Estimating inertia on old machines for adding electronic braking

[jraef]

Can anyone share some shortcuts to estimating inertia on older machines for which drawings and data sheets are not available? I need to come up with a replacement braking system for older candler roller machines and the machine mfr is long gone. My fall back position is that since it starts across the line, I can scope the starter to determine the acceleration time from a stop, then using the motor data to determine starting torque, extrapolate the likely inertia of the entire system. Worst case scenario I end up estimating higher and over-doing the electronic braking. But has anyone used something I haven't thought of?


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[ZeroSeq]

Having just run into this scenario recently, your online test is the best bet. The only trouble will be ensuring you can start the motor on full load. If not, you will basically be stuck in the same position of having to estimate the torque component during deceleration (assuming variable torque load).

Trying to acquire average inertia data of not only the motor but the load as well is a very difficult (if not impossible) task to get within any reasonable level of uncertainty.

 

[jraef]

Thanks for the feedback. Yes, in this case the machines are all starting across the line now. But I know what you mean, if the machine is using reduced voltage starting or even a VFD, estimating accelerating torque becomes a lot more challenging.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[itsmoked]

I can't find any references to a "candler rolling machine" unless it has something to do with eggs. Got a picture of one or a link?

If there are belts involved there are ways to use belt tension to calculate dynamic loads.

Since I don't know even vaguely know what this machine looks like (hold in hand or a block long) can you drag over a VFD and run it with that long enough to measure the inertia?

Depending on the machine you could just calculate it:
X weight on a conveyor at Θ slope.
+
Any rotating shafts bigger than an inch with their weight lumped in the biggest gear or pulley diameter.
+
the motor's rotor inertia
+
any parts that can come down with gravity

I'd bet the math method would give you a better measure than a scope and motor curve. Heck throwing rotten fruit at a brick fence would probably give better numbers...



Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

Keith,
My bad, I posted that from my iPad and failed to properly negotiate with the auto-correct from "calender" (the machine) to "calendar" (the thing with dates) and when I tried to fix it, ended up with "candler". Sorry.

A calender is a rolling machine for the rubber industry. I though tanneries use them, so you might be familiar with them.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[itsmoked]

Ah yah! They use them to grain the leather so some of the rollers are textured. Size of a pickup truck.

They seemed to me to mostly be a mass of friction more than a pile of inertia. They usually don't turn very fast. Though I remember them sucking in and spitting out a half-side in about.. Two seconds.

Knowing what they are now, still makes me think the math method would be the best. Looking at the roller ends would tell you if they're hollow or solid.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[LionelHutz]

So you need it to stop within a certain amount of rotation so it doesn't suck an operator in too far? That type of application I've been involved with weren't particularly challenging to get stopped in time. If you're using DC then getting the DC applied fast enough after removing line power will be the biggest challenge. If not, then plugging is another option. I saw that done at one site and the contactors had to be switched quite quickly to get it stopped in time.

It is an induction motor, right?

 

[jraef]

yes, induction motors, and they are currently using mechanical clutch-type brakes or on some, plug reversing. The clutch-brake systems are worn out and nearly impossible to replace now, the plug reversing ones eat through contactors at an alarming rate and when they fail, there is zero braking. But in both cases, the stop time is unpredictable and OSHA is giving them headaches about it. Watching the workers, I can see why... there is NO WAY I would do what they are doing, probably for low wages to boot...

https://www.youtube.com/watch?v=rrfSBnPqNdE

This is the same basic machine, although they are using it for a different purpose in this video. But this shows the same speed theirs are turning, and the rollers are over/under instead of oriented front to back like in the video, so the operators are feeding into the pinch. They are trained to use closed fists for their work of pushing material in, because lingering fingers get lost...


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[Stephen Nuchia]

I never worked on the calendar machine controls directly but I did switchgear maintenance in a couple of tire plants. The calendars are a lot bigger than a pickup truck, think something closer to a modest newspaper press. The good news is that for this kind of machine the stopping inertia is pretty close to the starting inertia, unlike mills, compressors, and a lot of other machines.

I would hook up a digital power meter and measure the total energy (NOT Volt-Amps) consumed during a start of the machine empty and full of product. I think product will contribute more friction than inertia in this case but still use the larger figure. Divide it by the allowable braking time and that's your target brake power, and you also know the heat sink capacity you'll need.

 

[waross]

I would be looking at an air brake on the drive shaft as close to the machine as possible.
Spring applied, air pressure released is more fail safe.
I would investigate using a brake drum and brake hardware intended for a large truck.
You will probably need some expensive machine work done to mount the brake, but the upside is replacements parts are readily available and reasonably priced.
The air pots most common now have provision for air applied, air release, or the action may be compounded for greater braking force.
You need about 120 PSIG air pressure.
You may be able to use the axle and hub and connect it to the machine with a drive shaft.
You may be able to install the brake on the non-drive end of one of the rollers.
These must be "U" frame motors to take plugging.
If the contactors are failing they may be undersized.
Plugging is not the best for safety.
A failure of one phase will leave the machine driving forward.
The plugging switch is a point of failure. One type of failure may leave the machine running backwards.
Another type of failure may lead to abrupt starting if the shaft is rotated slightly while the machine is at rest.
Picture a mechanic in cramped quarters pushing on a wrench on a coupling bolt. The switch latch has failed. A slight turn may lead to the motor going DOL in the opposite direction driving the wrench back towards the worker.
Even with a LOTO program this is not a safe installation.
(I have seen too many electricians follow the steps faithfully and lock out the wrong circuit.)
I vote for a spring applied brake, air to release.


Bill
--------------------
"Why not the best?"
Jimmy Carter