Electric Motor Generator Terminology

PMSM motor: A Brushless AC electric motor, similar to a brushless DC electric motor, whereby the magnetic field of the rotor is supplied by the permanent magnets rather than by electromagnets. However, the stator windings of brushless AC motors are sinusoidally distributed windings, while those of a brushless DC motor are salient field coils.


Direct drive: Drives that transmit power to the application directly without gears or other means of power transmission. 


Efficiency (motor): the ratio of output to input (effectiveness of energy conversion).


Efficiency (system): the overall efficiency of the motor, controls, electric cables, drive train and driven equipment. This is determined by multiplying the individual component efficiencies together. 


Power density: the power to weight ratio of a motor.


Torque constant: The rate at which torque increases with respect to current.


Back EMF (BEMF): the voltage generated when a permanent magnet motor is rotated. This is proportional to motor speed and is present whenever the rotor turns.


Neodymium magnet: A type of permanent magnet with Neodymium in the material structure (NdB Fe).


Samarium Magnet: A type of permanent magnet with Samarium Cobalt (SmCo) in the material structure.


Curie temperature: A characteristic property of a ferromagnetic material, often used as a reference point for assessing magnet capabilities.


Demagnetizing current:  The current at which the motor magnets will start to demagnetize.


Core losses: Losses in the core material hysteresis and eddy current losses. A core is the magnetic steel around which the windings of a motor are built. 


Eddy current: Localized circulating currents induced in any material by alternating magnetic flux, causing losses and heating.


Flux density: The density of the magnetic field at any point in space.


Magnetic flux: A measure of quantity of magnetism, taking into account the strength and the extent of a magnetic field.


Saliency: the variation of the inductance at the motor terminal according to the rotor position. Also referred to as inductance saliency or magnetic saliency.


Permeability: A measure of how easily a magnetic field flows through a material.


Saturation: non-linear property whereby it becomes increasingly difficult to force additional magnetic flux through a material.


Resolver: an electromagnetic feedback device which converts angular shaft position into analog signals.


Power Electronics Terminology

AC (Alternating Current): The commonly available electric power supplied by an AC generator and is distributed in single- or three-phase forms. AC current changes its direction of flow with respect to time (cycles).


Ambient temperature: Temperature of the surroundings.


Ampere (Amp): The standard unit of electric current. The current produced by an electromotive force of one volt in a circuit having a resistance of one ohm equals to one ampere.


Back-EMF: Electromotive force generated when a conductor passes through a magnetic field. In a motor, it is generated any time the armature is moving in the field whether the motor is under power or not. The term "back" or "counter" EMF is referring to the polarity of the voltage and the direction of the current flow as being opposed to the supply voltage and current to the motor under power. For details on back-EMF, click here.


Back EMF constant (V/RPM): The constant corresponding to the relationship between the induced voltage in the rotor and the speed of rotation. In brushless motors, the back-EMF constant is the constant corresponding to the relationship between the induced voltage in the motor phases and the rotational speed.


Bridge Rectifier (Diode): A diode bridge rectifier is a non-controlled full wave rectifier that produces a DC voltage at the output when supplied by alternating voltage at the input. For details on bridge diode, click here.


Capacitance: As the measure of electrical storage potential of a capacitor, the unit of capacitance is the farad, but typical values are expressed in microfarads.


Capacitor: A device that stores electrical energy. 


Closed-loop: Describes a system where a measured output value is compared to a desired input value and corrected accordingly (e.g. a servomotor system).


Controller: Used to describe collective group of electronics that control an electrical device (e.g. motor, drive, indexer, etc.).


Converter: The process of changing AC to DC or DC to AC or DC to DC. The term “converter” may also refer to the process of changing AC to DC to AC (e.g. adjustable frequency drive). A “frequency converter,” such as that found in an adjustable frequency drive, consists of a Rectifier, a DC Intermediate Circuit, and Inverter and a Control Unit.


Current: The flow of electrons through a conducting material. By convention, current is considered to flow from positive to negative potential. The electrons, however, actually flow in the opposite direction. The unit of measurement is the Ampere.


Direct Torque Control: A method of using current regulation circuitry to regulate torque instead of speed.


Drive: (Also called a Variable Speed Drive) An electronic device that can control the speed, torque, horsepower and direction of an AC or DC motor.


Eddy Current: Localized currents induced in a conductive material by a changing magnetic field. These currents translate into losses (heat) and their minimization is an important factor in motor design.


Efficiency: Ratio of output power over input power expressed in percent. In motors, it is the ratio of mechanical output to electrical input indicated by a percent i.e. the effectiveness with which a motor converts electrical energy into mechanical energy.


EMF: A commonly used acronym in motor design for electromotive force which is another term for voltage or potential difference.


Feedback: The element of a control system that provides an actual operation signal for comparison with the set point to establish an error signal used by the regulator circuit.


Field Weakening: The action of weakening the strength of the magnetic field in a rotating machine and thereby decreasing the back EMF that is produced. Consequently, the speed that the machine can be operated at can be increased. 


Filter: A device that passes signals within a desirable frequency range and attenuates all others.


Frequency: Frequency is the number of occurrences of a repeating event per unit time. The typical unit for frequency is Hertz (Hz). One Hz is defined as one cycle per second. The standard frequency that is used by North American power utilities is 60 Hz. Most of the rest of the world uses 50 Hz power.


Inductance: Inductance is the ratio of the voltage over the rate of change of current. The unit for inductance is the Henry (H). 


Inductor: An inductor is a passive electrical component, which resists changes in electric current passing through it by setting up a time-varying magnetic field within its core. The varying magnetic field induces a voltage in the conductor which opposes the change in current that created it.


Inverter: Typically describes an electronic device that converts DC voltage into AC voltage.


Laminations: The steel portion of a transformer or stator core of a motor make up a series of thin laminations (sheets) which are stacked and fastened together by cleats, rivets or welds. Laminations are used instead of a solid piece in order to reduce eddy-current losses.


Open Loop: A control system that lacks feedback.


Power: Work done per unit of time. Measured in horsepower or watts usually.


Power Factor: Power factor is the ratio of real Power (W) over apparent power (VA). In undistorted AC systems, it is also a measurement of the phase difference between the voltage and current. It is represented by the cosine of the angle of this phase difference.


PWM: Pulse width modulation. An acronym which describes a switch-mode control technique used in drives to control motor voltage and current. This control technique is used in contrast to linear or step wave control and offers the advantages of greatly improved efficiency.


Rectifier: A device that transforms alternating-current to direct-current.


Reluctance: The characteristics of a magnetic field which resist the flow of magnetic lines of force through it.


Resistance: It is the measure of opposition to current flow through a given medium [Ohm]. Substances with high resistances are called insulators and those with low resistances are called conductors. Those in between are known as semiconductors. The unit is the Ohm. 


Resistor: A device that resists the flow of electrical current for the purpose of operation, protection or control. There are two types of resistors - fixed and variable. A fixed resistor has a fixed value of ohms while a variable resistor is adjustable.


Resolution: The smallest distinguishable increment into which a quantity can be divided (e.g. position or shaft speed). For encoders, it is the number of unique electrically identified positions occurring in 360 degrees of input shaft rotation.


RMS Current: Root mean square current. Root-mean-square (RMS) refers to the most common mathematical method of defining the effective voltage or current of an AC wave. In an intermittent duty cycle application, the RMS current is equal to the value of steady state current which would produce the equivalent resistive heating over a long period of time.


Speed constant: Inverse of Back EMF constant. See Back EMF constant. 


Speed Range: The minimum and maximum at which an electric machine must operate safely under constant or variable torque load conditions.


Speed Regulation: In adjustable speed drive systems, speed regulation measures the motor and control's ability to maintain a constant preset speed despite changes in load from zero to 100%. It is expressed as a percentage of the drive system's rated full load speed.


Transducer: A device that converts one energy form to another (e.g. mechanical to electrical). Also, a device that when actuated by signals from one or more systems or media, can supply related signals to one or more other systems or media.


Transistor: A solid-state three-terminal device that allows amplification of signals and can be used for switching and control.


Total Harmonic Distortion (THD): A measurement of the harmonic distortion present and is defined as the ratio of the sum of the powers of all harmonic components to the power of the fundamental frequency. For details on total harmonic distortion, click here.


Voltage: The force that causes a current to flow in an electrical circuit. The unit is the Volt. 1. Volt is defined as the difference of electric potential between two points on a conductor that is carrying a constant current of one ampere when the power dissipated between those points is one watt.


Watt: The amount of power required to maintain a current of one ampere at a pressure of one volt when the two are in phase with each other. 


Work: A force moving an object over a distance. Work = force x distance travelled.


Magnetic Bearing Terminology

Magnetic Bearing: A type of bearing, particularly for rotating shafts, that uses electromagnetic forces to support the shaft with no mechanical contact.


Active Magnetic Bearing (AMB): A type of magnetic bearing in which non-contact support of a shaft is achieved by using closed-loop control. Such bearings use attractive magnetic forces where at least part of the magnetic force is produced by actively controlled electromagnets. The AMB includes a magnetic actuator, position sensor and magnetic bearing controller (MBC) with a control device, power amplifiers and power supply. Most industrial and commercial magnetic bearings are of this type.


Passive Magnetic Bearing: A type of magnetic bearing that does not require active control. While there are a wide variety of passive magnetic bearings, they have not found significant commercial applications due to low stiffness and low force density per unit surface area. 


Electromagnetic (EM) Biased Magnetic Bearing: A type of AMB in which the magnetic field has two components – a constant bias field and a variable control field, with the bias magnetic field being generated by an electrical current in a coil. The primary purpose of the bias is to linearize the applied force vs. coil current relationship of the actuator. Almost all commercial magnetic bearings are either EM Biased AMBs or Permanent Magnet (PM) Biased AMBs (see below). 


Permanent Magnet (PM) Biased Magnetic Bearing: A type of AMB in which the magnetic field has two components – a constant bias field and a variable control field, with the bias magnetic field being generated by a permanent magnet (PM). As with the EM Bias AMB (above), the primary purpose of the bias is to linearize the applied force vs. coil current relationship of the actuator. All commercial AMBs are either EM Biased AMBs or PM Biased AMBs. For details on PM Biased Active Magnetic Bearings, click here.


Radial Magnetic Bearing: Magnetic Bearing in which magnetic forces produce radial support for the shaft in two orthogonal axes.


Axial Magnetic Bearing (or, Thrust Bearing): Magnetic Bearing in which magnetic forces produce axial support for the shaft.


Combination Radial and Axial Magnetic Bearing (or, Combo bearing): Magnetic Bearing in which magnetic forces provide both axial and radial supports for the shaft. For details on Combo bearings, click here.


Magnetic Bearing Controller (MBC): Electronic hardware with all of the components needed to operate an AMB system. The MBC includes the electrical power supplies, power amplifiers, and analog and/or digital control processing components. All modern MBCs now use digital control as this provides the most flexibility for use with a wide variety of machines, but it also enables a wide variety of commissioning, diagnostic and monitoring capabilities. For details on Magnetic Bearing Controller, click here.


Magnetic Bearing Actuator (or simply, “Actuator”): A device capable of producing magnetic forces in a magnetic bearing.


Magnetic Bearing Position Sensor (or simply, “Position Sensor”):A device capable of sensing shaft displacement (position) with a wide bandwidth. In AMBs, information from the position sensors is used by the MBC to control the actuator coil currents, and thus the magnetic force on the shaft. Most often the position sensor will include a sensor head (transducer) in the machine adjacent to the magnetic actuator and drive/demodulation electronics in the MBC. For details on position sensors, click here.


Magnetic Bearing Speed Sensor (or simply, "Speed Sensor"): A speed sensor is a device capable of measuring rotational speed of the shaft about its spin axis. Information about the shaft speed is used by the MBC to ignore the synchronous component of the shaft position variations, thus allowing the shaft to spin about its center of mass. This feature minimizes transmission of the shaft vibrations due to unbalance to the housing of the machine and reduces the load on the magnetic bearings. 


Magnetic Bearing System: Actuators, position sensors, speed sensors and magnetic bearing controller, together form the complete control system – called Magnetic Bearing System or AMB System – to control the position of the shaft in an air-gap. In general, a magnetic bearing system has five control axes: four radial axes to control translation and rotation in two orthogonal planes and one axial axis to control translation in the axial or thrust direction. For details on how this system works to control shaft position, click here.


Back-up Bearing (also Auxiliary Bearing or Touchdown Bearing): Mechanical bearings, such as ball bearings or retainer rings, used to support the shaft and protect components from damage in the event that there is power interruption, there is a temporary overload of the magnetic bearing, or there is a failure of some component of the AMB system. For details on back up bearings, click here.


Levitation: Supporting a body (rotor) on a magnetic field. When an AMB is activated and controls the rotor to the desired position (the set point) via magnetic forces, the rotor is said to be levitated.


Magnetic Center of an AMB: Location in an AMB, typically near the geometric center, where the attractive magnetic forces are balanced in all directions and there is no net force on the shaft centered at this position.


Magnetic Bearing Negative Stiffness (or simply "Negative Stiffness"): The actuator position stiffness of the actuator due to bias flux at the nominal rotor position without an external load in an EM Biased or PM Biased AMB,. In a conventional spring/mass system, displacing the mass from the equilibrium position results in a spring force that pushes the mass back toward equilibrium – this is positive stiffness. In a biased AMB with no control, displacing the mass (shaft) from the quasi equilibrium position at magnetic center results in a force that pulls the shaft further away from the center – this a negative stiffness. Because of the negative stiffness, AMBs are unstable without closed-loop control.