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Bigger motor than VFD for hydraulic application ok?

JasonPAtkins

Hot Rolled
Joined
Sep 30, 2010
Location
Guinea-Bissau, West Africa
Hey all, I have just enough experience to be dangerous in two areas that are coinciding in today's work.

I posted recently about the compressed adobe block making machine I have over here in West Africa. The Yanmar L70V6 diesel engine that runs it was abused by my (untrained) guys, and has seized. That motor has given me so much trouble over the years, and I now have good solar electricity, that I'm done with it and going to convert the machine to electric. I'll send a replacement Yanmar over eventually so it can still be mobile if the need arises.

Unfortunately, not having planned for this conversion right now, I'm digging through my spare parts bins to put something together, doing the best I can with what I have. Hence my current question.

The Yanmar was 6.5hp (4.9kw) at 3600rpm. The motor drives a two-stage hydraulic pump.

In my spares bin, I have Nord motors of both 2hp and 3hp. I would prefer to use the 3hp, because I'm not certain 2hp of electric motor is equivalent to 6.5hp of diesel. I know you need less electric, I'm not sure exactly how much less for hydraulics.

My current limitation, however, is that my biggest single phase (which is all I have available at that site currently) VFD is 2hp. (I don't have any single phase motors, or I'd use that and skip the VFD.) I need to fab and machine a mount for the motor, so I'd rather use the 3hp motor so that later when my power situation is upgraded to 3 phase, I can switch to a 3hp 3 phase VFD without also having to make a new motor mount and swap the motor.

I did some cursory searching and found people asking about using a higher rated motor than VFD (using it at the VFD rating) and there were some comments about surges causing problems. I'm wondering though, for driving a hydraulic pump where there's no braking, and where the speed ramp up and down can be slow, if that's still a problem? Would it be safe to use a 2hp VFD to drive a 3hp motor if the motor is powering a hydraulic pump, or is that likely to cause problems? (A further note, the hydraulic pump wants to spin fast, so whichever motor I use, I'll be driving a 1740rpm motor as fast as I can. The Nord motors are rated up close to 3000rpm. The pump wants 3600, but I think the difference might change my block cycle time from 15 seconds to 17 or something, which is totally acceptable. The pump is also a 3000psi pump and the most the machine actually needs to operate is 2500psi.)
 
You're way off the deep end both in terms of HP and RPM.

A 6.5HP pump needs a 6.5HP motor if you want the full flow out of it. If you run it at 2HP, I doubt it will perform well. You'd have to use some sort of bypass or unloader to compensate.

VFDs are all about amps. It it only puts out enough amps for 2HP, that's your ceiling regardless of motor size.

A 4 pole motor at 2 pole speed? On a little motor you might get away with it but.....


The answer in this case is try it...but I doubt you'll like it.
 
You're way off the deep end both in terms of HP and RPM.

A 6.5HP pump needs a 6.5HP motor if you want the full flow out of it. If you run it at 2HP, I doubt it will perform well. You'd have to use some sort of bypass or unloader to compensate.

VFDs are all about amps. It it only puts out enough amps for 2HP, that's your ceiling regardless of motor size.

A 4 pole motor at 2 pole speed? On a little motor you might get away with it but.....


The answer in this case is try it...but I doubt you'll like it.

As Doug pointed out, I think it's pretty well established that an electric HP > than a gas HP.

Also, the motor is inverter duty, and lots of those have a higher max rpm than the normal. I have a 3hp Baldor that's 1740rpm and lists right on the nameplate that it can go to 3000rpm if driven by a VFD.
 
As Doug pointed out, I think it's pretty well established that an electric HP > than a gas HP.

Also, the motor is inverter duty, and lots of those have a higher max rpm than the normal. I have a 3hp Baldor that's 1740rpm and lists right on the nameplate that it can go to 3000rpm if driven by a VFD.

I would NOT overspeed a hydraulic pump.
 
Digger reread the post#1. the diesel engine and pump run at 3600 rpm. He is overspeeding the electric motor NOT the pump. In fact he is running the pump below speed.
 
Basically kw is kw. There is a slight difference between electric and fuel but it's insignificant for what we're doing here. 0.7457kw per Hp is the magic number. Your 2Hp is 1.5kw, the 3 is 2.2kw and your Yammer is 4.9kw. No matter which motor you use the VFD is the limiting factor at about 1.5kw. Quite a difference. If you restrict the FLOW of the pump at the same pressure the power requirements go down. Counter intuitive isn't it. Every centrifugal pump has a performance chart and that much restriction may cause it to slip and not reach the required pressure at all. My opinion for what it's worth, it won't work. If that's all you got, give it a try, you never how. Bob
 
I do not think it will work unless you gear the pump down. If you maintained amps and torque, then doubling the speed/frequerncy of a motor might double the hp, but as the VFD is rated in hp, I doubt this will be the case.

No, I don't think that anyone thinks a 2hp motor on a 2hp VFD is putting out 4hp just because it's spinning at 2x the motor nameplate hp.

What I'm hoping is that because you only need half the hp from an electric motor as gas to drive a hydraulic pump (which i thought was fairly common knowledge) (VFD or not, btw) that my 2hp is more like 4hp on gas.

But the point of the original question was, can I drive the 3hp motor on a 2hp VFD (at 2hp) so that later i can upgrade to a 3hp system just by swapping the VFD.
 
All the foolishness with gas vs electric HP is a reflection of game-playing done by OEM's of low-spec equipment. I promise if you buy a 5000HP pump, the pump maker will tell you it needs a 5000HP driver and they don't care if it's electric or gas or diesel. Power is power.

If a 6.5HP combustion engine is CONTINUOUSLY rated at 6.5HP, it will be applied as a 6.5HP driver same as an electric motor. But the game-playing is they nameplate it as a 6.5HP intermittent duty.

As for the points made on centrifugal pumps...they are valid. But I would strongly suspect the pump we are discussing here is some form of positive displacement pump.
 
But the point of the original question was, can I drive the 3hp motor on a 2hp VFD (at 2hp) so that later i can upgrade to a 3hp system just by swapping the VFD.

in this case i would advice you to run the 3hp motor. it may pull more amps for a given torque but it will probably be a higher efficiency motor than the 2hp will be.

now, regarding getting more than 2hp out of the vfd:

run the supply voltage as high as you feel safe doing so. 280 vac will produce 400 volts on the dc bus. sounds high, but that's probably the voltage the brake resistor is activated at.

then run the motor rpm as high as the amps limit will let you go on the vfd.

in order to get the most torque for the least amps, you need to do some trial and error with the volts per hz produced by the vfd. usually in the case of a bigger motor than the vfd, you drop the volts per hz. but since you are operating the motor on the field weakening side of the curve you may need as high a voltage as you can get.

in the event of a complete miss-match, to get 2hp out of a 3hp motor at 3600 rpm ( when the nominal voltage is 240v at 1750 rpm and you only have a 240v vfd)
you need to re-wire the motor for 138 volts delta, and the motor will be happy with the 260vac your vfd can deliver at 120hz.


basically I'm saying you can't get 2 hp out of a 1750 rpm 3 phase 240v motor with a 240v vfd at 3600 rpm. the motor won't have the volts it needs. but you might be able to make it if you run the vfd from 280volts.

you may be able to get 2hp out of a 3hp motor at nominal volts per hz from a 240v vfd, as high as 2500 rpm. but somewhere above this rpm the amps will start rising again and you'll hit the amp limit of your vfd.


some 2hp vfds can deliver 10 amps, others max out at like 7 continuous.
 
All the foolishness with gas vs electric HP is a reflection of game-playing done by OEM's of low-spec equipment. I promise if you buy a 5000HP pump, the pump maker will tell you it needs a 5000HP driver and they don't care if it's electric or gas or diesel. Power is power.

If a 6.5HP combustion engine is CONTINUOUSLY rated at 6.5HP, it will be applied as a 6.5HP driver same as an electric motor. But the game-playing is they nameplate it as a 6.5HP intermittent duty.

As for the points made on centrifugal pumps...they are valid. But I would strongly suspect the pump we are discussing here is some form of positive displacement pump.

Someone should probably tell that to all of the equipment manufacturers. For example, a Stone 9 cu ft cement mixer (the same model) is available as a 8hp Honda or 1.5hp electric. Do you think the electric mixes 4x slower? Is that just marketing BS and the electric option should have a 8hp Baldor on it to be equivalent?
 
My understanding is small engine makers calculate the theoretical mathematical displacement of the cylinders. This assumes perfect flow with no friction losses on the intake or exhaust sides. Then they multiply by a magic number of hp per cubic inch times rpm to get the supposed hp. Of course then they limit top end rpm below that number.
Similar to how car makers used to disconnect the water pump, alternator, oil pumps etc, fan etc before measuring engine horsepower. Now a days the engine has to be powering all its normal internal acceriseres. I do not think power steering has to be used?
Farm tractors have actual standardized tests that must be DONE BY INDEPENDENT OUTSIDE LABs to be rated as a tractor that bank loans can be written on.
Bill D
 
A long term solution may be a solar array DC voltage to 3 phase AC inverter.

MPP solar (A Taiwan company) has four inverters available. The smaller , 2.2KW inverter, works off of a maximum 420 volt DC supply. The larger, 7, 11, and 15 kw inverters, work off of a maximum 720 DC supply voltage.

https://www.mppsolar.com/v3/catalogs/SP PUMP INV.pdf

The 2.2 KW model is priced at $252
(ls) Single Phase Or 3 Phase 220v 230v 240v Mppt Solar Pump Inverter 2.2kw - Buy Mpp Solar,Single Phase Or 3 Phase,Solar Water Pump Inverter Product on Alibaba.com

The 7 kw inverter is priced at $320.
Sp 7.5kw 3 Phase (380vac ~ 400vac) Solar Water Pump Inverter - Buy Mppt Solar Pump Inverter,3 Phase,Solar Water Pump Inverter Product on Alibaba.com

These are sine wave inverters. They can be used on motors, such as used on submersible well pumps, that are not designed for VFD inverter service.

The inverters need to be purchased directly from MPP through the Alibaba website. There are copies being produced in China that do not have the quality control that MPP has. Purchasing directly avoids the risk of receiving a defective inverter and avoids the retailer add on costs. They have a customer support office that can advise on suitable inverter applications

I purchased my 8KW off grid inverter from MPP.
 
Someone should probably tell that to all of the equipment manufacturers. For example, a Stone 9 cu ft cement mixer (the same model) is available as a 8hp Honda or 1.5hp electric. Do you think the electric mixes 4x slower? Is that just marketing BS and the electric option should have a 8hp Baldor on it to be equivalent?

We've been over this heaps of times.

You need to consider the rpm of the IC engine - if it's not running at the design max HP/torque it delivers FAR less than nameplate.

On another forum we had a similar argument with a guy who wanted to build a diesel-electric drive for a boat. It took a LOT to convince him that running his Kubota engines at 1500 rpm was only going to deliver maybe 12HP despite them being rated as delivering 30HP. Which they would do - at 3000 rpm continuous rating.

So I find it easily believable that you might have a Honda 8HP engine required to substitute for a 1.5HP electric motor, depending on the rpm & gearing involved.

Anyway you've got your advice here - suck it & see. I think you're pushing shit uphill myself, but - go for it.

PDW
 
A long term solution may be a solar array DC voltage to 3 phase AC inverter.

MPP solar (A Taiwan company) has four inverters available. The smaller , 2.2KW inverter, works off of a maximum 420 volt DC supply. The larger, 7, 11, and 15 kw inverters, work off of a maximum 720 DC supply voltage.

https://www.mppsolar.com/v3/catalogs/SP PUMP INV.pdf

The 2.2 KW model is priced at $252
(ls) Single Phase Or 3 Phase 220v 230v 240v Mppt Solar Pump Inverter 2.2kw - Buy Mpp Solar,Single Phase Or 3 Phase,Solar Water Pump Inverter Product on Alibaba.com

The 7 kw inverter is priced at $320.
Sp 7.5kw 3 Phase (380vac ~ 400vac) Solar Water Pump Inverter - Buy Mppt Solar Pump Inverter,3 Phase,Solar Water Pump Inverter Product on Alibaba.com

These are sine wave inverters. They can be used on motors, such as used on submersible well pumps, that are not designed for VFD inverter service.

The inverters need to be purchased directly from MPP through the Alibaba website. There are copies being produced in China that do not have the quality control that MPP has. Purchasing directly avoids the risk of receiving a defective inverter and avoids the retailer add on costs. They have a customer support office that can advise on suitable inverter applications

I purchased my 8KW off grid inverter from MPP.

Thanks for the recommendation! My system is 3 x Outback Radian 7kw inverters. Together they'll make three phase, but as we're in the building phase I just have one installed, and am waiting to build my shop to set up the whole system (which will be the sole power source for the fab / machine shop and my house). I'm trying to avoid having to set up the other two 100 pound inverters until I can hang them in their real spot. If I have to though, to use the bigger 3ph VFD, I will.
 
Well, the truth is a pump KNOWS what is driving it. They're smart devices. If it senses an electric motor is connected to it, it will only require 1.5HP. But when it finds out that someone connected a gas engine to it...boy, does it get pissed. It refuses to do its job unless you up the HP to 8.

That's because if you read any engineering book there is clearly a section on electric horsepower and gas horsepower. While it's true there are no differences in gender, race, smarts, or looks....there are several different kinds of horsepower.
 








 
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