More than one motor on a VFD ?

[spiyda]

Hi,

I've retrofitted an old Engine Lathe for CNC

The spindle is powered by a 3hp 3 Phase motor powered by a VFD

I'm now at the stage where I want to add powered tooling..

I am building a spindle that takes ER40 Collets



As I already have a VFD powering the spindle, can I just connect a second smaller 3 phase motor in parallel... to power the spindle...

The Main motor will be running virtually without load whenever the smaller motor is connected.. (much of the time it would be not physically connected, but clutched out)

This arrangement would save money on a second VFD and mean there was no possibility of letting out the magic smoke by firing up the VFD with no load connected..

It would give me variable speed on my powered tooling

My question is this... Will it work ?

Will it work without the need to change settings on the VFD when connecting the second motor ?

or am I being too clever for my own good ?

[mike_Kilroy]

it will work fine as long as both motors are rated same voltage and frequency & the total load amps does not exceed the vfd rating; if the large one runs no load then u prob have enough current to run the other too; since the vfd will have current limit protection u will know w/o hurting anything if u dont have enough amps and it shuts off on error.

If u want to thermall protect both motors u will need to add thermal overload relays to each motor to protect them; hook up the thermal sw in each motor too if there. best to use TOR instead of fuses so u dont disconnect motor while running if pops.

[spiyda]

Cheers Mike...

I needed that sanity check ... thanks

[underthetire]

I'd use a contactor set to isolate one or the other, easy to do. I agree with at least fusing the smaller motor. If the max current is set for a larger motor, it can pop the smaller one, unless you change vfd settings.

[mike_Kilroy]

underthetire, i think he wants to run BOTH motors at the same time. and i began to write FUSE also then erased cuz last thing we want is to totally open the motor leads to drive connection while running...... but now that i say this, if both motors are on in parallel, fuses in line with the little one would still leave the big one in circuit.... so it would be ok!

[spiyda]

I'll fuse the smaller motor (probably 1 hp) and leave the bigger one (3hp) running in neutral..

They are both 50Hz 4 pole motors which I run at up to 100 Hz

It seems the cheapest safest way to do what I need..

and means I won't have to change the VFD settings every time I want to use the tooling. (removing an opportunity for me to mess up !)

The powered tooling with the smaller motor will be on a plug and socket as it needs to be removed completely from the machine from time to time..

And I'll add a disconnector for the main motor for the odd time I will be using the powered tooling on its own for very long periods.. in which case I will have to change the VFD settings.. and I'll site it so that it will be hard to operate with the main motor running..

Cheers Guys