Last VFD question?? hopefully :)

E

eddief

Eddie Fosnaugh
Okay, I have my VFD (http://www.automationdirect.com/adc...ts_(115_-z-_230_-z-_460_-z-_575_VAC)/GS2-22P0) wired up to my 1.5 HP vari-speed head Bridgeport.

Everything works great, but at low RPM, say 300 or less, I can stop the spindle with a tight grab of my hand. I understand the loss of torque with the unit because of the variable Hz output.

So my question is will it be okay to switch my Hi-neutral-Lo lever on the right side of the head to low if I need good torque at a low RPM??

I spoke with a tech today at Automation Direct and I do have everything programmed correctly. Thanks in advance!
 
Yes, your machine will work best if you use the gears and the VFD to get the speed and torque you need.
 
As HenryA says, yes, use the gears and increase the motor rpm to where it generates some torque.

I am always surprised that I see so many people who get these VFD's for their machines, yet never see where someone simply goes out and gets a DC motor with drive and does it once and properly. Yes, you can put an AC inverter motor on and a drive, but the cost is not any less than DC and you get vastly less rpm range with serious dropoff of torque at the low end vs DC that has none. Ebay has zillions of motors on there that are better suited to (and specifically designed for) variable speeds. Why not just go get one?


Now, I realize a vfd is cheaper than a motor and drive, but for about the cost of a mid range rifle action, you can have a hell of a nice drive motor w/control.

Maybe the reason I don't see posts from DC motor owners is, after they get a DC motor, they never ask again. :cool:
 
Well, this is my first Mill and VFD, so I'm learning. Before this I was very green as far as wiring, VFD,...etc.

I'm just glad I got everything wired up and nothing was smoked.
 
I understand, and that's fair enough. I wasn't attempting to single out you by any means.

As an aside, if you have a frequency display on that thing, you might try your best to keep it above 40hz output all the time. In my experience with variable freq motors (ones actually intended to be used that way), going below 40hz usually causes undue stress on the drive, and produces more heat than is healthy. If you ever talk to someone who's had a drive fail, it probably wasn't at near 60hz. Usually, it happens when running real slow. Try to avoid that when possible. Full speed but in low range is a great place to be.
 
VFDs and all the information you could ever ask for..

The HOME SHOP MACHINIST AND PRACTICAL MACHINIST webbsites have whole forums on VFS and phase converters. There is a webbsite called DEALERS DIRECT that has all kinds of VFDs for sale new and used.

Just some general information:

Older 3 phase motors are not as compatatible with VFDs as newer motors.They may run hot and burn up due to their windings.

Older motors require higher horsepower rated VFDs to operate with out significant torque loss. (plus 30 to 50% higher)

Newer American 3 phase motors and VFDs are more compatatible with equal horse power ratings.

Most if not all Chicom lathes and mills come with metric motors.

You can buy American NEMA metric motors so the chicom sheeves can be put right on the shaft with out adaptation.

Nat Lambeth
 
There is a whole lot of BS in this thread. Regarding Dealer's Direct website. Please bear in mind that they are in the business of selling VFD's and motors. They know that very few people know anything at all about VFD's, motors and their interactions. Consequently, they tend to advise in a very conservative manner for their own and their customer's protection. Some comments on the above follow:

1. Most( 90+%) quality motors, new or old, can maintain rated torque down to 30 Hz without overheating. Premium quality, industrial duty motors can go lower, maybe as low as 20 Hz. This requires that the low freq. voltage boost parameter in the VFD be set correctly. If the VFD has all the motor data required input correctly , it's internal overload protection function will protect the motor over the entire freq. range pretty well. The fact is the duty cycle in the avg. shop is a small fraction of that of an industrial continous process setting. This means that the casual user can drive a motor much harder than normal for short periods of time without problems.

2. 5. VFD driven motors are constant torque below 60 HZ in theory. If the motor can be properly cooled, it can produce full rated torque at zero speed. Since our machine tool spindle motors use shaft driven fans for cooling, cooling decreases as speed falls off, causing a temp. rise. There is, however, usually a good bit of thermal capacity unused because of the low duty cycle usually seen in the avg. shop. This allows short term high torque loads at low speeds, provided the VFD will permit it.

3. My J-head Bridgeport has one of the old style, open motors on it and has had a VFD on it since 1995. It is only 1 HP and my speed selection sets max spindle speed at 3000 RPM @ 120 HZ.. I have plenty of torque at 10 HZ, for routine power tapping up to 3/8" in steel. and enough to twist the head off a #10 machine screw. I don't remember ever using back gear. If the spindle stalls, I simply reduce the feed rate or deptth of cut.

4.. I have never seen a VFD that was not capable of overdriving a motor of the same HP rating. I'm not saying they don't exist, but I have never seen one by a reputable manufacturer. In otherwords, oversizing the VFD for the usual 4 or 6 pole motor found on machine tools is simply not necessary. I don't care about the age of the motor.

5. It is true that VFD's do stress the motor's winding insulation slightly more than than regular utility power. However, if a motor fails when a VFD is connected or shortly thereafter and the VFD/motor is set up correctly, then the motor was on it's last legs, anyway.

6. If you are concerned about overheating your motor, use your senses. if you can't hold your hand on the case without pain, it probably needs the load reduced, speed increased, or a shutdown for awhile. If it is an open motor instead of TEFC, you will likely be able to smell it when it gets hot. We all know the "dark brown" smell of overheated electrical insulation. There is usually ample time to shut down before things go critical.

A little about me: I'm a retired EE with many years of motor and VFD application experience. I wrote the first article to appear in "Projects in Metal" magazine, Dec '98, (companion magazine to "Home Shop Machinist) about retrofitting VFD's to home shop machine tools. I retrofitted my 13x36 Jet , my BP and my mongrel belt sander with VFD's. All the motors( except the BP) were purchased on eBay for very low prices. All the drives were purchased from Automation Direct which is a good, technically competent supplier whom I have no connection with other than as a satisfied customer.

RWO
 
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Sorry, I forgot. It is certainly OK to change the gear ratio as needed to get more torque. I suggest that eddie investigate his VFD settings related to "torque boost" or "voltage boost". VFD manuf. use both terms. Raising the setting up to max. might be neeeded to get enough low speed torque. As long as he does not run in high torque/ low speed mode for over 15 or 20 min., motor heating should not be a problem How can he know how much torque he is using? Read the motor current on the VFD display while under load and try to keep it below 125% of the motor's rated full load current ( on the motor nameplate). Roughly speaking, motor torque is proportional to motor current.

RWO
 
RWO,

So what exactly did anyone say that wasn't basically the same as what you said?

Motors and drives do overheat. It IS possible, and it IS more likely to happen when at low speed. You said as much yourself.

Something doesnt' jive about you saying motors basically have full rated torque at all speeds, and then talk about your own mill stalling the spindle when you go slow. ???? Am I missing something? I've never seen a spindle stall (tho I've seen a tool sent to the netherworlds more'n once)

In my experience, I've personally set up and used drives from Allen Bradley, GE, Hitachi, Omron, and for all I know there may be others I don't remember. Is ABB a division of Allen Bradley? I've used them too. In any case, every single one I have ever installed says right in the product info that use below a specific frequency should be avoided, and if load is to exceed xyz, the drive may shut down at low freq. Now, I'll assume they mean this, and since I've seen drives go into thermal protection at 35hz with very little load, I'll also assume there is some truth to what they say.

Now I'll confess that I do not deal with too many things where the duty cycle requirement is not 100%. But, to keep talking about "small shops" as if they have no requirements for 100% duty cycle, think about this. Lets say, one of these people decides on a Saturday they want to go do a bunch of work with their mill. And, let's say, they are not interested in working for 15 minutes and then standing around twiddling their thumbs for the next 45 waiting for the machine tool to cool down. They only get one Saturday per week, and this one is wasting away. Would it not be nicer to have a drive and motor that would allow them to keep working while they have time? Or, would it not be equally nice to have someone mention a few tricks that might get me more duty cycle out of my motor/drive? For me personally, I like stuff that runs when I want it to. A little overkill here and there isn't a big deal. Usually doesn't hurt anything. Certainly beats buying twice.
 
This is ABB,

HeritageBrands_195.gif


http://www.abb.com/cawp/abbzh252/a92797a76354298bc1256aea00487bdb.aspx
ABB offers a lot of components but is heavily into turn key, packaged robotics, medical equipment.
ABB was formed in the 1980's by the merger of Asea and BBC which makes it over 120yrs old.

AB, Allen Bradley is mainly into components, supplying OEM's and factory control, and of course their industrial software through Rockwell. They, both ABB and AB have very similar product lines
 
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Eddie,

I think you need to ask the technical man for some information of the VFD you are using now. You need to make sure that if this VFD has large torque at low frequency? Do you need to set the torque boost for your VFD?

Some VFDs perform well at low frequency (low RPM), say less than 300.
 
I agree with RWO and I have 2 Bridgeport J Heads with 1hp motors. I also had my Bud install resistors for instant braking. VFDs are wonderful, but everything has limitations. I wouldn't trade for mine.
 
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