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Load on electric motor

Load on electric motor

(OP)
I have a 1/3 HP electric motor that is coupled with a rotary vane pump and intended to pump gasoline. If I were to switch it's application, and say, couple it to an external gear pump for transferring more viscous fluid such as motor oil, what would I need to know to determine the max. viscosity it could safely handle?

I was looking at products currently offered and found this:

http://redlinepumps.com/

Their pump is capable of handling up to 500 cSt (heavy/cold oil), but the motor runs at only 1140 RPM. The motor I have runs at 1725. My concern would be if you connected it to a pump capable of handling very heavy fluids, it would put too much load on the motor and damage it. I found a couple of pumps that handle lower viscosity (around 90-100 cSt) which is basically good for lighter oils and warm weather only applications. Doing a gear reduction for lower speed and more torque is not an option because of cost, so the motor would essentially be running at only 1725 RPM.

Is it possible to figure out, if this particular motor was used to pump a 500 cSt fluid, how long could it pump before motor damage would occur? What is the max. viscosity? Would it actually damage the motor or just pump very slowly? Not sure what parameters or calculations I would need.

RE: Load on electric motor

Sell what you have.
Buy what you need.
Homebrewing, i.e., misapplying what you have, gets very costly very quickly.

Mike Halloran
Pembroke Pines, FL, USA

RE: Load on electric motor

Most of these problems can be reduced to flow rate times pressure increase. That gives you the net power required, minus efficiency. Throttle back the flow and recirculate if you need to use less power.

RE: Load on electric motor

Mike is right, buy what will do the job for you - give the pump supplier what you need and let them select- easy no hassle solution.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

RE: Load on electric motor

If a new motor is not an option could you not reduce the speed with a vee belt pulley drive which would be pretty low cost. A lot of modern motor oil is very thin even when cold, (although it depends on what you call cold I suppose).

RE: Load on electric motor

The answer lies in the motor specified for the pump - 1140 rpm, 1/3 hp. If you want to drive that pump at 1725 rpm, the power will increase by (at least) the square of the speed increase so 1/3 x (1725/1140)^2 = 0.76 hp.

je suis charlie

RE: Load on electric motor

Cube of speed change.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

RE: Load on electric motor

310toumad,

Looks to me like you need a crash course in pumps here, so apologies if you know this but here goes.

Rotary vane pumps and gear pumps belong to the positive displacement (PD) family of pumps.

This means the pump moves a certain volume for every rotation of the shaft.

So in your case going from 1120 to 1720 your flow will be 1.5 x what it was at the lower speed.

Now the problem is power. This depends on flow, but also the head or pressure the pump generates. For PD pumps this pressure depends wholly on what your piping system needs to be able to flow whatever is coming out of your pump. A short ( few metres) relatively large bore pipe with small head difference will be a low pressure, a longer pipe / tube with a smaller pipe will be a much larger pressure. Until you know that you cannot know whether the motor will become overloaded or not. So try and calculate how much frictional and pressure losses there are in your system to be able to calculate the power the pump needs. Viscosity will impact on this pressure required to flow at your flowrate. Higher viscosity leads to higher pressure for the same flowrate.

Your pump might have an in built pressure relief system which limits the max pressure the pump can produce. However you should aim to be below this limit in normal operation.

If you overload a motor it heats up more than it is designed for. If you overload it for a long time it could actually catch fire or start to smoke. Most decent sized motors have thermal cut outs to prevent this, but that size of motor might not.



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Load on electric motor

(OP)
Thanks for the input. I'm not concerned about losses due to the piping system, the oil will travel out of a 50 gal drum through a bung adapter, thru another short length of hose (probably a few feet), through the pump, then another few feet into a tank. To my knowledge this motor does have thermal overload protection, so it WILL shut off if it gets too hot. Obviously that would still present a problem if the user could only run the pump 10-15 minutes at a time, then have to wait for the motor to cool down. Not very efficient. I believe its only rated for 30 minutes of continuous use anyway.

RE: Load on electric motor

The point is the losses in the system is a key part of the power calculation you said you wanted to do. Be careful you don't pull a vacuum on your inlet line.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Load on electric motor

"Cube of speed change." That makes some assumptions about the characteristic of the load (hence I said "at least the square") but you are probably right.

The key point is - even using the most conservative assumptions, the OP needs a 3/4+ hp motor to spin the pump that fast.

je suis charlie

RE: Load on electric motor

grunt guru.

If the original pump was actually sized (I suspect it's just a stock motor) an operated at the top end of it's capacity and then NOTHING changed and you increased the speed, I would agree with you and artisi that the power would increase by a power of 3, more or less.

1.5 times more flow, frictional resistance proportional to 1.5^2 (ignoring static head) so power increase 1.5 ^3

However if the discharge pressure of his new gear pump is less than the previous maximum, then he might get away with it (especially if the flow output of his new gear pump is less than his old vane pump). sound like this is an oil transfer pump going a few feet from a 50 gal barrel to a storage tank so the discharge head might be very small.

I think the issue will be more can the small bung adaptor and short piece of hose provide a small enough resistance to allow flow through to the pump AND whether the increased pump speed will work with high viscosity oil (the referenced pump spec advises it doesn't -
"The MP3000RSG is a reduced speed transfer pump designed for pumping thick, heavy, or cold oil. It operates at 1140 RPM. Most pumps run at 1800-3600 rpm making them not suitable for heavy or cold oil transfer. ").

Generally with this type of fluid, the slower the pump speed the better.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Load on electric motor

(OP)
I researched several options for pumps, one is similar to the pump shown in that link. 10-12 GPM @ 1440 RPM for a fluid viscosity of 500 cSt. The others are 10-12 GPM @ 1440 RPM for a fluid viscosity of around 100 cSt. Max working discharge pressure is around 100 PSI for all. I figured the pumps capable of handling lower viscosity would be better suited for this motor running at a higher RPM. It would be marketed as more of a light oil/warm weather application.

RE: Load on electric motor

Even at 1440 at those specs I get a power of over 2/3 HP, based on the max pressure.

Add in more flow and higher resistance to that flow, you will need to reduce the max pressure down to about 30 psi to keep it within the power range of the motor.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

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