ML7 Main Shaft Slipping?

[SteviegtrParticipant @steviegtr]

The manual that came with the Omron is 160 pages & very technical. I used to fit loads of them but must confess, I retired a long time ago & even struggled with this one getting certain programs set correct. I did eventually get it how I wanted it to work, but what an experience.

Maybe the cheaper imports are better suited for the small workshop. I did a video of mine running at 1.5Hz. The chuck was rotating at about 2 rpm. I held the chuck & put pressure on it,(AT 2 RPM) & could not stall it. I know that is a stupid test but just proves my original rantings that an inverter was poor at low frequency. I am sure on a scale it would be very low.

My cheap dro will not register below 9 RPM. I have mine on the lowest ratio pulley. It will only go to around 900 RPM at 60hz. It has 4 pulleys so not a problem to speed up if required. One thing I did notice in the manual, it said when run above 50hz the torque will reduce.

Steve.

[MeunierParticipant @meunier]

Posted by Cornish Jack on 11/03/2020 14:08:42:

Make life simple – fit your VFD to a TriLeva – works a treat!

rgds

Bill

Now don't be putting ideas in my head Bill, I have the identical twin lathe of yours !
Will continue with the Brook Gryphon 2-speed motor but bear your comments in mind.
DaveD

[Anonymous]

Posted by Howard Lewis on 11/03/2020 17:44:05:

Is 0 Hz Zero volts, or full voltage DC ?

Neither, it'll be enough voltage to drive the rated current through the resistance of the winding. However, 0Hz is not applicable to simple VFDs. They have a minimum frequency of operation as well as a maximum. To maintain control at 0Hz one needs to use vector control rather than V/f open loop control. As an example for electric vehicle drives full torque at zero speed is useful. It's the equivalent of holding on a hill by slipping the clutch.

The V/f characteristic is quite reasonable as controls go, but tends to go wrong at low frequencies where the voltage drop due to coil resistance becomes a significant proportion of the back emf. Some VFDs allow one to modify the V/f characteristic, modelling it as a series of lines of varying slope. Torque boost simply means increasing the winding currents more than rated current in a controlled manner. And preferably for a short period unless the motor has extra cooling. It can be useful to get through a short term overload.

The experiment involving not being able to stop the chuck at low speed doesn't really prove anything. Below base speed, in simple terms, the torque is constant, and probably more than one can hold, whatever the speed. However, given that power is torque time angular velocity the power available for cutting is proportionally reduced. That's not useful if a large item needs to be turned at slow speed. That is why people say (correctly) that VFDs have some limitations at low frequency. A mechanical speed reducer (belts or gears) increases the torque at low speeds over and above that at higher speeds, keeping the power constant.

Andrew

[not done it yetParticipant @notdoneityet]

Well put, Andrew. When upping the current, I^2R losses increase exponentially. It is clear from Cos Theta that motors are running with the voltage and currents out of phase, so induction is playing a limiting part in the design. At zero rpm (zero Hz from the VFD, by definition) the I^2R losses would clearly be 100% resistive and with zero speed the cooling would also be zero. Not a good combination🙂.

While clearly being a bit theoretical, the analogy is pertinent. Reduced speeds alter the motor heat balance detrimentally, decreasing the speed to zero being the ultimate condition possible – particularly if, as you comment, the zero speed was while there were a load acting in the opposing direction. Clutch wearing drivers is a good analogy.

[Clive FosterParticipant @clivefoster55965]

#2 to what Steviegtr says about satisfactory operation between 35 rpm and 65 rpm. Generally that pretty much seamlessly closes the gap between two belt steps on a lathe so you can leave it on middle speed most of the time.

Way, way back when VFD boxes started becoming available (but not affordable) I had discussions with the suppliers about what performance could usefully be got out of them and what effect the natural motor characteristics had. Bottom line was that ± 1/3 rd of nameplate speed would give generally acceptable performance in any application with engineeringly sensible power margin. Which translates to 33 to 67 Hz, near enough. Which is probably where the figures used by Steviegtr came in.

Back in those days the boxes were of much simpler design without the clever circuitry that can extend the performance of modern units so the overall performance was much more strongly defined by what the motor itself could do and what its torque / supply speed characteristics looked like. Up to a point modern control algorithms can extend the useful operating range but ultimately you are driving a device designed to self stabilise close to nominal speed when driven at 50 hz (or 60 in the USA). Ultimately you cant fool physics. Once you get significantly outside that range with a normal induction motor you will have to compromise and accept power reduction over and above the theoretical constant torque (at low rpm) and constant power (at high rpm) losses.

In my view the best way to exploit a VFD on our sort of machinery is to swop the common four pole 1,440 rpm motor for a 6 pole 950 rpm or even 8 pole 720 rpm. Or at least change the drive pulleys to give a similar effect. High machining speeds are for small components so we don't need monster power so the losses above + 1/3 nameplate speed don't much matter. Far better to pull the natural drive speed down so we have power at lower speed where we can really use it.

The servo substitute motor – VFD combinations which really can develop serious power down to very low speeds are rather different to the common induction motor. The way they work has much more in common with brushless DC motors. Connecting one directly to the power line without its controlling VFD would be a disaster.

Clive

[old martParticipant @oldmart]

Further to the variable power outputs when running a three phase motor from a VFD, this 3/4 hp 1500 rpm motor is typical of the type fitted to Myford lathes, note the figures quoted by the manufacturer.

**LINK**

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