Re: 3 phase VFD

[kayzed1]

Hi again all,
   re: the VFD and 3 phase on the mill, i read some where that if running the motor slowly i should have an additional fan to cool the motor! do i need one? as i have an RPM meter onboard and it tells me the motor can go up to 2240RPM and as low as 88RPM,
so at what speed should i have to use another motor fan?
The remote has the speed control in it and at 750RPM on the remote the meter says 790-ish and is around 40 to 50 rpm above the pendant rotory control setting...or does it not matter>
Lyn.

[John Swift]

if you have a 4 pole 3 phase motor with a fan designed to run off a 440V 50 Hz supply
the motor will run at 1500 RPM

a VFD producing a 75 Hz supply will run the motor at 2240 RPM
  higher speeds can over stress the fan and the  increased eddy currents heating motor

at lower than the motors normal speed you will reach the point   the fan can not move enough air to cool the motor 
the exact speed will depend on the design of the motor especially if it reflects the cost of  copper for the windings , steel used for the magnetic pole pieces and the design of the fan

(  motors can be designed with more copper and less iron or less copper and more iron to minimise the cost )

without the extra cooling of an additional fan I'd fit  an over temperature switch or a  thermistor
to the motor to shut down the VFD or sound an alarm 

do you have a link to the VFD manual
I would of expected the remote speed display to be the same as on the VFD

may be details of the wiring to the remote control can give a clue to why the speed is not the same on both displays
( may be  an analogue signal to a millivolt meter calibrated in RPM ,  or a digital  display with the data corrupted by noise  from the VFD  ?)

    John

[kayzed1]

Hi John, thanks for the fast reply, i think the differing speed is maybe to do with the RPM meter being an ebay buy for £3.
I will borrow the lads race car hand held RPM thingy and use a paint mark on the Quill..i will check out the VFD info in the
morning as all papers and the CD are in the shed, i did read in the bumpf that you can blue tooth to/from the vfd via a
laptop or smart phone   must rear up on how to set up the brake..
Lyn.

[kayzed1]

https://www.inverterdrive.com/group/AC-Inverter-Drives-230V/Mitsubishi-750W-230V-1ph-3ph-FR-D720S-042-EC/

it is the FR-D700-sc model i have.
Lyn.

[bertie_bassett]

for short run times you would probably get away without adding an extra fan, but for peace of mind id just stick a fan on it running all the time.
older motors that aren't made for inverter use do tend to run a little hotter on an inverter then if on mains, so a bit of extra cooling wont do any harm, plus will help spread the heat around the workshop keeping you warm in the winter 

if you've got access to the equipment you might as well check the speeds and see which one is telling the truth. Have to remember that without feed back the inverter doesn't really know what speed the motor is doing, so its worth knowing how accurate its guess is.


from my experience very few setups will be 100% accurate on speed so iv never paid all that much attention to the speed the inverter 'thinks' its doing.  a few RPM's here or there doesn't really matter to me, I'm more interested in what the machine on the end of the motor is doing.  iv seen more then a few blocked pumps sit there humming away stalled , whilst the inverter thinks they are doing anything from 10-250 RPM.

braking shouldn't be that difficult to set up, just need the appropriate resistor and to make sure you have a reasonable time set for the stop.

[nrml]

I am going to use a bathroom extractor fan as a blower on my lathe motor. It has a convenient flange on which a flexible duct can be mounted and  directed to where it is needed. The air flow should be sufficient for non continuous use and it is cheap.

[bertie_bassett]

looking over the manual it seems you can do dc braking with out the optional resistor, page 104 of the manual seems to be the one you want.

looks simple enough to set up, only 3 settings, basically just when you want the braking to START (Pr 10), how LONG for (Pr 11)and how STONG  you want to brake (Pr 12).