Another vfd question

[John Stevenson 1Participant @johnstevenson1]

John,

Modern motors are very good with VFD's the ones believe it or not that suffer are the older heavier Brookes type.

It has to do with the percentage of iron to copper in the build up.

A few years ago i ran a 1HP Brookes up on a Town Woodhouse mill to see what it would do. It was fine up to 85 hz and then performance dropped off at 90 hz it wasn't happy and by the time it got up to 105hz it was getting quite warm and sounding like it had lost a phase. Once back to 85hz it cooled down nicely and ran well.

Now fast forward to today and the motors are designed to accept VFD's

This is a standard off the shelf TEC [ Chinese ] 1.1Kw [ 1.5Hp ] 2800 2 pole motor.

They are then stripped down and the spindle pressed out , rotor bored and a new spindle fitted with integral ER32 fitting.

The end cover which is very skeletal is replaces with a solid cover to support double row angular contact bearings.

Everything is then rebalanced and then it's ready to run.

These run all day at this speed.

The next step from this but more expensive to to take that same motor, do exactly the same mods but have it rewound to run at 200hz this gives you a maximum of 12,000 rpm but then transforms that 1.5Hp motor into one capable of about 5Hp

These get done for CNC high speed router applications where they might have 5 heads on one machine. Not done any of these for a while because the water cooled spindles from China are now far more cost effective.

[AjohnwParticipant @ajohnw51620]

You can buy motors John specifically intended for variable frequency drive. This can even include taking account of circulating currents doing nasty things to the bearings in the motor. There are plenty about where they have just added forced ventilation. Some get a better balanced armature. Costs escalate – too much as usual.

Brooke at the time I bought my motor – very cheap because it seems they have a shelf life before they should be relubricated – were talking to Mitsubishi, if I understood correctly along the lines of building inverters into the motors. Mine runs ok at 100Hz anyway but I have no need to drive it that fast and given the lack of hard info would wonder what might happen to it. I doubt if I could find another similar motor at the same price.

My main reason for posting what I did is that there are people who will assume that pulleys and gears aren't really needed when an inverter is fitted. One cropped up on another forum recently. Do away with all of the pulleys on a miller including in this case a pretty extreme reduction gear. The low speed end is the main problem. There are retailer that would say fine and sell a package to do exactly what he wanted to do and no mention of the sort of problems he could have. He eventually found the correct TEC catalogue and seemed to get annoyed because it showed him that he couldn't do what he wanted to do but still wanted to do it. Then comes a video of a demo from some one else – drills hole running normally then changes the cutter to sort of face cutter – more like a fly cutter, slows right down with the inverter and faces around the hole. Great but that operation took about 30 secs..

Most people who fit inverters and look into it fit a more powerful motor than the one that was fitted to try and get round the problems. One good thing about Brooke motors in my view is I get the impression that they are concervatively rated. I don't really have any interest in running at very high speeds. I went for a 6 pole to help get round the low speed problems. Some get attracted to 2 pole motors on the basis that 2,800 can simply be slowed down to what their 4 pole currently runs at. Most I am aware of stick to 4 pole but I decided to be different. For really low speeds I just have to swing 2 levers so went to 6 pole. It works out well for me. It on a Boxford ME10, 1HP i was sure that the frame size would fit easily until I actually put it in place. Finished up having to add some M4 fixing holes to do it – high tensile of course.

 

The front bearing cover is normally on.
 

John

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Edited By John W1 on 28/06/2015 17:36:22

[Steve PaveyParticipant @stevepavey65865]

Managed to get a handbook this morning, and what I originally thought was a clutch, and then decided was a spindle brake, is in fact referred to as a clutch by Harrison. That probably means that starting currents will be a little more manageble.

I'm not too worried about the variable speed control – I think it might come in handy for threading up to a shoulder. The ability to slow to a stop and then reverse looks useful for all those odd thread pitches that involve keeping the half-nuts engaged. But there is every reason to keep on using the gear selectors for most speed changes. I wonder exactly how many people here have actually overheated their motors terminally by running at slow speed through a vfd???

the thread has drifted a bit, but it's all interesting stuff – thanks for all the replies.

[Neil WyattModerator @neilwyatt]

> I wonder exactly how many people here have actually overheated their motors terminally by running at slow speed through a vfd

Very few, I expect. A decent VFD should, by defauilt, be able to make a pretty good estimate at how hot the motor is getting using standard parameters and reduce the power or stop. You can tweak the parameters to match your VFD.

My Tec motor has a thermistor in it, but they didn't reply to my email asking about its characteristics, which would allow my VFD to monitor the motor temperature directly.

Neil.

[MuzzerParticipant @muzzer]

My ancient Xtravert (>20y) VFDs have a simple thermal model in them and I doubt that is unusual. They can infer enough from the name plate data and the operating conditions even without a thermistor to do some derating and protection.

You'd have to be pretty special to overheat a motor in this manner. If you run it so far below base speed that the cooling suffers, you are going to be way down on available power. Very few of us have such massively over specified machines that we could do any meaningful machining there. If you are trying to get some tangible work from your tools, you need to use the gears to get in the right zone.

It's rather like manual gearboxes in cars – although admittedly you do come across a few people who should really take the bus….

Murray

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