Single Phase vs Three Phase Motor

[hanksmith]

Had a question come up at work and I thought I had the answer but was told differently,

We are looking at a 3 phase system, with a 600-208/120 transformer feeding a breaker panel. 208 phase to phase and 120 phase to neutral.

We want to hook up a 208V motor to this breaker panel, I could get a 3 phase motor and hook it up to phase A,B and C.

Now can I buy a single phase 208 V motor, only hook it up to phase A and B, I am being told this is a three phase motor, I said it can't be if it's not using all three phases,

Anyone have a simple explanation?

Thanks
Hank

 

[waross]

First, the applicable motor voltage for a 208 volt system is 200 volts. Look for 200 volt motors. 230 volt motors are also often used on 208 volt systems. Such motors are common in air conditioning systems.
If you have a three phase motor, hook it up to A,B,C.
If you have a single phase motor hook it up to any two of the above.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Skogsgurra]

Yes, a single phase induction motor actually is a three-phase motor that gets some help from a capacitor that fakes the third phase. Not as efficient and not as "strong" as a three-phase connected motor. But it works for lower HPs.

If you have got three phases, then use a three phase motor. Better, cheaper, simpler.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[itsmoked]

Far more reliable..

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Skogsgurra]

Amen.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[roydm]

HankSmith,
When I was an apprentice in NZ, too many years ago two phase motors were quite common, particuarly out in the country. I guess the justification was saving on copper. 2 phases of 230V (400 between) and neutral would cover either domestic or light industrial loads. The motors had 2 identical windings and no capacitor. I don't remember the motor specs but they must have been better than the regular single phase.

Regards
Roy

 

[tofu123]

hanksmith,
I also prefer to chose 3 phase motor. Before you looking for the motor, make a calculation for deciding how much the voltage required.
For example:
Suply voltage (208 V)=motor nameplate voltage +
voltagedrop/feet(if you use long cable)*lenght of the cable +
2.5%*transformer voltage loses

So if have 208 v suply voltage, the motor should be more than 215 volt.
Regards,
Tony

 

[waross]

Uh tofu123;
There are some indications that hanksmith may be in north America, and may be in Canada.
local standards would dictate that the voltage level is determined AFTER the transformer regulation is applied and no further adjustment is required for transformer losses.
North American codes limit the maximum voltage drop to 5% from the service to the point of utilization and the conductors will be sized to keep within this limit.
We generally subtract the voltage drops from the supply voltage rather than adding them.
The proper motor rating for a 208 volt system in North America is:
Single voltage rated; 200 volts.
Double voltage rated 200-230 volts.
Tripple voltage rated 200-230-460 volts.
For many years, 230 volt rated motors were used successfully on 208 volt systems. When the load on the motor is less than 87% or 90%, a 230 volt motor will be acceptable in almost all instances.

Bill
--------------------
"Why not the best?"
Jimmy Carter