# A happy discovery



## hman (Oct 7, 2020)

Lots of folks will tell you that a 3 phase motor can have problems when run at less than full speed, because the internal fan isn't running fast enough for good cooling.  I’ve been thinking about adding an external fan - possibly a 12 volt computer fan - atop the 3 phase motor on my mill.  But, being lazy, I didn’t want to have to bother with turning it on and off every time.  Gotta be a better solution!  

I had a wild hair last night and checked it out today.  Among the parameters available on a TECO VFD is #21, which sets the function of the “Relay” terminals (the first two screws on the left side of the low voltage terminal strip).  The default value is 3, which turns on the relay if there’s a fault.  I reset it to 2, “Motor is up to speed.”  The contacts are open when the motor is not turning, closed when it’s at the set speed, and open again when the motor has stopped (exact timing depends on ramp-up and ramp-down times).  So I can wire this switch between a 12 volt power supply and a fan.  Problem solved!!!


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## Flyinfool (Oct 7, 2020)

That will work.

What is option #1 on that drive?

Is there a terminal that will have the fan on anytime the VFD is on? This would allow the fan to continue to cool the motor between cuts while the motor is off.


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## hman (Oct 7, 2020)

What is option #1 on that drive?
Good question ... option 1 is called "Run mode," turns on the relay when the motor supposed to run.  This appears to turn the relay on even during the motor speed ramp-up and ramp-down.  Come to think about it, this might well be even better than option #2.  I'll be looking into it tonight.

As for keeping the fan on whenever the VFD is on, I suppose you could connect a separate relay to the 240 volt wiring that supplies the VFD.  But I leave the VFD, DRO, etc. powered up "all the time," and that's not how long I want the fan to run.  YMMV.


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## Eddyde (Oct 7, 2020)

You could also wire in a second, time delay relay, it would let the fan run for a set period after the motor stops.


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## hman (Oct 7, 2020)

Thanks! Definitely worth some thought.  I'm kinda hoping I don't have to add more hardware.  But I do have a TD relay or two on hand, so I'll be keeping an eye (actually, a hand) on the motor for overtemp.  I actually have a very similar motor on my round column mill.  It's been incredibly handy for power tapping, as well as saving lots of time versus belt changes.  Never checked the motor for overtemp, though.  But then, I never heard or saw it complaining.


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## vocatexas (Oct 7, 2020)

There are fans similar to computer fans that run on 240 volts. I bought one for my Lincoln welder. 









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## Dabbler (Oct 8, 2020)

@hman it is totally legit to wire the 240V fan that @vocatexas suggests to any of the two leads of the motor wiring, giving you single phase.

[edit]  as @hman points out below, this isn't actually a good idea...


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## hman (Oct 8, 2020)

I did a quick check of setting option #1 on function 21 tonight.  YEEEEEE-HAAAAAA!  Works perfectly!  The relay goes on when the "run" input is on, regardless of motor speed or direction.  MANY thanks, @Flyinfool !!!

Question for @vocatexas and/or @Dabbler - If the fan was wired across motor leads, wouldn't it be subject to the same frequency variation that the motor is?  Wouldn't this reduce the fan speed when the motor is running slowly?  Or would it just be happy to see 240 (208?) volts and keep running at full speed?

Just for fun, I'm including a screen shot of the "final and complete" (famous last words?) schematic I just finished drawing up.


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## Papa Charlie (Oct 8, 2020)

I am amazed that the small fan is sufficient to keep the motor cool or is it that it is a boost to the motor fan that is running at a slower speed?

Nice work identifying the solution. One of life's little joys when something is as simple as it should be.


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## NC Rick (Oct 8, 2020)

hman said:


> I did a quick check of setting option #1 on function 21 tonight.  YEEEEEE-HAAAAAA!  Works perfectly!  The relay goes on when the "run" input is on, regardless of motor speed or direction.  MANY thanks, @Flyinfool !!!
> 
> Question for @vocatexas and/or @Dabbler - If the fan was wired across motor leads, wouldn't it be subject to the same frequency variation that the motor is?  Wouldn't this reduce the fan speed when the motor is running slowly?  Or would it just be happy to see 240 (208?) volts and keep running at full speed?
> 
> ...


The back-gear auto direction is brilliant.  I have my "trained" (mostly) to use the reverse switch.  It still causes momentary confusion for me at times.


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## macardoso (Oct 8, 2020)

There are a number of performance motors with separately powered blower motors. This gives significantly improved low speed thermal performance over passive cooling or having a cooling fan powered by the motor shaft.

For example a high end servo from AB with a 208V blower fan on the rear which runs any time the motor is running.


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## Dabbler (Oct 8, 2020)

@hman - oops!  you are 100% right!  i forgot about that!

-- that auto-reverse switch is brilliant!!


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