Overcurrent Braking Trip

Toms Wheels · May 3, 2008

On occiasion when braking my lathe the VFD will trip Overcurrent.Generally with the 6 jaw chuck and a 5+ pound workpiece.
Monarch 10EE, 5HP Blackmax motor, Hitachi SJ200 VFD, 1000rpm+, 225w 50u braking resistor, decel time <.5sec.

I can prevent the OC by increasing the decel time. But I don't like the stopping time when threading in BG, so I then reset the decel time when threading.

If it OC should I get a higher Ohm resistor, and leave the 225w? Ideas?

Steve in SoCal · May 3, 2008

Tom,

Do you have the option of setting more than one decel profile? If so how about a micro-switch that changes depending on the position using the back gear lever as the actuator. More resistors would convert more current.

Steve

JST · May 4, 2008

OVERCURRENT isn't a matter of the resistor......

If you have an OVERVOLTAGE trip, THAT would be a matter of possibly too high a resistance, which can't drain the energy out fast enough.

Over-CURRENT sounds like trying to stop it too fast. That is somewhat like a "plug-reverse" that stops with the motor stopped, before it quite reverses. As such, it may draw lots more than running current.

You may be able to change the OC TRIP time. Some allow a range of OC trip times, from immediate to a short delay. That might allow you do get through the peak.

Failing that, slow the deceleration.

Pierce Butler · May 4, 2008

If you want instant stop, get a RPC and a reversing switch. That will bring that bad boy down to zero in a hurry.

You haven't lived until you tried doing a quick fwd-to-reverse on a single phase motor.

JunkyardJ · May 4, 2008

Well, there are some options to play with.

First, are you getting an overCURRENT trip (E02), or a braking resistor usage error (E06), or a DC bus overVOLTAGE error (E07)? If you get an E07, you can tell it to pause braking until the voltage drops down under control. Then it will slow the motor as fast as possible. You MAY be able to get a better stop time by going with a slightly LOWER (less ohms) resistor, with a higher wattage rating. That's ONLY IF you're getting an E07, E02 means it's getting rid of as much DC voltage as it can without blowing up the transistor inside for braking with the resistor you have. It's CHEAPER to buy a bunch of small resistors, and put them in parallel. Setting B130 is what you need to change to help with the overvoltage problem. Then you can play with the DC braking functions to get a little faster stop (A051, A052, A054, and A055 3-19 in the manual). If you get an E06, you need to tell it a higher percentage for B090. You can enable the overcurrent trip supression (B140), but I think that's a BAD idea. You SHOULD try to get the braking current under control.

Another option that MAY be possible, is to put an external DC braking unit on there that can take more current than the VFD's internal transistor for braking. I dunno if the smaller SJ200s can support such a thing.

JST · May 4, 2008

Will it even detect an overcurrent in the brake?

Many OC trips are on the phase outputs only. In that case, this phase current trip is evidently set low enough that it can't stop the motor as fast as it is being asked to without over-current on the phases.

J Lauffer · May 4, 2008

Are you using the DC Braking function along with the Dynamic Braking? Overcurrrent trips during braking are usually associated with DC braking, not dynamic braking (as others have mentioned, more common to get overvoltage with dynamic braking).

John

Toms Wheels · May 4, 2008

The fault is E02, overCurrrent on braking. The first programing I did had both Dynamic and DC braking, with a delay of the DC braking. It seems that Dynamic alone worked better than the combo so thats hows it has been most of the time. As stated I know that increasing the time will decrease the amps.

The trip on braking current is set at the max 150% as I recall of full load motor amps. This is an external resistor setup.

My thinking was to increase the ohm's would raise the voltage and decrease the amperage, Yes?

peterh5322 · May 4, 2008

"My thinking was to increase the ohm's would raise the voltage and decrease the amperage, Yes?"

Yes.

Each drive has a maximum current spec into the DB resistor, which sets the lowest value of resistance. A higher value of resistance reduces the current into the DB resistor until the resistance becomes infinite, whereupon the current into the DB resistor becomes zero.

The inductance in the motor feeder comes into play, too, as you cannot instantaneously change the current through an inductor.

The capacitance on the dc bus comes into play, too, as you cannot instantaneously change the voltage on a capacitor.

A pure resistor has neither of these properties, so the presence of a DB resistor allows the instantaneous and steady-state currents to flow into it, thereby dissipating considerable power.

I'm sure there is a happy medium, where some power is returned to the dc bus, where it may later be reused, while some power is dissipated in the DB resistor.

JST · May 4, 2008

OK, yes there is probably a best balance point of resistance where you get the fastest brake, and neither OV nor OC trips.

J Lauffer · May 4, 2008

"...there is probably a best balance point of resistance where you get the fastest brake, and neither OV nor OC trips."

Right, it is definitely worth playing around to find the best combination, but he may not end up much better than when he just lengthens the decel time.

Does the VFD manual say what the minimum allowable resistance is? If you are exceeding the current rating of the DB transistor then it would seem the 50 ohm resistance is too low, and eventually you'll burn out the transistor. If the manual doesn't provide that info, then I would call tech support to get it. Getting the resistance correct is the place to start, then play with the decel time to get the lowest setting possible. If the minimum allowable resistance actually is 50 ohms, then it is strange that you would get that fault. Maybe your resistor has a wide tolerance (+/- 10%) and is actually only something like 45 ohms.

John

JunkyardJ · May 4, 2008

You could get a bunch of smaller resistors.

I posted in a thread a while back about building your own braking resistor. I remember it was very cheap to use a BUNCH of smaller resistors in parallel. You can play with the number of resistors to vary the total resistance of the bank until you stop getting overcurrent trips. The recommended resistor for 2 and 3 horse VFDs was a 35 ohm 225 watt resistor. You can take 16 25w 560 ohm (part #25W156 from MCM electronics for $.69 ea) and put them in parallel to get a 400w 35 ohm resistor. If you put like 5 or 6 of them on little toggle switches, you could have a braking resistor with different stages of braking strength. You'd still have a 250w resistor even with 6 less resistors, but it would be 56 ohms which would result in just over half the current of the 35 ohm resistor. Put it all in a nice aluminum box with some wires going from the VFD to it, and you have a cheap multi stage braking resistor. I bet 4 stages would be plenty. You could even try different resistances to make for different strengths. Just remember, if you mix resistors, the ones with less resistance (lower ohms) will absorb more power. So, you want the majority of the resistors to be a lower resistance, then use higher resistances on the toggle switches for stages.

http://www.practicalmachinist.com/vb/showthread.php?t=153631&highlight=braking+resistor&page=2

http://www.mcmelectronics.com/product.aspx?productid=25W156

http://www.mcmelectronics.com/search.aspx?C=3829952

JunkyardJ · May 4, 2008

Oh, you're allready at 50 ohms.

You could put 20x 1k ohm 25w resistors (part #25W210 from MCM for $.69) and have a 50 ohm with finer steps. You could even put different numbers of resistors on the toggles to have a bunch of different combinations. I'd have 4 toggles with 4,3,2, and 1 resistor on it. With all the toggles off, you'd have a 100 ohm 250w resistor for half of the braking current you have now. You could adjust it in steps of approximately 10 ohms down to 50 until you get no more overcurrent trips. That would be my next try. :cheers:

http://www.mcmelectronics.com/product/25W210

JunkyardJ · May 4, 2008

What setting controls the current trip percentage?

What setting controls the current percentage for the dynamic braking overcurrent trip??? I didn't see that setting ANYWHERE in the manual, just the usage percentage. :confused:

Jraef · May 5, 2008

J Lauffer said:
...If you are exceeding the current rating of the DB transistor then it would seem the 50 ohm resistance is too low, and eventually you'll burn out the transistor. If the manual doesn't provide that info, then I would call tech support to get it. Getting the resistance correct is the place to start, then play with the decel time to get the lowest setting possible. If the minimum allowable resistance actually is 50 ohms, then it is strange that you would get that fault. Maybe your resistor has a wide tolerance (+/- 10%) and is actually only something like 45 ohms.

John

I think this is the most important point, pay heed to John here.

The OC tripping is indicative of the braking current being too much for the braking transistor to handle. Just changing the resistor is not necessarily going to "fix" the problem unless it was not sized properly to begin with. If you need to dump 10A of braking current into the resistor and your transistor is only rated for 7A, it isn't going to work. Lowering the ohmic value of the resistor will immediately allow more current to flow and the protection circuit will still trip off. That protection is there to protect the devices in the VFD, not the resistor. You may, as mentioned, be able to play with values a little to improve your situation, but you may just need a bigger VFD or learn to live with longer decel times.

JunkyardJ · May 5, 2008

Supposedly a 35 ohm resistor is recommended.

I looked up the 2 and 3 hp SJ 200s and they are supposed to use a 35 ohm resistor. The overcurrent trip is because the decel time is set crazy fast for the inertia. I guess the Hitachis will let you set the decel time to whatever it can do, and if you set it too low, you get an overcurrent trip. The circuitry is protecting the braking transistor from failure. He's got a 50 ohm resistor, and it's still tripping out. I'll be curious to see if the higher resistance will still allow the fast stop time, or if it will get a DC bus overvoltage. :confused:

Toms Wheels · May 5, 2008

What I have been doing when it trips is the smart thing, increase the stop time so it does not trip. It may be to much to ask from the unit, the almost instant stop, just nice. 35ohm is the stated min resistor from Hitachi, but no wattage suggestion, which seems odd.

The parameter refered to for the over current setting was B022. A more careful reading states it is for accel, and constant speed, no mention of Decel. Just assumed that it is used for Decel as well.

There is also a curve that can be entered into the decel profile, that may also be tried.

Thanks for the responses

JunkyardJ · May 5, 2008

The suggested resistor is a 225w 35 ohm.

Looking up the resistor by part number gives a 225w 35 ohm resistor. I forget where I looked that up, but I was curious about that a while back. I was thinking about sticking a braking resistor on my VFD so the mill would stop faster. Then I saw the INSANE price hitachi wanted for a resistor! The hitachi resistor was like $300, and the other cheaper brands were over $200!!!!! HOLY COW, I'm NEVER going to pay that much $$$ for a RESISTOR! Anything over $50 is RAPE. :icon_bs:

PeteM · May 6, 2008

FWIW, I've used strip heaters (about $30 from McMaster Carr) as braking resistors. McMaster supplies only the voltage and wattage, but you can figure the approximate resistance (I'm not sure if the value changes a bit as the "resistor" heats up) from that.

Toms Wheels · May 10, 2008

In further reading of the manual it states that the overcurrent for decc. is set at 200% FLA.

I forgot that this VFD has memory and pulled up the last fault of overcurrent on braking. 302V DC and 25.8 amps at 3.5hz. So it trips at about 100rpm. Guess that I am asking too much from the VFD, and will leave the settings at .8 sec.

Overcurrent Braking Trip

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