A load meter is a great accessory for machinery to get a sense of how near capacity a task is bringing the machine. In a previous post, a load meter was built for a CNC milling machine using a ammeter and a voltmeter. This approach is not ideal because apparent power is being measured instead of actual power. Because the power factor changes with different motor loads, measuring power with a ammeter and voltmeter is not ideal.
Specialized meters are required for measuring Watts, or actual power. Murata has a power meter product that uses a microprocessor to correctly time the sampling of current and voltage waveforms. The current is inductively sensed from a load wire passed through a current transformer. The voltage is sensed via a fused connection to the mains.
The power measurements while drilling a 1/2″ hole into aluminum are compared to computed estimates of horsepower requirements. The two are surprisingly close. The disadvantage of the Murata power meters is their slow update of roughly 1 Hz, making them unsuitable for quick response. However, the power meters are likely the best method for measuring the load on the machine during steady-state operations.