Thread: Variable speed on 3ph motor

What sort of machine? Just interested cause 20 is a lot of horses.
A VFD for that size motor will set you back about this much
http://www.ebay.com.au/itm/SAJ-18KW-...4a4000c&_uhb=1

The VFD can give you a calculated RPM but for an exact reading you would need to attach a tacho to the shaft. You can possibly feed that into the VFD to give you the RPM on the display but that would depend on the VFD and it's available inputs.
You do loose power as the speed decreases but your process is unlikely to need 20HP constantly. Most VFDs will try and maintain the output power but it does this by increasing the current which increases the heat and at lower speeds the motor cooling is much less effective. Running for extended periods without additional cooling is not recommended.
Sorry, maybe too much information there.

Originally Posted by Barterbuilt

Wondering if the sparks guru's could give me some advice....?

I'm looking at a machine with a 20 hp 3 phase motor.
What I'd like to do is make it variable speed and to know the exact spindle speed.
How would I go about this. ( I would of course get a sparky to do the job )

Good , 20HP motors is not something non-sparkies should be playing with.
Not all sparkys can do VFDs, you'd need to find one that has "industrial control experience"
Best way is to get a personal recommendation in your area (hopefully someone will pop up with that) but failing that something like Yellow pages would be where I would start.

And would you loose any hp by reducing the spindle speed.

HP is always lost when RPMs are reduced - what is more important not to lose than HP, is torque.
Depending how slow you want to go a quailty VFD should not lose any (or much) torque as the RPMs drop.
Your "industrial control experienced" sparky should be able to advise on getting a suitable VFD to meet your needs.

Thanks Fella's
Appreciate your input.

How close to the motor would this unit be mounted..... reason I ask it may be in a wet-ish invironment.
The unit pictured doesn't look like it's overly water proof.
Could it be mounted well away from the motor ?

Thanks again

Originally Posted by Barterbuilt

Thanks Fella's
Appreciate your input.

How close to the motor would this unit be mounted..... reason I ask it may be in a wet-ish invironment.
The unit pictured doesn't look like it's overly water proof.
Could it be mounted well away from the motor ?

They are not usually waterproof

The distance away from the motor you can mount it depends on the specific VFD but a couple of metres should be not problem.

It should be no problem to mount some water proof control controls right where you need them.

I don't see any major problems with what you are wanting to do, 15KW drives are readily available. I'd suggest you go for a drive that has sensorless vector control, that way you'll get better torque at the lower speeds.

It will need to be mounted in an enclosure, (to suit whatever "wet-ish" means ) and you will need to re-wire the stop/start and reversing controls on the lathe to control the VFD, so you will need to make sure the sparky you get knows exactly what you want.

I've used a lot of Danfoss drives over the years and always got reliable performance. Get a price on a 15KW Danfoss VLT series drive, I'd guess $2500 or so for the drive, plus same again to install and commission.

Ray