
> Electronic and automatism eng
>
>
>
> "Bob" <licku45@yqahoo.com> escribió en el mensaje
> news:od84d1lfflftmkebostclpm3v7q5fguqds@4ax.com...
>
> Quote:
> Actually, most VFDs do both. Dynamic Braking, i.e. using the braking
> resistors, is a good way to bring the motor down CLOSE to a stop, but
> as the motor slows, the energy in it becomes less andless, so the
> braking energy that can go into the resistors becomes less as well.
> So
> at some point, the law of diminishing return takes over, and you have
> to finish the job using DC injection braking.
>
> On Wed, 6 Jul 2005 18:47:57 +0100, "Ted Rubberford"
> rubberford@hotmail.com> wrote:
>
>
> "Fred" <KirkJohnson@shaw.ca> wrote in message
> news:syNye.1875618$6l.883601@pd7tw2no...
> Correct me if I'm wrong but I believe that AC motors are braked by
> introducing a DC current across the stator windings??
>
> Fred
>
> Read the post again boy, "motors controlled by a VFD"
>
> Teddy Rubberford.
> Bizarre Bugger...
>
>
> me@home.com> wrote in message
> news:dhbmc1hp68hq14o2a7m7gsvqlet9lmt358@4ax.com...
> The only place for motor energy to go is into the load or into the
> resistor. To slow the motor the drive the energy has to go somwhere,
> its only path is back through the IGBT or similar device back to the
> DC bus, where a resistor bleeds off the excess energy to keep the DC
> voltage under control.
>
> The AC/DC converter on the front end is different for a regenerative
> AC drive. SCRs vs. diodes. It lets you shut off or otherwise control
> the incoming power to control the braking.
>
> On Tue, 05 Jul 2005 23:58:49 GMT, "Dave" <feltcher@telus.net> wrote:
>
> Hello, can someone give me a explanation on how braking resistors
> work
> on
> AC
> motors controlled by a VFD?
>
> Thanks, Dave
>
>
>
>
>
>
>
>
>
> "Dave" <davco1@telus.net> escribió en el mensaje
> news:gJQNe.151327$wr.27141@clgrps12...
>
>Thanks. Yes the motors are driven by ABB drives with a braking resistor for
>each drive mounted in a vented enclosure on top of the VFD cabinets.
Dave
| |
| Ignacio Simón Yarza 2005-08-22, 5:21 pm |
| What do you mean by "how is the resistor used in the motor cct?"?
Are you meaning "how is the resistor used in the motor circuit?"?
If so, the explanation is given below, in the 4th point. Otherwise, may be I
would need that you make the question clearer.
Best regards.
Ignacio Simón Yarza.
Mech&electrical-electronics engineer.
"Dave" <davco1@telus.net> escribió en el mensaje
news:u4nOe.234797$on1.229560@clgrps13...
>
> "Ignacio Simón Yarza" <ignaciosimon@ya.com> wrote in message
> news:dec8oa$3ni$1@news.ya.com...
>
> Dave
>
| |
|
| On Sun, 21 Aug 2005 01:24:28 GMT, "Dave" <davco1@telus.net> wrote:
>Hello, can someone give me a explanation on how a braking resistor works on
>a VFD for a 480 V 3 ph motor?
>
>I know that it is used to dissipate the energy, but how is the resistor used
>in the motor cct?
>
>Thanks, Dave
>
The VFD must have a "chopper circuit" to feed the excess energy into
the resistor. It basically senses the DC bus voltage, and as
regenerative energy rises the DC bus, the chopper turns on to bleed
that energy off by burning it into the resistor as heat. Once the DC
bus level drops, the chopper turns off again.
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