Mill has 3 motors - Can I run it off 1 VFD?

My vertical mill is a Gorton, and it has separate drive motors for the:
a) 2hp quill
b) 1/8 hp quill downfeed
c) 1/2 hp table feed.





According to these threads, I can't run them all off one VFD, but I need confirmation.

http://www.practicalmachinist.com/v...hase-converter-5-hp-hendey-253888/index2.html
http://www.practicalmachinist.com/v...ild-buy-new-bigger-rpc-vfd-255336/index2.html

Thermite suggested static phase converters for the b) and c) motors which seems the cheapest way forward.

Is this a real problem? Can't I get around it somehow? I'm buying the VFD and the mill next week, so my time is getting short. TIA,

Katou

If you run only one speed, it may work fine, as the feeds are smaller. if you use a karger HP drive it should work... the sum of the feed motor start current (figure 6x run current) and main motor should be under the short term VFD current limit, usually 1.5x to 2x nominal continuous current.

Switching motors on and off is not an issue, regardless of what you read, unless the motor inrush is larger than the VFD can provide, as it would be with the spindle motor.

You won't be able to use the variable speed, so you might as well use an RPC as a VFD.

What voltage do all the motors operate at ? ---Trevor

Katou,

In my opinion the RPC (assuming you have one, if not, why not make one?) is the elegant solution here as it is the simplest (and cheapest) way out of your dilemma. Thermite's suggestion is more complicated but offers real advantages in that you can use the VFD to vary the spindle speed while the other two soldier on. If you had all three motors running from a VFD, varying the frequency would slow down all functions, whichever are being used, spindle and either feed, proportionally, a potentially nice feature as it maintains chip load.

JST,

My understanding of electricity tells me that the problem with starting/stopping motors with the VFD on isn't the start but the stop. If a motor is disconnected under load (switched-off or otherwise) the current flowing now has nowhere to go and creates a big voltage spike, which destroys the VFD's output drivers. If you are running a 2HP spindle, and that motor keeps running while you switch off another motor, you may be okay, as I'd guess the big motor can sink the 1/8hp motor's spike and probably the 1/2hp as well. The problem with this approach is that the overload protections and many other functions of the VFD are lost.

I have a big (10HP) VFD that I use to test run machines or to demonstrate them to potential customers. I have had impromptu connections come undone while small-ish motors (~1HP) are running and it hasn't harmed my VFD, but I'm sure it hasn't done it any good, either.

Steve

VFD is 5hp 440v

Spindle motor is 2hp, but it's an old motor, must be 400 lbs, might be able to pull over 2 hp for little bits.

I think what you're telling me Bill, is to stop fussing about and pick a solution. Fair enough.

So, where does one find quality Static Converters cheap?

Thanks,

Katou

ps. I'm staying away from RPC's for noise, space, and because I can get a VFD + transformer for $350 and I have not had any luck finding a cheap, big motor and transformer.

sa100 said:

My understanding of electricity tells me that the problem with starting/stopping motors with the VFD on isn't the start but the stop. If a motor is disconnected under load (switched-off or otherwise) the current flowing now has nowhere to go and creates a big voltage spike, which destroys the VFD's output drivers.

Click to expand...

Steve, just for the record, in 2012 you can now switch motor on and off on output of any decent brand VFD: the protection is indeed built in and no voltage spike will result damaging the vfd outputs. I cannot speak for dirt cheap imports. If this was 1990 then yep, you would blow the output of many vfd models.

We routinely today leave output contactors in circuit on large machine tool spindle retrofits as it is easier than teaching folks how to bypass their logic contacts and such, as well as it leaves the positive disconnect feature a lot of folks like for safety when changing tools. During initial startup after these retrofits it is not uncommon for those contactors to open and close at wierd times until the CNC gets it all under control again. We stopped worrying about hurting the vfd a few years ago and have not looked back.

Just to set the record straight on the old "disconnect switch" issue......

The VFD itself switches the output current though the IGBTs on AND OFF at a rate of many thousands of times every second. Each "turn off" is accomplished in a considerably shorter time than a switch opens, namely, in a few microseconds.

That rapid turn-off is an interruption of the output current at its maximum value.

Since engineers are practical folks, and know that it isn't a good idea to need to replace IGBTs at a rate of say 24,000 times per second*, devices known as "fast recovery diodes" are across every IGBT in the drive. These "commutating" diodes act to provide a path for the current after the IGBT shuts off, "clamping" the spikes of voltage to the "power supply rails" in the VFD in a relatively few nanoseconds.

A minute of run time provides a couple million potential "spikes" that are caught by the commutating diodes. A mere 10 or twenty thousand switch-offs over a few years is fewer "potential spikes" than one second of operating time.

Common "inverters" used in your car, truck, solar system, etc are made the exact same way, and have no trouble with motors and other equipment being switched on and off. it's absolutely routine so long as the starting current is within the inverter ratings.

The real problem is that the motor , if "dropped onto" the VFD output by closing a switch or contactor, will typically pull a current of roughly 6X its normal operating current. To stand that, the VFD must be rated for at least 3x, and more likely 6x, the normal current. That suggests you must use a very much higher current, higher power, and higher cost VFD than is actually needed when using a ramp-start as every VFD does.

Your small motors happily turn on and off, simply because they do not exceed the current limits on the VFD.

* 8 kHz x 3 phases gives 24000. Many VFDs operate above 8 kHz.

For what it would cost, get another VFD for the feed motors.

You can buy a new 1hp drive for $150 delivered... That gives you the option of independent variable speed control for the spindle and feeds.

I wouldn't even entertain an RPC when the advantages of infinitely variable feeds and variable spindle speed with braking are worth a lot. A VFD on the spindle with a dynamic brake is excellent for tapping since you have nicely controlled reversing, and you don't have to change the damn belt position from drilling to tapping.

You could also have push button rapid feeds if you set it up with an external pot for speed control. If you had button that overrides the pot and gives full voltage(usually 10 volts) to the analog input you would have a rapid button. Having the feed VFD set to 125%-150% makes it even better....

Chris

It's nice to hear people talking sense about power electronics instead of vodoo based on urban myths.

I have recently seen a mill with two motors actually working with a VFD supplying the three phase power. I wouldn't do it that way myself but what could I say? I suggested that some of the spare contacts in the control relays be used to disable the VFD except when power was actually needed. ie., like a clutch that prevents power from being applied to a transmission during shifting.

There are a few other considerations when switching a VFD's output while delivering current:

1.) the contactor may not be rated for interrupting the current at the VFD's switching frequency. I could see that the arc developed on opening could take longer to extinguish because the VFD in continually applying a high voltage and switching it back and forth.

2.) the contactor will definitely be damaged if it tries to open when DC injection braking is being applied. When switching AC, the arc extinguishes 120 times per second. With DC it won't and the contacts start to transfer metal and could even melt. I wouldn't think this would be a planned operation, but could happen if an E-stop button were wired in and the VFD set to brake when commanded to stop.

3.) the contactor may not interrupt the three phases at the same time. If one phase opens earlier, the imbalance could cause trouble. I had one chip conveyor that used a 1/2 HP VFD driving a 1/3 HP motor. One day it stopped and the VFD started throwing error codes on run. The motor windings weren't burnt and nothing appeared to have been overloaded. Turned out the motor had an open winding and the VFD had one shorted output transistor. Single phasing the output of a VFD definitely doesn't do it any good, hopefully there won't be any damage for short periods, ie. milliseconds.

Cheers,