PDA

View Full Version : VFD Braking Resistors - another question.



IanPendle
04-16-2018, 03:08 AM
There was a recent thread on VFD braking resistors, but Ill start a new one.

Im probably going to reveal a whole lot of ignorance of VFDs by asking this question, but here goes.

Using a VFD, you can slow and then stop the motor by setting the ramp down duration, or use a braking resistor or use DC injection. Lets ignore the DC injection method. If I set up the VFD to use a braking resistor (assuming I have set the parameters correctly for the breaking resistor use), does this override the ramp down settings? By this I mean, will it stop the motor by using the braking resistor only, or does it wait until the ramp down period has finished and then switch in the braking resistor?

Ian.

Paul Alciatore
04-16-2018, 03:16 AM
That sounds like a question for the manufacturer.

MattiJ
04-16-2018, 04:07 AM
There was a recent thread on VFD braking resistors, but I’ll start a new one.

I’m probably going to reveal a whole lot of ignorance of VFD’s by asking this question, but here goes.

Using a VFD, you can slow and then stop the motor by setting the ramp down duration, or use a braking resistor or use DC injection. Let’s ignore the DC injection method. If I set up the VFD to use a braking resistor (assuming I have set the parameters correctly for the breaking resistor use), does this override the ramp down settings? By this I mean, will it stop the motor by using the braking resistor only, or does it wait until the ramp down period has finished and then switch in the braking resistor?

Ian.

No and No.

It will stop the motor according to set ramp "rate" by dumping the "braking" energy to the brake resistor. Without brake resistor its not possible to use as fast ramp as you would be able when braking with resistor.
(Motor will act as a generator during dynamic braking and the generated energy has to go somewhere. Either brake resistor, backfed to utility network(only fancy expensive models) or used up by VFD losses and partly stored on DC bus capacitors)

IanPendle
04-16-2018, 04:24 AM
Hello Matti,

OK, understood.

"......Without brake resistor its not possible to use as fast ramp as you would be able when braking with resistor......"

That sentence makes it all clear.

Thanks.

Doc Nickel
04-16-2018, 05:40 AM
That's pretty much it- you need a resistor for either a heavy rotating mass, or to stop very quickly (or both.)

For example, my little Logan and Sheldon both are set for sub-2-second ramp down. I don't need a braking resistor, they seem to tolerate those speeds just fine, even with decently sized chucks and workpieces.

My big Springfield, however, if or when I convert that to VFD, will likely need a big braking resistor if I wanted to stop it under 2 seconds- especially if I had a big workpiece in it. But, if i dialed that back to a 3 or 4 or 5 second ramjp-down, said resistor likely wouldn't be needed.

Doc.

CCWKen
04-16-2018, 08:35 AM
My BP would error out on 2K rpm and above with a four second stop without a resistor. I just ignored it and pressed reset. With the resistor, I can't get below two seconds at 1200 rpm. I still have to tinker with the settings a little after understanding the language. By the way, my resistor didn't even feel warm to the touch after 7-8 timing experiments. But the weather has been cool lately.

If DC Injection is so bad, why are all the major YT players using it? I see (and hear) it used a lot.

J Tiers
04-16-2018, 08:43 AM
DC injection works very well.

The energy from the stop stays in the motor, it appears as heat in the rotor.*. so if you use it to stop very often, the rotor, and eventually the rest of the motor, will get considerably hotter than usual, which is generally bad.

* During the stop, large currents are induced in the rotor, which is essentially a short circuited winding. The heat comes from the resistive loss in the rotor, due to the (small) resistance of the rotor "windings". The currents are larger than the normal running current.

MattiJ
04-16-2018, 08:55 AM
My BP would error out on 2K rpm and above with a four second stop without a resistor. I just ignored it and pressed reset. With the resistor, I can't get below two seconds at 1200 rpm. I still have to tinker with the settings a little after understanding the language. By the way, my resistor didn't even feel warm to the touch after 7-8 timing experiments. But the weather has been cool lately.

If DC Injection is so bad, why are all the major YT players using it? I see (and hear) it used a lot.

Downsides of DC injection is possible overheating of the motor and limited braking torque. Good VFD +resistor set up correctly should be able to slow down the motor with dynamic braking in same time as it takes to speed it up = pretty fast.
I think DC braking is perfectly fine for applications with low/limited inertia and limited amount of starts per hour. Like a mill or most lathes. High rpm lathe with large steel chuck could be pushing your luck.
Dynamic braking excels when you have 60 ton load on a crane and you need to lower it 30 meters down.

CCWKen
04-16-2018, 09:09 AM
Is there any problem using the manual brake to assist stopping at higher rpms or with a heavier tool such as the Suburban fly cutter? Or will that foul the VFD's logic?

J Tiers
04-16-2018, 12:24 PM
The VFD slows by reducing the frequency at the selected rate. That makes the motor "generate", because it is then turning above synchronous speed, and as a result it puts energy back onto the DC bus, raising the voltage, and removing the energy fom the rotating load, slowing it.

All the VFD is doing is reducing frequency, and consequently, motor speed. If you use another means to slow the machine faster than the VFD does, the VFD may actually start to DRIVE the machine against the braking, which defeats the purpose.

Use one or use the other.

Noitoen
04-16-2018, 03:25 PM
I installed a bicycle disc brake on my motor pulley and setup a pedal actuator with the vfd set to freewheel stop. Normal stop I just lightly press the pedal to actuatte the stop microswitch and the motr coasts to a stop.

lakeside53
04-16-2018, 09:12 PM
With many VFD you can program them to use dynamic braking (resistor or external braking unit) AND foot brake. When you press the foot brake it overrides the dynamic braking and simply freewheels (coasts) to a stop under mechanical brake control.

My prior big lathe had an electro-maganetic brake - superb, and stops on a dime (unlike a vfd without DC injection). My current lathe has a foot pedal bar. I like the later but miss the electric brake. When I get around to converting this lathe to VFD, I'm going to implement both, and if I can get a vfd that will allow it, add DC injection at the last part of the dynamic braking cycle to make for a positive stop. The trick is to turn OFF the DC injection when rpm falls to zero, not just inject for defined fixed period. And... thermistor sensing (inside the motor) via the VFD to avoid overheating of the motor.

J Tiers
04-16-2018, 10:24 PM
If you can set an over-ride, then obviously there is no issue, you never have both at one time.

DC injection is stupidly effective at high speeds. At lower speeds it becomes less effective, but despite common warnings, it is pretty effective even at speeds that you can turn the motor just with your hand on the shaft. Try it sometime, and you will see.

the one thing it really does not do, though, is a "positive stop". Zero speed is zero braking.

And the DC that is injected is generally not enough to do much heating. The heating is actually of the rotor conductors, which heats the rotor, and indirectly, the stator and frame. I have never even tried to see what FLA as DC would do in terms of braking.... certainly a heck of a lot... Even a few hundred milliamps of DC does a very significant job of braking.

lakeside53
04-16-2018, 11:06 PM
Hmm... if you have a VFD available, play with DC injection. Unlike dynamic braking, DC injection is a superb at low speeds (and yes, can be very aggressive at any speed) - a very positive gear slamming stop - and it literally locks the rotor in place if left applied (bad idea). I'd have been happy if it "gave up" at zero speed, but without feedback it doesn't know what zero is.;)

My usage is all on various (three) Hitachi VFD. Hitachi specifically recommends that you use a motor-mounted Thermistor in conjunction with DC braking, so they are concerned. The problem I ran into was that it was not possible to define that the DC injection finished at zero rpm, only for a defined time and braking time varied, so often the rotor was "locked" for a short (or long if programmed as such) time after stopping. Maybe the latest VFD have more control.


For the rest of the audience,
http://electrical-engineering-portal.com/methods-braking-motor

http://www.machinedesign.com/technologies/give-me-brake

https://en.wikipedia.org/wiki/DC_injection_braking

Toshiba note for DC injection implementation

https://www.toshiba.com/tic/datafiles/9.0_H9_DC_Injection_Braking_6551.pdf

MattiJ
04-17-2018, 02:14 AM
Hmm... if you have a VFD available, play with DC injection. Unlike dynamic braking, DC injection is a superb at low speeds (and yes, can be very aggressive at any speed) - a very positive gear slamming stop - and it literally locks the rotor in place if left applied (bad idea). I'd have been happy if it "gave up" at zero speed, but without feedback it doesn't know what zero is.;)


This is where different manufacturer literature (and probably actual implementations) seem to differ. (and I can't try it out on my VFD's as ABB DTC drives have only dynamic braking and flux braking(what yaskawa probably calls high slip braking))

Yaskawa is talking about less than 100% nominal torque during dc braking
https://www.plantservices.com/assets/Media/1005/WP_SlipBreak.pdf

But Google finds me for example this:
https://i.stack.imgur.com/Jetsv.png
https://i.stack.imgur.com/HeSoj.jpg

So the most of the dc injection braking torque is available at relatively low speed, around 10-20% nominal and full speed braking torque is much less.

CCWKen
04-17-2018, 07:25 PM
I see the graphs but have no idea what they mean. What is a unit? And who runs a mill motor at 10-20% That's like 6-12hz for a 60hz motor. :confused:

When I'm on low pulley and back gear, the spindle is turning ~80 rpm but the motor is still turning at 1755 rpm at 60hz. How are all the guys power tapping on YT getting a quick stop? That's what I'd like to know. The graphs don't help me.

J Tiers
04-17-2018, 09:38 PM
The graph actually does make sense...... I had to remember the basics here.....because it seems a bit counter-intuitive.

When the RPM is high, the "slip frequency" of the DC braking is high, and the rotor current is less due to inductance of the cage conductors being a higher impedance at that frequency. That reduces braking.

At the same time, the voltage goes up, so the curve is not linear.

As the motor slows, the generated voltage is less, but the impedance is also dropping. At some point there is a maximum current point. Below that, the voltage is low, and the current again drops off.

However, don't let it fool you, the effect of DC braking can be pretty violent from high speed even so. And it can be very noticeable at low RPM also.

danlb
04-17-2018, 10:00 PM
On a semi related note, my hybrid car uses regenerative braking. It uses a similar concept to brake by using the electric motor as a generator. The generated DC goes back to the battery pack. The deceleration is very predictable right down to the last few MPH. That's when it switches over to mechanical disc brakes. From what I read, they did not go with DC injection for that last part because of overheating fears.

It would seem that using the same combination on a lathe would work well there too.

Dan

J Tiers
04-17-2018, 10:25 PM
On a semi related note, my hybrid car uses regenerative braking. It uses a similar concept to brake by using the electric motor as a generator. The generated DC goes back to the battery pack. The deceleration is very predictable right down to the last few MPH. That's when it switches over to mechanical disc brakes. From what I read, they did not go with DC injection for that last part because of overheating fears.

It would seem that using the same combination on a lathe would work well there too.

Dan

With the VFD that is in the car, they can program a very good braking curve. Standard VFD braking is not a heat issue because it is basically just running the motor in a normal way.

DC injection is really "eddy current braking", that induces currents in the rotor conductors. That energy goes into I^2 * R losses, and ends up as heat in the rotor. If you beat on a motor hard enough, you could actually melt the aluminum conductors. It would take a bit of doing, but is possible.

lakeside53
04-17-2018, 10:33 PM
I found it interesting that the Toshiba link I provided mentioned using DC injection it to drive moisture from the motor.

J Tiers
04-17-2018, 11:08 PM
I found it interesting that the Toshiba link I provided mentioned using DC injection it to drive moisture from the motor.

It clearly would work to do that. you need to use a fair bit of DC, maybe 10% or up to 25% of FLA, but it will warm things up for sure.

MattiJ
04-18-2018, 02:26 AM
I see the graphs but have no idea what they mean. What is a unit? And who runs a mill motor at 10-20% That's like 6-12hz for a 60hz motor. :confused:

When I'm on low pulley and back gear, the spindle is turning ~80 rpm but the motor is still turning at 1755 rpm at 60hz. How are all the guys power tapping on YT getting a quick stop? That's what I'd like to know. The graphs don't help me.
I think the units in the grahps are nominal speed ie 60hz = 1.0 and nominal current.

You should start by telling (again) what VFD and brake resistor you have and what are your settings. Or start separate thread specific to your hardware.
My bench drill would reverse "pretty fast" if would trust the pulley axle nuts to hold... oversized VFD that is capable of dynamic braking at around 400% nominal torque from full speed to zero speed and accelerate again at 400% torque to full reverse.

lakeside53
04-18-2018, 11:05 AM
With it set as you say in back gear and with 80 rpm spindle you can stop pretty quick with just dynamic braking (resistor); I could probably get a fraction of a second. BUT it's compromise. It will only stop as fast as your deceleration setting - and you will have to reprogram the deceleration/acceleration setting to be more practical for other speeds or you will trip out. If I add DC braking, my VFD ignores the deceleration times and "slams on the brake". That's one way to get a "quick stop" but...

This is why I want both mechanical and vfd braking (both dynamic and DC injection) on a lathe. The inertial mass varies massively on my lathe (collets, 6 inch to 16 inch chuck just for a start, never mind the work from grams to 100's of lbs); it's hard to find any single sweet spot for deceleration with just dynamic braking.

I need to play with the PLD on my vfds - in theory I can read a selector switch and auto-set parameters - would be nice to have multiple changes at the flick of a switch. I can already set 2 "different" motors, so I guess I could have two discrete sets of parameters entered.

wombat2go
04-18-2018, 11:34 AM
This effect ( DC injection) was a problem back in the days of the early induction motor vfds based on scrs
with forced commutation by capacitors.

If an scr failed to commutate, there was uncontrolled rise of stator current, basically into a circuit similar to Matti's #15.

On time as a young skinny guy I was testing one of those early vf drives into a motor about 35 kW
sitting unbolted on a pallet in the test bay.

On a decomm at full speed the motor lifted up into the air, started rolling over and crashed down onto the floor.

The 6 pulse dc drives of about that era had add on pack of dynamic braking resistor and DC bar contactor.
usually bolted on top of the cabinets in mesh housing. That was braking by resistance across spinning armature, not DC injection.

J Tiers
04-18-2018, 12:10 PM
With it set as you say in back gear and with 80 rpm spindle you can stop pretty quick with just dynamic braking (resistor); I could probably get a fraction of a second. BUT it's compromise. It will only stop as fast as your deceleration setting - and you will have to reprogram the deceleration/acceleration setting to be more practical for other speeds or you will trip out. If I add DC braking, my VFD ignores the deceleration times and "slams on the brake". That's one way to get a "quick stop" but...

This is why I want both mechanical and vfd braking (both dynamic and DC injection) on a lathe. The inertial mass varies massively on my lathe (collets, 6 inch to 16 inch chuck just for a start, never mind the work from grams to 100's of lbs); it's hard to find any single sweet spot for deceleration with just dynamic braking.

I need to play with the PLD on my vfds - in theory I can read a selector switch and auto-set parameters - would be nice to have multiple changes at the flick of a switch. I can already set 2 "different" motors, so I guess I could have two discrete sets of parameters entered.

You can KNOW what is possible and what is not.... if you care that much.

And, dynamic braking (meaning the regular deceleration via programmed stop) can simply be programmed for the desired speed.... It will NOT matter what the mass is, so long as it results in less than the maximum braking energy per unit time.

If you need to always have the optimum for whatever you have in the machine, then you can calculate it.

The braking resistor pulls a certain current, dependent on the duty ratio. And there is a certain current draw from the power supply, etc, plus any load current. That is the drains.

The energy content of the capacitors is 1/2 C*V^2, and you can figure the difference in energy content between normal voltage, and the trip-off voltage. The VFD will accept up to that total energy without tripping off.

The drains (brake resistor, etc) subtract energy at a given total rate, so the net energy per unit time that the unit will accept with a given resistor and duty cycle is knowable. That will give you the stopping rate that is possible, if you know the spinning mass (and effective radius) of the drive train and load.

In general you will not care very much for home shop applications, and it will just work if you add a resistor. Or maybe even if you do NOT add any "braking resistor". But in some industrial operations, with a given "process cycle", it is actually required to calculate that all out and verify that the cycle will actually work.

EVguru
04-18-2018, 12:32 PM
Some VFDs can be set to limit decelleration based on the DC bus voltage.

That's how the Huanyang on my Harrison L5 used to be set up; With the collet chuck fitted, it would stop withing the programmed decceleration time. With the three jaw fitted, you could hear the motor pulse a couple of times as it hit the DC bus limit. With the big 4 jaw fitted, it would still occasionally trip if stopped from high speed.

Now the VFD has the braking components and an external resistor fitted and stops fine all the time.

MattiJ
04-18-2018, 02:13 PM
Some VFDs can be set to limit decelleration based on the DC bus voltage.

That's how the Huanyang on my Harrison L5 used to be set up; With the collet chuck fitted, it would stop withing the programmed decceleration time. With the three jaw fitted, you could hear the motor pulse a couple of times as it hit the DC bus limit. With the big 4 jaw fitted, it would still occasionally trip if stopped from high speed.

Now the VFD has the braking components and an external resistor fitted and stops fine all the time.

And on some fancier VFD's you can set the maximum desired torque used for deceleration and/or maximum motor power limit for deceleration.

kf2qd
04-18-2018, 08:12 PM
Another thing to consider - If you demand rapid starts and stops you are operating in the range that most VFD manufactures would call Heavy Duty operation and they would suggest a larger unit, 1-1/2 to 2 times larger. This would also allow for faster stops as there are more capacitors to charge.

While you may be using the VFD for the convenience of running a 3 phase motor off single phase they are more designed for cost savings when operating motors. By running a motor on a pump or fan at less than full speed the cost savings can be enormous. Also - if you are starting and stopping often you may be in a Heavy Duty classification of operation and actually need a bigger VFD.

If you are using a braking resistor and it is not heating up it is either not installed correctly, the VFD is not configured correctly, or it really is not needed in your application.

H380
04-18-2018, 08:33 PM
If you ever get a chance to look over a Hass TL 1. They just use an old fashion spiral Electric stove heating element as the breaking resistor.

J Tiers
04-18-2018, 09:19 PM
If you ever get a chance to look over a Hass TL 1. They just use an old fashion spiral Electric stove heating element as the breaking resistor.

While that sounds cheap and effective, you need to be careful if you try that.

There is a minimum resistance that is allowed, because the discharge IGBT ("Braking resistor IGBT") is almost always smaller, and has limited current capability. If you pop that part, you are out of luck, because they cannot be repaired except by replacing a module that is usually as expensive as the whole VFD, if it is even available.

OK, maybe I and a few other folks could do something else, but in general it is just not practical....

lakeside53
04-18-2018, 09:59 PM
I use either 300W 120v (48 ohms) and 1500W 240v (39 ohms) elements on the Hitachi 3hp vfd. Dirt cheap, They work fine and exceed the 35 ohm minimum resistance requirements. The problem with using close to minimum resistance is that your braking duty cycle goes way down. Often it's better to pick one that is not as low (aggressive) and have better overall usage results. All depends on how you use your machine.

One thing NOT to do is blindly (no pun intended) use incandescent light bulbs. The resistance varies widely as they "light up". Could be pretty though.

boslab
04-18-2018, 10:39 PM
I remember using the heating element cartridge out of a tumble dryer once, long time ago, I can’t remember the resistance but there was a hell of a load on it, I think it’s still in use (it was a small reversing mill for samples)
Mark

CCWKen
04-18-2018, 11:58 PM
I need to play with the PLD on my vfds - in theory I can read a selector switch and auto-set parameters - would be nice to have multiple changes at the flick of a switch. I can already set 2 "different" motors, so I guess I could have two discrete sets of parameters entered.
Mine has the "two motor" option also. On my KOC100 I believe there's up to four start/stop time parameters available but their selection is via the DI (digital input) terminals. I'm not educated on how the DI terminals would be implemented. I guess you would need some external circuit to supply that. :confused:

lakeside53
04-19-2018, 12:05 AM
Most digital Inputs are pretty simple.. switch to ground (NO or NC) or +24 depending on how you program them. You might have to read the manual ;)

CCWKen
04-19-2018, 07:45 AM
That's for the Analog Input (AI) terminals. How would you input a value of "23" with a switch? :rolleyes:

lakeside53
04-19-2018, 11:19 AM
With difficulty. The switch(es) via digital inputs would be used to select preprogramed parameters, like "second motor set", or a sequence held in the internal PLD (not all VFD have access).

kf2qd
04-19-2018, 10:24 PM
I use either 300W 120v (48 ohms) and 1500W 240v (39 ohms) elements on the Hitachi 3hp vfd. Dirt cheap, They work fine and exceed the 35 ohm minimum resistance requirements. The problem with using close to minimum resistance is that your braking duty cycle goes way down. Often it's better to pick one that is not as low (aggressive) and have better overall usage results. All depends on how you use your machine.

One thing NOT to do is blindly (no pun intended) use incandescent light bulbs. The resistance varies widely as they "light up". Could be pretty though.

The Ohmic rating is only 1/2 half otr the data needed. The whatage is much more important for braking resistors. The Higher the wattage the more often you can use the braking funstion as a higher wattage rating has the ability to get rid of more heat. Resistors in parallel with give you a higher wattage.

lakeside53
04-20-2018, 12:19 AM
resistors in series give you increased wattage too! but seriously.. you need the right ohms and a minimum sufficient wattage.

Increasing the wattage of the resistor has little effect on duty cycle after a nominal point (i.e. "big enough"); it's more related to duty cycle capability of the VFD. The resistor part is easy - how hot can it get hot and survive? The VFD is much more limited.

1500w is HUGE comparison to what you need for a typical 3hp motor at any reasonably sustainable duty cycle. Advantage of elements is that they are dirt cheap and can can take "red hot" temperatures. 300W works well for 2-3hp, never gets "red" or even smokes on mine even if I use it as much as possible before the vfd limits.

Jim Stewart
04-20-2018, 12:55 AM
The Ohmic rating is only 1/2 half otr the data needed. The whatage is much more important for braking resistors. The Higher the wattage the more often you can use the braking funstion as a higher wattage rating has the ability to get rid of more heat. Resistors in parallel with give you a higher wattage.

My first reaction to this post was "HUH??"

No offense intended, but (in a technical forum (this one)) I'd expect as a licensed Advanced Radio Amateur licensee (judging by your handle) you would not make up your own terms.

There is no such thing as an "ohmic rating". As you certainly should know, it's resistance or impedance. There is no such thing as "wattage". It's power rating.

73, KF6R

lakeside53
04-20-2018, 01:06 AM
Somehow it slipped into the language

wattage
ˈwdij/Submit
noun
a measure of electrical power expressed in watts.
the operating power of a lamp or other electrical appliance expressed in watts.

Jerry : where are you when you are needed? get over here and argue both sides ;)

Jim Stewart
04-20-2018, 01:10 AM
Somehow it slipped into the language

wattage
ˈwdij/Submit
noun
a measure of electrical power expressed in watts.
the operating power of a lamp or other electrical appliance expressed in watts.

Jerry : where are you when you are needed? get over here and argue both sides ;)

:D :eek: