which VFD drive

[Karl_T]

I'm buying a VFD for my CHNC spindle. There's several choices at this site:
http://tinyurl.com/bhs5o


In particular, there's three Hitachi drives: SJ100, SJ200, and SJ300. Which should I buy? FWIW, I just installed a 7 1/2 3 phase motor for the spindle.

Karl

[Milo]

Do you have three phase available? If so 240V or 480V? Some drives will accept single phase in and produce 3 Phase out. But are usually limited in HP.

[Al_The_Man]

The 300 is a Flux Vector drive which is best for spindle control, also it has zero speed detection output which you will need.
Al.

[HuFlungDung]

Karl,

I have an opinion, but just take it for that. I don't really like the sensorless VFD's for machine tools that may require high torque at low rpm. Now depending on your application, I think that torque for lathe turning can be quite demanding. The sensorless VFD's compensate in a somewhat lazy fashion to changes in the load. At very low frequency, like 10hz or less, the motor may simply stall in the cut.

I used a sensorless VFD on my first mill retrofit, and the performance at low speed is what prompted me to go with a Baldor Vector drive on my second retro. A vector drive uses an encoder for feedback to the drive. The motor becomes much more responsive, like a motor in a servo system. This is because it has a speed command and a way to compare its current rpm to the commanded rpm.

Its a good tradeoff, but does not totally replace the torque multiplication that you get from changing gears. If you run a motor at 1/4 speed, you get 1/4 of the nameplate horsepower. The torque is supposed to be constant. Compare that to changing gears to a 4:1 ratio, which multiplies the torque of your motor by 4 times. This is why CNC machines have huge motors, because they have to get the torque for low rpm, heavy cutting from somewhere, since they often lack an extensive mechanical transmission to multiply what is available.

You don't need a particularly expensive encoder on your motor to do this. But, you might want to dedicate one encoder to the motor for this purpose, and use a different one for feedback to your CNC retro for threading information.

[Al_The_Man]

Hu, Was the first one a Magnetic sensorless flux vector VFD? As these are fairly good at low speed, although I will grant they are not as good as having sensor feedback at low speeds.
With a sensor F/V VFD, these VFD's are now used to replace DC motors in elevators and cranes as they have maximum torgue at low speed, and are capable of holding a load such as a crane lift at zero motor speed.
The Magnetic sensorless flux vector VFD, especially if it has auto-tuning, builds a model of the electrical characteristics of the motor and uses a microprocessor to calculate by current etc where the armature is, so it has in effect feedback. The problem is that at very low speeds, it gets harder for the calculations to be as accurate as a feedback type.
If you have gearing for the low speeds then this is not usually a problem.
Although you did mention in a previous post that you were putting an encoder on the spindle shaft, so you might want to look at sensor type, make sure the one you get has auto-tuning.
Al.

[Karl_T]

I'm wondering how far down the pike of creeping elegance I should go?

This machine originally had a 2 HP Servo. So. I looked for 5 hp. 3 phase and didn't find one that fit. So, I bought a 7 1/2 hp unit.

I read its best to oversize the VFD, so I'm looking at 10hp. units.

Now, we're looking at pretty spendy VFD units. Do I really need this? I just want to equal the original machine.

Another question, Camsoft (my control) allows encoders at both motor and spindle, so I'm doing that. I was going to have logic for motor below commanded speed (first reduce feed rate, then alarm) Is this a good route?

I'm quickly getting to the point that I should have just stayed with the servo spindle motor and replaced the dirve, cost wise.

Karl

[Al_The_Man]

In hindsight your best option might have been to go with a matching 2~3hp vector motor and VFD with pulse generator feedback already fitted to the motor, for the price so far.
There are quite a few manufacturers that supply matching vector grade motors. WEG, Mitsubishi, Baldor.
Al.

[HuFlungDung]

Al,
The first unit was a Poly speed PSV1-50-2, I think it is a simple sensorless unit.

Karl,

I'm wondering how far down the pike of creeping elegance I should go?

Nice catchy phrase And so true.

Well, it does sound as though you've amply oversized the motor compared to what the machine had originally. I think doing that will make up for any low gears that you may dispense with. I'm not familiar with that machine. Does it have gear ranges, or how will you handle that aspect?

If AL says that the flux vector is pretty good, then I would believe him. The catch is to know if you will be using the machine at quite low motor rpm or not. Let's suppose you can keep the motor speed above 500 rpm, then you might be alright with the flux vector or even sensorless. That motor is going to need external cooling perhaps if run at low speed for hours on end.

I would attempt to size any drive pulleys such that your useful motor speed range would be between 30 and 120hz. If you have to go outside that, then its time to shift gears.

That is something I have never dealt with in Camsoft, is how it handles the S command signal across various gear ratios. I only used M functions on my lathe retro, so I did not have to deal with that aspect.

Edit: I suppose you can limit the motor current to that large motor if you are afraid of too much torque going into your machine when running the motor at 1800 rpm. It should be good and peppy on accel/decel with a 7.5 hp twisting its tail. But that is a good thing, on a lathe.

[Al_The_Man]

Originally Posted by HuFlungDung Al, That is something I have never dealt with in Camsoft, is how it handles the S command signal across various gear ratios. I only used M functions on my lathe retro, so I did not have to deal with that aspect.

Usually the way it is done, you first have to have a gear shift or range detection switch to tell the control what gear you are in, then when a S value is commanded it looks to see what range you are in and if the S value is within the specified rpm for that gear, if not an auto gear change is implemented or a manual change is indicted to the operator.
Al.

[Karl_T]

I'm a bit of an expert on lathe gear shifting. My Mazak M4 is a 16 speed.There's more code in [[SHIFTER]] macro than anywhere else. BTW, threading does NOT work correctly on this unit.

Fortunately, the CHNC has no gear shift. The only time I'll run it slow is for threading, let's hope I can do that at 500 or 1000 RPM. This machine is for very small parts,4000 to 5000 RPM will be the typical speed. Right now, I'm using the original 1:1 pulleys and belts. After I debug threading, I want to re-pulley it for more top end.

I guess I'll order the VFD Al reccomends.

BTW, today I tore down the axis units. Need to make brackets for the new encoders. And order the encoders from US digital. The machine originally had resolvers and a HUGE prox switch for index pulse.

While I'm waiting for parts, I'll trace out all the air cylinders on the turret, limit switches, etc. and install this I/O to the galil and G4 Opto boards.

Karl

[Al_The_Man]

Originally Posted by Karl_T This machine is for very small parts,4000 to 5000 RPM will be the typical speed. Right now, I'm using the original 1:1 pulleys and belts. After I debug threading, I want to re-pulley it for more top end. Karl

One thing, if you intend running 1:1 with a standard motor, if it is 4pole (~1750rpm) then it I would recommended that you not run over 120hz which will give you something in the range of 3300 rpm. If you have a 2 pole motor then you may have to stick to 60hz max which will give you what ever the base speed is right now ~3500.
Of course if you gear it later then you can get the top end you need.
Al.