Thread: Replacing A Single Phase Motor With A Three Phase Motor....................

Motors are sometimes rated input power, and sometimes rated output power. It depends what type of motor and what its intended use is (industry grade or consumer grade).

For example, a standardised industrial motor will always and invariably be rated for its output power, regardless if it is single or three phase, regardless if it is continuous duty or otherwise. The nameplate will likely also list the motor's efficiency in %, so that its easy to calcultae its input power (output power in KW divided by efficiency in % multiplied by 100 = input power).

However, consumer grade motors used for example in a home duty air compressor, or in a domestic vacuum cleaner, or in a treadmill, are most likely to be rated by input power alone, and very often without there being a nameplate to let you draw any informed conclusions about its true output power or efficiency. Sometimes it is possible to find a motor manufacturer's data sheet with more information, but often ther will be no information available.

In the case at hand, we are I believe talking industrial grade TEFC motors. These are rated for their output or shaft power. It is true that a 2HP and a 5HP motor will both have comparable input power when loaded to 2HP. But during startup, the 5HP motor can and will draw a 2.5 times larger input current. Same when plug reversing. This is why the power supply and the motor protection contactor must be rated accordingly. Most small motor protection contactor have a screwdriver adjustable wheel to set the optimal thermal trip current for the motor (but some less expensive consumer grade lathes or mills may not have such a motor protection contactor at all). Also, the 5HP motor will likely be less efficient when only loaded at 2HP - and this not only because it still has to drive its larger cooling fan at the same speed as it would when loaded for 5HP. And this means it will have a higher input power when loaded 2HP, then a 2HP motor loaded at its rated 2HP would have.

Originally Posted by cba_melbourne

the 5HP motor can and will draw a 2.5 times larger input current.

Even taking into account that you've changed to 415V and 3 phase?

I'm not 100% sure that the current wouldn't go down(in fact my WAG says it would be a little lower and of course for about half as long)

Stuart

Originally Posted by cba_melbourne

. . . . . . . Also, the 5HP motor will likely be less efficient when only loaded at 2HP - and this not only because it still has to drive its larger cooling fan at the same speed as it would when loaded for 5HP. And this means it will have a higher input power when loaded 2HP, then a 2HP motor loaded at its rated 2HP would have.

My understanding is that Induction motors have a near flat efficiency curve above about a 20% load and for larger motors the peak efficiency is less than at 100% load.

e.g. from http://powerelectronics.com/motion-s...e-speed-drives.


Larger motors are also inherently more efficient than smaller ones and electrical standards also require them to be made such.
A 5HP motor has a typical efficiency of ~85% where as a 2HP motor is only required to be 78% efficient.
My measurements on motors show the typical additional current drawn by a motor to run the fan is 30-40 mA.
The additional current drawn by a larger fan on the 5HP motor is undetectable using my current meter (i.e. < 10mA).
At 440V thats < 44W or ~1% for a 4kW/5HP motor.

Combining all these and I doubt the 5HP will be any less efficient at delivering 2HP and there is a good chance that it might even be more efficient.

Originally Posted by Stustoys

Even taking into account that you've changed to 415V and 3 phase?

I'm not 100% sure that the current wouldn't go down(in fact my WAG says it would be a little lower and of course for about half as long)

Stuart

Stuart, I was simply comparing the OP using either a 2HP or a 5HP motor of same voltage, but utilizing only 2HP. The 5HP motor will draw a 2.5 times higher start current. Only once at speed, will the two motors draw similar currents foe similar loads. In the case of 3phase motors, the same will be true when plug reversing direction. And that is also assuming that no soft start devices are used.

Originally Posted by BobL

My understanding is that Induction motors have a near flat efficiency curve above about a 20% load and for larger motors the peak efficiency is less than at 100% load.

e.g. from http://powerelectronics.com/motion-s...e-speed-drives.


Larger motors are also inherently more efficient than smaller ones and electrical standards also require them to be made such.
A 5HP motor has a typical efficiency of ~85% where as a 2HP motor is only required to be 78% efficient.
My measurements on motors show the typical additional current drawn by a motor to run the fan is 30-40 mA.
The additional current drawn by a larger fan on the 5HP motor is undetectable using my current meter (i.e. < 10mA).
At 440V thats < 44W or ~1% for a 4kW/5HP motor.

Combining all these and I doubt the 5HP will be any less efficient at delivering 2HP and there is a good chance that it might even be more efficient.

You are right there:

efficiency_vs_load.gif

It depends a lot on motor size, and only some on motor make. And the fan is going to affect the smaller motors much more than the larger ones.

WOW !!!
Thanks guys, I have learned one hell of a lot about electric motors now......
I think I am leaning towards taking my motor out and having it checked at the rewinders, and if it is a dud I will look at getting a nice new 2hp single phase of decent quality.
As this is my second lathe, and my other one is smaller, I am also seriously thinking about adding a third much larger 3ph lathe to my arsenal.... lol Next thing I'll be in direct competition to Morrisman hahaha
But the work I am getting in these days requires me to have the ability to swing much larger diameters..

Again thanks guys

Matt

Hi Matt,

A second hand 3 phase motor and VSD might be a cheaper way to go, though staying with single phase would be easier. I havent used a VSD on a lathe yet but I love the one on my mill.

Originally Posted by cba_melbourne

Stuart, I was simply comparing the OP using either a 2HP or a 5HP motor of same voltage, but utilizing only 2HP.

I guess "In the case at hand" threw me.

Would a motor protection contactor which is set to the current of a 2hp 3phase motor really see a difference between for e.g. 20amps for 0.5sec and 10 amps for 1sec? Sure a VSD would see it, but a bimetallic strip?(or have motor protection contactors moved on and I only see bimetallics because of the age of the machines in my shed? lol)

Stuart

Originally Posted by Stustoys

Hi Matt,

A second hand 3 phase motor and VSD might be a cheaper way to go, though staying with single phase would be easier. I havent used a VSD on a lathe yet but I love the one on my mill.

Stuart

Ditto!

Unless the budget does not allow, or it would present a massive PITA to do, it would seem a shame to convert to a 3 ph motor and not take advantage of the benefits of a VFD.

Simon

Originally Posted by Stustoys

Hi Matt,

A second hand 3 phase motor and VSD might be a cheaper way to go, though staying with single phase would be easier. I havent used a VSD on a lathe yet but I love the one on my mill.


I guess "In the case at hand" threw me.

Would a motor protection contactor which is set to the current of a 2hp 3phase motor really see a difference between for e.g. 20amps for 0.5sec and 10 amps for 1sec? Sure a VSD would see it, but a bimetallic strip?(or have motor protection contactors moved on and I only see bimetallics because of the age of the machines in my shed? lol)

Stuart

Stuart, in the "case at hand", I was comparing the options of using a "2HP-3ph motor" vs a "5HP-3ph motor only loaded to 2HP". I can assure you that a bimetal protection contactor WILL see the factor 2.5 higher inrush current of the 5HP motor. Remember a manual lathe motor does frequently start/stop. The two motors require different contactors. Also because otherwise, the current protection would frequently trip whenever the 5HP motor was even for short periods loaded higher than the 2HP, say when taking a large diameter facing cut.

But yes, nowdays it would seem rather silly not to install a VFD, when converting a lathe or a mill or for that matter a drill press fro single phase to 3-phase. I keep saying that variable speed is the single most useful modification that one can do to a lathe, and since about the year 2000 VFD's have become a very very affordable option.

Originally Posted by cba_melbourne

I can assure you that a bimetal protection contactor WILL see the factor 2.5 higher inrush current of the 5HP motor.

Pity, I would have thought there was some amp V time thing going on with a bimetal switch.

Stuart

Originally Posted by Stustoys

Pity, I would have thought there was some amp V time thing going on with a bimetal switch.

Stuart

Stuart, there is some time lag, but a factor 2.5 times larger motor is pushing it a bit too far. An old fashioned motor "thermal overload relay" with a bimetal strip is designed to just about accommodate the brief high starting current of a motor, all the while protecting it from running for prolonged time at a current overload. Its obviously a fine balance being sought between the two. And it can only be achieved for a given motor power rating. I just had a look at an adjustable 3ph motor protection contactor/relay I happen to have on the shelf, it is only adjustable over a current range 1:2. The idea of these adjustable current realys is to allow it to be adapted to the usage of the motor. Like if driving a conveyor belt with a high inertia, starting may take 5 or 10 second and the relay can be set to a higher current to avoid it tripping. Or if a motor sees very frequent start stop cycles, a higher setting will prevent it tripping. But this adjustability is not intended to bridge the gap between different motor sizes (and most certainly not to bridge the gap over several motor frame sizes).

That is different from using a more modern solid state "soft start" motor controller. These can be freely programmed very much like a VFD. But at a cost of only about 30% of a VFD, they just do not provide variable speed. For smaller motors the price difference becomes so small, that for a one off application a VFD almost always represents the better value solution. Soft starters are more economical either in mass products, or where larger motors are involved.