13 replies to this topic

[sd40t-2]

Greetings to the group,

I'm a lift operator at one of the local resorts and I think I tend to stand out from the "crowd" because I tend to notice more and seek to learn more about the lifts which I operate.

I was asking a lift mechanic about a plausible scenario in which a lift designed for 100% bi-directional loading were to have a brake failure with a heavy downhill side. A gondola or people mover likely would be most likely to fit this criteria. It would seem most contingencies are designed to deal with a true opposite from normal direction of travel of the haul rope. What contingencies has anyone heard of for lifts which could have a heavy downhill side with a brake system failure and loss of power?

Dee

[Emax]

Very good question.

Remember, downhill loading is not restricted to Gondolas and people movers - there are many chairlifts rated to carry passengers down the mountain as well. Tramways rated for downhill traffic depend on three elements to prevent an overhauling load from overspeeding the rope. Let's think of "overspeed" as meaning "faster than the operator's selected speed" - which might be anywhere between 20% and 100% of maximum design speed. In order of system dependence, these elements are:

1. A regenerative (load-absorbing) motor control or other automatic, regulated braking device in the drive line.
2. The service brake (should the regen system malfunction)
3. The drive sheave brake (the last resort)

Under normal conditions, the regen drive will maintain the selected speed very accurately - providing driving or braking torque as needed. The drive itself will usually sense it's own failure to regulate speed and shut itself down. Next, the service brake (often an intelligent one) will come into play and attempt to bring about an orderly stop. In the rare event that the service brake is unable to effect a stop, the drive sheave brake will be automatically actuated by an overspeed sensor external to the drive system.

It is of course possible that all three of these measures could fail - but that possibility is a very remote one. I know of no such circumstance.

[sd40t-2]

Emax,

Thanks for the reply. I've watched the the speed readouts on various lifts and notice that the system is pretty good at regulating speed although it seems most PLC control screens seem more stable than the main drive motor control screen located on the control cabinet. Seems the motor control screen (on the lift I work the most is a Eurotherm DC drive control) jumps around more and has a much higher sampling rate. I've not had the chance to see the motor control screen when the lift is in more demanding conditions such as a full uphill or downhill load. Would the main drive control be monitoring speed only for speed control or a conbination of speed and voltage /amperage / motor Back EMF - inductive load?

Second question is do most lifts for regenative braking use resistive braking like diesel-electric locomotives, or actual inverter driven regeneration to put power back into the power grid?

In my original post I was envisioning a scenario on a lift with a heavier downhill side than uphill and by which the main power was out. For sake of the conversation let's say arrangements were being made to enable the APU (Aux. power unit) but it was not yet connected. If the service brake and bullwheel brakes were either not operating or unable (highly unlikely) to hold the bullwheel / high speed input shaft, there wouldn't be any sensing as the drive system would be dead. From my understanding, the 24V system can only kill power to the DC motor and cause the APU (an evac-only APU) to shut-down. I suppose that this scenario would be the one you mentioned in the final line of your reply.

My wording may seem a little disjointed but I'm not trying to cover every possible type and make of lift - Just one which would fit the criteria mentioned. I'm sure this "worst case scenario" is batting extremely poor odds but it makes for interesting topics.

Cheers,

Dee

This post has been edited by sd40t-2: 25 November 2005 - 03:22 PM

[Emax]

"Would the main drive control be monitoring speed only for speed control or a conbination of speed and voltage /amperage / motor Back EMF - inductive load?"


Well, yes and no. The end product of what the drive is doing is speed control, but to achieve this many very quick changes in motor torque are needed constantly. When you, as an operator, select a speed different from the current one, the regulator loop in the drive arranges for changes in the firing angles of the power semiconductors (probably SCRs) that will causec the actual motor speed to the requested one. All of this must happen in an orderly manner and at an acceptable rate of change. To make this happen, the drive considers not only speed feedback (probably a digital tach - but it could be the back EMF of the motor) but also current feedback (a signal derived from the actual motor current). The inductance of the motor windings is a factor, but not one that is actively dealt with.

"Second question is do most lifts for regenerative braking use resistive braking like diesel-electric locomotives, or actual inverter driven regeneration to put power back into the power grid?"

While it is possible to use resistors to dissipate overhauling energy as heat, not many drives use this approach. Most often, a second converter (SCR bridge) is connected in anti-parallel with the one that provides forward torque. When the regulator turns on this bridge (in a very carefully-controlled manner) it's output polarity opposes that of the back EMF of the motor - so the motor now produces torque in the opposite direction from its rotation. The energy from the motor is thus commutated back into the power grid (and is actually used elsewhere by beer coolers, french fryers, etc. ).

"In my original post I was envisioning a scenario on a lift with a heavier downhill side than uphill and by which the main power was out. For sake of the conversation let's say arrangements were being made to enable the APU (Aux. power unit) but it was not yet connected. If the service brake and bullwheel brakes were either not operating or unable (highly unlikely) to hold the bullwheel / high speed input shaft, there wouldn't be any sensing as the drive system would be dead. From my understanding, the 24V system can only kill power to the DC motor and cause the APU (an evac-only APU) to shut-down."

Yes, connecting the APU belts could be really ticklish if one were not confident in the service and drive sheave brakes. None of the anti-rollback devices are any use with an overhauling load.

The "24-volt system" not only shuts down any source of motive power - it is also in complete control of all braking systems. Regardless of the lift manufacturer, you can be sure that if motion is detected while the lift is supposed to be at rest - brakes will set.

I hope this helps

[Emax]

Tried to edit the goofs in the above but was denied permission. Here's the fix.

Well, yes and no. The end product of what the drive is doing is speed control, but to achieve this many very quick changes in motor torque are needed constantly. When you, as an operator, select a speed different from the current one, the regulator loop in the drive arranges for changes in the firing angles of the power semiconductors (probably SCRs) that will cause the actual motor speed to cahang to the requested one. All of this must happen in an orderly manner and at an acceptable rate of change. To make this happen, the drive considers not only speed feedback (probably a digital tach - but it could be the back EMF of the motor) but also current feedback (a signal derived from the actual motor current). The inductance of the motor windings is a factor, but not one that is actively dealt with.

[Kelly]

The "overrunning" scenario that you describe has happened more than a few times in lifts that are designed for uphill loading only. The down line was inadvertently loaded beyond the friction capacity of that lift to slow down by itself. Any over-speed limit circuitry or brakes were nonexistent. You might (should) see the downhill capacity posted at every top terminal on up-loading lifts due to the historical nature of this occurrence rather than current design (last 15 years) limitations of the braking systems.

As Emax implies the capacity for over-speeding in downloading lifts is quite high -this is quite recognized by engineers. This is dealt both electrically and mechanically and the systems are tested in both directions.

The problem arises on lifts that were not designed to expect that condition. The classic scenario could be summer time operation with large impatient groups wanting to get down the lift, or possibly a work crew (again impatient) wanting to get down the mountain… not that I have ever ah heard of the later happening but it does seem plausible.

Emax – I have seen the slow circuit being used to slow an over speeding DC drive lift with a large sandbag load on the downline – would you mind explaining to the forum readers how this is possible.
Thanks,
Ryan B

[sd40t-2]

Let me merge back into my original post and throw out this little simple scenario...

Let's say you have a Gondola which both uploads and downloads. It's late afternoon and most of your patrons want to download vs. ski/board down the mountain. Your bottom has performed a "last cabin" procedure so your uphill side is slowly becoming lighter as less people are on that side. All of a sudden you lose power from your power utility and everything stops. I'll throw a wrench into the works - A moderate wind begins to blow down the line. The service brake is not working as well as it should and bullwheel brake begins to slip. Although it's a very unlikely, From what I understand, there is nothing that would stop the rope from moving in the normal "forward" direction unlike a true "rollback" in the reverse direction other than braking surfaces (service & bullwheel brakes).

Hope this clears up my disjointed posts,

Cheers,

Dee

[Allan]

You are right, there is no other brake to stop the lift if it starts to run away in it's normal direction of travel. That's why we do stopping distance tests every morning - to see if the brakes are working as designed, but that's not to say oil couldn't get on both surfaces from a gearbox hose that decides to pack it in.

[Emax]

"Emax – I have seen the slow circuit being used to slow an over speeding DC drive lift with a large sandbag load on the downline – would you mind explaining to the forum readers how this is possible."

There are a few possibilities. Obviously you do not refer to a lift equipped with a regenerative drive, since they control overhauling loads for a living. That leaves five other drive possibilities:

1. a 2-quadrant DC drive system (provides torque in one direction only)
2. an AC inverter drive system
3. either of the above in conjunction with an eddy-current retarder
4. a non-speed adjustable AC drive (directly connected to the mains)
5. a hydrostatic drive

1a. A simple 2-quadrant dc drive with normal connections cannot absorb load. As the motor speed begins to exceed the speed command, the SCRs simply phase back to zero and the lift coasts - gaining speed until (and IF) frictional load takes over. Since you mention sand bags, I will assume that this is a load test situation and that there is an informed technician present. This being the case, the drive connections can be temporarily changed to allow the excess energy to be regenerated back to the utility grid. Any DC drive IS regenerative under the right conditions - one need only arrange for the motor torque to be applied in the direction opposite to the load-applied rotation. This is done by reversing the motor's rotation.

There is really only one correct way to do this: reverse the armature (A1 / A2) connection. Yes, I know the thing will turn the other way if the shunt field (F1-F2 / F3-4) is reversed, but this presents a conflict with the stabilizing windings (S1 / S2) that most motors in the industry are equipped with. Without getting into just why the S1 / S2 windings are there, accept that if they are in the circuit then F1 and S1 must be the same polarity if any serious motor current is expected. If they are not, the brushes will begin to "spark" as load increases - possibly damaging the commutator. This said, it is probable that the amount of torque needed to control the sandbag load on a downhill run will be minimal - since the frictional losses of the lift help out quite a lot. The actual maximum motor current that will be needed can be estimated with fair accuracy by subtracting the observed motor current when the lift was empty from the current observed when it was fully loaded. Most often, this will be 50% or less of the motor's full-rated armature current. At such a current, reversing the (much smaller) shunt field wiring is probably acceptable - just don't make it a habit.

Since you did say "DC drive", I guess the rest of this is moot - but I'll mention it anyway.

2a. An Ac motor controlled by an inverter drive can provide a degree of load absorption even if it is not equipped to be fully regenerative. A resistior bank connected to its DC bus can dissipate a modest amount of overhauling energy. Once the bus exceeds it maximum voltage, however, the controller will shut down - leaving the brakes to do the job.

3a. Some lift drive systems have been built with eddy-current retarders located somewhere in the drive line. These are the same units used on many large trucks and buses - they can be thought of "as anti-motors". A modified drive regulator circuit can control these devices to simulate the operation of a fully regenerative drive. Actually, I think it's superior to a regen because it can be arranged to "fail safe."

4a. The old "plain-Jane" AC motor - directly connected to the mains is a real trooper as speed control goes. It will do its very best to maintain its synchronous rpm whether it is driving or being overhauled. A 2-speed motor will "haul in" the sandbag load when switched from fast to slow - regenerating the load back to the mains. Connected to an inverter, it cannot do this unless a mess of extra (very expensive) components are added to the controller.

5a. Hydrostats are great brakes. When valved to run at a given speed, they will - come hell or high water. They're a lot like the "plain-Jane" AC motor scheme in trhis respect. Right now, I know of no lifts that use a hydrostat for both the primary and standby drives - but it might not be a bad idea if the intensive maintenance issue can be brougfht under control.

OK - I've gone too far (again), but I will not apologize. I think that some of this needed to be aired. I will apologize for any typos, etc. in this reply. I'm using my wife's obnoxious laptop, and it dislikes me almost as much as I dislike it. Almost.

Emax

[sd40t-2]

Emax, on Nov 26 2005, 10:32 AM, said:

5a. Hydrostats are great brakes. When valved to run at a given speed, they will - come hell or high water. They're a lot like the "plain-Jane" AC motor scheme in trhis respect. Right now, I know of no lifts that use a hydrostat for both the primary and standby drives - but it might not be a bad idea if the intensive maintenance issue can be brougfht under control.

Emax,

The only lift that I know of that is/was both pri/sec hydro drive (haven't seen it in a few years) is the people mover at the local amusement park "Lagoon" (in northern Utah). From what I can remember there is a moderate sized hydraulic motor on the bullwheel and then the drive motor is a AC motor with rather small gas engine on the back end of the electric motor as the APU. This arangement is small and at ground level. the motor and APU are about the size of three or four refrigerators on their side. I suppose since the lift is on the level and never has a heavy load (two seater) they can get away with "smaller potatoes".

Cheers,

Dee

[Emax]

"Let me merge back into my original post and throw out this little simple scenario...

Let's say you have a Gondola which both uploads and downloads. It's late afternoon and most of your patrons want to download vs. ski/board down the mountain. Your bottom has performed a "last cabin" procedure so your uphill side is slowly becoming lighter as less people are on that side. All of a sudden you lose power from your power utility and everything stops. I'll throw a wrench into the works - A moderate wind begins to blow down the line. The service brake is not working as well as it should and bullwheel brake begins to slip. Although it's a very unlikely, From what I understand, there is nothing that would stop the rope from moving in the normal "forward" direction unlike a true "rollback" in the reverse direction other than braking surfaces (service & bullwheel brakes).

Hope this clears up my disjointed posts,

Cheers,

Dee"

Please see my posts in both the "industry only" and "general" forums that suggest a braking system at the return terminal. There are imaginable performance scenarios that are not presently covered by the existing "code". (it isn't really a "code", but a performance standard - frequently adopted as "code" in the imaginative vacuum that exists in the industry).

[Aussierob]

Monster thread! I guess if I get right to the point the question is what happens if an overhauling load is unable to be retarded? The answer is - all kinds of things, none of them good. The situation you describe is nearly imposssible to happen. The chances of all sets of stored energy brakes failing is quite remote. We just load tested Solar Coaster at Blackcomb this year. It is rated for 100% download. All three drive systems are capable of controlling a full load up or down. As well all three brake systems must individually be able to stop a full downhill load and meet code (0.45m/s/s). They did. This is not a situation that concerns me. You have about a 100 times better chance of being killed on the highway on the way to the ski area, than in this scenario.

On another front, I am quite sure that DC "regenerative" drives do not put power back to the grid when they apply torque in reverse. This is technically plug braking but is frequently called regenerative braking. I could stand to be corrected though.

[Emax]

I am quite sure that DC "regenerative" drives do not put power back to the grid when they apply torque in reverse. This is technically plug braking but is frequently called regenerative braking. I could stand to be corrected though.


Sorry, but you are mistaken. Yes, "plugging" the motor can work too, but the excess energy is given off as heat, greatly limiting the amount of time it can be done. DC motors, when plugged, have their armature circuits shorted - usually through some resistance. Those resistors get plenty hot - very quickly.

Let's say that you are "braking" with a 400 hp regen drive at a constant 40% load - runnging sandbags down the hill. You are converting the kinetic sandbag load into about 250 amps at about 480 volts - some 120,000 Watts. That's a really big heater! Where do you supppose this energy goes? If the motor were simply "absorbing" it, it would burn down in an (extended) heartbeat.

[Emax]

I thought that more in the way of explanation regarding motor braking might be helpful. For those who are interested, the attached PDF contains text lifted from volume 3 of Samuel Heller's definitive work on motors and generators. Page 7 (PDF pg 7) offers a simple description of regeneration. Plugging is also described elsewhere in the text.

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