Vintage lathe meets 21st Century

[Mike PooleParticipant @mikepoole82104]

We all too often seem to forget that maintaining torque at low rpm is not maintaining power. Gearing down effectively multiplies the torque and ignoring losses maintains power. Working on large diameters at low rpm it is easy to not have enough power to make a decent cut. I would fit a VFD every time but at the very least have a gear or belt reduction to make large diameter work more practical.

Mike

[Paul White 3Participant @paulwhite3]

John, Not wishing to be pedantic but I think you have one" C" to many . NC does the job, see Joe Noci mod to his EMCO.

regards

paul.

[not done it yetParticipant @notdoneityet]

Yep, Mike is right. Power is basically torque multiplied by speed of rotation. Hence power at zero revs is zero as well.

Fan cooling volume is proportional to at least the square of the fan speed, so overheating may still be problematic at lower speeds, I would think?

[SillyOldDufferModerator @sillyoldduffer]

Posted by Mike Poole on 11/04/2018 09:21:27:

We all too often seem to forget that maintaining torque at low rpm is not maintaining power. …

Mike

Far worse than just forgetting in my case! I have hazy notions of the relationship between Work, Power and Torque and find the whole thing slippery. Doesn't help that 'work' and 'power' both have day-to-day meanings adrift from their precise scientific/engineering definitions. Or that torque is sometimes defined as 'turning power', I think(?), with power used in it's unscientific sense. Confusion abounds.

Here's my attempt at visualising what's going on in a Permanent Magnet Synchronous Motor:

• Unlike motors depending on eddy currents or field windings to provide a magnetic field, a permanent magnet is always full on, ready to go.
• Power is the rate at which the motor can do work. The work (scientific definition) doesn't vary, so a tiny motor running all day can do as much work as a large motor does in 5 minutes. But being less powerful it takes longer to do it. A stopped motor does no work safely. A stalled motor also does no work; instead it converts it's energy input into heat and magic smoke. Perhaps this leads to the (wrong?) idea that a stopped motor has no torque.  I believe it might.
• Torque is a measure of a motors ability to push, and it's rotor doesn't have to move to have force behind it. Imagine holding a breeze block over your head; you and it are not doing any work – no power is developed. But even though it's not moving the breeze block has the potential to do work, it is applying force. Drop it on your foot and different rules apply – work and power are delivered to your toes.

The ability of different types of motor to deliver torque across the range varies considerably. When stopped IC engines provide no torque at all: they have to be cranked up to start and run quite fast before they can turn anything. A clutch and gearbox are needed to make an IC engine's limited torque useful at the wheels. Likewise a steam-turbine; hopeless at low speeds, brilliant at high, and again the need for a gearbox. IC and turbines can deliver lots of power, but they ain't good at torque. Conversely reciprocating steam engines provide excellent starting torque, but aren't efficient, or easy to scale to deliver very high powers. Electric motors are even better than reciprocating steam, and the PMSM type is particularly good.

Although I find it easier to visualise the difference between torque and power when nothing is moving, torque and power coexist at all times. Once a motor is turning belts and gears can be used to increase torque (the ability to make slow deep cuts) or increase speed ( fast shallow cuts). We manipulate our machines to deliver a good combination of torque and power to do the job, fortunately without needing a University qualification.

I'm not too worried about the possibility of overheating when a PMSM is running slowly or stopped. Conventional 3-phase motors are delightfully robust but the best that can be done for them is a temperature cut out and current overload. Many installations are only protected by a fuse. You might say they are strong in the arm and thick in the head! In contrast a PMSM requires a controller with a brain the size of a planet. It's not difficult to imagine a controller clever enough to keep the motor inside it's safe zone.

Grateful for comments and criticism – I really struggle with this stuff. Wikipedia has a good entry on torque but the maths is way over my head. For me it's like trying to open a door without the key.

Dave

 

 

Edited By SillyOldDuffer on 11/04/2018 11:07:51

[Michael GilliganParticipant @michaelgilligan61133]

Posted by not done it yet on 11/04/2018 09:35:35:

Power is basically torque multiplied by speed of rotation. Hence power at zero revs is zero as well.

.

Very true … albeit perhaps of largely academic interest.

I venture to suggest that the more noteworthy aspect is that a mechanical speed-reducer is also a torque-multiplier, wheras the typical electronic speed-reducer is not.

MichaelG.

[John HaineParticipant @johnhaine32865]

Of course a stopped motor can have torque – otherwise it couldn't start turning. Torque is just the force that is exerted say on a belt running on a pulley at the pulley's radius – force in Newtons (10 to a kilo of apples) times radius in metres = newton-metres. If you pull back on the belt stronger than the motor it won't start, and may even turn backwards. Some motors are designed to just exert a force/torque, say to tension a thread or magnetic tape, but to be happy being stationary or even turning backwards.

That's the force also that was being exerted on the tool that was facing the chunk of tough cast iron on my S7 this morning (and it was noticeable that the lathe slowed down as it started to cut on the periphery). It's force that you need – and therefore torque – to make the tool deform the metal, as I found when I tried to hacksaw off a chunk (though I'd have been aware of the energy too had I persisted). As it deforms it gets hot (and the heat makes it plastic and helps it deforms), so the energy from the motor is converted into heat. Making the tool cut in the first place needs torque (force) but you need the energy to do the work on the metal to dissociate the bonds between the atoms holding it together.

Induction motors are very cheap to make and very robust. You can drive them to produce practically the same maximum torque right down to zero speed. When the rotor is stopped the fields in the stator will be buzzing round at probably about 10 Hz in order to generate currents in the stator to generate the torque. So the VFD is always generating AC, thought if its a vector control type it will be sensing the rotor position and speed and adapting its drive to optimise the torque. A conventional squirrel-cage induction motor can't generate torque with a "zero frequency" supply as the rotor has "constant reluctance" so it can't be "grabbed" by the stator field.

But if you had a permanent magnet rotor, you can exert a torque at zero speeds just by the stator applying a steady magnetic field that's "in front" of the rotor magnet, so the rotor tries to catch up to make the magnets align. If you can sense the rotor move, you can then advance the stator field to keep the angle between the fields, and the torque, constant. But this needs a different VFD architecture which can operate down to zero Hz, sensing the rotor position. A conventional VFD probably won't do this (to answer a question above).

Sorry to dredge the remnants of my electrical machines course, but hopefully this might be useful to someone.

[Neil WyattModerator @neilwyatt]

Torque is a rotary equivalent of force, with a circular vector instead of a linear one.

Work done = force x distance moved

So like with pushing a brick wall, you can exert a force and do no work if the point of application doesn't move.

As torque is equivalent to a force at a specific radius

Work done = torque x angle turned.

If your motor doesn't turn, it does no work.

Power is the rate of doing work.

So when the motor turns faster at the same torque, the power increases.

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