more power from motor

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junkhound

junkhound

Senior Member
Location
Renton, WA
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EE, power electronics specialty
need a 20 HP motor

have a 184 frame 1750 RPM 5 HP (240V, 60Hz, 26A) single phase motor.

Can I run it at 3600 rpm (or higher) with an inverter and use an old furnace fan to 'supercool' the frame to 'push' the motor to produce 20 HP?

Have been told this can be done.
 
junkhound said:
need a 20 HP motor

have a 184 frame 1750 RPM 5 HP (240V, 60Hz, 26A) single phase motor.

Can I run it at 3600 rpm (or higher) with an inverter and use an old furnace fan to 'supercool' the frame to 'push' the motor to produce 20 HP?

Have been told this can be done.
Click to expand...

Welcome to the forum.

Hopefully you are joking. . .
As an EE on the 2009 Code cycle, shouldn't you already know the answer?
 
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junkhound said:
need a 20 HP motor

have a 184 frame 1750 RPM 5 HP (240V, 60Hz, 26A) single phase motor.

Can I run it at 3600 rpm (or higher) with an inverter and use an old furnace fan to 'supercool' the frame to 'push' the motor to produce 20 HP?

Have been told this can be done.
Click to expand...

My first question is "What the...." followed quickly by "Who the...".

Then the thought is why go to all the expense and trouble even if it was/is possible.
 
Maybe if you use a liquid nitrogen cooling system, but there still might be mechanical force issues running the motor at twice its rated speed.
 
The 5 HP most likely has capacitors that would not like the frequency change required to hit 3600 rpm.
You would be increasing the current considerably requiring a significant change in wiring from the Service Entrance on down. The POCO may not be able or willing to supply 1ph power for that size of a motor load. IMO a furnace blower would not super cool enough.
 
To both the above established and respected members....
More than once I've seen comments and questions posted here that have given me cause to wonder....
People who become members here ought to have some basic knowledge of electrical matters.
The guy in the OP.......I don't wish to be unkind......
 
junkhound said:
need a 20 HP motor

have a 184 frame 1750 RPM 5 HP (240V, 60Hz, 26A) single phase motor.

Can I run it at 3600 rpm (or higher) with an inverter and use an old furnace fan to 'supercool' the frame to 'push' the motor to produce 20 HP?

Have been told this can be done.
Click to expand...


Who knows, maybe in theory it would work. If your goal is just to say that you made a 5 HP motor develope 20 HP.

In the real world I think the idea would be useless. You don't just choose a motor based on horsepower. What type of use do you have for said motor? Will it need to start under heavy load ( this would surely kill the idea)? Will you need speed control?

For military use you often see 400 cycle power used to build smaller and more lite-weight motors and then gear down from a higher RPM. If you are looking for something small with more HP and less weight (Military surplus).
 
My guess is that what you have heard is someone having completely misinterpreting something that they observed being done, but had no idea what they were looking at.

This is an old machine builder's trick employed when there is no other choice, because of the potential pitfalls.
THEORETICALLY, if you have a 3 PHASE motor that can be connected 230V or 460V, and you HAVE 460V available, you can connect the motor for 230V, then using a 460V rated VFD, you program the VFD to calculate the V/Hz patter to be that of a 230V motor. Then when you run the drive up to 120Hz, the motor maximum voltage becomes 460V at 120Hz, and all is right for the motor torque, because you are still at the correct V/Hz ratio. Therefor TECHNICALLY, because the definition of "HP" is Tq x RPM/5250, if you have the same rated torque and twice the speed, you have twice the HP out of the same motor.

General potential pitfalls:
1. The motor bearings must be rated for the doubled speed. Many are not.
2. The motor cooling must be evaluated; some fan types may fail to increase the cooling to cover the added losses in the motor, and/or may fly to pieces at the doubled speed.
3. this only works for constant torque loads. You CANNOT do this sort of thing with a centrifugal machine, i.e. one that uses a variable amount of torque depending on speed, like a pump or a fan. In a centrifugal machine, power required by the load increases at the CUBE of the speed change. So at 2x speed, a pump will require 8X the power, and you have increased the motor power by only 2X. You are now only producing 1/4 of the REQUIRED power for the new load.

Additional Problems specific to your application:
4. You must DOUBLE the voltage, so the motor must be CAPABLE of having the voltage doubled. A single phase 230V motor will not.
5. It must be a 3 phase motor. A single phase motor has capacitors, they will not survive this trick.
6. This can DOUBLE the HP, you are wanting to QUADRUPLE the HP. Missed it by a little bit...

Maybe you should start by saying why you think you need to get more HP at a higher speed out of your motor.
 
Besoeker said:
It's single phase. See the OP.
Click to expand...
I know, that's why I said it was a misinterpretation of something someone observed. The only reason I posted that was in an attempt to explain the likely source of the misinformation. Most myths have a tiny kernel of truth behind them somewhere and if you can find it, you can understand how they get to where they are.
 
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Saw some Westinghouse drive motors for a cable elevator array, 16 floors. The motors were relatively tiny except for the large continuously running blowers bolted onto the motors, pumping a lot of air through the windings. Probably DC, high torque, low speed, motors with additional gear reduction. Westinghouses's motto was "You can be sure if it's W...". I took one look at it and said "you can be sure it won't work".

We were there to A/C the elevator penthouse because the elevators kept tripping off the line in summer daytime.

When the boss says "chickens can pull trains", your job is to hitch em up.
 
example

example

Gotta admit, posted this just to see what type responses there would be. "Pop quiz' sort of post <G>

I was surprised that no one mentioned stray capacitive bearing current from an inverter running a non-inverter rated motor and causing very shortened bearing life.

Also surprised many folks thought there would be a mechanical bearing problem (well, the lifetime would be 1/2, just from total rotations in a given time period, but not due to overloading or speed per se, 8-10k is normal limit for most regualr motor ball bearings)

The comment on 'new service' was good, as if this were existing load on a new service around here, the poco would want to put in a 400 amp service just for flicker - but with a soft start inverter, that would be no problem.

Was surprised at the number of comments inthe 'can't be done' category though.


pic below of the motor/pump as now configured -
running at 10 HP powering mTm 4kpsi, 4 gpm pressure washer that originally had a 11 HP Honda engine.

Old furnace blower ducted to shroud around finned case motor and ducted thru windings, approx 1300 CFM air flow at high Reynolds number. Yep, the 'cooling system' is bigger than the motor, but liquid N2 not needed.

Motor runs at < 20C rise on the case, have not measured the temp of the windings, so temp no problem with the airlow.

Thinking of going to fatter nozzle and upping the flow to 8 gpm.

It draws 48 A/240V now at 10 HP (48A includes the 1/3 HP blower motor); need to run a 6 AWG feeder if I go up to 8 gpm.

I've gotten a 'kick' out of some comments from 'gurus'.

I used to teach some EE courses at work before I retired, one of the 'trick' questions was how to do this and what were the concerns.

Hint: For 20 HP, gotta build a 2 phase inverter, one phase for main winding, second for the start winding to be able to eliminate the capacitors, and incorporate soft start frequency control.

At 10 HP, the run cap is at 420 Vac, at 120 Hz, would need to change cap values to maintain correct phase shift or go with the custom inverter - so, the comments related to capacitor concerns were right on.


ry%3D480
 
You do realize a brand new pressure washer is $249 at Home Depot.

And you teach what, ... how to turn scrap metal into scrap metal ? The E in EE is quantative, doing it by the numbers. How would I save money doing it your way? Your students, tracked to scrounge scrap yards or know and follow industry standards. Is that a saleable product or a liability nightmare? 3600 rpm belt drive?

Anyways, welcome to the board.
 
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