single phase motor power on 3 phase system

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tom grocki

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Is it common for a single phase motor to run high on current when fed off of two phases of 208V three phase Y power? The motor I am talking about is rated 208V-240V, 23.5A - 21.0A, when running it continuously pulls around 30A. It is wired off two phases of a three phase Y system, 208V. The motor powers a compressor and the compressor is brand new, nothing holding it back.
 
tom grocki said:
Is it common for a single phase motor to run high on current when fed off of two phases of 208V three phase Y power? The motor I am talking about is rated 208V-240V, 23.5A - 21.0A, when running it continuously pulls around 30A. It is wired off two phases of a three phase Y system, 208V. The motor powers a compressor and the compressor is brand new, nothing holding it back.
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No, something is wrong. 208V single phase is almost ALWAYS 2 out of 3 phases on a Y system. So the fact that their nameplate says that the FLC at 208V is 23.5A, means it is a motor that is perfectly capable of running on that, and if you are drawing 30A, it is over loaded. Either there is a mechanical problem or someone selected too small of a motor. Did you check the voltage at the motor by the way?
 
The 208-240 designation generally means that it will operate satisfactorily BETWEEN those voltages. A 208volt system may routinely drop below 208 volts.
If you have a 208Y/120volt system, you should use a 208volt motor.
db
 
Jraef said:
208V single phase is almost ALWAYS 2 out of 3 phases on a Y system.
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J,
Just curious. :-?
He is using two of the three phases, around the 2pi cycle. .
That means power is available from the sine waves on 2 of 3 pulses.
I missed my coffee, so I may be wrong,
but it seems that would produce something
akin to 'single-phasing' or "chug-chug-gasp" power production.
My VW does this sometimes, running 3 of 4 cylinders "chb-chb-chb-gsp" .
Comments please. (^-^)
 
glene77is said:
J,
Just curious. :-?
He is using two of the three phases, around the 2pi cycle. .
That means power is available from the sine waves on 2 of 3 pulses.
I missed my coffee, so I may be wrong,
but it seems that would produce something
akin to 'single-phasing' or "chug-chug-gasp" power production.
My VW does this sometimes, running 3 of 4 cylinders "chb-chb-chb-gsp" .
Comments please. (^-^)
Click to expand...

You are reading into it too far. It is 208 volts RMS from line to line and that is what the load will see. If the load is designed to operate at that voltage there is nothing wrong with it. Same sytem supplies 120 volts line to neutral and those loads do not operate like a 3 cylinder engine only running on one. There is only one voltage sine wave between the two points of the system, and one current sine wave also.

Ok, there is only one current sine wave for each connected load for single phase loads.
 
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barclayd said:
The 208-240 designation generally means that it will operate satisfactorily BETWEEN those voltages. A 208volt system may routinely drop below 208 volts.
If you have a 208Y/120volt system, you should use a 208volt motor.
db
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A 208-240 volt motor is designed for connection to 208 nominal system and can deal with less than 208 volt.
 
Phases

Phases

Jraef said:
No, something is wrong. 208V single phase is almost ALWAYS 2 out of 3 phases on a Y system. So the fact that their nameplate says that the FLC at 208V is 23.5A, means it is a motor that is perfectly capable of running on that, and if you are drawing 30A, it is over loaded. Either there is a mechanical problem or someone selected too small of a motor. Did you check the voltage at the motor by the way?
Click to expand...

Just to clarify an important concept of this discussion, there are 3 wires in a "three-phase" system, and 2 in a "single-phase" system. When any "phase"(A,B, or C), then there is only a "single-phase" left, because loss of any "phase" affects two of the phases. An example of "two-phase" power distribution is rare to find, but when I was in the Navy in the 1980's we still had some onboard ships that had been abandoned. "Two-phase" power had 4 legs, and this seemed odd, but upon closer inspection makes sense.:)
 
glene77is said:
Comments please. (^-^)
Click to expand...
Instead of a non-symmetrical wave, the difference in timing is manifest by the fact that the line-to-line voltage is not twice the line-to-neutral voltage.

If you could "grab" the two 120v vector arrows at 120 degrees apart, and spread them apart until they're 180 degrees, the voltage would climb to 240v.

Now, if you include the neutral in this discussion, then you do indeed have two separate waves, which is how 2 phases can supply 3 in an open Delta.

But, even then, the voltages measured between any two conductors is the same: 120v from each line to the neutral and 208v measured line-to-line.
 
A single phase motor will not pull more current because it is on a 3 phase system. It doesn't know or care. If wired correctly, more current means more load on the motor. If your motor is rated at 23 amps full load and it is pulling 30 amps, it is significantly overloaded(~30%). I'd check voltage and current with another meter to be sure the readings are reasonably accurate. An overloaded motor's life will be shortened.
 
Do you part,

Do you part,

Uncouple the motor and verify the load amps. If in spec, should be lower than its listed FLA, the compressor is at fault. if its FLA or higher its the motor... Either way get it repaired/replaced quickly, prior to bad things happening.

I assume the reading were verified on some other quality meter...
 
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