Variable Frequency Drives: A Technology Primer

In my 20+ years in electrical engineering, the first time I came across a variable frequency drive was on an HVAC (Heating, Cooling and Air Conditioning) drive system, in a manufacturing facility. Since then I have continued, to hear, and read of increased use, and advancement, of the technology.

What is a Variable Frequency Drive (VFD)?

In layman's terms, a VFD uses frequency and voltage to control the speed and torque of a motor.

This technology, although first implemented in the 1950's as a result of solid state electronics, didn't really make it commercial appearance until the microprocessor revolution of the 1980's. The 1990's saw reduction in physical size and improvements in reliability and application potential.

Explainer: What is a dV/dt filter? -“dV/dT” refers to the change in voltage over change in time. For VFD's, dV/dT is explained as the quick change in voltage at the beginning and/or end of the squared wave pulses that build the pulse width modulated (PWM) output of a VFD.

The potential is endless, however, there are electrical characteristics that are inherent to VFD technology that have had to be overcome to provide the flexibility, performance and energy saving factors of these systems.

The Ghost Of Pulse Wave Modulation (PWM)

Variable Frequency Drives are well known for their ability to control motor speed, saving energy and extend the lifespan of motors. In as much, as they make things easier, there is a catch – Pulse Width Modulation (PWM).

PWM is the technique VFDs use, to control motor speed, by varying the width of voltage pulses. While this works in theory and practice, it comes with a dark side – transient currents.

Transient Currents : When the VFD is turned on, it sends pulses of voltage to the motor. These pulses create transient currents, which can wreak havoc on the motor's insulation and bearings. Over time, this can lead to a premature failure of the motor.

To mitigate these transient currents, output reactors or dV/dt filters are used. These components will help flatten out the pulses and protect the supplied motor from damage. Also, proper grounding and bonding practices are crucial to maintain system integrity.

Explainer: What is a Transient Current? - an oscillatory or aperiodic current that flows in a circuit for a short time following an electromagnetic disturbance. - Webster

Navigating The VFD Wiring

Unlike standard motor controls, VFDs require particular cable characteristics.

An uninformed cable choice can lead to electromagnetic interference (EMI), voltage spikes, and signal distortion.

When selecting VFD cables consider the following:

Shielding: Shielded cables provide EMI (Electro-magnetic interference) protection when it comes to VFD installations. Without shielding, control signals can be unreliable due to harmonics interference.

Segmented Grounds: In most VFD cases, segmented grounds are required. This means separating the grounding conductor for your VFD from other system grounds. It stops ground loops, and reduces the risk of interference.

Cable Sizing: Undersized cables can cause voltage drops, which badly impact motor performance, and can trip protective devices, like breakers. Refer to the National Electrical Code (NEC), Canadian Electrical Code (CEC) or local code for compliance to cable sizing.

Explainer: What is Electromagnetic Interference (EMI)? - Electromagnetic interference is unwanted noise or other effects from electromagnetic radiation. - Collins

Renewables and Energy Conservation Revolution

VFDs are playing a big role in supporting energy efficiency efforts. They allow precise control of motors, optimizing energy consumption in a multitude of applications.

HVAC Systems: In commercial and Institutional buildings (Hospitals, Courthouses, Fire Stations, Malls, Stadiums, etc..), VFDs are used to control HVAC systems. They easily adjust fan and pump speeds according to required demand, reducing energy usage, and operating costs.

Renewable Energy: VFDs are also major components in green energy systems. They help control wind turbines, solar system components, and hydroelectric, for reaching maximum energy capture.

Pumping Stations: Water Treatment Plants (WTP) use VFDs to control pumping systems efficiently. By varying the pump speed to match demand, energy costs are reduced, and the life of equipment is extended.

Factoid: James Clerk Maxwell, Scientist and Poet - 1864 Famous oral presentation: “Dynamical theory of the electromagnetic field” presented to Royal Society containing ‘Maxwell’s Equations’ states “.. that it seems we have strong reason to conclude that light itself (including radiant heat and other radiations if any) is an electromagnetic disturbance in the form of waves propagated ……according to the electromagnetic laws” - James Clerk Maxwell Foundation

Final Drive

Variable Frequency Drives are a catalyst in power distribution and renewable energy applications. However, to have them work as expected, the engineering characteristics of pulse wave modulation and cable design have been incorporated by engineers and designers.

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