Reducer to Direct Drive Energy Savings?

[Concentrator]

We are currently operating a 1655GPM slurry pump at 706rpm with 75' of head. A GE AC2000 drive runs the 400hp 460v motor at about 130A (30%). We are looking into removing the reducer and direct-coupling a 150hp motor to run more efficiently (estimated 10% increase just in the motor, not sure about the reducer). This pump runs 24/7 except about 3 weeks out of the year.

Is the payback in energy savings worth purchasing a $7000 motor and possibly a $5000 drive plus the time involved to convert this pump?

Thank you for any help or suggestions!

 

[77JQX]

What's your cost of power in $/kWh? What's the cost to install?What's your desired payback period?

 

[Concentrator]

We pay about $.06/kW. I'm not sure what the cost to install would be. Maybe $2000 or so?

 

[77JQX]

Assuming you get the 10% efficiency gain, I figure that would be about $4,350 per year in electricity, or a payback of just over three years. We'd do it. Of course, this depends on your reducer efficiency - you're going with a gear drive? Which also begs the question of why you're using a vfd now - do you need close control of output, and will you be able to achieve it with the new drive? And you should know - I've never done this type of conversion before, so this is all just theory.

 

[jraef]

Be careful on replacing gear reducers with direct drive. Don't forget that with a gear system, a decrease in speed means an INCREASE in torque. If you take it out and directly couple the same motor controlled by the VFD, you can have the SAME torque from the motor, but the system was designed for MORE torque than that. You may end up having to start with a bigger motor, and thus a bigger VFD, which changes the payback calcs.

That doesn't mean the system NEEDED the extra torque, I'm just pointing out that you may not be aware that the original engineer may have factored that into the design.

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[LionelHutz]

Your electrical input power minus the motor and gearbox losses would give the pump shaft power. So, if you are running the present motor at around 120hp then you have to make sure the motor you select will produce around 120hp.

Remember that the motor hp is proportional to the running speed divided by rated speed ratio. For example, you can't pick a 125hp motor rated for 1200rpm and then expect it to produce 120hp @ 706rpm since it would only produce 73hp.

 

[MikeHalloran]

I'd want to search very thoroughly for documentation about why such an >apparently< oversized motor was selected in the first place, presumably by someone who knew what they were doing.

I.e. does the pump need to run at a much higher power level during startup?
Does the slurry get drastically more viscous at a different temperature or concentration, neither of which have been encountered recently?
Was the pump/motor selected for a different product, which will never be handled again?




Mike Halloran
Pembroke Pines, FL, USA