Basic VFD questions

[Nemo1966]

Ok so I have a Denford Easiturn 3 - all converted to Mach4 with Pokeys, all working spendidly.

However the VFD is old and temperamental and I want to replace it. Looking at generic VFDs on websites and Ebay they all seem pretty much the same... but they are seem to be rated at 0 - 300hz output. But the KEB original says 0 - 130Hz.

It seems the new VFD has option to change the frequency...?

Basically has anyone got knowledge of VFD thast can give me a nudge in the right direction please?

thanks

[m_c]

The frequency is the maximum frequency the VFD can deliver. You don't have to use the maximum frequency of the VFD.

What you need to know, is what the maximum frequency is for the lathe, so you can then set that in the new VFD.
However, if the Easiturn is anything like my old Cyclone, then they're using a VFD rated motor, that is designed to operate above the usual rated 50Hz,and IIRC the current ratings were not the same as a typical induction motor.

Ideally, you'd want to get write down all the settings from the KEB, but I know with mine, it had a PIN set, which I could never get past.

[JohnHaine]

A lot of Denford machines actually use DC motors; and "KB Electronics" make DC drive controllers. One of their controllers might have a maximum mains frequency limit of 130Hz, so just to check that your lathe does use an induction motor and VFD? The electrical drawings seem to confirm that but just to be sure.

Assuming that it is a VFD, then just setting the max frequency of the new one to 130Hz shouldn't go far wrong. Or you should be able to work backwards if you know the maximum lathe spindle speed and any pulley ratios and how many poles the motor has. Look at the motor spec plate, if it says 50 Hz and ~2800 rpm it's 2 pole, or 1490rpm it's 4 pole.

The other possible gotcha is if the motor is star connected and expects to see a 415V supply, since the vast majority of the lower end VFDs only give a max voltage of 240V. I doubt your motor is 415V but worth checking. If it is it will still run on a 240V VFD but won't give the same maximum power.

I assume that you know the Denfordata site?

https://www.denfordata.com/bb/viewforum.php?f=34

[Muzzer]

If the motor is a conventional industrial product that is intended to run from 50Hz mains, there's little benefit in driving it with much more than 50Hz. The voltage applied to the motor is proportional to the speed of the rotor. The current is proportional to the torque applied. While you might gain a slight benefit operating up to 60 or 70 Hz, the torque (power) at that speed will be much reduced, as the VFD can't generate a voltage greater than the mains voltage it is operating from.

Also be wary of overspeeding the motor. Doubling the speed would quadruple the centrifugal forces on the rotor. While a rupture of the rotor may be contained by the stator and housing, it could write off your motor and you might "make mud" if you are standing next to it at the time.

The required motor plate data that you enter into the VFD setup should tell it most of what it needs to know to get going. That's the basically the voltage, frequency and number of poles along with the maximum phase current (tells it how powerful the motor is, so you don't over drive it). If the motor is designed for variable speed operation as m_c suggests, ignore this stuff about running above 50Hz.

[m_c]

@JohnHaine, the larger Denford lathes used induction motors with a KEB VFD, not to be confused with a KB-Electronics DC drive.

The issue I had, was the motor wasn't rated at 50Hz (I want to say it was something seemingly random like 63Hz - I thought I still had a photo of it, but I can't find it).
A standard VFD will by default be set to provide the rated voltage at 50Hz, however I was never sure whether the motor on the cyclone should have that point set to the rated speed, or the standard 50Hz.

[JohnHaine]

Aha! I didn't know that. Regarding overspeeding, many motor ranges share components between 2 pole and 4 pole types so it should be perfectly safe to run a 4 pole at 2 X plate frequency, but not perhaps a 2 pole.

[Nemo1966]

OK - thanks for all the replies. I would have replied earlier but email notifications to replies doesn't seem to be working.


Heres the info on the front of the Unit - seems to be single phase 220 in AC (UK) and 220 three phase out AC