Power Calculations with Fluke 1735 meter

Status
Not open for further replies.

mull982

Senior Member
I was using a Fluke 1735 power meter to record power on a 480V motor. The motor is on the secondary of a 3-phase wye 480/277 transformer, however the motor itself connected in delta.

On the meter you have to select weather the system that you are measuring is a wye or 3-element delta. Thinking that I was measuing on the secondary of the wye configured transformer I took all power readings with the meter set to a wye network. After I thought about the fact that the motor was connected in a delta configuration I began to question if I was connected correctly. I called fluke and they told me that the network configuration on the meter corrosponded to the load that you are measuring and not the source. So for a delta connected motor load I should have set the meter for for a 3-element delta.

My question is, will my power results that I obtained with the meter set for wye configuration be incorrect for this delta connected motor? What factor will they be off by?

If measuring a wye connected load I understand that the meter will measure the voltage and current on each line, multiply them to get a power and then add the three different power elements to obtain the overall power. The meter does not do any calculations with the sqrt(3) but simply adds the different powers on each phase element.

However for a delta load I do not see how the current will be calculated across each phase element. Obviously the meter will see the voltage across each element with the voltage leads connected, but with the CT's placed on the lines, how will the meter determine the current across each element to calculate the power across each element?

I know that the current across each element in delta is the line current/sqrt(3) and the total power in a delta load is given by sqrt(3)Vl * Il*cos phi. In other words the total power in delta can be calculated off of line voltages and current, however fluke says they do not perform this kind of calculation. So I am curious how they determine each power element and the total power in delta. Does anyone know?
 
I thought about this some more and even ran a sample calculation. From what I saw the results should be the same no matter how I had the meter configured. The power is the same across each winding, it is just a matter of a difference between line vs load power when looking at a Delta config.

The only thing that I can think that might make a difference is how the different meter configurations calculate phase angles and therefore PF and KW. Maybe this could skew the results?
 
I went back and set the meter up correctly for a three element delta. I noticed that the kW only changed about 2 or 3 kw so it wasn't that much of a change or difference as I suspected.
 
Status
Not open for further replies.
Top