Power is defined as P = iv, where i and v are the instantaneous values of the current and voltage.  For constant DC, power is simply the product of the voltage and the current.  For AC power it is not quite so simple.  Most watt transducers measure the instantneous power averaged over some time interval to obtain real average power.

Because watt transducers measure the rate at which work is being done they are an invaluable feedback device for discreet control of a process.  Examples include cutting, grinding, heating, welding, pumping and mixing.

Watt transducers have one or more elements.  Elements are also referred to as multipliers.

Remember Blondel’s Theorem:

For any (n) wire sysem, (n-1) wattmeters or elements are required to measure total power. For more information  please refer to the technical paper in our Tech Library entitled “Selecting and Using Transducers for the Measurement of Electric Power, Voltage and Current”.

Almost the entire output of the electric power industry in the United States is generated and distributed as polyphase power at 60Hz.  In a three phase system the three voltages are mutually out of phase by 120°.  A balanced  load will draw equally from the three phases: however, when one of the voltages is instantaneously zero the phase relationship shows that the other two must be at half amplitude.  At no time does the instantaneous power drawn by the load  reach zero.  In fact the total instantaneous power drawn by the balanced load is constant.  This is a big advantage for rotating machinery because it maintains a more constant torque than if a single phase motor were used.

Utility power is generally referred to as “Y” or “Wye” or a three-phase, four wire connected system.  To determine the total power each of the three branches must be monitored.  The total power delivered by the source load will be the simple sum of the readings of the watt meters on each branch,

OSI watt transducers for monitoring a wye-connected  load have three individual multipliers (elements) contained in the transducer housing.

Polyphase watt  transducers manufactured by Ohio Semitronics, Inc. are rated for voltage in two ways:

  • Three phase, three wire two element watt transducers are rated for line to line voltage.
  • Three phase, four wire three element watt transducers are rated for line to neutral voltage.

Measuring power accurately on non-linear loads can present problems.  Examples of non-linear loads include UPS systems, electronic power supplies, lighting ballasts, battery chargers, variable frequency motor drives, phase angle-fired SCR controllers, zero crossing SCR controllers and pulse wave modulated  controllers.  While these non-linear loads can create high neutral currents, voltage distortion, heat and reduction of system power factor it is possible to measure power accurately if the correct power transducer is selected.  You should consult with your OSI Technical Sales Specialist when considering the purchase of a watt tranducer which will be applied on a non-linear load.