Change old AC motor to BLDC motor in Sieg C6

[Michael GilliganParticipant @michaelgilligan61133]

I tried to post this last night, but the forum software was playing silly B’s again

Here is SIEG’s spec for the lathe:

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Methinks 5000 would be a risky bit of overclocking.

MichaelG.

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Credit: https://www.siegind.com/shop/c6-bench-lathe

[Stuart Smith 5Participant @stuartsmith5]

I can’t see any mention of 5000 rpm from the OP.

Stuart

[Jens Eirik Skogstad 1Participant @jenseirikskogstad1]

Hi, quite a few comments to read through here… My goal isn’t to reach tens of thousands of RPM on the lathe. Rather, the objective is to achieve good torque at low speeds, and to have more flexibility in setting the desired RPM for specific materials—based on the required cutting speed and diameter.

You’ve mentioned 3-phase electric motors and VFD-based speed control.
Are there any advantages to using a 3-phase motor compared to a BLDC motor?

[Jens Eirik Skogstad 1]

On Jens Eirik Skogstad 1 Said:

You’ve mentioned 3-phase electric motors and VFD-based speed control.
Are there any advantages to using a 3-phase motor compared to a BLDC motor?

Cheap, reliable, easy to set up, dozens of frame, mounting and shaft types.

That said, I am not overly familiar with brushless motors.

[Stuart Smith 5]

On Stuart Smith 5 Said:

I can’t see any mention of 5000 rpm from the OP.

Stuart

Mea culpa, it was John Haine who linked to a 5000 rpm motor. Spec above doesn’t mention motor speed. People seem to be confusing motor speed with spindle speed. If a slower motor is OK (3000rpm) then by all means 3 ph and vfd

[duncan webster 1Participant @duncanwebster1]

According to This the original motor is 550W AC squirrel cage, so vfd and 3 ph definitely on the cards

[Peter Cook 6Participant @petercook6]

BLDC motors are three phase, just driven by switched DC rather than AC. I have no experience of 3ph with a VFD, but BLDC motors seem to be much more compact than 3ph squirrel cage.

My 220W BLDC IS ABOUT 10% of the size  and weight of the original 1/4 hp single phase motor.

[Robert Atkinson 2]

On Robert Atkinson 2 Said:

For a small lathe you would be better using a 4 pole 1500 RPM motor and VFD and turning the frequency up to 100Hz to get 3000 RPM than using a 2 pole and turning it down for lower speed. You can’t increase the current at lower speed due to resistive losses so will get much lower power and torque.

My knowledge from AC motors comes from what I have picked up the last 20 years from reading forums, so maybe I am totally wrong.

A low pole count motor will run at a low max RPM but will deliver a more constant torque at a lower RPM.
A high pole count motor will run at a high max RPM but the torque will decrease significantly when running at lower RPM.
My low pole count AC servos deliver an almost constant torque from 0 to their max (3000) RPM.
My AC spindle motor with VFD delivers a high torque from 200 RPM to 2250 RPM.
My low pole count BLDC motors deliver an almost constant torque from 400 to their max 3500 RPM.

So for more torque at low RPM (threading) I would select a low pole count motor and compromise on the max RPM.

 

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