VFD for lathe (sensorless vector or not)

[Dan Keenan]

I need a VFD for the 3hp induction motor on my lathe. I will run the motor at a constant speed and change speeds via the gears. Do I need a sensorless vector drive or can I save some money and get a std. drive?

 

[kurcules]

I kinda vote for vector, because it's easier to get it to hold RPM as the load varies. Also if you decide you want to split gear ratios, vector can deliver solid torque well under and over 60 hz. I've found myself using the vector control rather than fiddle w/ gear changing.

 

[motion guru]

If you can get an encoder on the motor shaft or something that is a consistent ratio to the motor shaft AND you are not afraid of jumping into the details of setting up parameters - closed loop vector is hands down the best choice.

You can hold 1 rpm with full torque all day long without issue - and with vector control - motor heating at slow speed operation is not a problem so no fussing with muffin fans or concerns about running at slower than synchronous speeds.

 

[mike_kilroy]

...............and with vector control - motor heating at slow speed operation is not a problem so no fussing with muffin fans or concerns about running at slower than synchronous speeds.

Motion Guru, your comment here might get some folks in trouble. Maybe you should expand on what you mean?

Do you mean you can run down to 1/6th base speed w/o overheating on that standard motor by adding an encoder and closed loop vector control drive? Below that the motor will STILL overheat without added fans?

 

[motion guru]

Hi Mike,

You are correct - the real answer is "It depends" -

TENV motors - no cooling ever needed - even at 100% torque at zero speed continuous

TEFC motors - cooling should be used at less than 1/8 - 1/4 or ??? speed (dependent upon fan design / motor casing / load)

My experience with TEFC motors on a Planer Mill that I have - we run the X-Axis all day long at 50 - 75 rpm at near full load during milling and the motor is only warm to the touch. It is connected to a large gearbox with a C-Face adapter - no doubt heat is being pulled out via. the mounting.

Generally speaking - we have documented on numerous applications that we can typically get 15 - 20 percent more torque than rated from a motor when operated under ACVector control without any increase in motor temperature - we attribute it to accurate current control that maximizes torque for a given level of current.

For typical machine tool use - the amount of time the motor is loaded at rated current is generally a fraction of the time it is running except in the case of a spindle - as such, I don't use aux fans on the motors when using ACVector control.

 

[mike_kilroy]

Can I bug you for a tad more of your experience? You say it depends of course but generally spindles need to watch heat at lower than say 1/6 nameplate speed if TEFC, ok.

You mention an X axis on planar mill at 50rpm - 10hp or so ac induction motor or permanent magnet motor?

 

[motion guru]

Plain Jane used TEFC Baldor Induction motor . . . has a Northstar sandwich encoder between the motor and the gearbox.

 

[john11668]

Dan

The extra cost of advanced vector is trivial compared with the benefits .
Even with a good range of gears you will find the speed control useful.
Then you have the issue of reduced torque under load .

Basic VFD is something I bought when I had not had the experience of AV
I wont ever buy a basic VFD again

John

 

[precisionworks]

Basic VFD is something I bought when I had not had the experience of AV.
I wont ever buy a basic VFD again

+1

Today, there is almost no price difference between a volts per hertz drive and a vector drive. There is, however, a huge performance difference. Like John, my first VFD was V/Hz ... and the performance sucks. The next three VFD's are all vector drives, and deliver outstanding performance.

Spend the few extra dollars for a vector drive and smile every time you use the machine. Or buy my 3hp V/Hz drive for half of new price and be unhappy every time you use it. It's for sale for just that reason.

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Barry Milton

 

[mike_kilroy]

just curious; why are you unhappy every time you use your 3hp v/hz drive?

 

[precisionworks]

why are you unhappy every time you use your 3hp v/hz drive?

The wrong drive for the application

In a V/Hz drive the voltage applied to the motor is directly related to the frequency ... as the freq is decreased, so is the voltage. The net-net of this is that torque drops to a (sometimes) useless level at lower freqs. Sure, some drives have a voltage boost function, but that is far from a perfect solution.

My 3hp drive is used with a 3hp Baldor motor on a belt/disc sander (6" belt & 12" disc). From about 30 Hz and lower, it isn't too difficult to stall either the belt or the disc while grinding thicker metal plates. Contrast that to the 3hp SMVector drive that controls a 3hp Baldor on the Burr King sander. Impossible to stall or even slow down the belt, all the way down to 5 Hz.

The drill press & wire wheel machine (also with 3hp SMVector drives) work equally as well as the Burr King.

If the freq is held constant at 60 Hz, either a V/Hz drive or a vector drive will work well. The difference becomes clear as the frequency is decreased.

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Barry Milton