3 phase vs 1 phase motor currents


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Old 11-30-20, 08:46 PM
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3 phase vs 1 phase motor currents

Hi guys, few questions about motors cause i'm going cross-eyed...

I've got a project where I need to run a fan for about a month, it was on a single phase motor that I thought looked pretty cheap to run looking at its size but now I'm not so sure. The name plate specifies 3A, 240V single phase, 0.25hp which is what 186watts - this bit sounded cheap? But if it is pulling 720VA/apparent watts what the heck is it doing with the other ~530watts? - I'd be interested to have this explained aside from my question.

My question is, is it going to be cheaper to pull the hub and put it on another 3 phase motor I've found here, which specifies .18kW, 415V 0.8 amps(star). If i'm calculating power usage, is this simply 415*.8 making it 332VA and a heap cheaper than the single phase, or do I multiply the current by no. of phases?
 
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Old 11-30-20, 10:45 PM
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I did swap the hub out in the end, and I assume i've answered my own question by measuring the current draw at .72A, the only way it resembles 180 watts is without multiplying the current by 3 (phases). ~300VA on this motor vs ~700VA on the single phase, spinning the same hub at almost identical speed ~30RPM difference. What's going on with the single phase motor? If I can't feel heat where does it go...
 
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Old 12-01-20, 05:04 AM
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If your only running the fan for a month why worry? Just run it with the motor that's on there now. Switching from single to three phase power isn't going to do much for the power consumption and I certainly wouldn't do it for a month's use. Keep in mind much of the consumption is directly related to the work your getting out of the motor. If your fan only need 1/2 that power to turn the blades that's all the motor will consume. Even with a different motor it will still take about the same watts to turn the fan at the same speed.
 
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Old 12-01-20, 05:38 AM
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To optimize motor efficiency, load it at 75% of rated power +/-. You should see close to 90% efficiency on a 3 ph, and around 70% on a single phase. This assumes a capacitor run type 1 ph motor which has some efficiency loss due to the compromise made in capacitor sizing.
The motor nameplate is the rating; it has no idea of the loading. So, you have to measure "real power".
Also, use sq root of 3 to multiply one phase current to get the total on a healthy 3 ph motor. And, of course, there is some reactive power that is a component, that is not billable power.
 
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Old 12-02-20, 12:07 AM
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It's not a capacitor start type as I can't see any cap on it, I assume it's got a shaded pole or start winding maybe and is the reason it's inefficient.

that's interesting your last comment telecom guy, is out of phase current put back in the grid from a reactive load not billable?
 
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Old 12-02-20, 05:02 AM
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kWATT- hours are what gets metered in most cases, not kVA- hours, however, there can be penalties for high reactive loads, and some customers do power factor control.
Shaded pole motors have poor efficiency (~25%) and low starting torque, and so, are used only for low power apps.
 

Last edited by telecom guy; 12-02-20 at 06:18 AM.
 

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