OT_adding drive to pump motor

katratzi · Mar 22, 2007

A month or so ago I posted a topic about using 460volt 60hz
motors on a 400volt 50hz supply. The application is a hydraulic pump that applies pressure via a beam with cylinders that extend the beam to squeeze product together. The consensus
was that since the motors were running at 50 hz, the motor rpm would not be at rated value, and that max power output could not be reached. So, the process might suffer.

My thought was that since available connected voltage
was actually only 395 phase-to-phase, is that under load the motor would draw more current at max load than it was rated at.
Well, at high pressure settings on the hydraulic system, the pump/motor makes an ungodly noise and about 25% of the time trips the breaker that feeds its starter/overload. The breaker is a 15 amp rated unit, the motor nameplate says 9.2 amps max at 460volt.

My question (finally!

) is this: Would putting a drive on the motor and programming it to output 460v 60 hz supply help this situation any? At lighter loads, the unit presses ok, but when more pressure/larger loads of product to be pressed are needed, the pump/motor seems to be straining.

rsdoran · Mar 22, 2007

I had to think about this for a bit. A 480vac @ 60Hz will run 1800rpm (- slip). With 400vac 50Hz it will run 1500rpm (- slip).

The issue is a combo of things. There is a relationship with voltage, speed, HP, and torque. http://www.patchn.com/motorformula.htm

If your motor is rated say for 5HP at 480vac 50Hz then a 400v 50Hz supply will never allow it to obtain full load capabilities of 5 HP. In some cases, where speed is the issue that maintains the aspect of the load then a VFD may help

Leadfoot · Mar 22, 2007

If you will be running the motor at a reduced speed, you probably can pull this off.

I have seen 410 VAC L-L run a 460/60 motor. Your full load amps at rated speed will cause a larger ripple on the DC bus. You might be able to adjust the ripple detection to compensate.

BTW, the input freq and the out put freq ratings have no real relevance. Watts in to watts out is what you watch. Remember some VFD's can go to 400 hertz with just a program input.

CharlesM · Mar 22, 2007

Funny we were just talking about this at work. We just quoted a machine for Europe. Im not sure what the voltage would be but I know it would be 50 hertz. If the motor runs slow then we get less output from the hyd pump. The pump will adjust to the output required for the system but this would increase the HP required to turn the pump. So we think we would be ok with a few more HP.

Sparkz · Mar 23, 2007

A 460V-60Hz motor will have approx 13% less power running on a 400V-50Hz net. Luckily for us Europeans, the same calculation works in our advantage when exporting a 400V-50Hz motor to a 460V-60Hz country. ;-)

Since it's a hydraulic pump/motor setup, flow is reduced by approx 16% (1500/1800). So basically, all hydraulic movements are just a bit slower. Apparently the pump/motor assembly has been calculated too critically when delivering max pressure; an easy way of overcoming the problem could be done by lowering the pressure setpoint (on the bypass valve?), but this might have serious impact on the process (quality).

Sorry, but you can not have a drive output 460V if it's fed with 400V, no matter what frequency.

There might be a solution IF the motor is a star/delta type AND IF the motor is now connected in star. Then you could use a drive being fed 400V, having the base frequency set to 60Hz and the base voltage set to 460/1.73=266V. However, this VFD must be capable of delivering the rated 9.2*1.73=15.9Amps at 400V!

Or you could install a 400/460V step up transformer and a regular 460V VFD. But frankly, I just don't like the combination of VFDs and hydraulic pumps...

DickDV · Mar 23, 2007

The problem here is that we need to think in terms of torque and amps rather than hp or kw.

Any induction motor develops torque and draws load current as a function of the volts/per hz ratio. In this case we have a 460V/60hz motor. It's rated torque will be available from 60hz down to at least 20hz as long as the volts/hz ratio is held constant. In this case we are limited to 400V (since the drive cannot increase the supply voltage) which will limit the drive output to about 50hz. The available torque will not be reduced and the current drawn at those torques will be at normal levels as well.

Since the motor is now operating slower with the same available torque, the calculated hp will be lower by the proportion of 5/6 so, with a positive displacement pump, the volume will be reduced by 5/6 but the available pressure should be the same since the available torque is not reduced.

From the original data presented, it seems that the motor is drawing high amps but it is not clear at what speed this is occurring. The motor data in the drive should be entered as given on the motor nameplate but the speed limit should be 50hz. If the overcurrent occurs at 50hz, there is something wrong with the pump system (requiring too much torque). If the overcurrent is occurring at 60hz, the motor is starving for voltage and the max speed must be reduced to 50hz.

milldrone · Mar 23, 2007

What type of hyd. pump?

katratzi said:
So, the process might suffer.

Well, at high pressure settings on the hydraulic system, the pump/motor makes an ungodly noise

pump/motor seems to be straining.

If the pump is a pressure compensated piston pump, there may be a stroke limiter that can be adjusted to destroke the pump thus lowering the hp input. This may not work if the process speed will suffer excessively.

Sparkz · Mar 23, 2007

Dickdv said:
...Since the motor is now operating slower with the same available torque, the calculated hp will be lower by the proportion of 5/6 so, with a positive displacement pump, the volume will be reduced by 5/6 but the available pressure should be the same since the available torque is not reduced...

Exactly!

Essentially, the product 'flow x pressure' is a measurement for the required motor's kW, so if:
1. Flow drops approx 15%, because net frequency drops from 60Hz to 50Hz
2. Motor power also drops approx 15%, because the net voltage drops from 460V to 400V
So on both sides of the equation just one parameter drops 15%! This can only mean: Pressure remains unchanged! Pressure is allowed to raise to the same level as when the pump/motor would be running on a 460V-60Hz net. Like I mentioned before, everything is just going to be slower, that's it.

Or also meaning: You would have experienced the same problems running this pump/motor on a 460V-60Hz net. The motor was not correctly sized in the first place, OR the bypass valve setting is not correct.

It's not an electrical problem, it's purely hydraulic.

rsdoran · Mar 23, 2007

Sparkz said:
Exactly!

Essentially, the product 'flow x pressure' is a measurement for the required motor's kW, so if:
1. Flow drops approx 15%, because net frequency drops from 60Hz to 50Hz
2. Motor power also drops approx 15%, because the net voltage drops from 460V to 400V
So on both sides of the equation just one parameter drops 15%! This can only mean: Pressure remains unchanged! Pressure is allowed to raise to the same level as when the pump/motor would be running on a 460V-60Hz net. Like I mentioned before, everything is just going to be slower, that's it.

Or also meaning: You would have experienced the same problems running this pump/motor on a 460V-60Hz net. The motor was not correctly sized in the first place, OR the bypass valve setting is not correct.

It's not an electrical problem, it's purely hydraulic.[/QUOTE]

I read this but not sure how you came about that conclusion. In most cases if you have a certain sized pump then you need a certain sized motor to accomodate it. The details of the system have not been stipulated but I would bet your paycheck that they match.

The issue is the motor is rated to run at 60Hz 480vac, since the power supply does not provide that then the speed and HP will be decreased (derated), whatever term you want to use.

The simple solution would be to replace the motor with an equivalent 50Hz 400vac unit. It all depends on the size of the motor when replacing, if small i.e. 5HP or so then may want to step up or rewind to larger HP capability just in case.

The best thing is to learn what you load torque requirements are; then make sure the motor is capable of doing that on your power.

Sparkz · Mar 24, 2007

rsdoran said:
I read this but not sure how you came about that conclusion.

Maybe I'm jumping to conclusions, but due to the rather small motor I'm assuming it's a standard hydraulic gear pump and not a (pressure compensated or stroke adjustable) piston pump.

The flow is directly proportional to the motor's RPM; Since this is a constant, the max allowed pressure determines the required motor's kW.

Since the motor's power drops approx at the same ratio as the motor's speed (both about 15%), the max pressure is allowed to raise to the same level as if the motor was running at 60Hz.

rsdoran said:
In most cases if you have a certain sized pump then you need a certain sized motor to accomodate it. The details of the system have not been stipulated but I would bet your paycheck that they match.

Well yes, this should be the case! That's why I'm thinking the pressure is allowed to raise beyond its max level.

Sparkz · Mar 24, 2007

Let me rephrase a few sentences from above:
The flow is directly proportional to the motor's RPM; Since this is a constant (1500rpm at 50Hz or 1800rpm at 60Hz), the max allowed pressure determines the required motor's kW.

Because flow drops due to the drop in net frequency, the motor power is allowed to drop proportionally to obtain the same pressure level. Since the volt/herz ratio remains almost the same, motor power drops at the same ratio as the flow. Therefore the max allowed pressure level at this lower flow (50Hz, so 15% less rpm with 15% less motor power) is the same at higher flow (60Hz, with full motor power).

Tom Jenkins · Mar 24, 2007

Back to Basics

For a positive displacement pump:

HP = (gpm x psi x SG)/(1715 x EFF)

for 10 gpm @ 150 psi and oil @ SG (Specific Gravity) = 0.8 and EFF (Efficiency) = 0.85 (85%)

HP60 = (10 x 1500 x 0.8)/(1715 x 0.85) = 8.2 hp

If you cut the speed to 83% (50/60) the pump flow will be 8.3 gpm

HP50 = (8.3 x 1500 x 0.8)/(1715 x 0.85) = 6.8 bhp

Therefore your VFD and motor should be OK in terms of horsepower. As others have said, you can't get more voltage out than you put in, so that should be OK.

However, Torque = HP * 5252/rpm for torque in ft-lb

If the motor is 1750 rpm @ 60 Hz it will be 1460 rpm @ 50 Hz

T60 = 8.2 x 5252 / 1750 = 24.6 ft-lb
T50 = 6.8 x 5252 / 1460 = 24.5 ft-lb

The difference is round off error, and not significant.

As others have stated, motor torque is essentially a direct function of current draw. Therefore, for any positive displacement device the load on a VFD is constant torque at constant pressure.

If your motor was OK before, you should still be OK.

Changing the VFD settings probably won't help.

It is possible that the motor was slightly overloaded before, and operating into the service factor on occasion. You can get away with this on a constant speed motor where a short term overload won't trip the overload heaters. A VFD, however, can be set to limit output current and not allow operation into the service factor. It can also be set to operate at higher current, up to the drive output limit. This lets your run into service factor again. Note that cooling is reduced at 50 Hz, so your motor might run hot operating in this mode and damage itself.

Are you sure that nothing else has changed, such as product consistency or such, that is requiring higher pressure? That would account for your problem. Are you sure that the noise you hear isn't the relief valve opening?

My guess is that your problem is mechanical and not electrical.

DickDV · Mar 24, 2007

There is, of course, another option if you absolutely need to run with the higher flow rate and corresponding motor speed at 60hz.

Place a 400/480V 50Hz step-up transformer in front of the drive. This will give the drive the voltage it needs to permit the output voltage and Hz to rise to 480V at 60hz.

Now you have your motor running exactly as it would have in the US and, it seems to me, if it operated properly here it should operate properly there. As I recall, the drive is rated for 480V and the input frequency doesn't make that much difference.

Sparkz · Mar 25, 2007

Thanks for your post, Tom! Now I finally have some basic theory to back up my gut feeling.

Back to the subject: Katratzi mentioned 'cylinders being extended to squeeze product'. So if I understand correctly, increasing flow is not going to be helpful at all! Even now most of the oil flow is being dumped over the relief valve. It's pressure that is insufficient. Katratzi could try cylinders with a larger diameter. Or maybe using a gear pump for this application wasn't such a good idea...

katratzi · Mar 29, 2007

Thanks for all replies. Actually, I think the problem is not so
much getting enough flow or pressure, but we just need to get the
breaker to stop tripping when the pressure is set higher via an
adjustable valve. As long as the setting is low, the breaker
doesn't trip. There may indeed be a mechanical problem or design issue, but I need to determine the cause of the overcurrent condition. Unfortunantly, my knowledge of hydraulics and pumps is not what it should be. I appreciate the information shared here on this site.

Is it a fair statement to say that under the same load, the current used by the motor on a 400volt supply would be greater than the current draw on a 480 volt supply?

OT_adding drive to pump motor

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