When you turn on your car, the tachometer spikes up as the engine rumbles to life. RPMs skyrocket rapidly to around 1,500 before settling out at a comfortable 600 to 1,000 level. Think about the colder months when the car’s engine has to work a bit harder. That spike is even more exacerbated and grating. All motors require this spike in energy to get their gears shifting. Inertia eases the energy consumption, but starting from zero against gravity necessitates gumption. 

This gumption is what engineers call “locked rotor amps.” Locked rotor amperage is that large blow of energy within the first half second of a motor’s startup to get things moving. The aggressive overcharge takes a toll on its compatriots, but is understood as an expected and rationalized side effect. However, with VFDs, or Variable Frequency Drives, this problem can be solved now, not just cast aside as a future obstacle.

According to an article from the electrical company Jade Learning, these “inrush currents” or the high-voltage currents required for startup, reach levels 20 times higher than normal current levels. Once the motor starts rotating, the current is still fast but only four to eight times higher than a normal operating speed’s energy consumption.

Above: Inrush currents on motors with and without VFDs from Newark.

This jump in energy is okay for a single car or a couple of fan motors in one’s house, but imagine 50 people turning on their air conditioning units at the same time in a hotel. Each motor turned on is a jump in energy. Those numbers that reach heights 20 times higher than normal energy influxes add up rapidly to result in a surplus of energy consumption that isn’t even used for regular function.

Above: Variable frequency drive diagram from Newark

A VFD works by reorganizing the energy that goes into the motor, allowing it to gradually increase speed, no longer requiring locked rotor amps. VFDs are made of three parts: the rectifier, the filter and the inverter. AC, or alternating current (standard household electricity), is converted into DC (direct current) power in the rectifier using diodes. The filter reduces the amplitude of the rectified DC electricity and the inverter re-converts DC energy back into AC at a specific voltage and frequency as needed by the motor’s target speed. To simplify this electrical journey, standard electricity is compressed, slowed down and then reconfigured to your motor’s specifications.

Without VFDs, locked rotor ramps can have significant consequences from their inrush currents. According to a technical article from Electro-Mechanical Authority (EASA), locked rotor amps can cause overheating of stator windings and rotors, an overuse of protective troubleshooting devices and an overdraw from supply voltage. The sudden high-torque production can also wear down on the machinery and increase the exponential decay of performance outputs or startup ability. 

Motor-driven equipment accounts for 64 percent of total electricity consumed. The energy consumption that can be reduced with this technology not only saves surmountable expenses, but it also protects equipment to last longer and reduces carbon emissions, protecting our environment and working for a greater cause. 

Northrich Co. trusts Danfoss for their reliability and purpose to build a better future in HVAC. For more information on the VFD market and how to find one to best-suit your needs, contact Northrich company at 216-581-4750 or go to www.northrich.com.