New Input on Overspeeding Motors with VFD's

[SteveHGraham]

I have been cleaning up my shop and making it usable. I have been trying to decide whether to put up a second (air-conditioned) building. It has occurred to me that I could make better use of the current building, but for the perpetual presence of two tractors.

As a partial space-saving solution, I am considering parking my Kubota with the front end loader up. With the brush forks, the loader probably takes up 50 square feet. The other day I raised it and let it sit, and hours later, it was on the ground. I am told I should look at a valve under the control lever, but regardless, I need to be able to lock it in the up position without relying on hydraulics.

Kubota makes a brace for some of its tractors. It slips over one hydraulic rod. My tractor doesn't have one, so I am making one from 2.5" square tubing. I had to use plasma to cut one side out of the tubing to turn it into a channel, and I am fabricating things on the ends to make the brace friendlier to the paint and rod, and to make it less likely to fall off.

I hadn't used plasma in over two years when I started cutting yesterday, and I relied on some really bad advice RE air flow, so the first cut on the tubing was a mess. I ended up using an angle grinder to clean it up.

The photo makes it look much rougher than it is. The long marks inside the tube are light grinding marks.

I had to use my 2x72 grinder to make an end shield for the tubing. My mill is still far away. While I was in the process, I Googled information about motors and VFD's. When I built my grinder station, I used an 1800-RPM motor, expecting to wind it up to over 4000, and after I bought the motor, I got all sorts of conflicting information on the wisdom of speeding the motor up. Yesterday I saw something interesting which may be useful to other people.

A well-known machining forum participant named Forrest Addy said he had run a bunch of motors at 400 Hz for long periods with no problems. That's about 6.7 times the nameplate speed for a 3600-RPM motor. His position appeared to be that cranking a VFD up well past a motor's nameplate speed was not unsafe at all.

As I understand it, there are really two main concerns when you overspeed a motor. First, you don't want it to explode, and second, you may be very concerned about high-speed performance. If you have to have a lot of torque at all speeds, you may not be able to get it. Overheating can also be a problem in some cases. I think people conflate these issues and assume that a motor that is not rated for double speed is unsafe at that speed, when the manufacturer's concern is that it won't perform well.

Manufacturers of inverter-duty motors recommend certain top speeds, but they don't say whether they're protecting end users from mechanical failure or reduced performance, which are two different things.

I found this stuff interesting, because people had been telling me that if I ran my motor at 167 Hz, which was a frequency that generated a belt speed I had arbitrarily chosen, it would blow up and send shrapnel all over the shop. It appears now that they were speaking from pure ignorance.

I no longer plan to run the motor that fast, because it cuts extremely quickly at 120 Hz, and when you speed up too much, you can eat work up accidentally. Still, there might be times when I just want to waste metal, so maybe I will want to go faster in the future.

At 167, the grinder pretty much rips through metal.

I don't think reduced torque is an issue for me, because the motor is 3 horsepower, or about twice what you would expect from a manufactured unit like a Bader. I have a lot of torque to lose before it becomes a problem.

I don't know how many people here have 2x72 grinders, but I can't recommend them highly enough. They can shape metal in a real hurry, like no other tool short of a CNC cutting table. If you have a small wheel attachment and a big contact wheel, a grinder becomes extremely versatile.

To make the end shield for my brace, I cut a hole in it with the drill press, cut out a little piece of metal with the angle grinder to admit the grinder belt, and used the grinder to open up the hole into a nice U shape. You could do the belt grinder part of this job in 2 minutes with the right belt. I would need 10 minutes to do it on the mill.

My only complaint about my grinder is that I can't tilt it 90 degrees. I'm thinking about ways to do it. I might have to make a new chassis for it, but that's a very simple job, especially if you have plasma and a 2x72. It might be easier than trying to make a rig to tilt the existing grinder, which wasn't really made for it.

I'm glad I went with the big motor, which I chose because it was easier to find than a 2-HP motor. I think many people are going with undersized motors simply because they don't have 250V power.

People are intimidated by grinder prices, but you can get a very sturdy chassis for $500. After that, you just need a motor, VFD, and some pulleys, which you can make on the lathe. You could make a perfectly good grinder from plywood and two-by-fours. People have done it.

[whateg0]

I think the main things most people are concerned with when it comes to overspeeding motors is bearing life and heat. You get reduced torque because at frequencies other than 50/60 Hz, the windings never get close to saturating, the same reason that people using steppers run a much higher voltage than the motor is rated for, then PWM. While the possibility of a motor exploding certainly exists, I would say that is highly unlikely at the overspeeding that most users will do. I would guess that most never go much beyond 120 Hz. 400 Hz does seem a bit much to me, but on a well built, well balanced motor, from a reputable mfr, it might be ok. I'm reluctant to use anecdotal evidence as proof that it's ok, though.

I'd consider putting some sort of "remove before flight" tag on the valve for raising/lowering the loader. It's always been my understanding that those braces are primarily a safety thing for maintenance, so a guy does some work under the loader, dump bed, whatever, then removes the brace. Having it there, and forgetting about it when you start the tractor in a hurry to go pull somebody out of the ditch in bad weather on a late night, you or somebody else, might forget about it. Probably not going to hurt anything, but still a good idea IMHO.

Dave

[SteveHGraham]

I looked up the bearings for my motor, and I found a figure of 14,000 RPM, so 5000 is not scary.

[STRR]

While I can not make any comments about the high-frequency motor operation, I can make a few observations on hydraulics.

Many people automatically resort to blaming a valve leaking for hydraulic cylinders retracting. If you take a moment to use a little logic, you may come to a different conclusion. How far, in inches, does the valve travel every time it is operated? I would pose a guess of about 1/4 of an inch in each direction for a total of 1/2 inch. Compare that to the distance the piston travels in the cylinder every time it operates. Every operation can be up to 4 feet in each direction. Thus, the piston travels so much more than the valve, it is the MOST likely place the leak is occurring. The oil will travel from the place of higher pressure to the place of lower pressure. If it is seeping around the piston, the ram will retract (or extend depending on the orientation of installation) until equilibrium pressure is reached. Thus, the bucket will "fall" to the ground. Keep in mind, this is all internal and most likely there will be no external leakage. This is not to say the valve isn't leaking as well but it is saying the more likely problem is with the piston seals. I can not count the number of times I have experienced this.

As for the ram lock; great idea. As long as the lock is in place, the bucket can only fall until contact is made between the piston end and the cylinder housing. Most farm tractors do not have a downward pressure so there should be no problem if the lock is forgotten, except when you hit the garage door on the way out. For those tractors that DO have downward hydraulic pressure, the lock should be removed before moving the tractor. This will prevent the possibility of ejecting the lock into outer space when the lever is pushed to lower the bucket. I agree with a warning device being placed on the steering wheel or ignition switch to remind the operator the lock being in place. The ignition key could be placed on a hook attached to the lock so the operator would have to retrieve the key AT the lock thus reminding him of its placement.

Good Luck,
Terry

[RMinMN]

Your brace for the loader hydraulics should be strong enough to hold against any down pressure until the hydraulic relief valve bypasses and should have two pins to secure it to the cylinder rod so it cannot fall off or be ejected. I consider that to be the only safe way to keep the loader up when working underneath.

[Inspector]

I was wondering if a strut/pole placed under the bucket to the ground like the tail stands used on some airliners would be simpler. If the tractor was driven away the worst that would happen is the clang from the strut hitting the floor.

Pete

[SteveHGraham]

Many people automatically resort to blaming a valve leaking for hydraulic cylinders retracting. If you take a moment to use a little logic, you may come to a different conclusion.

I have a great reason for thinking the tractor's control valve is the problem, and here it is: the valve is easier to fix than the piston seals. My advanced logic may be too deep for some people, however.

If I figure out how to check the valve, I'll take a look at it.

I was wondering if a strut/pole placed under the bucket to the ground like the tail stands used on some airliners would be simpler. If the tractor was driven away the worst that would happen is the clang from the strut hitting the floor.

I can picture all kinds of problems with that. Hydraulic rod braces are standard for this purpose.

[John Hasler]

The shaft occupies part of the space behind the piston so as the piston moves into the cylinder the fraction of the cylinder volume not occupied by the shaft and piston must decrease. This can only happen if some oil leaves. Thus if your shaft seal is good and your fittings, hoses, and valves don't leak the cylinder will not leak down.

That is, it won't retract. A cylinder with a leaky piston will extend under load. This is a good reason to avoid installing cylinders in such a way that gravity puts them under tension.

[liveaboard]

Those control valves all leak a little, unless special expensive non-leaky ones are specified.
My FEL is 55 years old and leaks a bit more than new ones do.
For parking, I just prop up the FEL with a 2x4 and the bucket looms over my tablesaw.
I have a heavy hedge cutter that mounts on the FEL too, I used to cut the top of my hedge and raise it up every minute.
The hedge looked like a big ratchet.
I added a ball valve to the 'up' pipe, and that helps a lot; but it still drops over longer periods, despite having honed the cylinders and replaced the seals.

motor speed; I've been thinking about this lately.
Surely, the diameter of the rotor will play a role?
Instead of buying a 2hp motor, I might put a VFD on the old 1HP one and use a lower gear.

[STRR]

Steve,

The quickest way to determine if it is the valve is leaking or not is to do what liveabroad did, put a good ball valve in the line to isolate the operating valve. If the bucket still bleeds down, you know it's the cylinders.

Cranes use the same type of cylinders for boom lift and extension. BUT, we have an added component: Holding Valves. Think; hydraulicly operated check valves. You have to have hydraulic pressure supplied to the holding valve to make it open thus allowing oil into or out of the cylinder. My employer was concerned about a third party inspection stating the boom was leaking down and in. So, they spent $15000 for new holding valves, labor to install, a rental crane to do the required work while the company crane was being repaired, and lost/delayed maintenance due to the event. All the time, I had been telling them the cylinders were in need of a rebuild. You should know, the new holding valves did NOT stop the leakage down or in. WASTE of $15000.

To answer liveabroad's problem, your cylinders may have worn areas where the seals will leak. The piston head can only allow the seals to expand so much and then the oil will bypass the seals. The barrels can usually be machined, a slightly larger piston head made/bought, new seals installed to eliminate the bleed down problem. Another possible cause is the installation. Just because a man can take a thing apart, put in new parts, and get it to work again, doesn't mean the job was completed correctly. The seals may have been damaged during installation. I've seen it happen. On a farm tractor, you have to ask: Is it worth it? A little bleed down will not usually be a problem compared to the expense of getting it repaired.

Good Luck,
Terry

[SteveHGraham]

Thanks for all that information.