how to cool 5 HP motor to get 10 HP output

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junkhound

junkhound

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Renton, WA
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EE, power electronics specialty
How many different way can be used - adding water pipes, added forced air cooling, running at ony low ambient temperatures, etc.

ideas?

want to drive 4000 psi, 4 gpm pressure washer pump with 184T 5HP rated motor. e.g 25 A motor pulling 50A without overheating?

240V single phase.
 
I'm dubious that it can be done. If it can, it seems expensive and complicated.

Why would the complication be preferable to the right-sized motor?
 
I think the hardest part is going to be cooling the squirrel cage. I think you are going to have a lot of I2R going on there and to get any kind of cooling fluid through there is going to tough.

If the goal is small footprint and weight, why not start with a 3600RPM 3 phase motor, drive it above it's normal speed with a vfd, and adjust your pump displacement down?
 
If the undersized motor has enough torque to start and run the load (which may or may not be the case for a 2x overload), you might get 5 minutes of run time from cold to maximum temperature.

In slightly larger 3 (215T frame) three phase motors tested on a dyno with temperature monitors embedded in the coils, on a dynamometer, I saw run time of 15 minutes with a 2x overload.

So one technique is simply time limited operation.

The colder the motor starts, the longer the time limited run time; motors are more efficient when cold because the resistivity of copper goes down. But you can only go so far in pre-chilling. Bearing lubrication is a problem and the reduced resistance means increased starting current and possibly reduced starting torque.

-Jon
 
230721-0911 EDT

Use a permanent magnet rotor, that is a synchronous motor, use forced air cooling thru the rotor and field area, run at high RPM, and cool the forced air. Also do things internally to allow improved air flow.

Possibly add internal paths for liquid to be forced thru the rotor and field windings.

.

.
 
gar said:
230721-0911 EDT

Use a permanent magnet rotor, that is a synchronous motor, use forced air cooling thru the rotor and field area, run at high RPM, and cool the forced air. Also do things internally to allow improved air flow.

Possibly add internal paths for liquid to be forced thru the rotor and field windings.

.

.
Click to expand...
All in all though, it would be far far cheaper to just use a correctly sized motor…

As mentioned, you can do many things to cool the stator, but cooling the rotor is extremely difficult. I’ve seen it done on cryogenically cooled motors, ie the entire motor is immersed in a cryo application, but that’s not something you can do in a single phase motor application.

Pressure washer pumps are usually positive displacement pumps, so power and pressure will be generally linear. But when you run a motor over speed with a VFD, you are in “Constant HP” mode, meaning as the speed of the motor increases above base speed, the torque is diminishing and the power remains constant. If your goal here is to increase the pressure output, you cannot do that without increasing the work HP required. So that isn’t going to work.

Again, you need a bigger motor.
 
Sample calculation ofr cooling, push a LOT of air thru the 184T, say a furnace blower and appropriate ducting.
first few rows are to get a CFM airflow typical of 184T 70% efficient single phase motor, then see what temp rise would be for using furnace blower with ducting to end of 184T, remove existing fan. Blow air thru the motor and around the outside fins with a duct.
70%​
eff
3730​
watts out
5329​
watts in
1599​
heat W
5454​
BTU/hr
40​
deg rise C
72​
degF
303​
# air/hr
70.7​
CFM
Double motor HP, 4X loss
282.8​
CFM for 2x HP
1200​
blower CFM used
<72​
degF rise
 
There was and may still be a collage electric race car competition:
Formula SAE is a student design competition, that used AC electric motors at maybe 10 times their rated horsepower.
I recall that they oil cooled the motors and used much higher line frequencies. (I only saw one car).
At the races, they had football players change the battery packs in mid-race.
 
Besoeker3 said:
Precisely !
Click to expand...
Have a couple of 5 HP 184T motor on the shelf in the shop, big $$ (many benjamins) for a 10 HP motor these days. Also have furnace blowers and plenty of shop materials.

That is the why, $$ go for grandkids college expenses, lol...
 
junkhound said:
Have a couple of 5 HP 184T motor on the shelf in the shop, big $$ (many benjamins) for a 10 HP motor these days. Also have furnace blowers and plenty of shop materials.

That is the why, $$ go for grandkids college expenses, lol...
Click to expand...
Run the motors mechanically in parallel?

Or find a bigger 3 phase (15ish HP) motor in your favorite junkyard, and use a bunch of capacitors. I've actually seen guys use 3 phase motors on single phase with a rope start like a small engine
 
junkhound said:
Sample calculation ofr cooling, push a LOT of air thru the 184T, say a furnace blower and appropriate ducting.
first few rows are to get a CFM airflow typical of 184T 70% efficient single phase motor, then see what temp rise would be for using furnace blower with ducting to end of 184T, remove existing fan. Blow air thru the motor and around the outside fins with a duct.
70%​
eff
3730​
watts out
5329​
watts in
1599​
heat W
5454​
BTU/hr
40​
deg rise C
72​
degF
303​
# air/hr
70.7​
CFM
Double motor HP, 4X loss
282.8​
CFM for 2x HP
1200​
blower CFM used
<72​
degF rise
Click to expand...

Your calculation will show that enough airflow will keep the outside of the motor cool.

It won't tell you the internal hot spot temperature.

Since you have motors and blower essentially free, I'd say give it a shot. Put a temperature probe on the winding and shut down based on temperature.


Jon
 
Actually did the furnace fan setup 12 years ago. Have run continuously for over 8 hrs straight with no problems.

10HP pressure washer.jpg
 
NOT TE, open frame, 184T
There is a shroud over the external fins, air blown both thru and around the motor.

Have even run it on 208 Vac for full load hours continuously, no heating problem.
Same pump and nozzles, 48A at 240 V, 52A at 208, so power does drop a bit on 208Vac.
 
junkhound said:
NOT TE, open frame, 184T
There is a shroud over the external fins, air blown both thru and around the motor.

Have even run it on 208 Vac for full load hours continuously, no heating problem.
Same pump and nozzles, 48A at 240 V, 52A at 208, so power does drop a bit on 208Vac.
Click to expand...
You know I hardly even think of open drip motors anymore. All the industry around here is pretty much exclusively TEFC. The only open drips I've seen recently are on some single phase air compressors in automotive repair shops.

Well then there is the pre-U frame 5 HP 3 phase on my drill press

DP1.jpgdpm.jpg
 
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