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Braking torque

lazlo

Diamond
Joined
May 23, 2005
Location
Austin, TX
In my Hitachi VFD manual, it has a table of braking resistor values for 100% and 150% braking torque.
Is there any reason I wouldn't want to size the braking resistor for 150% torque?

This is for a VFD installed on a 2 HP manual lathe (Clausing 5914).

Thanks!

Robert
 
There's a huge difference between sizing a brake resistor to brake a high inertia load frequently, like loaded conveyor or some other machine that posesses a huge amount of inertial energy once when operating and sizing one for a lathe. A small lathe has relatively little energy contained within the rotating parts and can be brought to rest within a fraction of a second by bleeding this energy through a braking resistor.
Duty cycle is what needs to be considered.
If you want to stop your lathe quickly but intermittently you don't need a big resistor. You probably only need to size for a 10% or less duty cycle. As long as the resistance is within spec, standard cartridge heaters clamped to a aluminum plate work great and are cheap.
Braking torque is usually a drive parameter setting and is independent of duty cycle. Brake torque will determine how short you can set your decel ramp, the brake resistor duty cycle will determine how many times per minute (or hour) you can do this without frying the resistor.
 
The torque also affects the rate at which the energy is dumped to the bus. Too fast/too much and it may high bus fault....

PERCENT should be resistor value.

DUTY should be resistor wattage.
 
Duty cycle is what needs to be considered.
If you want to stop your lathe quickly but intermittently you don't need a big resistor.
Thanks Chris -- I'm trying to build a 10EE-style Electronic Leadscrew Reverse: I've build two carriage stops with microswitches to send a Reverse command to the VFD. So I'd like to be able to stop the spindle in under a second, if possible, since that will determine how close to a shoulder I can thread.

The duty cycle is a really good point -- depending on how long it takes to cut the thread and rewind the carriage, the duty cycle could be pretty high.

I grabbed a 121 ohm braking resistor module off Ebay that's made with 4 33 ohm power resistors in series. I'll try the 100% braking value first (90 9 ohms), and see how fast I can stop the spindle.

Thanks!

Robert
 
PERCENT should be resistor value.

DUTY should be resistor wattage.
Right, but the Braking Torque determines how often you can cycle the power resistor.

Duty Cycle = Tb/Tc * 100%

In other words, Duty Cycle is the braking stop time divided by the total cycle time. So the faster you stop, the lower the effective cycle time.

So like Chris says, you only want to stop as fast as you need to, or you'll adversely affect the cycle time. That's pretty hard to figure out for an ELR though -- I'll have to play with the values...

Thanks guys!

Robert
 
Sort-of.....

Braking harder also gives a higher rise to the bus voltage, requiring a lower resistance to drain off excess energy faster....... otherwise you bus fault.

Remember, if you brake slow enough, maybe you need NO resistor. That is because just the switching action of the IGBT bridge circuit drains some current off. The faster you stop, the faster the rise of bus voltage, and the more likely you need a lower value resistor to hold the voltage down.

The lower resistance needed to drain the bus faster also may need to be a higher wattage.

That is ASIDE from the duty cycle......

In fact, the faster stop, *unless it shortens the overall cycle*, may have little effect on "duty"..... because the dumped energy stored in the rotating parts is the same either way. But one way it almost ALL goes in the resistor, the other (slower) you may not need any resistor. To that extent it may need a larger wattage.

Every time you stop, you dump energy. The more times per minute you do it, the larger braking duty is, and the more wattage is needed.

Most controllers ONLY dump when the bus goes high, so they are sensitive to rate... slow stops maybe don't trip the dumper, fast maybe do, low energy stops don't, high energy stops do.

The "braking resistor" does NOT stop the motor......

Modulating the IGBTs does the stopping.

The function of the resistor is to waste the energy that was dumped back onto the high voltage DC bus by the stop, so as to avoid a bus fault. That energy shows up as charge on teh bus capacitors, and raises the voltage.
 
The "braking resistor" does NOT stop the motor......

Modulating the IGBTs does the stopping.
Right...the VFD ramps down it's output frequency (via the IGBTs), and the motor tries to follow. But if the ramp rate is too fast, the motor turns into a generator and pumps energy back into the VFD, which causes the DC bus voltage to rise and eventually leads to an overvoltage fault. The resistor allows the VFD to maintain control of the motor without tripping out.

John
 
the VFD ramps down it's output frequency (via the IGBTs), and the motor tries to follow. the motor turns into a generator and pumps energy back into the VFD,
I guess it depends on how you look at it: the braking resistor is allowing you to do dynamic braking by providing a dummy load to dump the back-EMF.

In other words, if you don't have a braking resistor, you can't do dynamic braking, since the IGBT's can't drain the back-EMF fast enough, and the bus overloads.

I still haven't found an explanation of the 100% and 150% braking torque values quoted in all the VFD manuals. 100% of what?

Like Chris says, it sounds like I have to just set up the lathe with a 100% braking resistor (93 ohms @ 4.2A in this case) sized for a certain duty cycle and see if the spindle stops quick enough for ELR, and try to calculate the duty cycle based on a typical threading job (maybe 6 passes a minute).

Can anyone with a 10EE with ELR tell me how quickly the spindle stops?

Thanks!

Robert
 
How about 100% or 150% of normal torque..... i.e. compared to accelerating torque?

No, this one I do not know, but that is at least reasonable, since there needs to be a reference....

Most of the semiconductors and other parts can take a short-term overload, which braking is, since there is somewhat limited energy involved.
 
How about 100% or 150% of normal torque..... i.e. compared to accelerating torque?
That sounds right JT -- just wish I understood what the baseline accelerating (or braking) torque they were talking about.

On the Powerohm.com catalog that someone here suggested eons ago, they show a linear Tb (time to brake) curve, but there's no reference to what the base acceleration or decleration curve is.

http://www.powerohm.com/catalog/DBCAT00.pdf

"The [braking resistor] resistance determines the braking torque and thus the deceleration rate of the motor. It is important that the resistance value must be within the allowable limits of the drive or braking module..."

We got all that. And I understand that the 150% braking resistor value is 50% higher current (% lower resistance), but I still don't understand "100% of what"
 
Lazlo, The ELSR does not reverse the lathe on a 10EE it mechanicly kicks the lever to Off, you then backoff the tool, then move the lever to reverse, it will then reverse until it hits the stop and again shuts off, ready to go forward again.

You would need to set the end point switches to stop the lathe, which would engage the braking resistors. Then select reverse, retract the tool, and run to the other stop. It sounds like you were stopping the lathe by putting the motor in reverse, which would also cause a tool problem as well as a vfd problem.

Unless I misunderstood your description.

my wheels don't slow me down
 








 
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