PowerFlex 70 Braking with External Resistor

dmned64

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I have a PowerFlex 70 with an external braking resistor connected. The braking function does not appear to be working. Attached is an image on the pertinent parameters from the VFD. Anything obvious about the configuration that would prohibit braking with the external resistor?

Comments welcome...

Capture.PNG
 
Is any of the on-board Inputs (P361 through P366) configured as "Stop Mode B" (=13) and is said Input '1' when the VFD is commanded to stop?

Stop Mode A (Ramp to 'Stopped' without DC Brake) is the default active Stop Mode unless an on-board VFD Input is configured as Stop Mode B (=13) and the Input is High when the drive is commanded to stop.
 
Is any of the on-board Inputs (P361 through P366) configured as "Stop Mode B" (=13) and is said Input '1' when the VFD is commanded to stop?

Stop Mode A (Ramp to 'Stopped' without DC Brake) is the default active Stop Mode unless an on-board VFD Input is configured as Stop Mode B (=13) and the Input is High when the drive is commanded to stop.


P361~P365 are all set to 'Not Used' and P366 is set to 'Enable' (and is High). I am uncertain of the bearing Stop Mode B has in this setup unless having it set to 'DC Brake' is what is causing this issue. Could the braking function if Stop Mode B (P156) is set to 'Ramp' without setting any of P361~P365 to 'Stop Mode B'? It seems the solution you suggest would require wiring one of the inputs.
 
It seems the solution you suggest would require wiring one of the inputs.

It is not a suggestion; it is the only way you could use a DC Brake with a PF70.

I presume it is some sort of safeguard against accidentally programming the drive for DC Braking without actually having an installed DC Brake; programming and wiring the controls presume a well thought intention.

If you don't have a hard wired Stop push-button (with a spare NO contact) or a PLC Output/Relay (which will be '1' when the VFD is commanded to stop) I'd suggest using one of the VFD's on-board DOs (P380 or P384), wired into the selected and programmed Stop Mode B DI, output which will be programmed as "DC Braking" (=16).
 
Is any of the on-board Inputs (P361 through P366) configured as "Stop Mode B" (=13) and is said Input '1' when the VFD is commanded to stop?

Stop Mode A (Ramp to 'Stopped' without DC Brake) is the default active Stop Mode unless an on-board VFD Input is configured as Stop Mode B (=13) and the Input is High when the drive is commanded to stop.

It is not a suggestion; it is the only way you could use a DC Brake with a PF70.

I presume it is some sort of safeguard against accidentally programming the drive for DC Braking without actually having an installed DC Brake; programming and wiring the controls presume a well thought intention.

If you don't have a hard wired Stop push-button (with a spare NO contact) or a PLC Output/Relay (which will be '1' when the VFD is commanded to stop) I'd suggest using one of the VFD's on-board DOs (P380 or P384), wired into the selected and programmed Stop Mode B DI, output which will be programmed as "DC Braking" (=16).

That's good to know about the PF70. The drive is controlled via the Ethernet/IP connection so not physical stop button is *not* being used. Thanks for the input, much appreciated.
 
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Check P153 Regen Power Limit

Sets the maximum power limit transfer from
the motor to the DC bus. When you are using an
external dynamic brake, set this parameter to its
minimum
(–800.0%) value. Overvoltage trips can occur if
set too negative and the connected brake is
unable to dissipate the energy.

You could also set Bus Regulation to both, db first.
 
I have a PowerFlex 70 with an external braking resistor connected. The braking function does not appear to be working. Attached is an image on the pertinent parameters from the VFD. Anything obvious about the configuration that would prohibit braking with the external resistor?

Comments welcome...

Have you checked that the brake resistor is the correct resistance? It`s just a resistor, and things get damaged in shipping ...

The brake resistor we have on the PF700 (not 70) has a thermostat wired as an input, so that when the temperature gets too hot, it will disable the dynamic brake. The thermostat has never gotten hot, but we did have a wire come off the thermostat terminal and the brake resistor functionality stopped working. The input is normally ON and goes OFF when the thermostat gets hot.

Perhaps this input is the Normally Closed signal that @dmargineau described. If so, I don't think that the input is optional, even if the drive is controlled via Ethernet. Ours is running on devicenet.
 
I know EXACTLY what your issue is...

DC Braking is totally DIFFERENT from Dynamic Braking, and it is only Dynamic Braking that uses the braking resistor! DC (Injection) Braking or DCIB means the VFD re-fires the transistors to pump DC into one winding of the motor, giving it a non-rotating magnetic field in the stator and the rotor attempts to follow it. DCIB then converts the kinetic energy in your spinning load into heat energy inside of the motor, so it should be avoided unless necessary. If you have to use it, you must count each Stop cycle as if it is a Start cycle when considering the "starts-per-hour" capability of the motor as well, otherwise you may burn it up.

Dynamic Braking is where the VFD keeps the motor energized so that the magnetic fields are still there, but lowers the frequency of the stator relative to the rotor so that your motor becomes an induction generator, pumping electricity back into the VFD. then once it is in the DC bus and raises the bus voltage above a threshold, that energy is shunted off into a resistor using a 7th transistor called a "braking chopper". So the kinetic energy of your load is REMOVED from the motor and put into the resistor to be burned off as heat.

Dynamic Braking can never completely stop a load however, because it suffers from the law of diminishing returns in that the slower the motor gets the less braking energy remains. So that's why VFDs include DCIB. You use Dynamic Braking to slow the motor down quickly without stressing the motor, then finish the job with DCIB once there is only a little kinetic energy left.

So you don't actually "turn on" Dynamic Braking, it's automatic. What you do is to set the Stop Mode to "Ramp", then set a very aggressively low ramp time. That will force the motor into regeneration and put the energy onto the DC bus. When the DC bus voltage increases above the threshold, the Brake Chopper transistor automatically fires to dump it into the resistor (if it's there).

In your case, you have Stop Mode 1 set for ramp, which is right, but you probably have too long of a ramp time set in parameter 142, which you don't show. The factory default is 10 seconds, but if your load has a Cost-to-Stop time of that or less, you will never see any Braking effects. If you want to stop it faster, the Decel ramp Time must be set at some value LOWER than the natural Coast time.


As to why your DCIB was not working (lucky for you because at those settings, you may have smoked something!): With the Stop Mode set to "Ramp", it is never going to employ the DC Braking, unless, in this case, you have some external input assigned to trigger "Stop Mode B". If you want to use DCIB at the end of your Dynamic Braking, set your Stop Mode to "Ramp to Hold" (a value of 2). Then for goodness sake, go back to 158 and TURN THAT DOWN! You likely need no more than about 10% of your motor FLA to finish stopping that, and set the DC Brake time to no longer than it takes to get it to fully stop. remember, DCIB is injecting HEAT into your motor windings.
 
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Originally posted by jraef:

Dynamic Braking can never completely stop a load however, because it suffers from the law of diminishing returns in that the slower the motor gets the less braking energy remains. So that's why VFDs include DCIB. You use Dynamic Braking to slow the motor down quickly without stressing the motor, then finish the job with DCIB once there is only a little kinetic energy left.

This is true with DC motors driven by single quadrant SCR drives where the resistor is tied directly between the armature wires. This is not the same thing that is happening with a VFD. The thing that limits the braking ability of a VFD is the motor control mode. If you are using vector control mode with an encoder (or DTC) you will be able to push all the energy you want energy back on the DC bus right down to zero speed. Scalar mode or most of the sensorless vector modes just can't get any torque out of the motor in any quadrant when the speeds get too low.

On thing I noticed is that you have your bus regulation mode set to Both- Freq 1st. This will cause the drive to adjust the frequency as the bus level increases in an attempt to keep the bus level down. If it can't control the bus level that way it will start to use the braking chopper. If you want the braking resistor to do anything select either Both - DB 1st or, better yet IMHO, DB only. The way I look at it I would rather know I can't control the decel event by having the drive fault than to have the drive basically ignore the decel rate I have it configured to follow. It's just easier to know you have a problem that way.

Keith
 
Thank you for all the good feedback. I have been working on another project and just now getting back to this thread. I plan to post what I believe is a solution based on the collective responses and welcome comments.
 
I have read back through the thread and note that some of the focus has been on DC braking which is not a method I am trying to employ. I have successfully used DC braking with PowerFlex drives on other occassions but did not want to use it on this application. The loads being driven by the drives are high inertial and I didn't want to use DC braking for an extended period. These loads naturally could take 20 to 30 minutes to coast to stop without braking.

So, for the purpose of this discussion, and not planning to use DC braking, it seems the need to properly configure Stop Mode B, et al is not necessary. @jraef mentioned significantly lower the value of P158 but this would only apply if DC braking were to be employed.

If I understand the discussion, with a brake resistor wired to the drive, the energy from the motor will transfer from the DC bus to the resistor after the Decel time has expired. On one of the drives I have the Decel set to 90 seconds which is far less than the natural coasting time of the load. Therefore I would have expected the resistor to begin absorbing the energy after the Decel had expired, a situation I don't believe we have experienced.

Several have mentioned P161 is set to Both-Frq 1st. I can easily set this to Both-DB 1st or Dynamic Brak which, as noted, should employ the DB resistor as desired.

@lanman mentioned setting P153 to -800% which I had already done based on some other threads I had read.

Finally, @thingstodo recommended physically checking the resistor integrity and for proper wiring, particularly of the thermostat. As I recall the resistors do have thermostats but I will need to verify and check if they were wired properly. I was not present for the installation of the resistors so I did not personally observe the wiring.

Comments and questions are welcomed.

SUMMARY
1. Make no changes to employ DC braking.
2. Change P161 to Both-DB 1st or Dynamic Brak.
3. Verify P153 on all three drives being set to -800%.
4. Verify P142 is far less than the natural costing time of the loads.
5. Verify the integrity and wiring of the resistors and thermostats.
 
If I understand the discussion, with a brake resistor wired to the drive, the energy from the motor will transfer from the DC bus to the resistor after the Decel time has expired. On one of the drives I have the Decel set to 90 seconds which is far less than the natural coasting time of the load. Therefore I would have expected the resistor to begin absorbing the energy after the Decel had expired, a situation I don't believe we have experienced.

No, DC Bus is not capacitor or battery, that will hold this energy and release it later. You constantly feed it from the inverter of the VFD. Then your motor starts decelerating, which starts to give power back to your DC bus. Voltage raises and your chopper releases this power to the braking resistor for short pulse, until it goes below the threshold, then raises again, again release. On the large resistors, you can even hear it, like a bzzz sound.

So maybe, you are in the situation Keith says, that on Both- Freq 1st, you just don't go so high on the DC bus and you can do what lanman proposed and adjust it to both DB first.
 

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