VFD for a spindle brake

I have a milling machine with a 5.5hp 3 phase motor. The machine is powered by a Phase Perfect unit. The motor does not currently have any sort of brake so the spindle coasts to a stop.

Would it be possible to use a VFD to provide a motor brake? If so how would this be configured.

You certainly could do this. Generally VFD's have parameters you set using the keypad. Usually 1 parameter to enable/disable spindle braking, and another parameter that tells the VFD the time period it should take to brake to a stop. You can also do the same thing when starting the spindle. Not all VFD's will support this, and if the load is big enough or the cycle times are short enough you may have to add braking resistors.

My milling machine has a VFD and I set it for 4 seconds to start and 4 seconds to stop. That is without a braking resistor ( Hitachi) Later on when I requested a braking resistor for the unit the supplier could not help me since it was an older Hitachi. If I have a really heavy cutting head spinning at higher speeds the VFD fails to stop it in 4 seconds and puts out an error code and I have to reset. I rarely used large cutting heads so I live with it.
The next time when I ordered a VFD to power my Voest lathe I ordered a resistor matched to the motor and VFD. I stop that unit in 2 seconds which is good enough for me. I don't like instant starts and stops. Some VFDs require an add on device along with the resistor.
There are formulas for sizing the resistor.

I guess part of my query is: I'm supplying the machine with three phase power already and don't need to vary the frequency to change motor speeds. Would the VFD just be wired in series on the motor supply lines? It would basically be just a pass through until the motor was shut off at which time the braking would kick in?

hobbyman said:

My milling machine has a VFD and I set it for 4 seconds to start and 4 seconds to stop. That is without a braking resistor ( Hitachi) Later on when I requested a braking resistor for the unit the supplier could not help me since it was an older Hitachi. If I have a really heavy cutting head spinning at higher speeds the VFD fails to stop it in 4 seconds and puts out an error code and I have to reset. I rarely used large cutting heads so I live with it.
The next time when I ordered a VFD to power my Voest lathe I ordered a resistor matched to the motor and VFD. I stop that unit in 2 seconds which is good enough for me. I don't like instant starts and stops. Some VFDs require an add on device along with the resistor.
There are formulas for sizing the resistor.

Click to expand...

Are you supplying the VFDs with 3 phase?
It's not clear to me what you are using the VFD for- varying speed, start, braking, other?

sneebot said:

Are you supplying the VFDs with 3 phase?
It's not clear to me what you are using the VFD for- varying speed, start, braking, other?

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In his case he may be using it for phase conversion ALSO, but it is a pretty standard feature in a VFD to have an acceleration time and a deceleration time. The accel is the time to reach full speed, and the decel is the time to reach zero speed, i,e, to stop, to brake, or whatever.

In your case, you would simply use the VFD to run the spindle. You would use controls FOR THE VFD to start and stop it. The VFD would speed it up to normal speed when started, and slow it down to a stop in the time you set. You would be using is essentially as a brake only.

Configuring would be to set the "accel" and "decel" times, set the thing to 60 Hz, and put its controls wherever you wanted them. Otherwise, 3 phase to VFD input, VFD output straight to motor.

You WOULD have to NOT use the machine controls, as start stop have to be with the VFD powered, so that it can be in control.

You might be as well off, or better, to put a brake unit on the motor, or even to use DC braking, if all you need is braking, and you don't want to change how the machine is operated.

JST said:

In his case he may be using it for phase conversion ALSO, but it is a pretty standard feature in a VFD to have an acceleration time and a deceleration time. The accel is the time to reach full speed, and the decel is the time to reach zero speed, i,e, to stop, to brake, or whatever.

In your case, you would simply use the VFD to run the spindle. You would use controls FOR THE VFD to start and stop it. The VFD would speed it up to normal speed when started, and slow it down to a stop in the time you set. You would be using is essentially as a brake only.

You WOULD have to NOT use the machine controls, as start stop have to be with the VFD powered, so that it can be in control.

You might be as well off, or better, to put a brake unit on the motor, or even to use DC braking, if all you need is braking, and you don't want to change how the machine is operated.

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So are you saying there is no way to do this with VFD without using the VFD for machine start and stop functions are well, yes ? I think Snee and I would prefer to use the original spindle on/off buttons (or lever in my case) but somehow the VFD would kick in for the braking function only.

In other words, I think we both want to basically have the VFD wired in but be able to ignore all of it's buttons and yet still benefit from it's braking functions somehow.

There are many ways to do dynamic braking without a VFD. DC injection braking is used frequently. You will need a rectifier and some contactors. Regen braking can also be done with simple contactors.

There are also soft start units that are not VFDs, but solid state motor starters. Most can also do braking.

Using the original controls should be easy with any unit, including a VFD. The controls should be low voltage 24VDC, or maybe 110VAC. The VFD or starter would just replace the motor contactor (or contactors if reversible). The controls would then be hooked to the control instead of the coils of the contactor.

The scariest system is the one used on some Blanchard grinders where you slow the motor by just holding the reverse button until it stops. It just puts the motor in reverse to slow it.

Milacron said:

So are you saying there is no way to do this with VFD without using the VFD for machine start and stop functions are well, yes ? I think Snee and I would prefer to use the original spindle on/off buttons (or lever in my case) but somehow the VFD would kick in for the braking function only.

In other words, I think we both want to basically have the VFD wired in but be able to ignore all of it's buttons and yet still benefit from it's braking functions somehow.

Click to expand...

Still doable. Bit of a PITA to preserve the OEM ergonometrics, but not a huge one.

One would have to utilize the OEM start & stop buttons to operate the VFD via its 'remote' option. It could then essentially be set-and-forget as to programming, buried in the bowels of the lathe or an enclosure 'somewhere', so long as not heat-trapped.

Downside is that I am hearing that a VFD does NOT brake nearly as well as a 4Q DC Drive.

Or perhaps it just needs a rather larger measure of upsizing than might otherwise be needed?

Might that come along as a 'freebie' if it were also being used de-rated as a single-phase to 3-Phase converter?

There must be at least a few VFD around that would be much better at this than others, no?

"DC Injection" braking is old stuff enough, but I have no current info on what is out there 'pre-packaged' so as to easily add it without design work, testing, and the accompanying rectal eversions, AKA "repeatedly jumping back and forth through yer own a**hole".

The ability of a VFD to do braking is just the limitations of the resistor connected to it. When the VFD does braking, the motor is a generator and the DC bus voltage rises. The deceleration time of the VFD is limited by the rise in DC bus voltage caused by braking. Hooking up a big resistor allows the bus voltage to stay below the limit despite the braking action.

The problem with VFDs is that many do not have the braking controls built in. They require a separate control to turn on the resistor during braking and turn it off the rest of the time.

sneebot said:

Are you supplying the VFDs with 3 phase?
It's not clear to me what you are using the VFD for- varying speed, start, braking, other?

Click to expand...

I needed the VFD to run 3 phase motor from single phase in the shop. The additional reasons I choose a VFD are complete motor protection, controlled stop and start times, and variable speed adjustment. If you match the right resistor you can have stop times that are in tenths of a second. I figure that could be hard on the motor but the menu is there.
If you already have 3 phase power in your shop the VFD only will do the three items I mentioned. You also do not need an oversized unit since you are not doing phase conversion.
Also I use all the old button switches only they operate on 24 volt current that comes from the VFD and signals for stop-start. A potentiomenter using similar low voltage controls speed.

If you use a VFD, and oversize it so it handles maybe 5x the motor full load current, then you can use the machine controls with almost no changes, powering the machine from the vfd. You would still have to somehow give it the stop signal without opening the contactor until after the motor stopped, , but it would stop the motor as fast as any electrical means can. The controls would need powered from before the VFD.

There are a number of possible ways to handle the operation, that require different amounts of changing to the machine controls.

The bottom line is that power would come to the motor through the VFD, and braking would be from it as well. What needs to be done depends on how "hidden" the VFD has to be as far as operation is concerned.

ewlsey said:

The ability of a VFD to do braking is just the limitations of the resistor connected to it. When the VFD does braking, the motor is a generator and the DC bus voltage rises. The deceleration time of the VFD is limited by the rise in DC bus voltage caused by braking. Hooking up a big resistor allows the bus voltage to stay below the limit despite the braking action.

The problem with VFDs is that many do not have the braking controls built in. They require a separate control to turn on the resistor during braking and turn it off the rest of the time.

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Almost, but not so. The resistor must be adequate, yes. But you can have a problem with decel before you run out of voltage , or before the resistor is overrun. Generally you can decel only about as fast as you can accel.

But the limit is the VFD in that one of a given size can only do a certain amount of energy transfer.... decel IS energy transfer, and is just like accel, except it is "negative".

The VFD is only connected to the load by the motor current, and that has a limit in the VFD. You will get an overcurrent limit if you decel or accel too fast. You can use a bigger VFD, which can handle the required current, but you still then can run into motor limits.

DC braking can be quite violent and stop fast. The problem is that it severely heats the rotor part of the motor. Repeated hard stops can even melt the aluminum rotor bars,although that takes a lot.

DC braking is effective but stresses the motor.

In a CNC, the motor is a specially made motor that (if not a servo) is designed to take high currents and stop fast. You cannot easily approach that with a standard induction motor.

As stated above, if not stopping a lot, a used Short Stop off ebay will do the trick. I use it on my direct drive woodworking machines that run 3500 rpm so the slow speed of a lathe should not cause any damage. Dave

beckerkumm said:

As stated above, if not stopping a lot, a used Short Stop off ebay will do the trick. I use it on my direct drive woodworking machines that run 3500 rpm so the slow speed of a lathe should not cause any damage. Dave

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Interesting.....now that I look at a few on eBay my brain is jogged to vaguely recall having one of those on a used woodworking machine many years ago, maybe a 30 inch bandsaw no wait,it was on a sliding table saw I think...seems like it did work for the purpose sure enough, thanks for the tip.

One wonders how it might react to a two speed motor on my lathe jogging the lever from slow speed to high...hopefully it would not react to that... I vaguely recall having to do electronic adjustments to get the stop reaction to my satisfaction on the table saw...but once adjusted to my satisfaction it worked well.

Monarchist, should I be concerned with "stressing the motor" if using one of these ?

The Ambitech website says the standard brake time for the short stop is 15 seconds (it is adjustable). This seems like an awfully long time. How adjustable is the time on these units?

sneebot said:

The Ambitech website says the standard brake time for the short stop is 15 seconds (it is adjustable). This seems like an awfully long time. How adjustable is the time on these units?

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Yeah I noticed that too....15 seconds is an eternity except for a 36 inch woodworking bandsaw. But seems like I was able to adjust the electronic stop on that table saw I had to more like 2 seconds brake time....but then I wonder about the motor stress Monarchist mentioned.

The brake time on my Schaublin 102N-VM is more like .001 second (but then, it's braking the transmission and not the motor)

Milacron said:

Interesting.....now that I look at a few on eBay my brain is jogged to vaguely recall having one of those on a used woodworking machine many years ago, maybe a 30 inch bandsaw no wait,it was on a sliding table saw I think...seems like it did work for the purpose sure enough, thanks for the tip.

One wonders how it might react to a two speed motor on my lathe jogging the lever from slow speed to high...hopefully it would not react to that... I vaguely recall having to do electronic adjustments to get the stop reaction to my satisfaction on the table saw...but once adjusted to my satisfaction it worked well.

Monarchist, should I be concerned with "stressing the motor" if using one of these ?

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That was JST's point. He isn't wrong, but I've directly-reversed too many 3-Phase motors on-the-fly, at full-gallop, under serious load, and of larger HP to see it as a significant issue for your probable use.

EG: "maybe" as aggressive as a sub half-second stop for testing, but backed-down to a one or even two second stop for daily-driver use.

if I needed anything more aggressive than that here, I'd modify Ray Behner's approach and put an electrically-tripped air-over hydraulic brake right at the load side of the 10EE's D1-3 spindle. Advantage is the whole shebang is where one can access it for adjustment or service without even bending-over, let alone crawling under and into.

You don't QUITE love that Schaublin enough to shred sweaty sheets with it anyway, do yah?

We have 3-phase for our shop and I still put VFDs on a few mills and lathes both for braking and also for soft starting. I have a Mori-Seiki 8020 lathe that had a really worn spline on the transmission input shaft that eventually failed from the motor starter slamming it up to speed.

I put a surplus VFD on it and wired all of the existing controls to relays and then to the drive to preserve the original control look and feel. The drive ramps quickly to 60Hz and quickly back down to zero. It is set up ramp up in 2 seconds and ramp down in 2 seconds and if the operator steps on the brake pedal (disk brake on the motor) then the drive just goes to coast mode and lets the operator take over braking. It took some futzing to get the right DB resistor - I bought a finned strip heater from McMaster with the right resistance / wattage and it works fine.

http://www.practicalmachinist.com/v...and-vfd/vfd-install-mori-mr2000-lathe-278320/

Same thing on a step pulley Lagun Mill . . . except that I put a variable speed pot on it to simplify things like power tapping as you can fine tune your tapping speed and direct reverse the motor with the drive and get predictable stopping points.

motion guru said:

I bought a finned strip heater from McMaster with the right resistance / wattage and it works fine.

http://www.practicalmachinist.com/v...and-vfd/vfd-install-mori-mr2000-lathe-278320/

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Good 'un, but you never did explain or photo the last 'right resistor' critter, BTW. Left us hanging with it due in 'next week'. Or are those flat bars, centre, bottom the 'strip heaters'?

Same thing on a step pulley Lagun Mill . . . except that I put a variable speed pot on it to simplify things like power tapping as you can fine tune your tapping speed and direct reverse the motor with the drive and get predictable stopping points.

Click to expand...

Any braking resistor on that one? Or no need?

I put a finned strip heater on the lathe (and the mill also has a strip heater to dissipate excess energy from the bus during braking)

We use the 240V AC units with offset terminal version -> McMaster-Carr

I realize that these are getting hit with 700+VDC, so I cannot officially recommend them for UL installations. We have however megohmed them to 1500V with no failures and we have used them in quite a few systems without a failure (quite a few is more than 1000) . . . we install a thermal click switch to detect over temp as well as a fuse and use 1000V rated high temp teflon insulated wire.