VFD break 1

[krsh]

Dear colleagues,
I design an application where railcars are positioned over a loading bunker with an electrical winch. The question is if i use a VFD do i need an electromagnetic break for the motor to stop the railcars or the VFD can meed this functionality too. The breake is used only to slow down the railcars not to hold them.

 

[waross]

Is this a special winch with an endless cable? Most winches will not hold back rail cars when the winch slows down and the cable goes slack. If you winch uses an endless cable and is capable of holding back, the VFD is probably a much better choice. If there is any slack in your system, applying a brake may result in severe mechanical shock as the slack is taken up each cycle. The VFD may be programed for an initial gentle slowdown until the slack is taken up, followed by more aggressive decelleration.
Wait for our VFD experts to discuss the configuration of the VFD.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[krsh]

Yes, it is a endless cable. A winch at 1 end and a roll back at the other end. I will program the VFD to ramp up to nominal 50 Hz in approx 10s not to damage the teeth of the gear but i have doubts about stopping the railcars because of the tehnological requirements of 1 meter tolerance. Time between 1 start and stop is >= 10 s.

 

[DickDV]

You may use the dynamic braking capability to decelerate the railcar to zero speed but you will need a mechanical brake to hold it in place if you ever come to need that.

Remember that braking requires hp or kw just the same as motoring so your motor and drive must be sized large enough for whichever is larger. And, of course, the braking resistor set must also be sized for the braking kw.

If you don't require a precise ramp time to zero, it might be cheaper to use DC injection braking. Most VFD's come standard with that capability and you wouldn't have to buy the resistor set.

Be careful about doing motor braking thru high ratio gearing. Some gearboxes will not transmit negative torque but will, instead, lock up (as in worm gears over 28/1 ratio). If you have that kind of power train, you will not be able to use the motor to stop the railcar.

 

[krsh]

The gear ratio is 63.

 

[jraef]

If, as DickDV mentioned, the gearbox is of a type that can handle negative torque, I would consider a Regenerative Drive as an option instead of dynamic braking resistors or DC injection braking if your cycle time is as short as I interpret your posting. Regenerative drives are more expensive, but instead of putting the kinetic energy from the moving load into resistors to be burned off as heat (dynamic braking) or into the motor as heat (DC injection braking), a regen VFD pumps the energy back into the line, essentially recouping that energy used to stop the load. Higher initial cost, much lower operating cost.

"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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[DickDV]

OK, the gearbox ratio is 63 but that doesn't establish whether it can handle reverse or braking torque. Generally, worm gears will not, while spur or helical gears will. Very important to know for sure.

 

[krsh]

It is a helical gear.
jaref does a Regenerative Drive needs additional equipement besides VFD?

 

[lectricwes]

Using the VFD for the sole source of braking is fine unless there is a chance for a "runaway". During an inadvertent power loss the VFD loses its braking capabilities. Using a electromechanical brake that opens at power-up and closes at power-down in addition to VFD braking might be something to consider.

 

[jraef]

No, a line regenerative VFD will be self contained. It is more expensive than a regular VFD and not everyone offers them. Assuming you are not in the US, ABB and Siemens both offer full line regenerative drives that I know of, both are good products. Siemens product is the Sinamics G120 + PM250, it is slightly more efficient than the ABB version, but stops at 100kW. ABB's product is the ACS800-17 and goes all the way up to 1600kW. I also believe you can use side-by-side Control Techniques Unidives and configure them as a line regenerative drive package, but I don't know much more about them, however others in this forum do and maybe they can shed light on that.

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[ScottyUK]

jraef,


Correct on how CT implement regeneration. I think economics would weigh against this in a single drive application, but in a multi-drive system with a common DC bus it might actually be favourable.

This link might be of interest.

http://www.controltech.com.tr/Documents/Unidrive Classic/uni_regen iss2.pdf

I've never used this drive family in regenerative mode, but as a product line they were pretty good: a lot of capability at a reasonable price. Not sure what effect, if any, the Emerson takeover has had, although generally I find Emerson's products to be toward the top end of the market.


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[krsh]

I spoke with Siemens representative and they suggested to go for the regenerative drive. It is a better solution for this application because of the inertia caused by the weight of the railcars require more than a breaking resistor.
Lectricwes, thanks for the advice i will ask for a electomagnetic brake for the motor as a fail safe solution in case of a power drop.

 

[jraef]

I take it your motor size is within the range of that product then? Be careful, the local Siemens people around here constantly forget that annoying little detail. They sell the concept to people, ask me to design and integrate the system for them, they I get stuck having to break the news that it is too big for the Siemens solution.

ABB loves that however, because I move them to that solution and it's a lot more expensive and complex: better for me.

"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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[krsh]

The motor size is 30 kW so its in the range. Thank you all for your answers.

 

[DickDV]

At only 30kw and 10% or less braking duty cycle, I'd be surprised if regen braking can be economically justified.

A brake resistor set for this application would be rather inexpensive and the energy would probably not amount to enough $ to save either. A simple motoring drive with a snubber resistor package might be the most economic choice.

It deserves a look, anyway.

 

[ozmosis]

Jeff
just to pick up on your comment about Siemens regenerative units.
The PM250 range of the G120 range is regenerative but is not an active front end; it uses IGBT's but with two comparators sensing the +/- phase to turn the IGBT on/off. Not actively controlled via a PWM gate circuit.
Siemens do have higher power Active Front End drives up to the MW range. This would be covered by the G150.

Having said this, I agree with DickDV. For 30kW and 10% braking duty, I would use a brake resistor based system.

 

[krsh]

The VFD is not only used for breaking but it is used to ramp start the motor to protect the gear teeth at startup.

 

[wcarter]

krsh,

Here is an article interview with another company than was mentioned above, but it generally covers what you asked, and has benn answered. Just a little extra support.

http://www.hesya.com/html/Electric motors motor controls engineering/20080415/22506.html

 

[jraef]

wcarter,
Welcome to the Forum! I see you're relatively new here, so just FYI that "hesya" website you linked to is nothing more than Chinese plagiarizers of Eng-Tips! So that entire thread actually took place in here, they just copied it and made it look like their own. They vex the Eng-Tips management but have that "Chinese immunity" meaning that since our government owes so much money to China, they are toothless to do anything about the copyright violation.

I know you didn't know, but in the future if you come across that site with Google, realize that you can also find it in here in it's original form.

Keep coming back!


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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[wcarter]

jraef,

Thanks. I DID notice the two had a particularly simular look to them, only that this one looked nicer. I thank you for the information. I've been reading on here on and off for over a year, but finally decided to put in a few cents. So good to know. I appreciate it.