VFD Question

[f350ca]

There's been a few discussions about VFD's here but the search engine hasn't improved and I don't think this concept has been brought up.
Im thinking (this is where my problems start) of building a large wood lathe for turning bowls. Variable speed is essential with possible a stepped pulley arrangement to get ranges. A VFD seems to be a no brainer for speed adjustment.
 Single phase input VFD's seem to max out at 3hp. A wood lathe shouldn't need more than 1 maybe 1 1/2 hp think my current Rockwell has a 1 HP. I have a high quality 5 HP 3 phase motor that was on my compressor when I bought it. With soft start on the VFD and maximum current setting capabilities could you run that 5HP motor with reduced load. Would save me buying a motor (remember I like recycling) and the larger frame size should make it run smoother (same as the super large frame 1HP motor on the Hardinge lathe.
With soft start ramping the speed up will the current draw not be lower than the surge current to instantly start a motor. Possibly within the capabilities of the 3HP VFD. In the event of an overload (me going into orbit off the end of a chisel if I take too deep a cut) would the current settings in the VFD not protect itself. 
Would appreciate some input before I talk to suppliers and sound like the fool I aspire to be.
Thanks

[stevec]

Greg,

With my previously displayed lack of knowledge of VFDs I won't attempt to advise you on anything beyond DON'T BUY A HUANYANG
VFD. even though they are pretty cheap.

Aside from need of help to work through the chinglish manual I have heard horror stories of dissatisfaction with the HUANYANG models.

Steve

[12345678910]

I like the KB electronics vfds

good manuals, good support

[oldgoaly]

if I read you post right? you are asking if a 3hp VFD could run the 5hp motor with the 1hp load? if the starting load is low enough, the VFD only sees resistance / inductance , current draw, counter-EMF, so if it is in the parameters the VFD is designed for it should work. I have Lense, Toshiba, ???FM50 can't remember the name? on various machines, knock on wood they all work fine!

[dallen]

the FM50 I a Teco, to start the 5 HP motor with a VFD for 3 HP you may have to ramp the start so that instead of it throwing all the starting voltage at the motor it will ramp up the load and not overload the VFD.
Should run fine as long as you don't put more then a 3 HP load on the Drive
DA

[f350ca]

When you set the VFD to ramp up the speed, how does it do it? By reducing the voltage or by bringing up the frequency?
The only load the motor would have on start up would be weight of the blank, i.e. building momentum.

[dallen]

Greg that's one I can't answer been too long since I had to work on any where I had to bring the motors up slow to stop over current conditons, damn 500 HP fan drive.

You might get away without ramping the start, by putting in a longer start trial time.

A whole lot of what you will be able to do will depend on the brand of drive that you get.

DA

[Terry S]

I'm a big fan of using VFD's for getting 3PH power for home use.

Having said that, I get a bad feeling about trying to power a 5 hp motor from a 3 hp VFD.
During set up, you enter things like  rated RPM, HP, and "full load amps" into the drive parameters. The drive later uses these parameters to control the motor start-up, to control torque boost at lower than rated speeds; to decide when to shut down on high load current, etc.

With a 5 HP motor, you will have to "spoof" the 3 HP drive into thinking it is running a 3 HP motor. I am afraid that many of the "preset" parameters will then not be valid. The VFD is going to see more electrical reactance than it is expecting. You still may be able to make it work, but you may have to do a lot of tuning on drive parameters that you would otherwise not have to mess with. Most of these drives have dozens of adjustable parameters - and the tuning of one can often affect the tuning of another.

If you are careful in setting up the drive - I don't think you will hurt it by experimenting with the 5HP motor. If the 5 HP motor proves unworkable, you can then invest in a 3 HP (or smaller) motor. In any case, get a Korean or Japanese drive with good documentation supporting it. You probably have to spend $200 - $300 for such a drive. Personally, I like Westinghouse TECO EV "Microdrive" series (but not the FM series which is an older drive). Others have mentioned other good drives.

Another note: even with a VFD, the capability of a stepped pulley system is a good thing. VFDs and motors like to run near their rated speed and down to perhaps 50% (maybe as low as 30% rated speed if you are careful with motor cooling). The stepped pulley system allows you to turn large diameter bowls as well as small diameter spindles and still keep the motor at a reasonable speed with good torque.

Terry S.

[Terry S]

Greg,
Forgot to address your other question about how the VFD goes about ramping up the motor (AKA "soft start").

You probably already know that there is a direct relation between frequency and motor speed. The motor reaches its rated speed at rated frequency (60 HZ here in North America). If you run the motor at 30 HZ, it will run at about 1/2 rated speed, but you also have to reduce the voltage to the motor so as not to over-current. The drive knows how to figure this out. With older VFDs, when you ran at less than rated frequency, the available torque fell off quickly.  The newer drives have "torque boost" capabilities that are able to give the right "kick" at the right instance with cycle to preserve more torque at lower speeds.

On a "soft start" the drive starts at a low frequency (like 5 hz) and ramps the frequency up to the setpoint (along with the voltage). You get to adjust how long this ramp is. I have my mill and lathe both set at a 3 second ramp. I don't do any machining in the first 3 seconds anyways. This makes for a quiet, gentle start. Pulls less current than "across the line." Puts less strain on belts and gears.

You also get to select the type of stop you want. You can have the VFD "ramp down" the speed on stop or "coast to a stop." I use "coast to a stop" on both of my machines. This allows me to use the mechanical brake that each machine has. If you were to try to use the mechanical brake while the drive was ramping down, you would be fighting the drive.

I don't make use of the "E-Stop" feature. E-Stop requires some additional electrical components for proper use.

Terry S.

[oldgoaly]

It seem everyone of the VFD's have slight differences, just like 3hp motors some draw more amps than others @ full load. So if you lathe was powered by a 1hp it should be well under the 3hp of the VFD.
One good thing "knock on wood" VFD's are overload protected, they just don't let the smoke out like somethings. I have the teco/FM50 on a Simpson sand muller 3hp motor draws 14 amps, vfd is 12 amps, when I over fill the muller the VFD will shut down. Give the "OL" error code. Not a big deal, sand doesn't mull right when it's too full.
speed presets should be fine, ramp speed setting you will have to see, oh the small print in the "owners manual"
in chinglish too!