single phase vs 3 phase

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electricalist

Senior Member
Location
dallas tx
I went to a customer yesterday to re locate his 2 fork lift chargers.
He ask why were they wired 1 single phase and one 3 phase.All the wiring was existing I only re connected them to the disconnects..
Whats the best voltage to wire equipment as far as cost and understanding for me as well as the customer,,,I know higher volts lowers amps.
208/240/480
39/ 34/ 17
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
I believe you will find with certain equipment such as that incorporating motors, the three phase will be more efficient. For a lot of equipment its simply a case of an overall reduction in amps per phase allowing for more equipment on a feeder or service or branch circuit of a specific size.
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
150507-0938 EDT

A three phase motor has a relatively constant amplitude rotating magnetic vector that drags the rotor around with little torque variation. A single phase motor has an oscillating +/- magnetic vector in only one direction, and thus, a large torque ripple. A single phase motor is like a swing that you add energy to the swing in synchronized pulses.

A three phase motor is self-starting, whereas a single phase motor has to be at least a 2 phase motor for starting, and some are 2 phase for running.

A local bearing manufacturer had used DC motors in durability test stands for possibly close to 100 years. Their life tests were based on this historical information. Then someone had the bright idea to change to AC motors, most likely single phase. Now bearings were exhibiting much shorter life times in the new test stands. There were no changes in the manufafcture of the bearings. All this change was because of the AC motor torque ripple.

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Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
On something like a battery charger, the transformer portion of an equivalent capacity charger circuit will be physically smaller for 3 phase than for single phase, which may have influenced the original buying decision. Other than that, if you don't have 3 phase available, you buy the single phase version. If you do have 3 phase available, the 3 phase versions would likely cost a little less initially, but you can use single phase if you want to. So most likely the single phase versions were bought used from someone who didn't have 3 phase available, then the 3 phase version was added later.

Now after the fact, it makes no difference, they are what they are, you can't change how they are connected.
 

junkhound

Senior Member
Location
Renton, WA
Occupation
EE, power electronics specialty
The single phase version would have worked for what they do.

Have you ever welded with 300A+ dc ?
A simple transformer-rectifier 3 phase dc welder lays down a much nicer bead than single phase, look at the waveforms of 3 phase vs. single phase rectifier to see why.

Note on chargers - Unless the chargers are PWM dc-dc converters, battery cycle life will be slightly longer with TR 3 phase chargers.
 

electricalist

Senior Member
Location
dallas tx
Thanks for all the good info.
When a customer ask me a question and I don't know the answer, that's what I tell them. I appreciate all the help I get here, I don't take credit for anything other than being smart enough to see There are good electricians here.
 
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