This piece was originally published in the June 2016 issue of ei, the magazine of the electroindustry.
Harvey Eure, Product Manager for Variable Speed Drives and Power Quality Products, Delta Products
There are initiatives bolstered by the emergence of the Industrial Internet of Things (IIoT) that involve monitoring and improving the efficiency of variable-frequency drive (VFD) systems and improving the performance of individual power components.
Today, VFD products have embedded computational capability that provides information that is IIoT ready. Improving the efficiency of components and reducing the overall power usage of the integrated drive system is important. The trend is to use the computed information from IIoT-ready products to improve overall production system efficiency.
A trend in industrial automation is to closely match motors with VFDs and other electrical equipment to improve system efficiency as an integrated drive system. Integrated drive systems include the motor, drive, and controls as one entity, from the electric utility to the production floor. The system may include a transformer, switchgear or circuit breaker, a VFD, an AC motor, mechanical coupling, and a pump or other machine providing work for the process. The integrated system efficiency is the ratio of output power to input power.
Affinity for System Efficiency
When improving system efficiency with industrial automation and VFDs, most engineers can relate to the affinity law of physics for centrifugal loads such as a fan or pump for energy savings.
As a pump or fan increases to 100-percent motor speed, less power is required to keep it operating at that speed because of the variable load. Therefore, reducing the motor speed significantly reduces the voltage and current used. For example, a 20-percent reduction in motor speed saves as much as 49 percent in electricity costs.
There are other ways to improve system efficiency with VFDs. A VFD uses variable voltage and frequency to control the speed of an AC motor. Typical AC motor types include asynchronous or induction motors, synchronous motors, and permanent magnet motors. A VFD with 97-percent efficiency experiences losses due to its electrical energy. Heat is generated through the resistance of its components.
In large transformers, energy is dissipated in the transformer primary and secondary windings and surrounding structure. Energy losses are the result of several factors, including eddy currents and winding resistance, and are given off as heat. Heat inside a transformer can rise as high as 338° F (170° C), depending on the transformer construction, before causing damage. Therefore, it must be cooled by circulating air or liquid through the transformer.
For transformers, copper is a more efficient material for electricity than aluminum, due to its higher conductivity rate. Aluminum wiring may develop oxidation over time and cause increased resistance. Copper does not experience the same degree of thermal expansion and contraction as aluminum; therefore, it is a more stable material to prevent arcing at electrical connections.
VFDs convert DC voltage to AC voltage to produce sinusoidal waveforms for the AC motor. A VFD contains power electronics, called insulated gate bipolar transistors (IGBT). When the IGBT is turned on, voltage and current flow to the AC motor. An IGBT is a power transistor that has high-speed switching and voltage output characteristics of a metal oxide semiconductor (MOS) field effect transistor (FET). The IGBT (figure 1) shows a bipolar transistor with an MOS gate structure and freewheeling diode. The freewheeling diode helps protect the IGBT in the event of overvoltage fed back through the emitter.
IGBTs switch on and off at a very high speed, called the switching frequency. The higher the switching frequency, the less distorted the sinusoidal wave output to the motor. Switching the IGBT off and on generates heat losses.
Switching speed for the IGBT is an important parameter affecting efficiency or loss. The IGBT experiences a loss when providing voltage and current for an inductive load such as motor windings. Heat losses that occur when the IGBT is in the on state are called conduction losses. Current flows through the substrate between the emitter and collector inside the IGBT and generates heat.
VFDs with active front-end devices can regenerate voltage back to the building’s electrical system. When a motor continues to turn without voltage applied, it generates its own magnetic field inside that produces voltage, similar to a generator. This voltage can be fed back to the electrical grid to offset electricity consumed.
Trending toward Better Power Factors
Improving electrical-system efficiency by improving power factor is a trend. Power factor is the measure of real current that produces real power to the motor and is expressed as a percentage of real power to apparent power. A VFD with power factor 95 percent or higher makes better use of its input line current therefore producing more efficient, real power to the AC motor.
AC motors require voltage in a sinusoidal wave form to reduce torque pulsation. The VFD waveforms can produce undesirable harmonic distortion current. The harmonic current component creates additional heating in motors and could disrupt electronic equipment in the building. Harmonics can be reduced with the use an active power filter for the building’s electrical system.
VFDs monitor input and output voltage and current to increase the efficiency of the drive. Motor protection management systems allow real-time monitoring of input and output voltage and current, power factor, harmonics, and motor temperatures. VFDs provide analog and digital signals to production network SCADA systems to allow the real-time monitoring of the drive functions. With real-time monitoring and efficiency and power factor improvements, integrated drive systems can reduce overall operating costs.
The IIoT allows access to data, such as kilowatt-hours, used in real time to make better economic decisions regarding the production process, rather than viewing historical data from energy dashboards. VFDs have real-time clocks and built-in programmable logic functions that allow data to be computed and then shared over industrial Ethernet IP networks. Ethernet IP allows access to drive functions and will allow more access by the industrial network systems.
Measuring and reducing power consumption for the integrated drive system is the new metric in improving system efficiency. Improving the overall integrated drive system efficiency with IIoT will produce greater results for the bottom line.
Mr. Eure handles variable-speed drives and power quality products for Delta Products in Research Triangle Park, North Carolina.