Open Delta and frequency

[Strathead]

171102-2126 EDT

To repeat our shop has a two transformer 240 V three phase open delta with a wild leg.

Unexpected first big surprise was how clean the sine wave was. No major flattening of the peaks as I see at home. Probably many fewer computers and other capacitor input power supplies on the shop substation compared to the substation supplying my home.

The sine waves from both the 120 V circuit, line to neutral, and the wild leg to neutral were clean, and basically undistorted. The wild leg appeared to be very close to a 90 degree shift from the 120 V, as expected.

Played a little more with the Fluke 87. I believe the frequency measurement problem with the Fluke, and possibly the original poster is the following:

Using AC voltage mode. Power on to AC volts. Probe the voltage. Let auto-range pick the correct range by measuring the voltage. While in this state press the frequency button. The frequency measures correctly.

Alternatively, manually set the range. With no applied voltage press frequency button. Apply voltage and frequency reads correctly. Remove voltage, go to a different voltage of sufficient magnitude, and frequency reads correctly.

I believe what happens with the Fluke is that: if range is not set for an appropriate voltage setting before going to frequency mode, then that range remains at its setting at the time of selecting frequency mode. If that voltage range is low, and a large voltage is applied, then amplifier saturation occurs, and frequency measurement gets screwed up. This is present conjecture.

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Thank you for bringing it back to my original question. I have a Fluke 87, but the person doing the reading has a Klein that has a dial setting specifically for frequency, and the reading was consistent with the other phases reading ok, so I am not sure this is the issue in this case.

 

[gar]

171103-0851 EDT

Strathead:

When my 87 wasn't reading 60 Hz, then the readings were not consistent and jumped all over.

If you have reason to follow up on this problem, then I would first read the voltage from C to N. Then I would load C to N with two 120 V 100 W incandescent bulbs in series. Room temperature these are about 10 ohms each and about 140 ohms at 120 V.

If the bulbs light properly, this would mean voltage is about correct at light (little) load, then measure the frequency. If frequency reads correctly, which it most certainly should, then the circuit is probably OK.

If bulbs don't light correctly, then there is probably an open circuit on C.

To get stable oddball frequency readings there needs to be a source. At this point why you got the readings you had is really a mystery. It may not be important to you to find out why, but it would be interesting to know if it is a meter problem or something else.

I was surprised that my measured phase shift was so close to 90 degrees. This mean't that the voltages were well balanced. However, it was after 5 PM when I looked at the waveforms. I don't know the loading on this substation, but it is more stores, offices, and limited light industry. At the time of my measurements none of our machines were on.

The substation serving the shop is only 1 to 2 miles from the one supplying my home, and the waveforms are quite different.

.

 

[Strathead]

171103-0851 EDT

Strathead:

When my 87 wasn't reading 60 Hz, then the readings were not consistent and jumped all over.

If you have reason to follow up on this problem, then I would first read the voltage from C to N. Then I would load C to N with two 120 V 100 W incandescent bulbs in series. Room temperature these are about 10 ohms each and about 140 ohms at 120 V.

If the bulbs light properly, this would mean voltage is about correct at light (little) load, then measure the frequency. If frequency reads correctly, which it most certainly should, then the circuit is probably OK.

If bulbs don't light correctly, then there is probably an open circuit on C.

To get stable oddball frequency readings there needs to be a source. At this point why you got the readings you had is really a mystery. It may not be important to you to find out why, but it would be interesting to know if it is a meter problem or something else.

I was surprised that my measured phase shift was so close to 90 degrees. This mean't that the voltages were well balanced. However, it was after 5 PM when I looked at the waveforms. I don't know the loading on this substation, but it is more stores, offices, and limited light industry. At the time of my measurements none of our machines were on.

The substation serving the shop is only 1 to 2 miles from the one supplying my home, and the waveforms are quite different.

.

I will pass this on. I don't think it is important to the customer so I doubt we will be following up, but if he does go back out he will have you suggestions. I am not even his supervisor, he just comes to me as his "person" when he runs across anomalies.