Which VFD for my vertical mill?

I want to install a VFD on my old US machine Tool vertical mill (looks like a round column Bridgeport). I have the original 1 HP 3 phase 220 volt 1725 rpm motor that was on it when I got it about 10 years ago. I had removed it and replaced it with a 120 vac single phase 1725 rpm motor. Now I want to put the 3 phase motor back on and get a VFD so I can go slower than the 375 rpm spindle speed that the 1725 rpm motor can give me. Like at least 1/2 of that. I have plenty of 220 single phase power around the place. So, I look at E-bay and there are lots of them. But I can't tell whether they are the "constant torque" kind that I think I will need or not. I get the impression that some VFD's can go down to very low speeds and some can't. The opposite also seems to be true. I'd like to set up the new drive by simply using the lowest speed vee pulleys, as I usually do now and use the speed control instead of belt changing to go either way. So, I want to down to MOTOR rpm's of about 800 (400 would be even nicer)and up to MOTOR rpm's of about 4000.

I also hear that some brands make lots of noise. There must also be caveats for other reasons, too.

Question 1: Could you folks give me some brands/ model numbers that will work well in my application?

Question 2: Is it true that the kind that say "variable torque" have low torque characteristics at low speeds that would make them unsuitable for vertical mill use?

Question 3: The curent 3 phase motor is connected wye. Must I reconnect it delta for a VFD?

Pete Stanaitis

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Reply to
spaco
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According to spaco :

O.K.

Good!

AFIK all *can* -- but the question is whether or not *you*

*should*. The problem is that when you slow the motor down, you reduce the cooling from the built-in fan, while the current in the windings (and thus the heat generated) remains constant at the lower speeds. (The current drops above the motor's rated speed with constant voltage, while the voltage drops below it to maintain constant current.

So -- if you *do* want to run for extended times at particularly slow speeds, you need to put an external fan on -- powered from the wall power -- to keep the airflow through the motor adequate for proper cooling.

Hmm ... with a 1725 RPM motor, you probably don't want to more than double it (120 Hz instead of 60 Hz) which should be the standard speed for a two-pole motor using the same rotor. Exceeding that speed would risk "centrifugal force" slinging out parts of the rotor. So plan on a maximum RPM of 3450 RPM -- not your 4000. An unless you have a booster fan for the motor, probably don't go below 862 RPM (at 30 Hz).

It is usually the *motor* which makes noise with certain units, the magnetic structure or the wires singing at the chop frequency of the VFD. Some VFD allow you to change the chop frequency, while others do not.

My most recent experience is with Mitsubishi VFDs which were rendered obsolete and replaced with newer ones about three years ago. So I'll let others suggest currently available ones.

I don't know. I think that the "variable torque" motors adjust the voltage/frequency ratio instead of maintaining a constant ratio. This would suggest that you can (for short periods at least) get higher torque at the lower speeds. Remember the warning about motor overheating at slower speeds -- it is more likely to do so if you are boosting the torque by boosting the voltage.

That should not matter -- as long as the Wye wiring is for the stated output voltage of the VFD (240 VAC) Changing it to a Delta configuration would change the voltage at which it should be operated. And -- if my Bridgeport Series I 2HP motor is a reasonable example, you

*can't* get Delta wiring with that -- just the choice of 240 VAC or 480 VAC. There are six windings, but only nine wires brought out. One set of three windings has one end of each connected to the other two inside the motor, and not brought out. The other set has both ends of each winding brought out.

To wire this type of motor for the lower voltage, you connect the proper end of these three windings together and insulate the connection. You then connect the other end of each winding to the proper one of the three already connected together to put the winding sets in parallel.

To wire for the higher voltage, you connect the proper end of each of the second three windings to the free end of the corresponding winding and insulate. Do the same with the other two windings, and bring the free ends of the windings out to connect to the outside.

As to how to tell which windings to connect to which, the wires are numbered, and there are drawings or lists on the motor's nameplate showing which numbers connect to which.

To wire in Delta, you would need access to both ends of all the windings --- including the ends which are joined and buried inside the motor -- so don't plan on that.

Now -- UK motors tend to have only the three windings, but all six ends are brought out, so you wire them in Wye for one voltage, and in Delta for the other voltage (which do *not* relate on a 2;1 ratio as the US ones do.)

There will proably be a lot of others jumping in on this thread. Read them all before you jump.

Good Luck, DoN.

Reply to
DoN. Nichols

Automation Direct is a supplier of VFD's with very good tech support. They carry Hitachi and their own house brand. The Hitachi line is made in sensorless vector and V/Hz while the GS line is V/Hz only.

Sensorless vector units(sometimes called constant torque incorrectly) will provide more low speed torque. V/Hz units(so-called variable torque) don't provide as much low speed torque. Both will go down to essentially zero speed although that is seldom needed with machine tools.

I have a Hitachi SJ100 (2 HP-sensorless vector) on my 13" lathe and it's performance is superb. I have a GS2 (1 hp-V/Hz) on my Bridgeport and it performs equally well. I have a GS1(V/Hz-1 hp) on my belt grinder. The units are several years old now and have been trouble free.

The main difference I saw between the Hitachi and GS lines was in the number of auxillary programmable functions. The Hitachi is loaded with just about every conceivable feature and all are programmable. It is really designed for industrial use with computer interface if needed. The GS is much less complicated and the price shows it. Both handle manual control-machine tool applications with ease.

I have no business interest in Automation Direct, but if I needed another VFD, I would look there first based on the service record of those I have.

1800 rpm and slower motors will normally tolerate 120 Hz safely but YMMV.

Randal

Reply to
Randal O'Brian

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