Closed Loop Hydraulic System

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.


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 (… ) 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).

Above is a snippet.

As an electrical engineer myself that has worked with smaller hydraulic systems, I would have asked about flow control valves in the bridge lowering side of the circuit. But alas, I did not see the original post and discussion at the time.

1 Like

If the motors are typical 1760 RPM rated motors, The slip is 1800 - 1760 or 40 RPM.
With an overhauling load returning a torque equal to full load torque, the slip will be positive instead of negative and the motor will develop full load torque at 1840 RPM.
As long as the lowering returned torque is equal or less than the raising torque you should have no problem other than a possible maximum speed increase of about 4.5%
You may want to provide some form of automatically inserted restriction in the event that the motors lose power for any reason.