# motor speed vs gearbox question



## nolabama (Oct 3, 2007)

Got to the Godforsaken bridge today, emergency call out. Stuck open. Drives were enabled, but no hz output to motor. 0.04 hz was displayed. Voltage was present in peckerhead. Checked voltage prior to realizing hz was what it was. Not thinking frequency I assumed a failed motor and went to hydrolic backup motor. Hydraulic motor spun against gearbox a few revolutions and stopped. At this point I am thinking gearbox, so I uncouple the motors. Hydrolic motor spins when turned on, but the electric motor does not. It may have moved a quarter of an inch when power was applied to it. We then hook a boat to the bridge and attempt to pull it shut. The boat is not powerful enough to make the last movement. I try the electric motor and it ramped up to a preset -4 hz which was correct. This enabled the bridge to shut. We then open and shut the bridge with electric motors from control panel like normal. No problems. I was wondering what would have prevented that drive from spining the motor uncoupled. Powerflex 700. Also for info I broke tleads and motor ohmed samed phase to phase nothing ohmed to ground. Motor has a motor brake but has been mechanicly disabled. I think that was one of the first things I did to this bridge was turn the brakes off. I hate a motor brake.

I believe the motor encountered an above normal resistance in the geartrain and VFD protected itself. I believe we cleared this when the boat spun the gearbox. This does not explain the motor not spinning when uncoupled.


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## John Valdes (May 17, 2007)

I am having a hard time really understanding what happened. When you say hydraulic motor, what exactly do you mean. Any hydraulic system requires a motor of some sort to pump the liquid through the system. So tell us what you mean by hydraulic motor vs electric motor as they should be the same unless you have a gas powered porta power setup? (gas /diesel over hydraulic)?

From what little I understand, the bridge was jammed? If this correct, the VFD could not produce enough power E/Hz to turn the gearbox and just sat there. You said 4 hz was normal? 4 hz is not going to move anything unless its a torque control and can produce full torque at zero speed. The addition of a gearbox increases the torque, but at 4 hz, how much could we expect? So what is the expected motor hz at bridge travel speed? 

And yes, the electric motor being driven by the VFD can stall at this speed if it is a standard volts per hertz drive. Even if connected to a gear reducer. A torque control would try and spin at 4 hz until it tore the gearbox out of its mount.

Please explain the operation sequence and scope.


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## wildleg (Apr 12, 2009)

pics !! we need pics !

I'm guessing this is a rotary bridge ?

the fact that the hydraulic motor jammed tells me there was a mechanical problem ? maybe a narwhal was resting against the bridge and you scared it away ?


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## BBQ (Nov 16, 2010)

I have been involved with some messed up stuff but I have never tried to close a bridge by towing it with a boat.

My hats off to you, you are the man.


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## nolabama (Oct 3, 2007)

Hydraulic motor is part of the geartrain. We have two motors plumbed into the gearbox via a chain drive. One is electric, and one is hydraulic. The bridge was jammed. The hydraulic motor did not move the geartrain. I call it a geartrain instead of a drivetrain cause its really huge gears. Oh and here is a pic for ya. WWII era boats







.
So called again. Seems that a surge protector has been damaged by most prolly lightning the other day. Intermittent control power loss on three circuits. That still in my mind does not explain why the motor would not ramp up uncoupled. I mean the VFD was enabled and delivering voltage, just no speed. Oh and four hertz does move the thing just not very fast.
Tourque is multiplied serveral times both by a gearbox that is not pictured and this gear. Pic is old and gearbox is removed for repairs.







.

One the other side of that electric motor attached to that same chain is a hydraulic motor. Yes, just for conversation the hydraulic power plant has an electric motor drive that pumps the pressure up. It also is powered by a VFD and was functioning fine. Expected motor hertz is four for creep, round twelve for slow, and round twenty for not so slow. Fast is disabled due to ongoing mechanical damage to rack circle that does not need to be worsend by tourqe being transmited at those speeds.


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## JRaef (Mar 23, 2009)

I assume it's in Open Loop (Sensorless) Vector mode right? Even if it is, 4Hz is pretty slow and the slower you run, the harder it is for the drive to maintain the motor model it needs to calculate the vectors properly. It's not impossible, but there are risks. One risk is that it the load does jam and the drive goes into current limit, it may be difficult for it to come out of it because you are working with very small amounts of power at that speed. In other words it may not have realized that the motor was disconnected, it may have looked virtually the same because of the margin of error in the motor model at that low amount of power.

If it happens again, I would seriously consider going to Closed Loop Vector by adding an encoder feedback. That will guarantee that the drive always knows exactly where the motor shaft is so it knows exactly what to do to make it move.


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## nolabama (Oct 3, 2007)

It is sensorless vector. I have an encoder/resolver in place. Wonder if I can convince engineers to do this.


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## jmsmith (Sep 10, 2011)

nolabama said:


> It is sensorless vector. I have an encoder/resolver in place. Wonder if I can convince engineers to do this.


Hey Nola--
The info you got from JR is spot- on. The systems that we installed for BNSF when I worked for another outfit incorporated vector control. When messing around trying to swing that much weight, going into creep mode too soon can be just as rough on the equipment as driving it too fast... Of course I figure you know that already! Hopefully they have a rebuild for the gearing in your future! You can only build welds up for so long...... Have a good day!
-Jim


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## JRaef (Mar 23, 2009)

nolabama said:


> It is sensorless vector. I have an encoder/resolver in place. Wonder if I can convince engineers to do this.


If you already have the encoder in place, why not?


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## jmsmith (Sep 10, 2011)

JRaef said:


> If you already have the encoder in place, why not?


You need to see some of these railroad bridge setups..... Makes you wonder about some of the engineers! :blink: :lol:


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## JRaef (Mar 23, 2009)

jmsmith said:


> You need to see some of these railroad bridge setups..... Makes you wonder about some of the engineers! :blink: :lol:


I've done 2 bascule bridges and a double swing arm, both for roadways, no rail bridges. The swing arm was done hydraulicly, but we used an encoder and a motion controller to make sure it worked smoothly and reliably. Maybe because they were roadways the engineers were very very particular about doing everything right. They don't want a school bus carreening off of the end of a swing arm bridge in the fog I guess...


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## Awg-Dawg (Jan 23, 2007)

BBQ said:


> I have been involved with some messed up stuff but I have never tried to close a bridge by towing it with a boat.


I was thinking the same thing Porkchop.

If I were in the same position and they asked me to fix it.......


This is all they would see


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## jmsmith (Sep 10, 2011)

JRaef said:


> I've done 2 bascule bridges and a double swing arm, both for roadways, no rail bridges. The swing arm was done hydraulicly, but we used an encoder and a motion controller to make sure it worked smoothly and reliably. Maybe because they were roadways the engineers were very very particular about doing everything right. They don't want a school bus carreening off of the end of a swing arm bridge in the fog I guess...


Railways for the most part are very particular about these, also. Especially when a barge knocks one just inches off-center. It is no easy task boating one back into position. But even down-south here, it has been a challenge just to get either railroads or transportation offices to even upgrade from the older relay logic panels!
Have a good day there, sir.


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