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joahmon
03-31-2005, 09:19 AM
My old head seem to recall a posting about using light bulbs as a braking resistor for a VFD, but my searching has found no such posting. Has anyone done this or heard about such a thing?
Bob

MikeHenry
03-31-2005, 10:04 AM
I seem to recall that someone (Forrest?) suggested using an electric water heater element for that purpose.

Mike

Michael Moore
03-31-2005, 12:04 PM
From a post by a Haas employee at the PM forum:


<font face="Verdana, Arial" size="2">Yup, we do use oven heating elements for dissipating the heat generated from slow the spindle and axis motors. Why do we use these? Two reasons; cheap and they are incredibly effective at taking electrical current and dissipating it. This is what an oven heating element is designed to do. We use to use ceramic coils for this purpose, but they would heat cycle over time and short out. These oven eyes are designed for exactly this purpose and are far, far less expensive.</font>

cheers,
Michael

John Stevenson
03-31-2005, 12:39 PM
Carefull.

This thread comes up quite oftem and I wonder in the HSM use why a braking resistor is needed.

First off for the unwashed let me explain acceleration and braking and other salient points for someone who hasn't used a VFD before.

This is general and generic but covers 90% of the the VFD's HSMer's are liable to come across.

VFD's take in 240v single pahse and do a bit of juggling and spit out 240 volt 3 phase.
This is all done internally and electronically with the added bonus that we get a variable frequency and the motors needs are taken care of electronically and internally in the VFD.

This means we get variable speed and loose all those coils, thermals, trip switches and 17 miles of rats nest.

Most of these features can be programmed from the keypad on the VFD. Two of these many options are acceleration and braking. To my knowledge all modern VFD's have this function, some of the early ones had small pots on them to do this.
Settings are usually between 0.1 and 10 seconds for both, average default is usually set at 5 seconds.

Now bearing in mind we are talking about a driven motor of between 1/2 and 4-5 HP these don't have a large mass, neither does the average lathe.
Acceleration need to be reasonable fast or a tool could jam in the cut as it comes up to speed if it's too slow. Three seconds is ideal for an average user, fast enought to prevent problems and slow enought to act as a soft start and help the transmission on the machine be it gears or belts.

Now braking, because we don't have any large loads to stop braking doesn't need to be fierce.
From experiance, and I've supplied and fitted well over 500 VFD's in the last two years going below one second can cause problems in that the back EMF or something like that can trip the VFD out and it will require a reset.

Keeping between one and two seconds will stop the average HSMer's machine in, well, one to 2 seconds, a big improvement over the old stop button and coast to a standstill.

Braking resistors only come into the equasion when the load is that high that a one second setting turns into 20 seconds. The resistor is needed to handle the extra power dissaptated.
Fitting resistors where not needed will only cause problems with the machine.
Got a threaded spindle? good party trick, try 1/2 second and see if you can let go the stop switch in time to catch the chuck http://bbs.homeshopmachinist.net//biggrin.gif

Stopping a machine dead isn't clever it's dangerous and causes untold damage.

I have seen a double ended tennoner where the sparky set the braking up to zero and the motor stopped dead but the 12" cutter block shot across the shot like a hub cap of Bodiceas charriot.

Be carefull, first of all try the internal braking settings in the VFD before obtaining and wiring in an external resistor.
The majority of the VFD's I fit are under 5hp and I have never had to fit an external resistor yet.

John S.

joahmon
03-31-2005, 04:16 PM
John, you're probably right for most applications in a home shop, but when I am running the mill at high rpm and either shut it down or slow it too rapidly the VFD goes into error mode and I have to "reset" it. If I extend the slow-down-rate, it takes "forever" for the machine to come to a stop during regular operations. The VFD is in an enclosure and it is a PITA to open it up to reprogram or reset it.

I am assuming (yes, I know the adage) the built in resistor/transistor is being overloaded and I want to experiment with an external resistor. I normally don't have the problem with the lathe as I'm not turning at such high rpm.

BTW, I forgot to mention in my original post the the VFD is a Teco-Westinghouse 1 HP model. The recommended resistor is 200 ohms at 60 watts. Heating elements are rated at low resistance and high wattage. Does that make a difference to the health of the VFD??



[This message has been edited by joahmon (edited 03-31-2005).]

John Stevenson
03-31-2005, 04:47 PM
joahmon,
OK I see your point.
Sorry but I can't offer any advise on the resistor perhaps one of the other guys who understanads this amps watts stuff can help.

I may be telling you how to suck eggs and not knowing what model VFD you have but on most you can take a line from CM or PW+ whatever it's setup for and fit a simple normally open switch between that and the reset terminal to save opening up the case.

Others have a programming option to use the stop buton after so may second to reset as well.
It's really down to model and manual.

John s.

larrynicks
04-01-2005, 12:09 PM
I have a Teco 1 HP VFD and have been using a braking resistor for three years with no problems. I got mine surplus from C&H Sales. Go to

http://aaaim.com/cgi-local/shop991/shop.pl/page=start_shopping.htm

Search for RS2003

This is a 200-ohm, 100-watt ceramic coated wirewound; about 3/4-in dia by 7-in long. And they are only $4.25. C$H does have a $30 minimum or a $5 handling charge, but I've never failed to find more than 30 bucks worth of stuff that I couldn't live without.

larry

stiven
04-01-2005, 02:48 PM
I had trouble with my mill when i first installed the vfd. It kept tripping out when i would stop it. I lengthened the braking time and it solved the problem. I hardly notice any difference. I have used old machines where you have to wait for it to coast or use the hand brake. Waiting five seconds is nothing after using those old dogs. By the way I have a Hitachi on a 3hp birmingham.

joahmon
04-01-2005, 04:49 PM
well, I tried a water heater element today. Lit up like a christmas tree when I applied power to the VFD ( did not press the start button). I killed power and when things cooled down removed the heater.

Measured the voltage across the "resistor" terminals on the VFD and got 316 volts DC. Same measurement on the sister VFD gave 6.7 volts DC.

Don't know if I f%^%ed up the VFD or if it was defective all along. No apparent change in function though so I guess I'll live with the longer coasting.

BTW, all the parameters in the two units are the same. Very tedious to check but there it is.

Thanks for all the replies.

ibewgypsie
04-01-2005, 08:40 PM
I have some real braking units. For shipping costs I will pick one out and send it to you.

It has, a real resistor, heat sinks, power transistor, boards. You'd have to clip out the resistor and use it by itself on your application thou.

I can't think of a reason to complicate my metalworking machines with added hardware.



------------------
David Cofer, Of:
Tunnel Hill, North Georgia

j king
04-01-2005, 10:28 PM
joahmon. Iwent thru the same thing a couple of years ago.I have a hitachi and it kept tripping at high speed stops.I wired in a resister and turned on power.I heard a tic tic tic noise and saw a bit of smoke come out of vfd.I was sick.Unhooked resister and all has been ok.Lucky I guess.Now it is programed for free run stop.

J Tiers
04-01-2005, 10:47 PM
Ok.....Braking resistors, need and reason.....


The VFD supplies voltage to the load in pulses. The inductance of the load integrates the voltage pulses into a steady current. Steady in terms of the VFD, which may be supplying 200 pulses or more per half cycle of AC.

When you apply the VFD brakes, the motor is operating as a generator. The energy in the motor etc rotation is absorbed by the VFD, and what happens is that the power supply capacitors inside the VFD are charged up by effectively "returned" current.

Dumping current into a capacitor will cause its voltage to rise. Unless the energy is released, the voltage will keep rising until the energy has all been absorbed and stored in the capacitor.

But, the voltage can't rise forever, at some point that will trip the overvoltage limiter, and cause the VFD to shut down. You have to dump the energy somehow to discharge teh capacitor. The VFD isn't powering the motor, so that path isn't working, putting the excess energy into resistors works and is cheap.

I work with Class-D amplifiers, and the equivalent sort of behaviour is called "pump-up". We sometimes use dump resistors to get rid of it, but we hate doing that because it wastes power, and we are after efficiency.

Anyhow, that explains why the resistor is needed, and why not having one can cause a trip-out.

Slow braking does not dump energy in so fast, and internal losses in teh VFD may be enough to prevent over-voltage and shut-down.

If there was 316V across the resistor terminals, the "braking dump transistor" was "on". Either the VFD was in braking mode for some reason, or the transistor is kaput and may be shorted. Since the stove element draws a bunch of current, especially at over its rated voltage (300+ DC vs 240RMS), its possible that the transistor was damaged.

Normally, only a relatively little current needs to be drawn to prevent over-voltage. The amount can be calculated if you know the motor constants, the voltage trip point, and the internal capacitor value.

If you want to use the resistor, either size it per the manufacturer chart, or start with lower wattage lamps and go up. The voltage will indeed be 300 volts plus, depending on your line volts. So it will take probably 3 120V lamps in series to avoid over-voltaging the lamps.

joahmon
04-02-2005, 08:10 AM
J Tiers,
Where were you before I tried my experiment...sob...sob...sob..sob
Bob
P.S. that's sob(cry) not SOB!

J Tiers
04-02-2005, 11:48 PM
If it lit up when power was applied, something was already wrong....the VFD, settings , or the connection.

There isn't any reason for the brake resitor to just "be on", when the VFD isn't producing output. It should be able to sit there and do nothing happily w/o braking resistor.

So maybe there was already a problem, and you happened to find it.