single-phase motor on 217V ( two legs of three phase ) ?

[beavis2]

Hi Forum. I have a motor which the manual states is rated for 230V single phase, but the shop where I want to install has a panel with two hot leads, neutral, and ground. When I measure between each hot and neutral I see 125V. When I measure between hots I see 217V. So I need to describe this power to a technician who supports the product, a compressor. What is the proper way to describe this power ?

In describing it as two-phase, 217V, two legs of three phase, the technician says to try it, but I'm hesitant because my understanding of AC motors says you don't mix single and poly phase. If the motor nameplate specifies Voltage Range as 208-240 does it matter that in this case L1-L2 are not in phase ?

Sven

 

[growler]

When I measure between each hot and neutral I see 125V. When I measure between hots I see 217V. If the motor nameplate specifies Voltage Range as 208-240 does it matter that in this case L1-L2 are not in phase ?

Since you read 125V from hot to neutral I'm assuming the voltage is reading high rather than low so this is a 120/208V system.

What you have available is 208V single phase so if the nameplate voltage range is 208-240V then everything should be fine.

You will need the correct double pole breaker. Make sure the motor is tapped for 208V because it may come wired at 120V (default).

 

[ptonsparky]

Try it. How big is the motor and what is the application?

 

[Strathead]

You will need the correct double pole breaker. Make sure the motor is tapped for 208V because it may come wired at 120V (default).

I respectfully feel that since there is an apparent need to clarify this, my recommendation is call an electrician. If the original problem does not make sense to you, you should rely on an expert, not an on line forum.

 

[bullheimer]

dont confuse single phase with being only one phase. still dont know why but that's life.

 

[beavis2]

Thanks for replies. My understanding is that single-phase or in-phase voltage sources are just added. So as I measure 125V between each voltage source and neutral I should see 250V between in-phase voltage sources, but instead I see 217V. So aren't these voltage sources out-of-phase ?

Motor is 5HP and 230V single-phase. There are lots of supporting documents and evidence of that. I've attached the wiring document for this air compressor. "N" in diagram is incorrectly labeled, should be "L2". Company support person is less concerned with motor voltage than control voltage on secondary side of compressor transformer which I'll measure when connected.

Sven

 

[jim dungar]

Thanks for replies. My understanding is that single-phase or in-phase voltage sources are just added. So as I measure 125V between each voltage source and neutral I should see 250V between in-phase voltage sources, but instead I see 217V. So aren't these voltage sources out-of-phase ?

Kind of.

The practice 'single-phase' and three-phase' really describe the number of Line-Line voltages we measure, when used like this when defining systems they do not include the neutral.

Single phase 120/240 can come from a center-tapped 240V transformer (common from utilities) or a series connection of two 120V transformers (common from customer owned units). Adding the voltages can be done with simple math.

However, we can also get 120/208 single phase from two legs plus neutral of a wye connected transformer (sometimes called an open wye). Adding the voltages requires us to consider the phase difference. These systems are common in large housing complexes as well as in small 'strip malls'.

As previously mentioned, it looks like you have a nominal 120/208 that is running a little high.

 

[Besoeker]

Thanks for replies. My understanding is that single-phase or in-phase voltage sources are just added. So as I measure 125V between each voltage source and neutral I should see 250V between in-phase voltage sources, but instead I see 217V. So aren't these voltage sources out-of-phase ?
Sven

I don't know how it would be described there assuming there is USA. Here, UK, it would be described as a single-phase line to line voltage from an arrangement like this:



The voltages come from:

V_LL = sqrt(3) x V_LN

217 = 1.732*125