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View Full Version : Can i install a vfd on my mill



mdred68
02-22-2011, 10:47 PM
I have a grizzley milling machine with a single phase motor. Can I install a vfd on this machine? Has anyone out there done it? Does any one out there have a good source for parts? Thank you.

squirrel
02-22-2011, 10:48 PM
If you have a single phase motor the VFD will not work.

J Tiers
02-23-2011, 12:09 AM
If you have a single phase motor the VFD will not work.

Almost surely true........ BUT a slim chance it could, IF your motor is a "PSC" type.

Almost certainly it is not, but there are single phase VFDs made to work with PSC motors, which are motors with a single small motor run capacitor. Often found as fan motors or gearmotors. Rarely found on machinery, but it is possible, since they are very smooth, unlike the usual sort of single phase motor which is very "pulsy".

SGW
02-23-2011, 10:22 PM
AS the others said, almost certainly not. The starting circuit of a typical single-phase machine motor does not like it when the motor slows down. A 3-phase motor doesn't need a starting circuit.

johnnyd
02-24-2011, 07:24 AM
Single phase motor speed controlers do exist.
However...the only ones I have seen have all been 220 volt.
I would look for one from one of the european suppliers.
I found one on ebay a few years ago. They are indeed rare.

A "google" turned this up .... http://www.anaconsystems.com/text/opti_e2.html
(they surely ain't cheap)

dian
02-24-2011, 08:29 AM
i had a conversation with a vfd suplier this week and he said he was able to set up the vfd to drive the singlephase motor on my drillpress. the main question probably is, how much torque you loose when you slow the motor down. at 150 $ i just might give it a try.

Jim Shaper
02-24-2011, 08:35 AM
Convert the device over to a 3ph motor and not lose anything... 3ph surplus motors are CHEAP.

J Tiers
02-24-2011, 09:39 AM
Johhyd and dian.........

The Invertek (UK) drives sold by Anacon are to drive only the type of motor I described above, the "PSC". They will also drive a shaded pole motor, but those are normally not candidates for machine tool usage, and I am disregarding them.

From the Invertek datasheet (download from Anacon website, or from Invertek):

"Designed to be cost effective and easy to use, the Optidrive E2 Single
Phase is for use with PSC (Permanent Split Capacitor) or Shaded-
Pole Single-Phase induction motors."

I happen to know quite a bit about those drives, since the company I work for is associated closely with the US representative for Invertek, who sells to Anacon and others.

As a rule, they, or other VFD type motor drives, will NOT work with split-phase motors having start circuits.

For one thing, the speed range is small before the start switch cuts in as you slow down, making the "VF" part of very limited use. For another, the start current is quite high, requiring a very "over-built" drive to start them.

A PSC type already has a high start current, the regular so-called "split phase" motor is typically worse.

Not only that, but in order to get decent start torque, all split phase (capacitor or resistance types) AND PSC etc, rely on a phase shift at 50/60 Hz. VFDs normally like to start at low speed and accelerate, but at low frequency, there is no phase shifted start winding current. So a single phase drive must start at or near 60 hz and then shift to the speed wanted, just in order to get starting torque to start the motor turning. So your load needs to be able to accept that.

One MIGHT be able to over-size a drive to run a regular split-phase motor, but as mentioned the speed range is smaller, and the start requirements are a potential issue. And you will have to start with a larger VFD.

dian
02-24-2011, 01:00 PM
i addmit i dont know much about this. was relying on the info i got. your statement will make me investigate further. thanks.

what exactly do you mean by "the start switch will cut in"?

johnnyd
02-24-2011, 05:16 PM
Thanks for the education.:o
I knew that I was in over my head,...but I felt the need to help.

At least I learned something.:)
(at my age,...that's significant):D

sch
02-24-2011, 05:47 PM
"the start switch will cut in"

Typical machine tool single phase motors have a centrifugal
switch in the end bell that cuts the start coil/capacitor out
of the AC line when the motor rpm gets up into the low
hundreds rpm. The start coil/cap combo provides the initial
phase shift needed to kick the motor into motion. It is not
designed to carry line voltage and will over heat if not cut
out by the centrifugal switch and removed from the power
feed. Typically it is 'on' for tens of milliseconds. If you listen
when the motor is cut off you will hear the click of the
switch closing as the motor slows to a stop. Cheap chinese
motors are well known for failures of these switches, which
in well made motors last years to decades.

J Tiers
02-24-2011, 10:22 PM
Correct.

And the switch typically opens once the motor is "up to" a speed where it has good enough characteristics running single phase that it will finish accelerating to full speed.

However it is a "dumb" switch, it only understands speed. If you slow down the motor below the point where the switch operates, it may close again, applying a huge start current load to the source (mains direct, or VFD).

The speed at which the switch operates is typically somewhere above half speed, and isn't super-precise anyway. There is a tolerance, so the range of possible slowing is limited before there is a danger of the switch closing (cutting in) and loading the source heavily.

The load on the source at the start may be very heavy. I had a problem getting an air compressor to start once, with a longer-than-desirable extension cord. The air compressor as I recall, normally drew about 8 amperes. With the extension cord, it would spin, but not come up to speed (yes the unloader was good). The centrifugal start switch would not open, it was still too slow for that.

I observed the 8A motor to draw over 75 amperes, as measured with a clamp-on meter, and the current was still increasing when I shut it off. VFDs which will supply a starting load of 75A are not common. Most can produce very short term currents of perhaps 2x normal motor current.

dian
02-25-2011, 05:15 AM
o.k., i got it cocerning the switch. i talked to to technician again, and what he said, as i suspected, is that torque will drop off rapidly, when you slow down the motor. if you slow it down by 50%, you get 50% torque (or even less). but you can overdrive the motor as a 3 phase. so that gives you a operating range of 50% - 200% (or even 300% for a short time) from a regular motor, assuming you dont need all your motors torque. i consider this as usefull and just ordered a 1.1 kw unit for roughly 160$. if it doesnt work out, i just use it on some 3 phase motor.

J Tiers
02-25-2011, 09:51 AM
o.k., i got it cocerning the switch. i talked to to technician again, and what he said, as i suspected, is that torque will drop off rapidly, when you slow down the motor. if you slow it down by 50%, you get 50% torque (or even less).


NO...... that guy has it wrong, that's bad information as far as three-phase. For single phase it may be slightly more accurate, but still not really true in the general sense.

Motor TORQUE stays about the same, it is POWER that is reduced.

Torque is dependent on motor CURRENT. That doesn't change if a VFD slows a motor. (single phase motors don't have smooth torque, which can result in a slightly different result)

POWER is dependent on torque AND speed. When you cut speed, you cut power, even though torque remains the same.

This is easily provable by physics..... "power" (watts) is the rate of doing "work". Work, or energy, is proportional to force x distance it acts through. Distance here is in revolutions (circumference).

So if the torque, which is the force, is the same, but the speed in revolutions per minute is less, then the same force acts through less distance per second, less total work is done every second , and the rate of doing work is less...... i.e. the power is lower in proportion to speed.

dian
02-25-2011, 11:54 AM
sorry, i didnt make it clear, i was talking about the 1 phase application. for some reason the torque drops off rapidly, apparently the motor only starts to turn at 10 - 15 hz. anyways, in a week or so i will find out how it is.

J Tiers
02-25-2011, 10:30 PM
sorry, i didnt make it clear, i was talking about the 1 phase application. for some reason the torque drops off rapidly, apparently the motor only starts to turn at 10 - 15 hz. anyways, in a week or so i will find out how it is.

Well, so was I............

As I mentioned a couple posts ago, the single phase motors that you CAN easily control with a VFD are ones that create a second phase with a capacitor, or an inductive effect. they depend on that second phase for torque.

Both those are frequency dependent...... the current though the capacitor drops off with frequency, which means speed, and so motor CURRENT, which creates torque, also drops off. That's why they start at 50 or 60Hz, and only then adjust to desired speed.

But those are special cases.

In the absence of a start circuit, for instance if spin-started manually, a split-phase type single-phase motor would work to a lower speed, with decent torque.

but due to the "pulse type" single phase power, a standard split-phase type single phase motor has to "coast through" a zero or very low torque interval every half cycle. At quite low speeds the mass is not enough to coast well if there is a load, and torque can suffer at rather low speeds, depending on motor design.

generally this is no issue because the motor has the start switch, which cuts-in the start circuit unless the speed is well above the bad torque range. And the motor is never used with a VFD anyhow.

I might mention that if the PSC type motors (which are VFD -compatible) were to have a larger capacitor installed, they would work with good torque to a lower speed..... but you would have to switch capacitors to cover the whole range of speeds.

it is not the motor itself, but the value of an auxiliary part which limits the speed range.

dian
03-20-2011, 04:59 PM
so today i played around with the new little vfd. i installed it on 6 single phase motors, two one capacitor grinders, a one cap. saw, one two cap. drill press and two other two cap. motors.

it works. it seems only two of the motors have a centrifugal swich. the trick with those is to start them at 40 hz or higher. below 25 hz the swich cuts in and the motor doesnt like it, gets warm too. on the others i can run 10 hz, but they vibrate a bit. anyway, below 30 hz the torque is gone, i stopped one of them at 20 hz by grabbing the shaft (550w).

so ill be getting a slower (six pole) motor for that drill press (preferably three phase) if i can find a good deal and overdrive it once in a while.

on the grinders the vfd works very well, no vibs down to 10 hz, interesting for buffing/polishing/lapping maybe. i have several single phase motors and now am considering to use them with small diamond wheels for grinding. a 100 mm wheel at 6000 rpm should work well.

rohart
03-20-2011, 05:08 PM
Well my take on the original question is to say yes - Strip out all the switch gear and the motor. Put in a three phase motor, connect up the VFD, connet up a panel with one pot, one on/off switch and one forward/reverse switch as per the instructions and you're good to go.

In the UK I would make sure the motor is modern and has the facility to be wired either star or delta. Delta wound would let a 440v motor develop full power with a 240v VFD. If you get an old star wound motor, make it bigger to get enough power at half voltage.

To get the same speed as the original, get a motor with the same number of poles. Alternatively, if you want to keep the original top speed, you could go for a slower motor and get the VFD to double the frequency (to 120 in your case) to give you the original top speed.

The switch panel is a breeze if you're comfortable with basic electronics.

If you really have these single phase motors running with a VFD, I'm sure we'd all like to know the details. I'd have thought it wouldn't be very good for a VFD to have only one of its phase pairs in operation.

dian
03-21-2011, 12:33 PM
maybe this is of interest. the vfd was not set to vector drive from the factory. when in vector drive, the 1 phase motor has more torque at 20 hz than it has at 50 hz normally (gets a little hot though). i am very pleased with this little unit (1.1 kw for 160$).