New VFD will only display frequency, not RPM

[Smiffy]

just had to replace my VFD as the old one went bang.

new VFD will only display frequency, not RPM.

Is there a simple Hz to RPM calculation?

Spindle is 2.2Kw water cooled chinese.

This is the chinese A2 VFD as pictured below

30974

[Clive S]

just had to replace my VFD as the old one went bang.

new VFD will only display frequency, not RPM.

Is there a simple Hz to RPM calculation?

Spindle is 2.2Kw water cooled chinese.

What VFD do you have? link to manual?

[Smiffy]

Hi Clive

Isacon A2

http://www.et.byu.edu/~sorensen/A2%20VFD%20Manual.pdf

[m_c]

Parameter PN01, however the manual isn't very clear on what it means by synchronous speed.

Normally you'd set the number of motor poles, which for this kind of spindle IIRC is 2.
However given the high range for the parameter, it may be expecting the maximum speed at full frequency.

Looks like you'll have to do some setting and testing to see what it should be set to.

[Doddy]

Smiffy,

It's not that difficult. The frequency describes the rotational speed of the magnetic field provided by the controller to the motor... so top whack of 400Hz = 400 cycles/second, or 24000 cycles per minute.

However, motors are typically 2 pole or 4 pole - describing either a magnetic N/S pair (180 degrees apart) or a N/S/N/S 2-pair (90 degrees apart). When considering the actual RPM of the rotor, divide the rotational speed by the number of pole-pairs. So, 24000 cycles/min = 24000 RPM for a 2-pole motor, or 12000 RPM for a 4-pole motor.

That's why you'll find single-phase mains AC motors at 1500RPM or 3000RPM, depending on if they're 4-pole or 2-pole (respectively).

Some/many spindle controllers will allow you to program the number of pole-pairs - that's necessary to determine the RPM from the frequency.

[JohnHaine]

But a slight wrinkle is that induction motors have to run slightly below the synchronous speed, at say 85 - 95%, so the actual speed will be slightly lower than that set by the inverter frequency. The difference is the "slip", and your VFD might allow this to be programmed as well.

[Smiffy]

Sorry for the delay responding chaps, had a rush job on which I just "ran by ear". Only cutting perspex so ran a scrap through at what I thought sounded right, checked it cut okay, and got the job done.

I've since had a play with the VFD settings and for PN01

1 - the display shows Hertz.

To display RPM you set PN01 to the maximum speed of the spindle. In this case I've assumed 24000rpm

To prove it, with PN01 set to 1, I then set the speed to 100Hz.
Then set PN01 to 24000 and ran the VFD.
Display then showed 6000 (rpm)

So I'm set now, and hopefully this thread may help anyone else who is confused the manual!

[Clive S]

I am surprised that there is no setting for number of motor poles

[Smiffy]

It's a cheap chinese drive, so I guess you pay for what you get.

for a 4 pole, you'd just set it for 12000 and be done.

I've assumed my 2.2Kw spindle is 2 pole and does spin at 24000rpm, it ties in with what the old VFD displayed (set up by previous owner). The guesstimated speed the other day ties in with the speed I used before for perspex.

I suppose the only real what to check it would be a strobe tacho on the spindle itself.

But, it's working, and earning, again.