Closed loop hydraulic system 4

[NKBhinge]

Hi,

I am working as an electrical engineer in a bridge design firm. I am currently working on a bascule bridge design which uses a closed loop hydraulic system. 2, 50KW hydraulic motors drive the pump. The pump motors run in the same direction while opening and closing the bridge. The speed and direction control is via a gear motor and swash plate arrangement. I have a question when the bridge is closing. Our mechanical engineers are saying that while closing the bridge, there is an overhauling load and the pressure in the system will cause the pump to run faster which will in turn cause the motor to run the faster. They want the hydraulic pump motor to hold the bridge down, basically make them run at the same speed.
Our worry is, the motor is seeing this overhauling load. This will cause the motor to run as a generator. So we basically need resistor banks and drives to dump this excess energy. At the same time we want the motor not to go above sync. speed when the bridge is coming down. So we use drives to make the motors run at the sync. speed always. Is this correct? Am I understanding this correctly? Do we really need drives? Or is the motor going to run at synchronous speed always no matter if the pump runs faster.

I would really appreciate some guidance here.

Thanks a lot.

 

[hydtools]

A hydraulic system schematic would be helpful.

What do you understand closed loop to mean?

Ted

 

[TimSchrader2]

Hello
Not sure if the "gear motor/swash plate" is a gear type flow divider to keep flow the same to both pumps. Are you using hydraulic counterblances? Or meter out flow controls to apply some back pressure to keep it (or minimimze) from doing what the above says? Or 'press compensated' flow controls which maintain same speed regardless of Press drop over the valve? But not to be used in proportional systems unles the vales itself is proportional with feedback.

 

[Jacc]

A gear motor in the electrical world seems to be a motor mounted to a reduction gearbox of some sort and not a gear motor as in an a typical hydraulic gear motor.

Since OP is electrical I assume a "gear motor" in this case means any hydraulic motor mounted to a reduction gear.


I'm guessing Closed Loop refers to the return oil from the motor going right back to the pump low pressure side and a feed pump replenishing the pump/motor leakages. In other words, your typical hydrostatic drive system. Sometimes also called Closed Circuit (to not confuse it with automatic control).

Closed loop systems should not contain any counterbalance valves as it is likely to become unstable (

https://www.sunhydraulics.com/tech-...ance-valve-closed-loop-transmission-circuit- 

) and can cause incredibly fast heat rise due to the small volume of oil.

A closed loop hydraulic system will allow the energy from lowering the bridge to be transferred back to the electric motors. This also seems to be implied in the original post so I assume that it is indeed a closed loop hydraulic system.

Assuming a closed loop system, no counterbalance valves, the question is not about hydraulics at all, it's about whether the electric motor will overspeed when lowering the bridge or not.

I am really uncertain about the answer but my bias is towards the electric motors will stay in synch (minus slippage) and act as generators, no drives required.
If you want to make the electric motors over speed you have to effectively increase the frequency of the entire electric grid (pretty much impossible unless the size of the motors corresponds to dozens of nuclear power plants), or just force the motors out of sync (which I think will require a lot more power than what they are rated for).
I hope some electrical guy will join in on this discussion and provide a better answer.

 

[ScottyUK]

Assuming it's a standard induction motor we're talking about, the speed/torque curve above synchronous speed is the same shape as it is below it. This essentially means that an overhauling load will regenerate into the source if the motor is connected to the supply. This also assumes that the torque applied to the regenerating motor doesn't exceed the pull-out torque, i.e. the maximum torque the machine can develop, because if you exceed pull-out torque the machine will run accelerate uncontrollably.

You might want to consider the effects of an AC supply failure during a lowering operation. It's an infrequent but entirely foreseeable scenario which your design should be able to handle gracefully.

 

[Jacc]

Good answer and good point about the AC power supply failure. I think the AC power supply failure situation can be handled by having the spring applied brakes held off by the feed pressure close to the motor low pressure port.

Follow up question; in my head AC motors can usually handle something like twice the torque that comes from their rated power but only for a short moment because of heat generation.

Is this about correct? Are we balancing on a cliff edge here when lowering the bridge or will the motor handle any unforeseen torque peaks with ease?

I know the answer is in your "the pull-out torque, i.e. the maximum torque the machine can develop" -phrase but I don't want to get lost in unknown datasheets this late in the day and also I'm looking for the "generic" or "typically-" answer.

 

[xnuke]

Have you consulted AASHTO's LRFD Movable Highway Bridge Design Specifications? I believe it has some discussion on the motors and drives used for movable bridges.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[LionelHutz]

A drive would have to be regenerative capable or have braking resistors, but it's a complete waste to use a drive on an application where the motors will only run at rated speed.

Directly line connected is the way to go. That way, the motors will regenerate just fine and will run at synchronous speed plus or minus the slip rpm. In other words, if the motor is 1750rpm rated, the slip rpm is 50rpm. It would run at 1750rpm under full load and at 1850rpm under full regen.

The pull-out torque can be up to 250% of the motor rated torque but I would never design something like this to use greater than rated torque as part of its normal operation.

 

[NKBhinge]

All,

I really appreciate everybody's invaluable responses. They are really helpful. Jacc you are bang on. That's exactly what I was trying to say. The mechanical designers are ensuring that the motor runs at 100% torque plus and minus the slip. The motor sync speed is 1500rpm. If we consider 50rpm of slip, the motor will run at 1550 rpm while overhauling. This will be for significant duration. I am guessing for over a minute. Wouldn't this be the generating mode? This application is in Chile and we are not sure if we can just let the motor supply to the grid. Hence we are inclined towards using VFDs and braking resistors and then control the speed of the bridge using VFD which would eliminate the gear motor/swash plate.

Thanks

 

[Jacc]

Generating mode it is indeed. If the "grid" is just a dieselgenerator setup to run the bridge and a few lights then no.
If on the other hand it's part of the full grid that is powered by a large hydropowerplant then it's probably ok to regenerate for indefinite time (hey, it's "free" energy, everyone wins!). The water turbines will just back down a tiny bit automatically to maintain the grid frequency. Same as when you take power out of the grid, then the turbines also have to make a small adjustment to maintain thegrid frequency. This happens all the time automatically. Again, free energy for a minute is not a bad deal as far as I can tell but by all means, check with the people running the grid.
We now have some heavyweight electrical guys in the thread as well so I'm sure they have good answers on this as well.

Also there are a lot of losses in the hydraulic system (and mechanical, wires etc? ) that all help to dissipate the energy from lowering the bridge. Mechanical friction in pump and motor, pressure drop in pipes and hoses, internal leakage in pump and motors, feed pump for closed loop are all losses. If some winch arrangement is being used with multiple falls than that is also a source of losses, bearing frictions in every sheave, bending the wire around the sheaves takes some energy. If 100kW is required by the electric motors to lift the bridge then I would be surprised if you get more than 70kW back during the lowering phase. I'm not sure but if you measured only 50kW going back I would believe that too.
Also, by not allowing full swivel angle on the pump during lowering you can even further reduce the power to be dissipated. Not that you need to but you can.
Also, if the pumps are power regulated (a common feature on hydraulic pumps, all hydraulic) then it is possible (I think) to have different power settings going up or down, then going down can then be set a bit more conservative. Again, you can but I don't think you need to because of all the losses.

As for my previous comment about feed pressure and brakes, that is of course to secure against hose breakage. The electrical controls should be designed to quickly swivel the pump to neutral during power failure as well as set the brake the second later.