Using VFD to lock lathe spindle

I was watching a video on John Saunders NYC CNC channel where he visits a very talented young machinist in Germany. The young man retrofitted his lathe with a VFD and he was showing John the various controls for the lathe he did, one of which was a switch to lock the lathe spindle. Does anyone know how he did that?

Thanks

Jim

Possibly applied DC to the main motor and that would only be effective if the gear train was set low. The logic to go between the DC and the VFD would interesting.

It is also possible that the spindle of the lathe was modified for a mechanical lock such as a pin in a hole.

Anytime the gear train is used to lock the spindle is an opportunity to break some gear teeth.

Tom

I know the VFD I set up had DC injection braking, but it is momentary

A switch?

probably a brake or pin

AC servo you could do it of course

DC will not actually "lock" the spindle, because it has to move to generate the braking. But with a suitably high ratio it might appear to be locked.

With a VFD, one might set up the logic for the auxiliary relay which many include. That could be set to activate a lock or brake when power is on but neither FWD nor REV has been activated.... essentially when in "stop".

One would simply set it to open the N/C contacts when in FWD or REV or if "jogged". The N/C contacts would set a brake when closed.

JST said:

DC will not actually "lock" the spindle, because it has to move to generate the braking. But with a suitably high ratio it might appear to be locked.

With a VFD, one might set up the logic for the auxiliary relay which many include. That could be set to activate a lock or brake when power is on but neither FWD nor REV has been activated.... essentially when in "stop".

One would simply set it to open the N/C contacts when in FWD or REV or if "jogged". The N/C contacts would set a brake when closed.

Click to expand...

I think maybe you are thinking of Dynamic Braking in meaning that the motor must move to have any braking. DC injection braking is different and will indeed "lock" the motor shaft, to a fashion. With DCIB you put DC on one stator winding and create a non-rotating (stationary) magnetic field, which then creates a magnetic field in the rotor that tries to come into alignment with it. Once the rotor stops, there is still a stationary magnetic field created in it that will align with the stationary stator field and exert a holding force.

The thing with this is, it can only do it for a brief time. Once the rotor stops moving, there is no more mutual induction taking place between the stator and rotor. Mutual induction is what creates impedance, which from an electrical circuit standpoint is the equivalent of resistance. So without that, the resistance of the circuit drops to a very low level, essentially just the resistance of the copper wire in the stator or the rotor bars in the rotor. So using the basic E=IR formula, if the voltage (E) is fixed and the resistance (R) drops to a low level, the current (I) is going to go high (I = E/R). Bottom line once the motor actually stops, the current will increase rapidly and at the same time, the motor's ability to cool itself has ceased, so you can burn up the motor winding VERY quickly and/or damage a rotor bar. As a gross general rule, we say 3 seconds of DC hold, max. After that you are putting your motor at risk.

Such is the danger of some of these YouTubers... they know not what they do, but they tell everyone how to do it anyway!

Well, in theory, and in my experience, the use of a DC current through a winding or an induction motor will NOT "lock" it in the sense that appears to be wanted.,

The stationary field does nothing unless a conductor moves in it. The rotation of the "rotor cage" produces a current, and a braking effect. So the result is that the rotor will move, but has a somewhat damped response... the amount of damping depending on the magnetic field established. It presents as somewhat similar to a "viscous" damping, with less resistance to slow movement. That is exactly what you would expect from theory.

You CAN hold that current all day, if you like.... by using a low voltage, and not attempting to run FLA at DC, which is not generally needed. Yes, the stronger the field, the more nearly locked the motor is, but it still moves. Just a fraction of FLA will produce a very considerable viscous braking effect in many motors.

The use of the relay in the VFD to operate a standard brake or locking mechanism is both more positive, and may be easier. Of course it could also operate a contactor for a DC source, which would naturally need to be interlocked with the power contactors.

An AC current would potentially do some "locking". A single phased motor that is not turning can be very hard to turn, and does not seem to give that same "viscous" sensation.

jmgalvin said:

I was watching a video on John Saunders NYC CNC channel where he visits a very talented young machinist in Germany. The young man retrofitted his lathe with a VFD and he was showing John the various controls for the lathe he did, one of which was a switch to lock the lathe spindle. Does anyone know how he did that?

Thanks

Jim

Click to expand...

Vector drive with position feedback "locks" the spindle awhen set to zero speed as long as you don't exceed the maximum motor torque.
Or there could be mechanical brake driven by auxilary output of the VFD.