f5 errors powerflex 4

[realolman]

I don't know if this belongs here or not ... if not, I guess the mods can move or delete it (like they need my blessing )

I have three identical motor applications side by side. Elevators that lift boxes from one level to another

The operator is complaining that on one of them motor quits.

He says he used to pretty much be able to depend upon it happening at startup, but lately it has become more frequent. He called me out today ( first I've heard of it ) and the powerflex was displaying f5

The manual states:

OverVoltage DC Bus voltage exceeded maximum value.
Monitor the AC line for high line
voltage or transient conditions. Bus
overvoltage can also be caused by
motor regeneration. Extend the
decel time or install dynamic brake
option.

The three powerflexes are mounted right beside one another in the same cabinet. three phase AC motors
they all show the same settings in the parameters that seem to me might be relevant, but some of the settings I don't understand:

P036 Start source 2 “2-Wire” I/O Terminal 1 “Stop” = coast to stop.
P037 Stop Mode 4 “Ramp” Ramp to Stop.
are these two contradicting each other?

A080 DC brake time 0.0
A081 DC brake level 0.1
maybe this doesn't matter if the time is 0.0 ?

A082 resistor sel 0 Disabled

I don’t know what’s going on for sure here, but we have three “things” on top of the cabinet, that I thought were braking resistors hooked up to the BR terminals… Long boxes about 1” thick 3” high and a 8” long, that have heat sink fins, and looks to me like a thermistor sticking out the front ……does the A082 setting make any sense?

I changed a couple settings A 92 and A 93 auto restart tries, and auto restart time, to try to keep the thing going, but I don’t know that that was the right thing to do, and it also seems to affect other errors that maybe I don’t want to affect

Although I don't recall the decel time.... when I looked at it it seemed reasonable... plus with some of these settings I'm not at all sure the decel time is being ramped at all or if it's just coasting to a stop

To sum up …. I would like the motor to run correctly and safely, and get rid of the errors ….I hope someone will steer me in the right direction…. Thank you.

 

[Jraef]

Let's start here first:

...
P036 Start source 2 “2-Wire” I/O Terminal 1 “Stop” = coast to stop.
P037 Stop Mode 4 “Ramp” Ramp to Stop.
are these two contradicting each other?

A080 DC brake time 0.0
A081 DC brake level 0.1
maybe this doesn't matter if the time is 0.0 ?

A082 resistor sel 0 Disabled

I know this may sound confusing, but stick with me here.
P036 ONLY controls the function of Terminal 1, then P037 controls the Start-Stop method, which MAY or MAY NOT include P036. So if P036 = 2 for two-wire control, then Terminal 1 becomes strictly a Coast to Stop "override" so to speak, and P037 then determines the NORMAL Start-Stop functions. But if P036 were set to 3 for three-wire control, then terminal 1 would become the Stop button input. So in your case with P036 being set to 2 means terminal 1 is essentially going to function as an override of the NORMAL start/stop as implemented by P037. In short, this might be where you put in an E-Stop button (but that gets complicated too, so let's not go there) or anything else that may NOT want a ramp down as it normally functions. So a lot is going to depend on exactly how your control functions are wired into the terminals. MOST OF THE TIME in a two-wire control scheme, terminal 1 is left with the factory jumper( or wired to a NC contact in an E-Stop) and terminal 2 is wired to your Run-Stop switch of some sort.

The function of P037 is basically then your "NORMAL" start/stop function setting as a two-wire control scheme using terminal 2, and you have it set for Ramp to Stop. That means when you close Terminal 2, it is going to ramp up in Fwd, and when you open terminal 2, it is going to ramp back down to Zero. That also then means when you open terminal 2, any regenerative energy in the motor is going to try to come back into the VFD.

Ignore A081 and A0822. DC Injection Braking (DCIB) is not related, that is a separate function from Dynamic Braking (DB), which is where the braking resistors are used.

However if you have A082 set to 0 for Diabled, then your DB function is not working at all. So if your "things" on the top, which by the way do seem to be resistors from your description, are wired to the BR+ and BR- terminals, then you have useless Braking Resistors. Someone has apparently decided they didn't want to use them.

That may indeed be exactly why you are getting OV errors but again, it is going to depend on what your control circuit looks like. If it works as I descibed above, then when they turn off the elevator and it is going down (presumably), the regen energy from the motor is charging up the DC bus and without the DB resistors enabled, has nowhere to go.

Let me know if that didn't make sense.

 

[realolman]

thank you very much.. most of what you said did in fact make sense. If something didn't, I am sure it is my shortcoming and not yours. I am home now so I'll have to look into the particulars tomorrow.

I think with the A082 setting of 0, DB is disabled indeed ...and probably never did work... wouldn't surprise me at all with this particular company's record. I don't know why one of three would cause problems, though... but I guess weirder things have happened.


Could you ( or anybody else who would care to ) give me a "dynamic braking for dummies" explanation of what is going on with the dynamic braking and the VFD?

How does a three phase induction motor (re?)generate energy that needs to be dissipated by resistors? I was unaware they could do that.

 

[Jraef]

"Dynamic braking for dummies"

"Dynamic braking for dummies"

Any AC squirrel cage motor can also be a generator. In fact a lot of grid connected generators, such as Induction Wind Generators, are in fact just AC motors.

To become an induction generator, an AC motor needs 2 things:

1) an overhauling prime mover, meaning something driving the shaft FASTER than the synchronous speed of the motor, and

2) magnetic excitation of the windings, because it is an electro-magnet, not a permanent magnet. So going back to the wind generator example, the windmill over-drives the AC motor faster than the synchronous speed, and connecting to the grid supplies the excitation of the windings.​

So when these conditions are met, the induction generator pumps the same amount of energy it could have used as a motor BACK into the grid as a generator. Because it is excited BY the grid, it is inherently synchronized WITH the grid.

Now how this ties into a VFD;

A VFD is changing voltage and frequency together at a pre-determined rate to maintain the same V/Hz profile as the motor's original synchronous speed design. So in effect, the VFD is changing the synchronous speed of the motor. To that end then, if the LOAD is spinning already, and you LOWER the synchronous speed of the motor with a VFD's controlled Decel function to a theroetical synchronous speed below that of the spinning motor, then you are satisfying condition #1, and by virtue of the VFD keeping the motor energized for deceleration, then you are satisfying condition #2. The result is, the motor has and continues to be a generator as long as both conditions are met.

The energy that the motor now regenerates is going to pass back into the VFD through the gated transistors. In that process, the transistors will act as a rectifier and pump that energy back into the DC bus in the VFD. The DC bus capacitors can only absorb a very small amount of that energy, and if it has one, the capacitor bleed resistor can dissipate a little bit more. But it's not much, so what happens is that the DC bus gets charged up and there is nowhere for that energy to go. To protect itself then, the VFD trips off before that voltage starts flashing over inside the VFD. That means the transistors turn off, which cuts off the excitation of the motor and it coasts to a stop, but no longer regenerates back into the VFD.

Enter a Dynamic Braking system. For a DB to work you need two (new) things:
1) resistors, called Dynamic Braking Resistors or DBRs, and 2) a switch that controls WHEN the resistors are connected. This switch is another transistor and is called a "Chopper" because it functions to fire the DC energy into the resistors in a pulsed, or chopped, fashion. The DB Chopper circuit also monitors the DC bus voltage level and will only turn on when the DC bus rises above the VFD's capacity,, otherwise the VFD would be heating up the resistors all of the time instead of powering the motor. DB choppers are typically built-in to small drives because they are part of a single power "chip" called an IPM (Intelligent Power Module) with all of the diodes and transistors together with their firing circuits. On larger VFD where the power components are all discrete (separate), the DB Chopper is often a separate stand alone module that attaches to the DC bus and mounts to the side, with its own heat sink, PC board etc.

Back to the DB resistors. They will be outside of the VFD because they get hot. The VFD's DB system is converting the kinetic energy of the spinning load into electrical energy, then the resistors convert it again into thermal energy and get rid of it into the air. Most of the time, you are stuggling to keep a VFD cool all by itself, you don't want the added burden of trying to find a place for the MOTOR thrmal energy too, so you put the resistors outside of the box.

So when you "Enable" the DB function on the VFD, you are telling it that you want the DB chopper to monitor the DC bus voltage and fire that excess enery off into the resistor banks to be burned off as heat rather than trip the VFD off-line.

 

[realolman]

well, thank you very much for that... I'm going to have to chew on that a while. Hopefully a reasonable portion will soak in.

I will never look at this vfd/ cabinet / resistor / motor combination the same again.

funny how that is.