Single phase vs Three phase

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jerjwillelec

Senior Member
Location
Nevada, IA
Had a farmer debate with me about why he doesn't want to bring three phase power to his upgrading grain handling system. He doesn't think three phase is worth it. Anybody got a solid answer as to why it is?

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charlie b

Moderator
Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
It's all about the bucks. I suspect the utility will charge more to install 3 phase, and will also charge more per KWHr for 3 phase. The cost savings would come from more efficient operation of the motors and less maintenance. Much depends on how many motors there will be, and how big they are. Most of the buildings I help design have 3 phase already. So I tell the mechanical and plumbing engineers that if they have pumps or fans of 1 HP or larger, I want them to select 3 phase motors.

Can you tell us what types and sizes of motors this facility will employ?
 

jerjwillelec

Senior Member
Location
Nevada, IA
It's all about the bucks. I suspect the utility will charge more to install 3 phase, and will also charge more per KWHr for 3 phase. The cost savings would come from more efficient operation of the motors and less maintenance. Much depends on how many motors there will be, and how big they are. Most of the buildings I help design have 3 phase already. So I tell the mechanical and plumbing engineers that if they have pumps or fans of 1 HP or larger, I want them to select 3 phase motors.

Can you tell us what types and sizes of motors this facility will employ?
No, I can't. This was in a passing by moment as we were just actually meeting each other. He mentioned he was putting in a grain leg, we had a very brief conversation and he had to leave. I'm not "quick thinking" when it comes to debates so I didn't have much to say. Hoping to get some good argumentative lingo here

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Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
No, I can't. This was in a passing by moment as we were just actually meeting each other. He mentioned he was putting in a grain leg, we had a very brief conversation and he had to leave. I'm not "quick thinking" when it comes to debates so I didn't have much to say. Hoping to get some good argumentative lingo here

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It's just not that simple. He might be right if he only has a few motors, all small, not running all of the time. 3 phase motors are not automatically more efficient just by being 3 phase alone, but it often works out that way. It's just because the motor mfrs spend more time and effort on making 3 phase motors more efficient as they tend to be used on continuous loads, whereas single phase are more often intermittent.

So if you pick out a NEW motor from a catalog, let's say 5HP 230V and look at the single phase and a 3 phase versions. The 3 phase will be a few percent more efficient., ie. 89.5% vs the single phase at 86.5%. If your motor runs once per day for a couple of hours, the cost to bring in 3 phase just for that would never pay you back. Even if that motor runs 24/7 5 days a week, 50 weeks/year, that 3% of 6.7kW works out to around 1700kWH, so at $.15/kWh that's only $250 or so. If the PoCo charges him $5,000 to bring 3 phase into his farm, it would take him 20 years before he even paid for it, let alone see benefit.

But if he has 20 such motors, it's 1 year. NOW it makes sense.
 

jerjwillelec

Senior Member
Location
Nevada, IA
It's just not that simple. He might be right if he only has a few motors, all small, not running all of the time. 3 phase motors are not automatically more efficient just by being 3 phase alone, but it often works out that way. It's just because the motor mfrs spend more time and effort on making 3 phase motors more efficient as they tend to be used on continuous loads, whereas single phase are more often intermittent.

So if you pick out a NEW motor from a catalog, let's say 5HP 230V and look at the single phase and a 3 phase versions. The 3 phase will be a few percent more efficient., ie. 89.5% vs the single phase at 86.5%. If your motor runs once per day for a couple of hours, the cost to bring in 3 phase just for that would never pay you back. Even if that motor runs 24/7 5 days a week, 50 weeks/year, that 3% of 6.7kW works out to around 1700kWH, so at $.15/kWh that's only $250 or so. If the PoCo charges him $5,000 to bring 3 phase into his farm, it would take him 20 years before he even paid for it, let alone see benefit.

But if he has 20 such motors, it's 1 year. NOW it makes sense.
Perfect! Thank you!

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