3 Phase motor question

[James Boster]

I have a question thats been on my mind for some time and I thought I would post here and get some opinions. I run both single and three phase motors in my shop machines so I not here to discuss phase conversion, I already understand that principle. I have been loooking at getting a wide belt sander for my shop. Most sander are offered in single phase or 3 phase. Now I know and understand that 3 phase motors are much more effecient sp?. But all of the sanders i have looked at and other machines that offer both types of motors offer for instance a 5hp single phase or a 7 1/2 three phase. Why do they all go up one size for the three phase moters if they are more effecient?

[tod evans]

james, i`m no electrician so take this as a laymans opinion. if you use a similar number of windings on each motor, one for 3 phase the other for single, you will have a more powerful motor running that third leg hence the higher hp rating for the same dollars. .02 tod

[Steve Stube]

I'll offer a couple of thoughts (and that is all they are because I don't know what the real answer is).
1.) It may be a ploy to reach down the consumer chain to the home shop or those shops where 3 phase is not available and 5HP is about the "normal" cutoff for single phase motors HP wise. Yes there are bigger single phase motors up to 10 HP but they are at a real premimum $$$ wise.
2.) A true 5 HP motor (i.e. ~28 amps @ 230V) has more inertia (heavier rotating member than does a 5 HP 3 phase motor so perhaps in increasing the 3 phase motor size to 7.5 HP inertial loads would better/closer match. How this difference in inertia (flywheel action) may play out in the sander operation is not all that clear to me at this moment (just pulled an all-nighter). Interesting for sure.

[lou sansone]

I have a question thats been on my mind for some time and I thought I would post here and get some opinions. I run both single and three phase motors in my shop machines so I not here to discuss phase conversion, I already understand that principle. I have been loooking at getting a wide belt sander for my shop. Most sander are offered in single phase or 3 phase. Now I know and understand that 3 phase motors are much more effecient sp?. But all of the sanders i have looked at and other machines that offer both types of motors offer for instance a 5hp single phase or a 7 1/2 three phase. Why do they all go up one size for the three phase moters if they are more effecient?

I think one of the issues with single phase vs 3 phase is the actual amperage required for a given HP. What you will find is that the required amperage is reduced for 3 phase by 1.73 ( if my memory serves me correct). That means that for a given motor starter and wiring set up you can put that bigger motor on and still keep the same size starter and feed circuit.

lou

[Bob Weisner]

If you buy a machine that has a 3-phase motor and wanted to take that motor off and replace it with a 1-phase motor, are there any other electrical parts that would need to be replaced also?

[Bob Wilkerson]

Three-phase motors have a higher range of efficiency than single-phase motors, and also have better starting torque. For single-phase motors to achieve good starting torque, they require more components like multiple capacitors and a centrifugal switch.

Three-phase motors have no inherent torque pulsations. Single-phase motors do have inherent torque pulsation, although this is usually not a noticeable problem under about 10 horsepower.

Three-phase motors are much more durable as there are no special starting circuits or capacitors involved to get the motor turning or keep it running. Lower amperage per leg is also a benefit in wiring costs. The only negative in 3ph that I can see is in the event that you loose one leg the motor will lock and eventually burn out. We had that happen with a 15hp motor at work when the utility company blew a fuse in one leg of a transformer (I think) that left our building with 2 phase legs hot but the third dead. A phase loss sensing breaker can solve that problem.

Replacement costs for 5hp and larger motors are significantly less for 3ph than 1ph. These days I'll buy equipment without regard to its initial motor characteristics but will often replace 5hp single phase motors with 3ph if it goes bad. Some changes are required to the starters in this case.

My standard wiring in the shop uses 4 wires for single phase 240v and 5 for three-phase. This allows me to obtain 240v and 120v at each machine as necessary. My drum sander originally had two plugs: 1 for the 5hp 240v single phase motor and one for the 120v belt motor. Now I just have one plug in the wall to provide both.

Bob Wilkerson - Addy protocol invoked - I'm an interested amateur.....

[Chris Rosenberger]

If you buy a machine that has a 3-phase motor and wanted to take that motor off and replace it with a 1-phase motor, are there any other electrical parts that would need to be replaced also?

It depends on the electricals. If the voltages & amperages are the same between 1 & 3 phase, then the electricals should work. If the voltages are different, then any transformers & coils may need to be changed. If the amperages are different then the overload heaters would need to be changed.