How do I measure the energy used by a motor?

Answer by Hugh Falkner

There are several options for estimating the energy consumption of a motor, the choice of which depends on the accuracy needed for the answer.  The question of accuracy is important, as whether a payback on an energy saving measure is 5 or 6 months (+/-10% tolerance) is arguably of little consequence; it is still a good thing to do. If less accuracy is indeed sufficient, then this means that a quicker and easier method to estimate the energy consumption can be used.

The first step is to measure the power:

A 3-phase power meter is the most accurate method, but requires connection of current and voltage probes to all 3 live phases, making it time consuming. In addition, taking any electrical measurement is potentially hazardous, and so any method that requires connection to the electricity supply should only be undertaken by a qualified person.

Above half load, the amps can be considered proportional to the power consumption.  This method has the benefit of not requiring connection to a live conductor.  For quick estimates this method is sufficient, but for more accuracy the results can be refined through use of the rated current and power factor shown on the nameplate.  This data can be used to calculate the rated out of phase current, and so with this information the line current at any load can be split into in phase (real) and reactive loads.  Since the reactive out of phase current will remain steady independent of load, the in phase current can then be used to directly scale the actual load current relative to the maximum rated current.

A simple no-contact method is to measure the speed of the motor using a stroboscope or tachometer, and compare it to the rated speed (nominal speed minus slip). The big advantage is that this does not require any electrical connection, and if using a stroboscope, no physical connection at all.  Where the supply voltage is known, this can be used to refine the estimate.  On larger and more efficient motors this method becomes progressively less accurate, as both the slip becomes less, and the 5rpm rounding used when declaring slip becomes more significant.  Repaired motors are also likely to have higher losses and hence a higher rated slip, but the value of this slip will not be known.  This method is only of limited accuracy, but this is often sufficient to give an approximate indication of the load.

Multiplying the Power by the time of interest gives the Energy consumption, and multiplying this in turn by the cost per kWh of electricity gives the cost of energy.  The time of interest might be a year, a week, a day or just a process cycle. 

A logging Power meter is the most accurate method, as it will also record any variations in power over the logging time.  Without this the estimate will be limited by how well you can estimate the actual running hours over the period of interest.   

Variations in power consumption over time can yield a rich harvest of information on the system being studied, but in addition to the difficulty of connecting the meter, the meter will have to be left possibly unattended and then collected again at a later time. 

An easier way of assessing running hours is to use a simple motor state logging device, which simply detects the magnetic field to log when it switches on and off.  This is much lower cost, and for the many applications with steady loads, can give quite adequate data.

Measuring energy and power gives invaluable information on the motor system being studied.   By careful consideration of the accuracy of the information that you actually need, it is possible to significantly reduce the costs of collecting data.

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