VFD on hydraulic pump

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Twoskinsoneman

Senior Member
Location
West Virginia, USA NEC: 2020
Occupation
Facility Senior Electrician
This is a very open-ended sort of question. Just hoping to get some knowledge about typical VFD theory with hydraulic pumps.
I was having a chat with another electrician and he was telling me that a recent system he serviced had been setup to try to reduce hydraulic press in a system with VFD freq changes. Basically a large door was moved using hydraulics, as the door reached almost closed the system pressure is cut to 1/3 normal in order to slow the door to keep it from slamming. He said they had issues burning up motors.
His conclusion is that the theory of using a VFD in that situation (to accurately addjust the hydraulic system pressure) is fudamentally flawed.
Is this accurate? I thought it could just be that the motors were not designed for the application.

Any thought?
Thanks
 
This is a very open-ended sort of question. Just hoping to get some knowledge about typical VFD theory with hydraulic pumps.
I was having a chat with another electrician and he was telling me that a recent system he serviced had been setup to try to reduce hydraulic press in a system with VFD freq changes. Basically a large door was moved using hydraulics, as the door reached almost closed the system pressure is cut to 1/3 normal in order to slow the door to keep it from slamming. He said they had issues burning up motors.
His conclusion is that the theory of using a VFD in that situation (to accurately addjust the hydraulic system pressure) is fudamentally flawed.
Is this accurate? I thought it could just be that the motors were not designed for the application.

Any thought?
Thanks

This sounds like a constant torque application. Your hydraulic pump is most likely a positive displacement pump. Varying the speed at the beginning - ramping up along an S curve - and slowing down at the end of the movement - ramp down, same S-curve - should produce a smoothly operating door. You can probbaly trigger the ramps with a pair of limit switches.


It sounds like that the existing motors are burning up because the pressure is choked down on the pump outlet, therefore the pump is greatly overpressuring thus overloading the motor.

The hydraulic pump itself can be equipped with an internal bypass, that reduces the overpressure and even to control the output volume on a constant pressure/speed application. Of course the bypass itself would ned to be triggered somehow and would not necessarily provide the softness of the S-curve ramping, or energy saving.
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
I agree. Typically this type of action is done with a proportionately controlled valve. The valve could theoretically be REPLACED by the VFD, but if it is left in place you could overload the motor. But typically people don't want to eliminate the valve because unlike a VFD it still works when the power fails (for lowering anyway, think hydraulic elevators). So unless someone did a THOROUGH analysis of the controls and not just slapped a VFD on there, it was doomed to fail.
 
I agree. Typically this type of action is done with a proportionately controlled valve. The valve could theoretically be REPLACED by the VFD, but if it is left in place you could overload the motor. But typically people don't want to eliminate the valve because unlike a VFD it still works when the power fails (for lowering anyway, think hydraulic elevators). So unless someone did a THOROUGH analysis of the controls and not just slapped a VFD on there, it was doomed to fail.

Well, I do not see how a PD pump controlled by an ASD with integral P,I,D function is significantly different than a valve that is also subject of control source failure.
 

Twoskinsoneman

Senior Member
Location
West Virginia, USA NEC: 2020
Occupation
Facility Senior Electrician
Thanks for the thoughts.
This door is massive. I've seen it. It is supposed to be many many tons. It only swings laterally like a regular entrance door. It is the craziest thing. It is an enormous nuclear blast resistance hanger door.

This fix was to use a spindle valve to redirect through a regulating orifice to drop the pressure and discard the VFD.
 
Thanks for the thoughts.
This door is massive. I've seen it. It is supposed to be many many tons. It only swings laterally like a regular entrance door. It is the craziest thing. It is an enormous nuclear blast resistance hanger door.

This fix was to use a spindle valve to redirect through a regulating orifice to drop the pressure and discard the VFD.

A poorer and less accurate choice. Sound like the doors at Iron Mountain.
 

iwire

Moderator
Staff member
Location
Massachusetts
Twoskinsoneman,

Consider many of the scissors lifts that we rent use variable speed motors to run the hydraulic pump so that travel and lift speed can vary, it is certainly a proven technology.

Another way to handle it would be a variable displacement hydraulic pump but I bet that would be more expensive than the VFD.
 

Twoskinsoneman

Senior Member
Location
West Virginia, USA NEC: 2020
Occupation
Facility Senior Electrician
Twoskinsoneman,

Consider many of the scissors lifts that we rent use variable speed motors to run the hydraulic pump so that travel and lift speed can vary, it is certainly a proven technology.

Another way to handle it would be a variable displacement hydraulic pump but I bet that would be more expensive than the VFD.

So I guess the theory is sound. Could a pump be burned up by using it with the VFD in the fashion described?
 

GeorgeB

ElectroHydraulics engineer (retired)
Location
Greenville SC
Occupation
Retired
So I guess the theory is sound. Could a pump be burned up by using it with the VFD in the fashion described?
Varying FLOW via VFD is tried and true. A positive displacement pump with a constant pressure load (the load determines the pressure, not the pump in most all cases) is a constant torque load to the motor. Many motors are TEFC ... catch that FC part. Below efficient fan speed, full torque will result in near full current and heat ... that's why we will blower cool for long term operation below (general numbers) half speed.

Ramping to a stop and cutting off ... PROBABLY ok, but there is little cooling below half speed and "none" below 10%. By ANY chance is the drive maintaining "holding" current to keep the motor stopped.

You've many good answers and suggestions.

Concerning the proportional valve ... rarely used on fixed displacement pump systems in simple form ... load the pump externally and don't operate slowly ... maybe ... but to get speed control via a TYPICAL prop valve requires blowing the relief, thus full torque. When we do that ... and we do ... heat load will GENERALLY be in the 30-50% of motor HP range.

I'm not tooting my horn, but your local hydraulic distributor probalby has an application specialist who can help you run the numbers given the requirements.
 

CONTROL FREQ

Member
Location
OHIO
I tried to answer this last night but 'technical difficulties'... There are pressure reducing valves, pressure relief valves, counterbalance valves, and COUNTLESS other valves to "meter in" OR "meter out" hydraulic flow OR pressures. I would tend to agree with your associate that the concept is flawed in its origination. It seems to me, in this particular application you want the motor and pump to run at a constant speed. The only reasons I can see to EVEN bother with a VFD would be:
1) To ramp up to speed avoiding a sudden Amp spike in your system (basically a poor man's soft start set-up)
2) To attempt PF correction in a facility without a capacitor bank at the point of service.

best advice would be: call Vickers or Rexroth or SOMEONE with a rep that will hook you up with a GOOD hydraulic engineer so you can stop BEATING that poor motor and pump to death!

PS, The duty cycle of the motor could make a huge difference as well. If for example you are ramping up and down a "continuous duty" motor, you're asking for trouble. Try an "intermittent duty" motor. Or even something else. BUT in my experience, it's best to control hydraulics with hydraulic controls... pneumatics with pneumatic controls, and let ELECTRICITY be the silent orchestrator of EVERYTHING!:)

Best of luck, hope I helped.
 

hurk27

Senior Member
While I have worked with hydraulics in the past, the first thing I learned is hydraulics is all about volume and pressure.

volume of fluid flow is the control of speed, while the pressure is the work force to do the job at hand.

if you have a cylinder that has lets say a 4" diameter piston, and you feed it with a 1/2" line, at a pressure of 3k psi and produce X amount of tons of work force, it should move fairly fast, now reduce that feed line to a 1/8" and it will move much slower but still have 3k psi and will still exert the force of the 1/2" line so the piston will still close the door.

But the problem of reducing the pump speed is that now you have reduce the available volume and pressure, the problem with this is, if the door requires X amount of force to close what is there to tell the VFD to speed up the pump to apply enough pressure to get the job done?

Seems to me that in this application there would need to be some PID's to tell the VFD where the door is at, and what speed it is closing at, and when it has reached the latching/open stop, of course the amperage of the pump motor and pressure transducers and flow rate sensors could be used?

And in all of this, as was pointed out, rating of the pump motor, cooling, and how far the distance conductor run between the pump motor and VFD's will all have an impact of the longevity of the motors.

Jraef has helped me out in many of the understanding of VFD's and their limits, as this is kind of a new field for me, and one I will be going to school for with my new job, but I do love the design work something like this presents.
 

Twoskinsoneman

Senior Member
Location
West Virginia, USA NEC: 2020
Occupation
Facility Senior Electrician
Or the The Greenbrier in WV, or site R, or the many other hidden places, or could just be a large bank vault door;)

I couldn't say too much more about specifics but it is a large, impressive door. It takes 1 minute 40 seconds to close. Around there. Part of the reason he was there was to see about shortening the time.

Wild see that gigantic steel door swing on two barrel hinges.
 

mei163

New member
Location
shenzhen, China
This is a very open-ended sort of question. Just hoping to get some knowledge about typical VFD theory with hydraulic pumps.
I was having a chat with another electrician and he was telling me that a recent system he serviced had been setup to try to reduce hydraulic press in a system with VFD freq changes. Basically a large door was moved using hydraulics, as the door reached almost closed the system pressure is cut to 1/3 normal in order to slow the door to keep it from slamming. He said they had issues burning up motors.
His conclusion is that the theory of using a VFD in that situation (to accurately addjust the hydraulic system pressure) is fudamentally flawed.
Is this accurate? I thought it could just be that the motors were not designed for the application.

Any thought?
Thanks


Frankly speaking, I think his thought is absolutely wrong.

I don't know the exact application at the moment. But I think there may be some problem of the wrong installation of VFD on the motors.
 
So I guess the theory is sound. Could a pump be burned up by using it with the VFD in the fashion described?

Constant torque loads, especially if the duty cycle and how long will they dwell at low - below 30% - speeds is definetly an issue. External forced air cooled motor should be used, built for the purpose. The external fan is on almost all the time, unless the motor is not expected to run for days on. Larger motors may invest in RTD and control the cooling fans on temperature rise.
 
I tried to answer this last night but 'technical difficulties'... There are pressure reducing valves, pressure relief valves, counterbalance valves, and COUNTLESS other valves to "meter in" OR "meter out" hydraulic flow OR pressures. I would tend to agree with your associate that the concept is flawed in its origination. It seems to me, in this particular application you want the motor and pump to run at a constant speed. The only reasons I can see to EVEN bother with a VFD would be:
1) To ramp up to speed avoiding a sudden Amp spike in your system (basically a poor man's soft start set-up)
2) To attempt PF correction in a facility without a capacitor bank at the point of service.

best advice would be: call Vickers or Rexroth or SOMEONE with a rep that will hook you up with a GOOD hydraulic engineer so you can stop BEATING that poor motor and pump to death!

PS, The duty cycle of the motor could make a huge difference as well. If for example you are ramping up and down a "continuous duty" motor, you're asking for trouble. Try an "intermittent duty" motor. Or even something else. BUT in my experience, it's best to control hydraulics with hydraulic controls... pneumatics with pneumatic controls, and let ELECTRICITY be the silent orchestrator of EVERYTHING!:)

Best of luck, hope I helped.

....as the Lone Ranger rides off to the sunset on his dinosaurus.:grin:
 

CONTROL FREQ

Member
Location
OHIO
....as the Lone Ranger rides off to the sunset on his dinosaurus.:grin:

And some grumpy 'ol fart rides in on a hov-a-round with a powerflex 700!:grin:;)
Just expressing my opinion about the VFD, all funning aside, I LOVE technological advances... I just think some engineers are a bit overzealous and tend to "OVER engineer", for crying out loud--- its a REALLY big door. Not a scissor lift. I'm just saying... wasn't trying to step on anyone's toes. Sorry if I did. Been a LONG time since I was the FNG!:roll:
 

GeorgeB

ElectroHydraulics engineer (retired)
Location
Greenville SC
Occupation
Retired
CF, you obviously have extensive experience here, and there is very little you say with which I disagree. What I'll add is that there is an active movement to maximize energy efficiency at almost any cost. If I'm going to do a system design, there are numerous options which depend on the system requirements.

But, in general ... for a multiple "axis" system ...

fixed speed fixed displacement ... simplest, least efficient
fixed speed, variable (pressure compensated) displacement, next efficient, sometimes simpler system
fixed speed, controlled displacement, more efficient
variable speed, fixed displacement, most efficient

The HVAC guys, with SEERs of 14 and up, usually have gone the variable speed, fixed displacement route. When we do a centralized system, we often will have some energy storage (accumulators) with pressure feedback to a PLC to multiple HPUs (different pressures to minimize reducing valve use)

In industrial hydraulic pumps commonly available, I have a hard time beating an internal gear pump for efficiency. A fixed piston unit is darn good too, but there are fewer available. If I need over (ballpark) 200 bar or 3000 psi, the piston units come into significant consideration. Your recommendation of Vickers (Eaton) and Rexroth (Bosch) is good. While my own experience is heaviest in those, I'd add Parker to the list; they are a major line as well.

I regret that some who profess great (electro) hydraulic understanding do not show it. But that is not unique to any field ...
 
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