| Across-the-line Start | When an AC motor is started with utility power. The initial inrush is great when performing an across-the-line start, which is why softstarters or VFDs are used instead. | Article: 4.6.4 Motor Start-up or Inrush Currents | | |
| Active Front End (AFE) | A VFD input configuration that uses IGBTs to reduce harmonics and improve power factor. | Rockwell Automation – AFE Overview | | |
| Active Power | The real power consumed by the motor to perform mechanical work, measured in watts (W). | Schneider Electric: Differences between Active Power, Reactive Power, and Apparent Power | | |
| Adaptive Tuning | A control method where the drive automatically adjusts its parameters in response to changes in load or operating conditions. | Rockwell Automation Video: PowerFlex 6000T MV Drive – Adaptive Tuning Training Tutorial | | |
| Adjustable Speed Drive | Another term for “Variable Frequency Drive” | | | |
| Affinity Laws | The Affinity Laws of centrifugal pumps and fans essentially tell us: Flow is proportional to speed, torque and pressure is proportional to speed change squared, and power is proportional to speed change cubed. In a nutshell, on a variable torque load, a small reduction in speed is an exponential reduction in power consumed. The Affinity Laws explain how VFDs can be used to save energy on variable torque loads. | EMA Blog Post: Energy Savings For Medium Voltage Applications Using VFDs | | |
| Amps | Short for “ampere”, amp is the unit of measure for electrical current. Typically, it is represented by “I” in equations. When sizing VFDs to motors, it is a good practice to size by amps. The VFD’s amp rating should meet or exceed the amp rating of the motor. | | | |
| Analog Input | An analog signal has an infinite range of values, it runs from a minimum to a maximum range and everything in between. In VFDs, analog inputs are used most often to give the frequency reference to the drive. Most common analog signals are voltage signals in 0-10VDC and current signals in 4-20mA. Because there is an infinite range, analog inputs can be scaled via the VFD parameters for better precision or to make up for inaccurate signals. | Engineering Essentials: A Closer Look at the I’s and O’s of VFDs | | |
| Analog Output | An analog signal has an infinite range of values, it runs from a minimum to a maximum range and everything in between. In VFDs, analog inputs are used to give feedback to a user’s control system and/or PLC of drive operating conditions such as current, frequency, and voltage. Because there is an infinite range, analog outputs can be scaled via the VFD parameters for better precision or to make up for inaccurate signals. | Engineering Essentials: A Closer Look at the I’s and O’s of VFDs | | |
| Apparent Power | Apparent power is measured in volt-amperes (VA) and reflects the total power flowing through the circuit, including both the real power being used and the reactive power required to maintain voltage levels. While active power directly contributes to performing tasks, apparent power accounts for the total power capacity needed in an electrical system. | Engineering Essentials: A Closer Look at the I’s and O’s of VFDs | | |
| Arc Flash | A rapid release of electrical energy that results in extremely high temperature conductive plasma and gases. An arc flash incident can cause severe burns resulting in serious injury and/or death | VIDEO: Dangers of arc flash | | |
| Arrestor | In electronics, arrestors are devices used to protect critical equipment from voltage surges. Typically, arrestors give energy a better path to ground instead through the items you’re trying to protect. | Surge Arresters: fundamentals of surge arresters | | |
| Automatic Restart | A VFD feature allowing the drive to restart automatically after a fault or power loss once predefined conditions are met. | Yaskawa Tech Note – Automatic Restart Feature | | |
| Bearing Pitting | A phenomenon where bearing damage is caused by undesirable shaft currents due to the non-sinusoidal output of variable frequency drives. Some common ways to mitigate bearing pitting issues are: grounding rings for the motor shaft, insulated bearings, and special motors designed to prevent bearing pitting. | Article: Why Do VFDs Cause Bearing Damage? | | |
| Bearing Pitting | Often caused by VFDs, bearing pitting occurs when unwanted electrical currents build up in the rotor and discharge through the shaft, causing premature wear or failure. | Video: Prevent VFD Induced Motor Bearing Damage | | |
| Braking Torque | This is the torque required to stop a given load. If a high amount of braking torque is needed on a load driven by a VFD, various braking methods may be necessary to prevent over-voltage trips. | Brake chopper circuits in VFDs | | |
| Bypass | VFD manufacturers utilizes bypass circuitry to remove the drive from controlling the motor and bring to motor across-the-line. In smaller, low voltage applications bypass is accomplished by contactors to bring isolate the drive’s output from the motor and bring utility power directly to it. In larger applications, a synchronous bypass may be used for a “bumpless” transition | Article: How to Bypass a VFD and Run a Three-Phase Motor Using DOL Starter | | |
| Capacitor | A device that stores electric charge. On a VFD, capacitors bridge the gap between the converter and inverter section and help smooth out the voltage on the drive. Capacitors are often electrolytic but there are also film-type and oil-filled capacitors. | VIDEO: Capacitors Explained – The basics how capacitors work working principle | | |
| Carrier Frequency | The switching frequency of the PWM signal used by a VFD to control motor voltage and current. | Article: VFD Switching Frequency | | |
| Cascaded H-Bridge Topology | Cascaded H-Bridge Topology, when referring to Medium Voltage VFDs, is a tried and true topology that uses low voltage components to create a low voltage output that is then cascaded to create a medium voltage output. This topology was patented by Siemens/Robicon until somewhat recently. Since then, many other manufacturer’s drives have entered the market using a similar topology. | EMA Blog Post: Medium Voltage VFD Topology-Many Ways To Create An Output | | |
| Cell Control Board | This board on medium voltage VFDs controls individual power cells and is often mounted to it. They contain the power supply and driver components necessary for the cell to work properly. | Find Cell Control Boards | | |
| Choke | In the VFD world, a choke is typically in series between the rectifier section and the DC bus . A choke is a single phase inductor that is used to smooth out ripple of the DC bus and resist sudden change due to voltage disturbances like sags and swells. They also provide harmonic mitigation. | Motion Control Tips: What is a DC bus choke and why is it used? | | |
| Circuit | A closed path through which electric current can flow. | Britannica: Electric Circuit | | |
| Circuit Breaker | A circuit breaker is an electrical switch that protects a circuit from overcurrent, overload, and/or short circuit. It breaks the path of current flow in such events to protect devices upstream and downstream of the circuit. Semiconductor devices within VFDs like diodes and IGBTs, typically short circuit in a very short amount of time so a tripped circuit breaker is typically an indication that a catastrophic failure has taken place. The VFD should be static checked at the very least BEFORE resetting a breaker. | Article: Circuit Breaker Fundamentals | | |
| Closed-loop Control | A control system where motor feedback (e.g., speed or position) is continuously monitored and used to adjust drive output. Feedback is typically given by an encoder or a resolver. | KEB Blog – How Does Closed Loop Control Work in a VFD? | | |
| Common Mode Voltage | A negative result of the switching produced by VFDs. The signwaves produced by VFDs are not sinusoidal, which results in damaging common mode over voltage. | MTE – Common Mode Noise Overview and Reduction Guide | | |
| Constant Horsepower Load | A type of load driven by a VFD. Constant horsepower applications run above motor base speed on applications like spindles. As the speed changes, the load torque will decrease at a rate inversely proportional with speed. | 3 Types of Loads (Constant Torque/Constant Power/Variable Torque) | | |
| Constant Torque Load | A type of load driven by a VFD. In constant torque loads, torque loading is NOT a function of the speed so using VFDs to reduce the speed on constant torque loads does not increase energy savings like a variable torque load. Constant torque loads draw full load current at any speed. | EMA Blog Post: An Overview of Variable Frequency Drives | | |
| Control Board | Also called the “logic board”, this is the part of the VFD that accepts “real world” commands and controls the drive to accomplish those commands. It is typically where the drive’s parameters are stored. The control board is the “brain” of the VFD. | | | |
| Current | Current is the speed at which electrons flow through a conductor. Measured in amperes AKA “amps”. | VIDEO: Basic Electricity – What is an amp? | | |
| Current Limit | Current Limit is the drive attempting to clamp the current to the motor in order to avoid overload or overcurrent. One variable torque loads, the drive will often “roll back” the frequency in an attempt to lower the current | EMA Blog Post: Common Faults On Medium Voltage Drives And How To Troubleshoot Them | | |
| Current Source | Current Source drives utilize a very mature topology. As opposed to voltage source topology, current source topology does not use a dc bus. Also called “active front end”, current source drives use large inductors to regulate current ripple and store potential energy for the motor. The output of these drives is composed of GTOs or SGCTs that turn off and on, regulating the output to the motor | Article: Inverter Topologies: Voltage-Source or Current-Source | | |
| Current Transformer (CT) | CTs are used to measure current draw. They work by reducing the current running through them to a much lower level so that they can be read by instrumentation such as meters and the circuitry of VFDs. | Electronics Tutorials: The Current Transformer | | |
| DC bus | Also called the “DC Link”, it refers to the part of the drive that supplies the desired DC voltage and current to the inverter section. This is typically made of capacitors to smooth and store the DC voltage. | | | |
| DC Link Capacitor | See “Capacitor” | | | |
| Digital Input | A digital signal is either on or off (sometimes called “high” or “low”). In VFDs, using digital inputs is a very common method to control VFDs. By utilizing relays, switches, and other devices, the drive can be “told” what to do. Digital inputs can be used to tell the VFD a variety of commands like start, stop, enable, shut down on safety inputs, etc. DI’s are typically easy to troubleshoot because it is either off or on. | Engineering Essentials: A Closer Look at the I’s and O’s of VFDs | | |
| Digital Output | A digital signal is either on or off (sometimes called “high” or “low”). In VFDs, using digital outputs is a very common method to give status or feedback to user’s control systems and/or PLCs. Modern drives typically have 1 or more relays that change state depending on how they’re programmed. Drive manufacturers also utilize photocouple-type digital outputs that are not rated as high as a true relay. Usings digital outputs is a great way to get status feedback from a drive such as run and fault status. | Engineering Essentials: A Closer Look at the I’s and O’s of VFDs | | |
| Diode | A device that allows current to flow in one direction and impedes it in the other direction, much like a check valve. Diodes are used in the converter section of a VFD | VIDEO: Diodes Explained – The basics how diodes work working principle pn junction | | |
| Disconnect Switches | Safety devices used to de-energize an electrical circuit. There are many different types available including ones integrated with circuit breakers. When working on VFDs, a disconnect upstream of the drive should be locked out and tagged out for safety. | Video: What is a disconnect switch | | |
| Drive Isolation Transformer | A transformer placed between the VFD and power source to isolate the drive and reduce harmonics or common mode currents. | Eaton Drive Isolation Transformers | | |
| Driver Board | A driver board is the generic term for the circuit board that “fires” or “drives” the transistors on and off to create an output to the motor. If a driver board fails, it can cause a misfire which can damage its associated transistor. If you find a failed IGBT in a drive, it is advisable to also change its associated driver board | EMA Parts | | |
| Dynamic Braking | A method for stopping or slowing a motor by dissipating energy through a resistor rather than returning it to the power supply. | Yaskawa eLearning Module: Braking Methods of Electric Motors | | |
| Eaton | A major manufacturer of not only variable frequency drives but also switchgear, components, and other industrial automation. | Eaton Drives and Controls | | |
| Electrolytic Capacitor | A capacitor is a device that stores charge and are the primary component used on voltage-source VFDs for the DC bus. Electrolytic capacitors are made of organic material that have a useable life of around 7-10 years. If your VFD is within that time frame, it may be time to think about changing the capacitors. | Electrolytic Capacitors: The Ticking Bomb | | |
| Electromagnetic Interference (EMI) | Unwanted electromagnetic signals generated by VFDs that can disrupt nearby electronic equipment. | Article: Introduction to electromagnetic interference with VFDs | | |
| Encoder | A sensor that provides feedback on motor position, speed, or direction, used in closed-loop VFD systems. | Article: What is an Encoder? | | |
| Fault Codes | Many VFDs give fault codes, which are designed to help with troubleshooting. Fault codes or “error codes” differ by manufacturer but help users narrow down what the problem is within or outside the VFD. | EMA Blog Post: Common Faults On Medium Voltage Drives And How To Troubleshoot Them | | |
| Field Weakening | A control technique allowing the motor to operate above its rated speed by reducing magnetic flux in the motor field. | Article: Basics – Field Weakening | | |
| Flying Start | A feature that allows a VFD to start a motor that is already spinning without causing a fault or damage. | LS Electric Video: AC Drive iS7 Flying Start Function Setting | | |
| Fuse | An electrical safety device that protects circuits from overcurrent by opening and interrupting the circuit. Fuses are designed to prevent fire, explosion, and safety. They do not necessarily protect semi-conductor devices from failing. Fuses typical blow as a result of a shorted semi-conductor. | VIDEO: How Does A Fuse Work? | | |
| Ground Fault | A fault that occurs when the drive’s circuitry detects leakage to ground from any point in the motor circuit. This is done by calculating the output summation current or by a ground fault CT. | Yaskawa Solutions Center: Troubleshooting a Ground Fault | | |
| Harmonic Filter | Equipment used to reduce harmonic distortion created by VFDs. They can be passive or active. | EMA Blog: Cutting Through The Noise: A Short Discussion On Harmonics | | |
| Harmonics | according to IEEE are “A sinusoidal component of a periodic waveform or quantity having a frequency that is a multiple of the fundamental frequency.” In layman’s terms: they are the unusable multiples of the useable fundamental frequency | EMA Blog Post: Why VFD Cause Harmonics And How To Mitigate Them | | |
| Heat Sink | A device attached to power electronics to dissipate heat and maintain operating temperatures. | EMA Blog: You May Be Killing Your Medium Voltage Drive | | |
| Horsepower | Horsepower is a measure of the rate at which work is being done. When a force is applied in a manner that produces motion, work can be measured. One horsepower = the force required to life 33,000 pounds, one foot in one minutes. 1 HP = 746 watts. | VIDEO: What Is Horsepower? | | |
| I/O | This is the anonym for “Input and Output”. I/O refers to the input and output controls to and from the VFD. I/O is typically how one operates the drive. Understanding what I/O is coming from the drive’s control system is important when troubleshooting and startup up VFDs. | | | |
| IEEE | Short for “Institute of Electrical and Electronics Engineers, IEEE is the world’s largest technical professional organization dedicated to advancing technology. | IEEE Homepage | | |
| IGBT | “Insulated Gate Bipolar Transistor”. IGBTs are used in the inverter section of VFDs to switch the voltage to the motor. | Insulated-Gate Bipolar Transistors (IGBTs) | | |
| Industrial Automation | Using controls systems, like VFDs, to automate processes and work more efficiently | Industrial Automation: How it Works, Types, and Benefits | | |
| Inrush Current | The initial current drawn by an electrical device when it is first turned on. Inrush is typically much higher than normal, operating current and is greatly reduced on AC motors by using softstarters and VFDs. | What is Inrush Current? | | |
| Insulation Resistance Testing | A diagnostic test to assess the health of motor insulation by measuring resistance levels to ground. | Fluke Video: How To Measure Insultation Resistance With The Fluke 1587 | | |
| Intermittent Load | A type of load drive by a VFD. Intermittent or “impact” loads have intermittent torque requirements which are not a function of the speed. This type of load is typical of punch presses and equipment that uses a flywheel. | | | |
| Jump Frequencies | In order to avoid continuous operation at a speed that causes resonance in driven machinery, the Drive can be programmed with jump frequencies that will not allow continued operation within specific frequency ranges. If a speed is commanded that falls within a dead band, or Jump Frequency, the Drive will clamp the frequency reference just below the dead band and only accelerate past it when the commanded speed rises above the upper end of the dead band, for increasing references. Similarly, the Drive will clamp the frequency reference just above the dead band and only decelerate past it when the command speed falls below the lower end of the dead band, for decreasing references. | | | |
| Liquid Cooled Cells | See “Power Cell”. In large drives with large heat loads, liquid cooling is used instead of air cooling because of size restrictions and overall efficiency. Repairing these cells is often very time consuming and requires special testing equipment. | EMA Blog Post: Repairing Siemens/Robicon Liquid Cooled Cells | | |
| Load Torque | The actual torque required by the mechanical load connected to the motor, critical for drive sizing and control. | Toshiba Tech Note- Speed Torque Curves | | |
| Lock out / Tag out (LOTO) Procedures | LOTO is a safety procedure to protect individuals from working on dangerous, live equipment. For equipment, like VFDs, the main power source suppling the VFD should be energized off, locked by all individuals who are working on that equipment, and tagged so that everyone knows who is performing the work. LOTO prevents anyone from accidently turning on equipment while someone is working on it. | OSHA Overview: Control of Hazardous Energy (Lockout/Tagout) | | |
| Logic Board | Also called the “control board”, this is the part of the VFD that accepts “real world” commands and controls the drive to accomplish those commands. It is typically where the drive’s parameters are stored. The logic board is the “brain” of the VFD. | | | |
| Low Voltage Drive (LVD) | A variable frequency drive in the 200 to 1,000 volt AC range. Typically 230V and 460V. | EMA Blog Post: YES, We Have 480 Volt Drives In Stock! | | |
| Medium Voltage Drive (MVD) | A variable speed drive in the 2,400 to 69,000 volt range. Most common are 2,400 and 4,160V. These drives operate much like low voltage drives but are typically more modular and require special safety training. | EMA Product: Medium Voltage Drives | | |
| Motor FLA | Stands for “Full Load Amps” and refers to the maximum current a motor will draw when running at full load . In the VFD world, you should always size VFDs to the motors they are running based on the FLA of the motor, not horsepower. The drive you’re matching the motor to should meet or exceed the FLA rating of the motor. The drive can always have a higher FLA than the motor, but never lower. | Article: How do you size a VFD? | | |
| Motor Protection Relay | A relay device that monitors motor operating conditions and trips the circuit during faults like overload, phase loss, or overheating. | Product: Multilin 869 | | |
| Multi-level Inverter | A VFD topology that uses multiple voltage levels to produce a smoother output waveform that is much more “motor friendly”. | EMA Blog Post: Medium Voltage VFD Topology: Many Ways To Create An Output | | |
| Multi-Meter | A digital multimeter is a tool used to troubleshoot and test electrical circuits. Typical DMMs test voltage and resistance but can have functions to test current, capacitance, inductance and various other things. | Fluke Multimeters | | |
| NEMA 1 | Indoor use to provide a degree of protection to personnel against access to hazardous parts and to personnel against access to hazardous parts and to provide a degree of protection of the equipment inside the enclosure against ingress of solid foreign objects (falling dirt). | NEMA.org | | |
| NEMA 12 | Constructed (without knockouts) for indoor use to provide a degree of protection to personnel against access to hazardous parts; to provide a degree of access to hazardous parts; to provide a degree of protection of the equipment inside the enclosure against ingress of solid foreign objects (falling dirt and circulating dust, lint, fibers, and flyings); and to provide a degree of protection with respect to harmful effects on the equipment due to the ingress of water (dripping and light splashing). | NEMA.org | | |
| NEMA 3R | Enclosures constructed for either indoor or outdoor use to provide a degree of protection to personnel against access to hazardous parts; to provide a degree of protection of the equipment inside the enclosure against ingress of solid foreign objects (falling dirt); to provide a degree of protection with respect to harmful provide a degree of protection with respect to harmful effects on the equipment due to the ingress of water (rain, sleet, snow); and that will be undamaged by the external formation of ice on the enclosure. | NEMA.org | | |
| NEMA 4 | Indoor or outdoor use to provide a degree of protection to personnel against access to hazardous parts; to provide a degree of protection of the equipment inside the enclosure against ingress of solid foreign objects (falling dirt and windblown dust); to provide a degree of protection with respect to harmful effects on the equipment due to the ingress of water (rain, sleet, snow, splashing water, and hose directed water); and , p g , ); that will be undamaged by the external formation of ice on the enclosure. | NEMA.org | | |
| NEMA 4X | Indoor or outdoor use to provide a degree of protection to personnel against access to hazardous parts; to provide a degree of protection of the equipment inside the enclosure against ingress of solid foreign objects (windblown dust); to provide a degree of protection with respect to harmful effects on the equipment due to thee ingress of water (rain, sleet, snow, splashing water, and hose directed water); that provides an additional level of protection against corrosion; and that will be undamaged by the external formation of ice on the enclosure | NEMA.org | | |
| NEMA Rating | The standard for degrees of protection for enclosures per NEMA (National Electrical Manufacturers Association) | NEMA.org | | |
| Oil-filled Capacitor | Oil-filled capacitors are often seen on small motor capacitors on things like HVAC units. In medium voltage VFDs, some manufacturers use oil-filled capacitors instead of the more common electrolytic capacitor to make up the DC bus of the drive. Oil filled capacitors have a much longer useable life (around 20-30 years) compared to electrolytic capacitors. They are heavier and more expensive than electrolytic capacitors but come in a variety of voltages and capacitance ranges. | | | |
| Open Circuit | An open circuit is the opposite of a short circuit. Opens occur when no current is flowing between two points due to a break in the circuit. This is typically from a broken wire or connection point. Resistance is infinite in an open circuit | What is an Open Circuit: A Deep Dive for Engineers | | |
| Overcurrent | This fault code alerts the user that the drive is being asked to provide much more current that it is rated for (typically 200-250% of its rated current). A shorted motor or motor wiring can cause overcurrent as well as failing output transistors and/or driver boards. | EMA Blog Post: Common Faults On Medium Voltage Drives And How To Troubleshoot Them | | |
| Overload | This fault code alerts the user that the motor is drawing more current than it is rated for. This has to be programmed properly into the drive for the drive to properly protect the motor. Overload typically occurs when something mechanical or environmental is increasing the load on the motor, therefore raising the current. | EMA Blog Post: Common Faults On Medium Voltage Drives And How To Troubleshoot Them | | |
| Overvoltage | This fault code alerts the user that the DC Bus of the drive is higher than it is rated for. Overvoltage is typically caused by regeneration, line spikes, or failing bus capacitors | EMA Blog Post: Common Faults On Medium Voltage Drives And How To Troubleshoot Them | | |
| Phase Loss | A fault condition where one phase in a three-phase system is lost, causing imbalance and potential motor damage. | Yaskawa Solutions Center: Troubleshooting Output Phase Loss | | |
| Potential Transformer (PT) | PTs are used for converting higher voltages to lower voltages. In medium voltage VFDs, PTs are often used to convert the medium voltage used on the incoming feed of the drive to low voltage like 120VAC for use of the controls | Video: What is a potential transformer | | |
| Power Cell | A.K.A “Power Arm” or “Power module”. These are typically used within medium voltage VFDs to make them more modular and easier to repair. Each cell is similar to a low voltage VFD with a DC bus and inverter section; some cells have the converter section as well. | EMA Service: Shop Repair | | |
| Power Factor Correction | Methods or equipment used to improve the power factor, reducing reactive power and improving efficiency. | Powerside: Power Factor Correction | | |
| Precharge | “Pre” or “Soft” Charging the DC Bus capacitors reduces the inrush current from current-hungry capacitors and reduces damage to transformers, diodes, and other devices. This is extremely common in low voltage VFD topology but some medium voltage drives do not utilize pre-charge circuitry and it can cause issues. | EMA Blog Post: Pre Charge on Siemens Robicon Perfect Harmony VFD | | |
| Preventative Maintenance | Performing regularly scheduled maintenance activities to prevent unexpected failures in the future. For medium voltage drives, we suggest a PM at least once a year. | EMA Service: Preventative Maintenance | | |
| Pulse Width Modulation | The method that modern VFDs use to switch the DC bus into a variable voltage, variable frequency sine wave to the motor. This is typically accomplished by switching IGBTs. | Video: How Pulse Width Modulation works in a VFD | | |
| Reactive Power | Reactive power is the alternating current flowing back and forth in an electric circuit. The unit used for reactive power is volt-amperes reactive (VAR) or kilovolt-ampere active (kVAR). R.P., a term also used for imaginary power or wattless power, is the form of energy stored and released by the reactive components within current flowing through an electric circuit–most commonly inductors (inductance) and capacitors (capacitance). This power is not useful to the circuit. | NEMA.org | | |
| Reactor | In electronics, a reactor is a passive device that provides additional circuit inductance which helps protect downstream devices from line spikes and surges. We often see reactors in series upstream of a VFD (line reactor) or in series between the VFD and the motor (load reactor). | Line and Load Reactor Basics | | |
| Rectifier | An electronic circuit that converts AC to DC, commonly used in the converter section of a VFD. | EMA Blog: Medium Voltage VFD Topology | | |
| Regenerative Drive | A drive capable of feeding energy back to the power grid during braking or deceleration. This is common in loads that require immediate stops like elevators, unwinders, and conveyors. | Video: Regenerative Solutions | | |
| Relay | An electrically operated switch. Relays are used to turn on and off low-power circuits. The most common type of relay is an electromechanical relay but there are many varieties available. Relay logic is often used to control VFDs and many VFDs have their own internal relays for status. | Wikipedia Relay Article | | |
| Reliability Centered Maintenance (RCM) | A preventative maintenance strategy designed to optimize maintenance programs by establishing safe minimum levels of equipment upkeep | Reliability-Centered Maintenance: What It Is & How It Maximizes Uptime | | |
| Resolver | An electromagnetic transducer that provides feedback to VFDs; they are typically used on servo applications. Resolvers are more precise than encoders. | Article: What is a Resolver? | | |
| Ride-Through | The ability of a VFD to continue operating during short voltage sags, avoiding nuisance trips. | ABB Paper: Power Loss Ride-Through In A Variable Speed Drive System | | |
| Robicon | The Robicon Corporation was a manufacturer of VFDs (among other things) that was purchased by Siemens Energy and Automation in 2005. Robicon’s Perfect Harmony Series Medium Voltage drives were extremely popular and many are still in use today | RobiconTechSupport.com | | |
| Rockwell | Rockwell is a major player in the variable frequency drive industry. Their Medium Voltage drive offerings include the Powerflex 6000 and 7000 series drives | Rockwell Automation | | |
| Root Cause Analysis | Sometimes referred to as “RCA”. Root cause analysis is the process of looking at a problem that has already happened and determining what caused it for the purpose of reducing or eliminating the chances of it happening again. RCA for VFDs often requires equipment, like Drivescan, that monitors the conditions before, during, and after the event in question. | Article: Why do variable speed drives fail and how do we test them? | | |
| RTD | Short for “Resistance Temperature Detector, RTDs are devices that measure temperature, often used in large motors to protect them from overheating. It is a resistive device that comes in all types of configurations. As temperature changes, the resistance of the RTD changes, which can be read by devices to determine temperature. | Video: What is an RTD | | |
| Scheduled Downtime | A planned “outage” is typical of an operating plant that wants to halt all processes for a period of time typically so that preventative maintenance can be performed on the equipment. This is a recommended practice to keep VFDs running. | EMA Blog Post: Things To Check When You PM Your Medium Voltage VFDs | | |
| SCR | Short for “Silicon Controlled Rectifier”, an SCR is similar to a diode but a gate has been added to turn it on and off. SCRs are used in the rectifier sections of VFDs for pre-charge as well as in softstarters and DC drives. It is interchangeable with the term “thyristor”. | Science Direct: Silicon-Controlled Rectifier | | |
| Sensorless Vector Control | A control method that estimates motor speed and position without physical sensors, using feedback from current and voltage. | Automation Direct Video: WEG CFW500 Sensorless Vector Control | | |
| Serial Communication | Serial communication is a method by which one can control and “talk to” a drive by using a network protocol such as BACnet, DeviceNet, Profibus, etc. Many drives have certain built-in protocols or an additional option card may need to be purchased. When choosing a VFD, it is important to make sure it has the ability to speak the protocol you plan on using. | | | |
| SGCT | Short for “Symmetrical Gate Commutated Thyristor”. SGCTs are used in the Rockwell Powerflex 7000 medium voltage drives to provide output to the motor. | EMA Blog Post: Powerflex 7000 Shorting Issue | | |
| Short | In electrical terms, a short or “Short Circuit” is when a low resistance connection is made between two conductors supplying power. Short circuits can cause a litany of issues and damage equipment due to the almost unlimited current flow that takes place. Proper upstream protection like circuit breakers and fuses are designed to open, interrupting the flow of current when a short occurs. | VIDEO: What is SHORT CIRCUIT – Explained with Example | Basics of Electronics | | |
| Siemens | Siemens is a major player in the variable frequency drive industry. They recently set up Innomotics, who now runs the motors and large drives division previously run by Siemens. | Siemens/Robicon Medium Voltage VFDs | | |
| Sinewave Filter | A sinewave filter is a LC circuit designed to change the square-wave type output from a VFD into a sinusoidal output that is much more “motor friendly”; sinewave filters are put in series between the output of a VFD and the motor. Sinewave filters significantly reduce the effects of dv/dt issues caused by the PWM output of VFDs. Sinewave filters are often used in applications where long motor lead length is necessary. | Video: TCI MotorShield Sine Wave Filter | | |
| Skip Frequencies | See “Jump Frequencies” | | | |
| Snubber | Snubbers are designed to impede voltage transients in electrical systems. In VFDs, there are often snubbers used to protect IGBTs from turn-on and turn-off voltage transients. There are a variety of waves to achieve this. | Article: What is a snubber and why do you want one? | | |
| Softstarter | Softstarters are devices that are used to reduce the initial inrush current drawn by an AC induction motor. A softstarter typically uses SCRs to “softly” start a motor by reducing the initial voltage. Most softstarters use contactors to bring the motor across the line once the motor is up to operating speed. | EMA Blog Post: VFD vs Softstarter: Which one do you need? | | |
| Switch Gear | Simply put: switchgear is equipment used in the transmission of electricity such as contactors, breakers, disconnects, and fuses. Typically there is some sort of switchgear upstream of medium voltage VFDs to help with power isolation. Switchgear is a broad term that covers all shapes, sizes, types, and voltage. | EMA Product: Switchgear | | |
| Synchronous Bypass | See “Bypass”. Also called “Synchronous transfer”. This process is often found in medium voltage drives that want to transfer control of the motor from the VFD to the utility line. This process allows a drive to bring multiple motors (at different times) up to speed and put them across the line. | Video: Sinamics PH Drives: Synchronous Transfer | | |
| Thermistor | A temperature-dependent resistor used in motors and drives for thermal protection and monitoring. | Omega – Thermistor Basics | | |
| Thyristor | See “SCR” | | | |
| Topology | Topology, when referring to VFDs, simply means how the drive is designed to operate. Current source and voltage source are two common topologies utilized by VFD manufacturers | EMA Blog Post: 5 Things To Consider When Choosing A Medium Voltage Drive | | |
| Torque | Simply put: Torque is a twisting force. Torque applied tends to produce rotation but there is such thing as static torque, where there is no motion produced. Torque is measured in foot pounds typically. | What is torque? | | |
| Torque Boost | A VFD feature that temporarily increases torque output during motor start or heavy load conditions to improve performance. | Video: Explaining The Torque Boost Drive Setting | | |
| Toshiba | Toshiba is a major player in the variable frequency drive industry. Their Medium Voltage drive offerings include the T300MV2. | Toshiba Medium Voltage VFDs | | |
| Total Harmonic Distortion | Also called “THD”, total harmonic distortion is the amount of voltage or current distortion due to harmonics. This is typically measured at the point of common coupling and utilities an fine or shut down facilities for high THD because it affects devices upstream. VFDs create harmonics inherently so a variety of harmonic mitigation tactics are used such as line reactors, DC Chokes, multi-pulse drives, and passive and active harmonic filters. | Why VFDs Cause Harmonics and How To Mitigate Them | | |
| Transformer | An electrical device that takes voltage and changes it to a different voltage. This is accomplished by electromagnetic induction. | Electrical Transformers What they are and how they work | | |
| Transistor | A transistor is a semi-conductor device that switches electrical signals. In VFDs, transistors are used as switches to regulate current and voltage to the motor. | ROHM Semiconductors: What is a Transistor? | | |
| Troubleshooting | Troubleshooting is the art of taking a large set of potential reasons for a problem and methodically narrowing it down to come to a reasonable conclusion as to what the problem is. This is a skill that some people inherently possess more than others. | EMA Blog Post: Medium Voltage VFD Troubleshooting Tips and Tricks | | |
| Undervoltage | This fault code alerts the user that the DC Bus of the drive is lower than it is rated for. This can be caused by sags in the line, failing capacitors, or single phasing the drive. | EMA Blog Post: Common Faults On Medium Voltage Drives And How To Troubleshoot Them | | |
| Unscheduled Downtime | When a piece of equipment fails unexpectedly and takes the process down. This often greatly affects operations and costs money. | How Remote Monitoring Medium Voltage Drives Saves Time and Headaches | | |
| V/f Ratio | The Voltage to Frequency (V/f) or Volts to Hertz (V/Hz) ratio refers to the amount of voltage the drive sends to the motor at a given frequency. Since VFDs vary the frequency as well as the voltage to the motor, this ratio changes depending on the speed and application of the motor. The V/f curve can be manipulated to increase or reduce torque for operational purposes. Most drives have built-in V/f curves that often work great but can be manipulated for more precision. | Application Note: The Effects of Adjusting V/Hz Patterns on Torque Production for Variable Torque Applications | | |
| Variable Frequency Drive (VFD) | A device used to vary the speed of a driven load; typically AC induction motors. A.K.A. “VFD”. EMA has been repairing, applying, and troubleshooting VFDs for well over 30 years! No One ANYWHERE Is Better At Drives Than We Are! | Video: What is a VFD | | |
| Variable Speed Drive | Another term for “Variable Frequency Drive” | | | |
| Variable Torque Load | A type of load driven by a VFD. In variable torque loads, torque loading is a function of the speed and it is an exponential relationship. Because of this, VFDs are used for energy savings on variable torque loads such as fans and pump. | EMA Blog Post: Energy Savings For Medium Voltage Applications Using VFDs | | |
| Voltage | The difference in electrical potential energy between two points. Measured in volts | VIDEO: Basic Electricity – What is voltage? | | |
| Voltage Sag | A short duration decrease in RMS voltage, which can affect VFD operation and cause trips. | | | |
| Voltage Source | Voltage Source topology is the most commonly used topology in VFDs and used almost exclusively in modern, low voltage drives. Voltage source topology utilizes diodes to rectify the AC voltage to DC which consists of capacitors. The inverter section draws power from the DC bus to create an output to the motor. Because of the larger physical size of a DC bus, current source topology is often used in very high horsepower, medium voltage applications. | Inverter Topologies: Voltage-source or current source | | |
