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Thread: Questions from a VFD newby
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20th Jun 2013, 01:08 PM #16Most Valued Member
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Hi Chris,
No you can turn it(stop ramp) off.
As I understand it.
Dynamic braking only works on the Decel ramp if the spindle is above the rpm for that point on the ramp. Sure the spindle is slowing down but its still has full power for what ever point it is at on the ramp. A long ramp isnt the same as coasting to a halt, as Bob said above, if for what ever reason the spindle trys to slow down faster than the "ramp" the VSD will try to keep it going.
Yes IF* you can and IF you have set it as such you may have immediate DC braking**.
But the OP already has a brake, sure its not fitted yet but while he was changing motors would be the perfect time.
I dont have anything against controlled stops IF there is a method of overriding them. i.e. a brake or a clutch or.....whatever. I dislike the idea of having to decide how fast I want to stop the machine before I start it, as I could go from wanting a nice slow stop to wanting a very very fast stop in not very long at all. Without a method to override I choose coasting stop......but as machines get bigger that may well change.
Stuart
*The Taco manual says DC braking cant start above 10Hz.
The Huanyang manual also states 10Hz as the max "stopping Frequency" (the point at which the VSD will either stop running or start DC braking). Now I'm pretty sure the Huanyang can be "set higher" than 10Hz but I dont know if it actually effects the braking.
**I'd assume you wouldnt really get DC braking at full speed?
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20th Jun 2013, 03:08 PM #17Cba
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> No you can turn it(stop ramp) off.
You think so? I like to think you cannot turn the ramping up/down itself off. Think about it. Imagine at startup you were applying 100HZ to the motor without ramping the frequency up. The motor would simply hum in a stalled condition. The only time when ramping is really turned off, is when the VFD trips and essentially shuts itself off.
> As I understand it.
> Dynamic braking only works on the Decel ramp if the spindle is above the rpm for that point on the ramp.
> Sure the spindle is slowing down but its still has full power for what ever point it is at on the ramp.
>A long ramp isnt the same as coasting to a halt, as Bob said above, if for what ever reason the spindle trys to slow down faster than the "ramp" the VSD will try to keep it going.
Yes, that is a correct description.
> Yes IF* you can and IF you have set it as such you may have immediate DC braking**.
Most VFD I know of will turn on DC injection only very close to standstill. It is to lock the motor once stopped, not to brake it. Consider, that activating DC injection means giving up control - I do not think you can have both DC injection AND a controlled ramping at the same time. Maybe larger VFD's can? I am mostly familiar with smaller VFD's
> But the OP already has a brake, sure its not fitted yet but while he was changing motors would be the perfect time.
Correct if I am wrong.... but a lathe brake is something quite brutal, near instantaneous emergency stopping? A threaded chuck would be guaranteed to come loose and fall on your toe. A heavy workpiece on a faceplate may break loose.... like in a cartoon, mlod screeech, everybody in the hall looks up, and the lathe may take a small jump if not bolted down....
Or is it controllable like a car brake, the harder you press the pedal the more it brakes the spindle? Sorry if I am asking something silly, I have no experience running larger lathes..... I remember during my apprenticeship at BBC large lathes horizontal and vertical, but I just marvelled at these, never used (BBC was making stuff like the turbines and generators for your Snowy electro scheme and nuclear powerplants to give an idea, nowdays the company is named ABB).
Anyway, I digress. If the OP's lathe had a mechanical brake, would this be a reason not to use the VFD's dynamic braking capability? Aren't the two more complimentary than mutually excluding?
> I dont have anything against controlled stops IF there is a method of overriding them. i.e. a brake or a clutch or.....whatever.
> I dislike the idea of having to decide how fast I want to stop the machine before I start it, as I could go from wanting a nice slow
> stop to wanting a very very fast stop in not very long at all. Without a method to override I choose coasting stop......but as
> machines get bigger that may well change.
Sure its nice to have a brake.... But if you cannot have or not afford a brake, why not use the next best available?... It is a bit like arguing "if I cannot have real airbrakes like a truck, then I rather have no brakes at all and coast to a halt in my delivery van"
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20th Jun 2013, 03:33 PM #18Philomath in training
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I need to disagree with that one - VFD's typically reject a fair amount of heat (the reason for large heatsinks). That power has to come from somewhere - therefore for the same output they must use more power. Some time ago I remember looking at a VFD for a motor application I had. To drive it without a VFD would take close to 15A (single phase) but if I put a 3 phase motor in with VFD it would go over that, needing a special circuit. While there may be times that the VFD better matches the load, the power consumption is more likely to increase I would have thought.
Michael
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20th Jun 2013, 03:58 PM #19Most Valued Member
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Michael,
the VFD I just bought dissipates 45W, so that obviously adds a small amount to the total consumption. However, I'm sure I read somewhere that a VFD can reduce motor power consumption because it drives the motor optimally. This could be complete bollocks and I'm sure Chris will set me straight if it is.
Chris
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20th Jun 2013, 04:17 PM #20Most Valued Member
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I said "(stop ramp)" for a reason. Yes you must have a start ramp(though both my VSD manuals say 0.1 sec is minimum, I would guess as long as it got a kick in the right direction it would start, it just might take longer. Never tired). But you dont need a stop ramp. Coasting stop, no DC braking like pulling the plug out the wall(on a non VSD machine). Yes just like the VSD had tripped.
Ok so it sounds like I shouldnt try upping the DC braking then. All my VSD's are only 2.2kW.
None that I am aware of.
Yes, All lathes brakes I have seen operate just like that, from a light press of the pedal(or lever**) tripping the motor, no braking and it coasts to a stop, up to stand on the thing and its stops almost instantly(mine its quiet that good depending on rpm,chuck,etc*). Now a really large lathe isnt going to stop that quick either or as you say things will come "out the floor"
Also you of course have the option to change your mind, if you flick the pedal and the lathe is coasting down but you then decide you need it stopped NOW! jump on the brake. With a ramping down VSD you're just going to have to wait it out.
Not at all
Yes certainly
As long as the brake can stop the spindle even if the VSD doesn't agree(and it would have to be a pretty wimpy brake if it couldn't) I have zero problems with stop ramps.
Because "as I see it" its not
"if I cannot have airbrakes on my truck, then I rather have no brakes at all and coast to a halt"
Its more like "if I cannot have airbrakes on my truck, then I rather use engine braking.......and maybe have throttle stuck half open"
Stuart
p.s. The other thing I like about it is(as I am sure you are aware, I say this for others). All my stop buttons(pedal) are in the one place and its the one I use all the time. Its the one reflexes will have me going for. The level of desperation varies the pressure on the button(pedal) . I might use the emg/stop button(which has a lock on) if I am doing a lot of setup. I've never used it to turn the lathe off. I almost never use the carriage lever to turn the lathe off)
*my brake is on the motor so I would be limited by the vee belts. If I recall correctly the only larger lathe I know enough about the insides of to comment about, the brake is on the spindle side of the belts.Last edited by Stustoys; 20th Jun 2013 at 04:32 PM. Reason: p.s, **
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20th Jun 2013, 04:34 PM #21Member: Blue and white apron brigade
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It can reduce over overall power consumption because it drives a motor optimally to maintain a specific condition or load (eg temperature control by an air conditioner, or flow via a pump) as opposed to running flat out for a certain period and controlling the temperature or flow by turning the motor on and off. I'm not sure that this has much meaning for a machine lathe as the chances are just as great that the motor will be run >50 Hz as much as it is run slower. I guess it depends on what gearbox combo you choose to normally run with. I'd be interested to hear from those who know more about these things what the power consumption is likely to be.
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20th Jun 2013, 05:09 PM #22
Three phase motors are more efficient than single phase motors, the load current on a single phase motor doesn't change as much as load current on a three phase motor as the mechanical load changes. What this means for a lathe is that when you have a light load the 3 phase motor will draw much less current than a single phase motor.
In general, wherever you have variable mechanical load, like pumps, fans, lathes etc... three phase motors are the best choice (if you have a choice that is).. there is also another more subtle advantage, and that is smoother running, and less vibration. Which in some critical applications like grinding can affect quality of the finish,
Regards
Ray
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20th Jun 2013, 09:08 PM #23Member: Blue and white apron brigade
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Thanks Ray. I wan't thinking single versus 3 phase but VFD versus no VFD.
My understanding is using a VFD on a 3 phase motor to control an external process uses less power than toggling the 3 phase motor "on-off"
But I'm not sure that applies that much to a machine like a lathe