What's new
What's new

Bed Mill Spindle Motor Protection Strategy

markz528

Hot Rolled
Joined
Sep 25, 2012
Location
Cincinnati
I was not sure if I should post this in drives section or CNC section but decided to start here.

I have an Atrump cnc bed mill with a 3 hp spindle motor. Clearly not the top of the line stuff, but fits my needs.

The drive is a Delta VFD037B23A. Its a 5 hp drive, single phase input, running a 3 hp motor.

The machine is less than 2 years old and I decided to back up all the drive parameters. I found the drive setup to be an utter mess. I fixed most of it to my liking but there was zero motor thermal or overload protection. I did set it up to reduce rpm if motor amps exceed 200%, and added a load meter to the cnc controller screen, but otherwise there is no spindle motor thermal or overload protection.

So my question is how is this handled on other CNC machines? Could this be by design to limit spindle motor trips to prevent crashes? I would think that the machine would go into estop mode when spindle drive trips, which would not be great when taking a big cut, but what is the risk?

How would you set it up?
 
Motor overload monitoring and shutdown are handled by the spindle drive on higher end systems. Multiple methods with most all systems using 2 or more of the following. The motor has a thermal switch providing an OH input to the drive. The drive has thermal switches or sensors on the power transistor heatsink and regen resistor. The drive monitors current to the motor, both running and IOC. The drive monitors bus voltage. Drive monitors phase loss. Cooling fan operation monitored. Probably others, this is what comes quickly to mind.

markz528 said:
I did set it up to reduce rpm if motor amps exceed 200%, and added a load meter to the cnc controller screen, but otherwise there is no spindle motor thermal or overload protection.

IMO, not a good idea. Reducing RPM and not reducing feed creates even more loading. Kind of a death spiral at that point. I don't know the Delta drive but find it hard to imagine that it does not have a setting to enable and set a level for electronic thermal overload detection.
 
IMO, not a good idea. Reducing RPM and not reducing feed creates even more loading. Kind of a death spiral at that point. I don't know the Delta drive but find it hard to imagine that it does not have a setting to enable and set a level for electronic thermal overload detection.

I thought of that but my rationale was that the spindle is rated for 4200 rpm. Anything above 1800 rpm or so is in field weakening (constant hp range) so running at reduced torque. Since I am usually running above 1800 rpm, reducing frequency reduces speed but torque increases. Torque is what does the cutting. So my thought (might be flawed) is that as the speed decrease and the torque increases they would come to a happy equilibrium.

But I think that you convinced me - I will take that out.

The drive has plenty of capability to protect the motor - its just not enabled. The motor does not have any thermals in it, but if I ever have to rewind the motor I will add some.

On the high end machines, what do they do with the motor overloads as far as stopping the machine - orderly stop or estop?
 
What I would do is run the E stop wires thru the VFD's isolated relay. Then program the VFD to open the relay if amperage exeeds (X) limit. It would be a second E stop, essentially.
 
.......On the high end machines, what do they do with the motor overloads as far as stopping the machine - orderly stop or estop?

Depends on the builder. The best do an orderly stop if threshold is reached. Feedhold and spindle stop enabled and an alarm generated. Some will just do an e-stop. For an IOC, e-stop is activated in all cases because that is usually the result of some kind of failure of the system or a crash, not a cutting overload.
 
Biggest issue with all that on a low end machine, if the drive trips and the machine goes into eSTOP, you have no idea why it stopped until you play detective and find the reason because at that point, even the control does not know why things stopped. Somewhat higher end machines will usually throw an error message so you know what the issue is.
 








 
Back
Top