CT kWh Meter Question

Hi gabobe, welcome to CR4.

A watt hour meter reads the actual power not the voltage multiplied by the current. Remember that with AC the voltage and currents are vectors and the relative angle needs to be taken into account. As a result the only time that a watt hour meter will read the same as multiplying the current and voltage is with a purely resistive load. The equations look like this

Power = Current x Voltage x Cos(Ø)

Where Ø is the angle of the current relative to the voltage and Cos(Ø) is know as the power factor. You can therefore calculate the angle from what you have by

Power Factor = Current x Voltage ÷ Meter Reading = Cos(Ø)

I hope that helps.

Is this 3 phase or single phase? Phasing becomes important on 3 phase kwh metering. If the potential transformer and current transformer, phasing and rotation are not correct, the reading will be in error.

Be sure the Current Transformer (CT) ratio Primary Current / Secondary Current (usually 5 A) - nominal values - is proper for kWh - meter /recorder (scaling & calibration),

Of course it would be very "primitive error", sometime it happens!

A little warning that I forgot to convey is that with a current transformer driven watt hour meter never leave the CT coil open circuit when swapping the meter. You can get tremendous voltages across the open terminals of a CT so if you remove the watt hour meter put a jumper across the CT terminals beforehand.

I have a CT kWh meter, when I connect the meter it reads less than expected. I tested two meters, the problem is the same, some times when change wiring it runs backward as fast. Is the problem reading or may connection is wrong?

You don't provide enough information about the circuit and how you tested the meters. kWh meter measures energy and not power. Energy is a time related unit. Assuming you are talking about a singlehase circuit. The reading on the meter is multiplied by the CT ratio. Example a 400:5 CT would have a multiplier of 80 times the meter reading.

Reverse rotation of the meter is caused by improper phasing of the CT.

One more comment:

Since both the meters are working wrongly, pls. check the polarity of the CTs & VTs that they are properly connected.

thanks.

Single phase (it seems to be your case if you have only one CT) can have following problems:

1. Wrong kWh meter rating. For example voltage in the system (grid?) 240 V, meter calibrated for 480 V. Also kWh meters calibrated for different nominal current then CT currents' ratio . If CT had 400/5 A so kWh meter must have the same numbers on its nominal plate description.

2. Of course switching (crossing) wires from CT will create negative torque and reverse motion.

Three-phase systems can have more problems. First you have check diagram appropriate to your system (4- wire with three CTs or 3- wire with two CTs).

Check polarity of current and voltage circuits as well as proper CT and, if used, VT (voltage transformers) ratios for your kWh meter calibration (data on its nominal plate).

There are a lot of schematics available on Internet.