230 three phase question...

[luke frisbee]

I wanna make sure of this....
does a three phase electric motor that has a plate that reads 4.9 amps at 230, use the same amount of electricity as a single phase motor that reads 4.9 amps at 230?

[junglegeorge12]

nope, it uses more. Think of it like three tubes of water, the amprage being the gph. But, you can get a 3 phase motor running lower amps to do much more work than a single phase will, they also last longer on average. The rule of thumb used to be 10-20% savings in power for using 3 phase.

[luke frisbee]

Well that is sneaky...and thanks

[Rich L]

I havn't worked with those numbers in over 30 years, but I believe the factor for estimating the three phase power with a single phase power is 0.6.

In your case work done by a 4.5A single phase motor will draw 2.7A as a single phase motor. The horse power of the three phase motor will be a little higher.

[junglegeorge12]

so 4.5 x 2 (single phase)= 9 amps of total current

while 2.7 x 3 (three phase) =8.1 total amps of current so the savings for the same amount of work would be about 10% for using three phase witht hose numbers. I believe that was the old rule of thumb, it is probably a little better now.

Also, the three phase will run cooler and with less stress, which means longer life for the motor windings, as long as the bearings are good, it should last much longer (I would guess 25-35% on average) than a single phase. So if a single phase motor would last 6 years on average, a three phase for the same application should last about 8-9.

By the way, how are you going to get three phase in a house? Homes only have 220v single phase to them in the states. Businesses can get both.

single phase is really two phases, 115v is a postive current with a common, while 220v single phase is two lines of 115v, and three phase is three lines of either 115v or 230v.

[saratogatan]

I thought I have burried those days when I built power station and sub-station. keep them coming, folks.

stan

[luke frisbee]

I was gonna use a VFD

[Koi Acres]

Quote: Originally Posted by luke frisbee I wanna make sure of this.... does a three phase electric motor that has a plate that reads 4.9 amps at 230, use the same amount of electricity as a single phase motor that reads 4.9 amps at 230?

W3-phase = E I / 1.732 (3 phase motor) 240 x 4.9 / 1.73= 679w

W = E I (single phase motor) 240vx 4.9a= 1176w

W = power (Watts)
E = voltage(Volts)
I = current (Amperes)
1.732 = 3 phase phase differential multiplier
If you were using the above E & I numbers single phase motor would be a 1.35 hp and the motor on a 3 phase system that consumed = watts (power) would be rated at 2.33 hp

It is cheaper to run a 3 phase motor with a VFD than a comparable single phase motor. Keep in mind that you have to factor the initial cost of a 3 phase motor & VFD vs single phase motor cost.

[saratogatan]

One litle thing I like to kick in:

Volt * amp is called VA, not watt, in the power industry, at least in the british common-wealth system, when I built power stations.

VA * power factor ==> watt

Most home appliance simply says VA is the same as watt, which is not true.

stan

[Koi Acres]

Stan,

You are correct originally I had PF & EFF in formulas, I left them out to keep it simple & avoid confusion.