PowerFlex 755 Dynamic Braking Monitoring

I haven't done many applications with braking resistors, but we just put one in for a customer on a 60HP application for a centrifuge. We're monitoring amps and DC Bus Volts and trying to figure out the best way to maximize the deceleration without tripping the overvoltage. The DC Bus Volts max is 790vdc. We're currently seeing around 740vdc in decel. In theory, when we speed up the decel, will that increase the DC Bus Volts, or is that regulated and we should be watching something else?

First with a buss voltage of 740 on decal then you are not controlling the decal at all
It sounds like the drive is just turning off the output transistors on a stop command and you are still coasting to a stop
You need to enable ramp to stop in the drive then set the decal ramp to a value you can control
Normally the buss loader (Braking ) module turns on abut 780v and off about 750v
I hope you installed the recommended resister if not you could damage the drive
A better option would be to change out the drive for one that can line regenerate then you can get 100% breaking torque at much faster rate

GaryS said:

First with a buss voltage of 740 on decal then you are not controlling the decal at all
It sounds like the drive is just turning off the output transistors on a stop command and you are still coasting to a stop
You need to enable ramp to stop in the drive then set the decal ramp to a value you can control
Normally the buss loader (Braking ) module turns on abut 780v and off about 750v
I hope you installed the recommended resister if not you could damage the drive
A better option would be to change out the drive for one that can line regenerate then you can get 100% breaking torque at much faster rate

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It's absolutely controlling decel. In coast mode, it takes about 30 minutes to come to a stop. We are stopping it in 3 minutes with decel. The resistor bank, which was properly spec'd, gets warm during this period.

I just need to know which parameter we should be looking at while playing with the decel rates so we don't come near an overvoltage condition.

I am not sure what you are trying to do
You have no control of the buss voltage other than enable Buss Loader the manufacture has set up all the parameters the on off voltage levels
When you measure the voltage level you are measuring the average voltage. The transistor is pulsing on and off way too fast for you to measure it without a high frequency scope.
Some drives will allow you to display the average buss voltage on the display
It should be close to what you will read with a meter
All you can change is the decal time if it is too short and the buss voltage goes to high the drive may fault and the motor will coast to a stop or it may just coast to a stop not depending on the drive control.

Something about you voltages doesn’t look right 790v is low for a 480v system, those capacitors are all rated well over 1,000 v
480 x 1.414 = 670 average buss voltage, 480 x 1.717 = 824 peak voltage with no motor load ( motor rnot running) I would expect to see voltages very near to the peak voltage range
I would have expected your measure voltage to higher well above 750 even at full braking
The only thing pulling power off the buss is the loader transistor and load resister
I have provided some links that may be helpful
As I have said before a line regen drive would be a better choice it would give you the 100% control you are looking for


Variable Frequency Drives: Viewing the DC Bus (testandmeasurementtips.com)
VFD Overvoltage Fault - troubleshooting variable frequency drives (precision-elec.com)
tps://voltage-disturbance.com/variable-frequency-drive/troubleshooting-vfd-problems-how-to-measure-dc-bus-voltage-and-dc-ripple/

DC Bus Volts are around 630vdc while running, with load. That goes up to ~740vdc in our current decel profile. But I think you just indirectly answered my question. So if we speed up the deceleration, the DC bus volts should go higher? If that's the case, then that is what we'll monitor. We'll pull that parameter into the PLC through a datalink and display it on the HMI.

Since time = $$, the idea is that we'll get the deceleration as fast as we can while staying a reasonable bit below the over-voltage trip.

The limiting factor is the resistor, when you dont change your setup drastically.

phuz said:

So if we speed up the deceleration, the DC bus volts should go higher?

Click to expand...

I think that the brake chopper will activate more often, and the resistor will be heated up faster. So you should monitor the resistor overtemperature sensor if there is one. The ones I use always have an overtemperature switch.
I think the only way you can figure out how much braking action you can apply without overheating is by experimenting.

JesperMP said:

The limiting factor is the resistor, when you dont change your setup drastically.
I think that the brake chopper will activate more often, and the resistor will be heated up faster. So you should monitor the resistor overtemperature sensor if there is one. The ones I use always have an overtemperature switch.
I think the only way you can figure out how much braking action you can apply without overheating is by experimenting.

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The resistor bank only has an overtemperature switch that opens, if I understand correctly. I'll have to ask what that temperature is, specifically, and maybe have the customer hit it with an IR gun during braking.
I agree that experimentation is going to be the only way to land where we need.

Even if you use an IR gun, what are you going to conclude by that ?
If you go to the very limit, I can guarantee you that the brake resistor will get smoking hot. You can also consider going for a bigger resistor. The recommendations of the VFD vendors are usually on the low side.

edit: With experimenting I meant brake harder and harder until you find the brake resistor goes on overtemperature before coming to a full stop. Then dial the braking back from that point.

edit edit: Also consider the necessary cooling time after each stop. It will take some time. The IR gun will definitely be helpful for that.

JesperMP said:

Even if you use an IR gun, what are you going to conclude by that ?
If you go to the very limit, I can guarantee you that the brake resistor will get smoking hot. You can also consider going for a bigger resistor. The recommendations of the VFD vendors are usually on the low side.

edit: With experimenting I meant brake harder and harder until you find the brake resistor goes on overtemperature before coming to a full stop. Then dial the braking back from that point.

edit edit: Also consider the necessary cooling time after each stop. It will take some time. The IR gun will definitely be helpful for that.

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We can find the thermal limit by using the IR. We actually oversized the resistor for the application. Based on theoretical values with the spec'd resistor, we could stop in 30 seconds, and we're currently at 90.

Not overly worried about cooling because this will perform a decel no sooner than once per hour, so the duty cycle is extremely low.

If you are showing a buss voltage of 630v while running then your supply voltage is a little low. That will limit the available power out of the motor but if your not having a problem with that it shouldn’t be a concern. But that works in you favor in braking because the buss had to raise more before the braking starts.
It sound like you have covered just about everything I don’t understand what more you are trying to do
There are limits to everything
Just something to think about, shifting a motor from high torque motoring to high torque braking can cause the windings in the motor to shift in the slots and brake down the winding insulation you might consider having the motor VPI ed to stabilize the windings it would improve motor life

GaryS said:

If you are showing a buss voltage of 630v while running then your supply voltage is a little low. That will limit the available power out of the motor but if your not having a problem with that it shouldn’t be a concern. But that works in you favor in braking because the buss had to raise more before the braking starts.
It sound like you have covered just about everything I don’t understand what more you are trying to do
There are limits to everything
Just something to think about, shifting a motor from high torque motoring to high torque braking can cause the windings in the motor to shift in the slots and brake down the winding insulation you might consider having the motor VPI ed to stabilize the windings it would improve motor life

Click to expand...

480VAC going to the drive. It's not low.

I was merely asking what parameter I should be watching in the list of 1000 parameters available that would give us some idea when we are approaching the limit of how fast we can slow the drive down without going into a DC bus overvoltage. That's all.

I would think you still could monitor motor amps, kw or torque, and come to a pretty good conclusion on the wattage you are dissipating.

You could also just measure the DC volts and amps going into the resistor (perhaps with a recording scope), and figure the watts.

Gene Bond said:

I would think you still could monitor motor amps, kw or torque, and come to a pretty good conclusion on the wattage you are dissipating.

You could also just measure the DC volts and amps going into the resistor (perhaps with a recording scope), and figure the watts.

Click to expand...

Which parameter shows the amps for the DC bus?

I was meaning motor amps. You can then roughly figure kw if the drive can't give you that. I have no idea of the parameters on AB drives, I try to avoid them.

If you want the fastest decel possible, with zero issues, get rid of the DB Resistors, and put in a Bonitron Line Regen unit.


I've been using them here for several years now, from 7.5HP motors up to 300 HP, and eliminated all of my OV trips on decel, even the occasional ones on steady state running on large inertial loads.