Why You Should Never Put a Load Monitor After an Inverter

Why this matters for panel builders and system integrators

In modern control panels, variable frequency drives are everywhere. They improve energy efficiency, give you speed control, and help meet tightening process and energy targets. It is completely natural to ask a simple question: "Can I put my load monitor after the inverter so I can see true motor load?"

From a wiring point of view it looks tempting. From a measurement and protection point of view it is the wrong place to be. If you mount a traditional kW or current based load monitor on the output of a VFD, you are asking a sinusoidal measuring device to make sense of a chopped, high frequency PWM waveform. The result is not better protection. It is unreliable protection.

What a load monitor expects to see

Most external load monitors, whether they are current based, power factor based, or kW (real power) based, are designed on one simple assumption: they are connected to a fixed frequency, fixed voltage, sinusoidal supply.

Under those conditions, a load monitor can:

• Measure voltage and current cleanly
• Calculate kW, kVA, and power factor with good accuracy
• Track mechanical load in a predictable and repeatable way
• Detect underload and overload against stable thresholds

As soon as you introduce an inverter between the supply and the motor, that neat world disappears on the motor side.

What actually comes out of a VFD

On the input side, the drive sees a normal three phase sinusoidal supply. On the output side, it produces a very different waveform:

• PWM (pulse width modulated) voltage instead of a clean sine wave
• High dv/dt edges at the switching frequency of the IGBTs
• Variable frequency as the drive accelerates and decelerates the motor
• Harmonic rich current that depends on both the motor and the cable
• Common mode noise and leakage currents through motor and cable capacitance

To the drive this is normal. It is designed to control and monitor that waveform. To a traditional external load monitor it looks like electrical chaos.

Why a load monitor cannot measure accurately after an inverter

When you place a load monitor on the VFD output, several technical problems appear immediately:

• Frequency is not constant. Many measurement algorithms assume 50 Hz. When the drive is at 12 Hz, 35 Hz, or ramping quickly, the monitor's averaging and timing are no longer valid.
• The waveform is not sinusoidal. Harmonics from PWM switching distort both current and voltage. A device expecting a sine wave calculates incorrect kW and power factor values.
• CTs and voltage inputs are stressed. High dv/dt and switching noise can push transformers and measurement inputs outside their intended operating range.
• Noise appears as "false load changes". Common mode currents, cable effects, and filter components can make the measured power jump about even when mechanical load is steady.
• Trip points become meaningless. Underload and overload thresholds that work perfectly on a direct-on-line motor become unstable or impossible to tune on a VFD output.

The bottom line is simple. The monitor has no trustworthy reference. Whatever value it shows, you cannot be confident that it represents true shaft load.

Real world consequences for your panel

When a load monitor is installed after an inverter, panel builders and integrators routinely see:

• False trips during acceleration and deceleration
• Unstable analogue outputs that are impossible to scale sensibly
• Missed faults because thresholds have been widened to avoid nuisance alarms
• Increased commissioning time while engineers chase problems that are not mechanical at all
• Customer frustration when a "clever" protection scheme behaves unpredictably in the field

In extreme cases, the high frequency content on the drive output can also stress measurement inputs to the point where premature failure is more likely.

Where a load monitor should be installed in a VFD system

There are two distinct electrical environments in a drive fed system:

• Supply side (line side) - fixed frequency, sinusoidal, generally clean
• Motor side (load side) - variable frequency, PWM, high dv/dt, noisy

External load monitors belong on the supply side. When installed on the mains feeding the VFD they can:

• Measure clean voltage and current
• Calculate kW with known accuracy
• Track changes in process load indirectly through input power
• Provide useful alarm and analogue signals to the PLC or SCADA system

They cannot directly see shaft power on the motor. For most applications this is acceptable if all you need is a good indication of whether the process is running light, normal, or heavy.

How to monitor load on an inverter driven motor correctly

If you really need detailed motor side information, the correct place to start is not an external monitor but the drive itself. Modern VFDs typically provide:

• Estimated torque and load percentage based on internal motor models
• Motor current, voltage, and frequency as live values
• Built in underload and overload protection with configurable time curves
• Digital and analogue outputs that can be mapped to load or fault states
• Fieldbus or Ethernet communication for higher level monitoring

For specialist applications where neither drive data nor line side power is sufficient, you can look at:

• Dedicated shaft power transducers
• Torque sensors on the driven equipment
• Process feedback such as pressure, flow, or tension

These solutions are designed with VFD environments in mind and do not rely on trying to make sense of a PWM waveform with a sinusoidal meter.

Design checklist for your next VFD panel

When you specify protection and monitoring around an inverter driven motor, use this simple checklist:

• Install any external kW or current based load monitor on the supply side, not the drive output
• Use the VFD's own protection functions for motor side overload and underload
• Bring the drive's load, torque, and alarm signals into your PLC or smart relay
• Consider process variable monitoring (pressure, flow, level, torque) where appropriate
• Document clearly in your panel drawings that no external monitor is to be wired on the VFD output

Need help choosing the right monitoring strategy?

If you are unsure how to combine external load monitoring with VFD based protection in your next panel design, our team at Charter Controls can help you match the right devices to the right side of the system.

Contact Charter Controls for technical support and application advice