VFD conversion on my Harrison 600

[HughF]

I have a very nice harrison 600 milling machine that I run from a rotary phase converter... However I would like to convert it to a VFD.

I have a suitable VFD, a nice Optidrive E3 220-415 unit.

The mill has a nice multi-speed gearbox and a clutch/brake unit with continuous running motor. I'm after an opinion, would you:

1. Run the motor all the time through the VFD using the existing start/stop buttons and leave the clutch/brake alone.

2. Use the interlock microswitch on the clutch arm to start/stop the motor and clutch the spindle in/out.

The benefit of option 2 being that I can shut the motor down when not running the spindle and save noise/electricity etc...

I plan to use a 2nd VFD (which I also have) to run the power feed motor (I don't have the change gears for this so have a fixed speed presently) and use a capacitor on the coolant pump motor.

 

[MattH]

I would constantly power the motor.
If you start stop it you might have to look at external braking resistors etc.

 

[daleyd]

I’d be tempted to do away with the clutch unit - one less thing to go wrong and also as you say when the lathe is stopped then you won’t be running the motor.

You shouldn’t need braking resistors unless you are frequently stopping heavy loads, a sensible ramp up/down time on a lathe should be well within the capabilities of any decent inverter.

 

[fizzy]

I’d be tempted to do away with the clutch unit - one less thing to go wrong and also as you say when the lathe is stopped then you won’t be running the motor.

You shouldn’t need braking resistors unless you are frequently stopping heavy loads, a sensible ramp up/down time on a lathe should be well within the capabilities of any decent inverter.

It's a mill

 

[daleyd]

It's a mill

You’re right of course, I just saw Harrison and assumed it was a lathe - that’ll teach me for not reading the detail!

The fact it’s a mill should mean it’s even less likely to need any external braking...

 

[AdrianH]

I would always suggest braking resistors on a VFD if possible for the emergency stop, these are times when you can not have a ramp down to stop so need it to stop a fast as possible.

I have a basic Harrison milling machine and retro fitted power drive to the table, so it is separate to the mill spindle VFD. I am very cautious when using it as yet I have not slaved the output of the table drive from the VFD. Ideally I want some way to stop the table drive when I hit the panic button on the mill drive otherwise I could still be driving the work into a stopped cutter. The mill spindle VFD just runs at 50 Hz. As you say there is a good speed range on the Harrison mill, so I tend not to change it via the VFD, it just has a second or two ramp up and down when normal start/stop is used. The VFD was smaller and quieter than my home made rotary converter and does not trip the breaker on start up.

Adrian

 

[maz0]

I have a very nice harrison 600 milling machine that I run from a rotary phase converter... However I would like to convert it to a VFD.

Does that mean your rotary will be for sale?

 

[daleyd]

I would always suggest braking resistors on a VFD if possible for the emergency stop, these are times when you can not have a ramp down to stop so need it to stop a fast as possible.

I have a basic Harrison milling machine and retro fitted power drive to the table, so it is separate to the mill spindle VFD. I am very cautious when using it as yet I have not slaved the output of the table drive from the VFD. Ideally I want some way to stop the table drive when I hit the panic button on the mill drive otherwise I could still be driving the work into a stopped cutter. The mill spindle VFD just runs at 50 Hz. As you say there is a good speed range on the Harrison mill, so I tend not to change it via the VFD, it just has a second or two ramp up and down when normal start/stop is used. The VFD was smaller and quieter than my home made rotary converter and does not trip the breaker on start up.

Adrian

For a lathe or mill at home I personally don’t think they are necessary and would be overkill - the existing machine wouldn’t have one, and an inverter can dump a lot of energy back into the bus before a separate resistor would be required. A lathe with a vfd can stop a lot quicker than the same lathe running a standard induction motor.

I’ve used them on hoist or tension control appilcations where you have a high torque mass or constant/sustained braking force required but I wouldn’t bother for something like this.

 

[AdrianH]

It is all down to what you want, I would rather have the chance to stop something sharpish.

As an example my lathe which is a 3 phase 3HP Flashcap was built with an emergency button that when pressed switched a phase around and stopped the motor almost instantaneously, when spinning a chuck at around 2,000 rpm it was frightening to see and hear.

When I converted to VFD for home use I gave myself the option of a switch I could use by my feet just in case I did something daft, for the extra cost of a few large resistors it was a no brainer.

For my 2 HP Mill the brake resistor is 100 Ohms at 300 Watts, that is less than £25 from CPC. I may be paranoid at times, but working on my own in the workshop means an accident could be more serious then one at work when there are others around to help.

Adrian

 

[daleyd]

Of course if you want to then there’s no problem in that - it’s my experience however they won’t be needed on a home lathe or mill - eg on my lathe when it was fitted with a single phase motor there was no braking at all - now I have a vfd fitted I can stop it dead almost instantaneously without any braking resistor if I use the quick stop function (it injects dc into the rotor to effectively lock it), though the normal ramp down is set for a couple of seconds - still quicker than it would stop as standard.

A braking resistor doesn’t actually stop the motor any quicker, it will however allow you to dissipate the energy from stopping and prevent the drive from tripping on for example an overcurrent (or sometimes called overbraking fault) during deceleration.

 

[HughF]

Does that mean your rotary will be for sale?

Almost certainly not, I still need to power my 300a AC/DC Tig somehow

 

[HughF]

I would always suggest braking resistors on a VFD if possible for the emergency stop, these are times when you can not have a ramp down to stop so need it to stop a fast as possible.

I have a basic Harrison milling machine and retro fitted power drive to the table, so it is separate to the mill spindle VFD. I am very cautious when using it as yet I have not slaved the output of the table drive from the VFD. Ideally I want some way to stop the table drive when I hit the panic button on the mill drive otherwise I could still be driving the work into a stopped cutter. The mill spindle VFD just runs at 50 Hz. As you say there is a good speed range on the Harrison mill, so I tend not to change it via the VFD, it just has a second or two ramp up and down when normal start/stop is used. The VFD was smaller and quieter than my home made rotary converter and does not trip the breaker on start up.

Adrian

Being able to power the table when the cutter is stopped is something I want on the retrofit, coming from years of running Bridgeport's with the rapid button on the power feed. Having the Harrison's power feed interlocked with the spindle clutch is a total pain...

 

[HughF]

Thanks for everyone's input, I think for ease of rewiring and my own sanity, I'll stick with the clutch/brake arrangement and power the motor continuously.

I'll look at a braking resistor and see if one is required.

 

[RichardM]

I bought a braking resistor for my lathe and it's still sitting in the cupboard not wired in. Certainly stops quicker than my old BUD single phase used to without the resistor.

 

[HughF]

I've got a perfectly good clutch/brake to stop the mill spindle...

 

[Sarto]

@daleyd
If I may ask (the VFDs are almost a magic thing, for me ):
What are the rules on sizing the correct resistor for an application?
When you say "sensible time" for the start and stop ramps, is there a rule of thumb
to find the minimum time? Or the installer has to do strange things like measuring the currents
on the motor Bus ,etc ?

Have a nice day.

Sart0

 

[daleyd]

@daleyd
If I may ask (the VFDs are almost a magic thing, for me ):
What are the rules on sizing the correct resistor for an application?
When you say "sensible time" for the start and stop ramps, is there a rule of thumb
to find the minimum time? Or the installer has to do strange things like measuring the currents
on the motor Bus ,etc ?

Have a nice day.

Sart0

A decent drive would have recommended sizes for the resistor to use in their manuals, eg see attached for an extract from one of schneider altivar manuals. With a cheaper drive they may not have one recommended but pick a similar sized “good make” and go for the same.

https://www.schneider-electric.com/.../en_US/ATV312_migratingATV31 to 312Letter.pdf


Sensible time to me means the drive isn’t trying to ramp up/down stupidly quick, it really depends on your application - but for home use on a lathe or similar then a couple of seconds ramp is a good starting point and you can tweak it from there. If you start getting faults like “overbraking fault” or suchlike then it’s generally an indication the bus voltage is too high during ramp down and you might need to slow it down or fit a resistor. There are so many variables it’s impossible to say for certain what would work in all applications but you can adjust almost anything so there’s plenty of scope to play.

 

[Pete.]

Your VFD manual will give you the resistor sizing, some don't even allow for a braking resistor. The VFD itself will provide some braking by pumping the energy back into the DC bus elevating the bus voltage. When the bus reaches a set voltage that's when it will dump energy into the resistor.

Read the attached excerpt from my VFD manual it will explain it better.

 

[Seadog]

Beware ramping down too quickly if you have screw-on chuck.

 

[Farside]

Beware camping down too quickly if you have screw-on chuck.

Polari?