setting Hitachi SJ200 / WJ200 VFD Dynamic braking parameters

[CRM]

Has anyone set up a Hitachi inverter for dynamic braking using an external braking resistor on a typical Bridgeport style spindle motor?
I've had my VFD successfully running the spindle for about a year now, but I have gotten E-stop fault trips when programming spindle speed changes that are significantly slower than the current one if I'm running in the upper range (2,500+ rpm) due to the VFD throwing a DC Bus overvoltage alarm. I also have the VFD set for FRS (free run stop) currently to avoid that problem when turning off the spindle, but I want to get to some decent decel rates now that I have the braking resistor installed and wired.

From what I can tell, parameters B097, B090, B095, B096, and B091 are 5 parameters related to dynamic braking. Are there any others? Does anyone have a list of values that would help get me in the ballpark?
As always, Thanks in advance!

[martyscncgarage]

I have never used Dynamic Braking. So I am interested to hear what Tom and Marc might have to say.
Simply I install the braking resisitor and I am then able to bring down the Decel times without tripping the VFD.
I use the max spindle speed as the worst case scenario to test.

Marty

[tblough]

The ABB VFDs that I use have a setting that combines dynamic and resistive braking. It uses the internal dynamic braking up until it starts to exceed the DC Bus max voltage, and then brings in the resistor. This helps keep the resistor from heating excessively during repeated speed changes like during rigid peck tapping 240 holes in an array

[BodeRacing]

Those appear to be the correct parameters. I am not familiar with he Hitachi drives. Are you sure you have a built-in braking transistor? Can you post your drive model number?

What values do you have in those parameters? Can you post all your parameters? Make sure that DC braking is disabled.

How fast do you want to stop it? My Atrump has a Bridgeport style spindle and from the factory they had the parameters all screwed up. Dynamic braking was not enabled. I was able to stop from 4200 rpm in 3 seconds and never tripped on DC bus overvoltage. With dynamic braking I did not change it - can go faster but its more abusive and I don't need it to stop faster.

[cncsnw]

Some corrections:

1) If you have your VFD programmed to ramp (decelerate) to a stop, then you are using dynamic braking.
2) Dynamic braking takes regenerated energy from the motor back in through the output transistors and puts it onto the drive's DC bus. This adds charge to the bus capacitors, increasing the voltage there.
3a) If the voltage gets too high and there is no resistor connected, then the drive has no choice but to fault and switch off all its transistors.
3b) If the voltage gets high and there is a resistor connected, then the drive can switch on its braking transistor, allowing current to flow from the DC bus, through the braking resistor. The effectively dumps excess power into the resistor.

[cncsnw]

In my experience with GS3 drives, and SJ700 drives with external braking units, dynamic braking performance can be significantly improved by lowering the voltage threshold for activating the braking transistor.

You just have to take care not to set the threshold so low that power is dumped to the resistor all the time. That would happen if the braking threshold were at or below the bus voltage that you see when the drive is powered up and idle (which in turn depends on your service voltage).

[martyscncgarage]

Some corrections:

1) If you have your VFD programmed to ramp (decelerate) to a stop, then you are using dynamic braking.
2) Dynamic braking takes regenerated energy from the motor back in through the output transistors and puts it onto the drive's DC bus. This adds charge to the bus capacitors, increasing the voltage there.
3a) If the voltage gets too high and there is no resistor connected, then the drive has no choice but to fault and switch off all its transistors.
3b) If the voltage gets high and there is a resistor connected, then the drive can switch on its braking transistor, allowing current to flow from the DC bus, through the braking resistor. The effectively dumps excess power into the resistor.

Great information Marc. Thanks for the explanation!
Marty

[BodeRacing]

Just for a clarification.... An induction motor works by having slip between the stator and rotor. Slip means that the stator electrical frequency is higher than the rotor frequency. This causes the rotor bars to cut magnetic lines of force causing current flow in the rotor bars which speeds up the motor. The more load you have the more slip you have. The motor is always working in equilibrium. Theoretically at no load the rotor spins at synchronous speed and the stator and rotor frequencies are the same so no current flows in the rotor bars as there is no magnetic lines of force to cut.

This is called negative slip.

However, an induction motor can also be used as a generator. Many times they are. For a motor to be a generator, the rotor must spin faster then the stator electrical field. This is called positive slip. Works just like a motor but in reverse.

This is where we get in trouble on a VFD. The inertia of the load (including motor rotor inertia) is keeping the rotor speed higher than the commanded stator frequency. As stated, this causes the motor to act as a generator and pumps the voltage up on the DC Bus. The dynamic braking resistor turns on to bleed the energy (we see it as voltage) off the caps and into the dynamic braking resistor by pumping current into the resistor. Positive slip increases in the motor (now a generator) but is limited by the the energy the inertial load has to generate power. As a user, you will never be able to tell that this is happening - it just works.

An alternative form of braking is regenerative braking where the energy is put back on the electrical grid. Very rarely would you see regenerative braking on our machines - usually reserved for large drives.

Another form of braking that we have in most drives is DC braking where DC current is injected into the motor windings. It does work but not as well as dynamic braking and is a bit abusive to the motor so should not be used for full braking on a regular basis.

I don't know if this makes any sense or not...........

[CRM]

Wow! Slip away for a little weekend of 4-wheeling and I get an avalanche of great advise.
First, THANK YOU to Marc for a great explanation of what is going on inside the "black box". That helps me a lot to use my intuition in deciding what parameters may need adjustment.
From the array of responses, it looks like I might not have been clear with what I CURRENTLY have configured, and what I want to CHANGE TO.
CURRENTLY: I have the Hitatchi VFD set up to Free Run Stop. I wanted to get the machine proved out before fine tuning the VFD parameters. The machine is now operational, so last week I mounted an EXTERNAL BRAKING RESISTOR that I acquired a month or two ago and wired it to the "RB" & P/+ terminals of the VFD with #10 ga wire.

Now, hopefully no one is going to ask me what the resistance and wattage specs are for the resistor. I can't remember off the top of my head, but I do remember the resistance was within the required specs for the inverter, and the wattage was significantly more than required. I'll have to find my notes if you must know.
These are pics of the actual inverter:

I understand that the motor and spindle drivetrain have inertia and on decel generates current that raises the DC bus voltage. I understand that is the reason my drive will occasionally fault out and trip the e-stop circuit. Up until now, when I was focusing on sorting out the servo system I lived with it and tried to avoid those decels that would cause a trip. Now I'm focusing on getting the VFD and spindle parameters tweaked into shape. I didn't turn up much about external resistors and how to set them up on a Hitachi drive on the Centroid forums, so I created this topic. I did find someone who shared some good (I hope) information on a different forum (practical machinist, I think), so I took a screenshot of the parameters and values he suggested. Just as importantly, he described in "real world" terms what those parameters are controlling. If anyone is familiar with the Hitachi SJ200, do the values look reasonable as a starting point?

[CRM]

Those appear to be the correct parameters. I am not familiar with he Hitachi drives. Are you sure you have a built-in braking transistor? Can you post your drive model number?

What values do you have in those parameters? Can you post all your parameters? Make sure that DC braking is disabled.

How fast do you want to stop it? My Atrump has a Bridgeport style spindle and from the factory they had the parameters all screwed up. Dynamic braking was not enabled. I was able to stop from 4200 rpm in 3 seconds and never tripped on DC bus overvoltage. With dynamic braking I did not change it - can go faster but its more abusive and I don't need it to stop faster.

According to the Hitachi manual, there is very limited Dynamic braking available without the external resistor, but the transistor is there to enable it, which now makes sense to me from Marc's explanation. I can't tell you off the top of my head what the values are in the parameters, it's been over 5 years since I initially set the VFD up on the machine; I did that while the old prototrak control was still on it because I needed to run from single phase power source. All I can remember is that I DID disable dynamic braking and set the drive to Free Run Stop because at the time I did not have an external braking resistor. I am only now getting back to revisit the VFD parameters and get dynamic braking to work, now that the Centroid control is pretty well sorted out and servos are tuned reasonably well.
I will document the parameters in the drive as they are now, and do an initial configuration for dynamic braking in the next couple of weeks. (This is my personal machine in my garage, the day job machinist gig and my 6 acre yard take precedence at the moment)