Standard_Drives_Guide

Industry White Papers Glossary– Drives Terminology

Linear Acceleration/Deceleration: A circuit that controls the rate at which the motor is allowed to accelerate to a set speed or decelerate to zero speed. On most drives, this circuit is adjustable and can be set to accommodate a particular application. Linearity: A measure of how closely a characteristic follows a straight line function. Locked-Rotor Current: Steady-state current taken from the line with the rotor at standstill (at rated voltage and frequency). This is the current when starting the motor and load. Locked-Rotor Torque: The minimum torque that a motor will develop at rest for all angular positions of the rotor (with rated voltage applied at rated frequency). Mechanical Safe Speed: Defined by the operating limits of the driven equipment being controlled by the adjustable frequency drive which should not be exceeded. The manufacturer’s specifi- cation is to be followed carefully. Megger Test: A test used to measure an insulation system’s resistance. This is usually measured in megohms and tested by passing a high voltage at low current through the motor windings and measuring the resistance of the various insulation systems. NEC: The National Electric Code contains recommendations of the National Fire Protection Association and is revised every three years. City or state regulations may differ from code regulations and take precedence over NEC rules. Negative Feedback: A condition where feedback is subtractive to the input reference signal. Negative feedback forms the basis for automatic systems. NEMA: The National Electrical Manufacturers Association is a non-profit organization organized and supported by manufactur- ers of electrical equipment and supplies. Some of the standards NEMA specifies are: horsepower ratings, speeds, frame sizes and dimensions, torques, and enclosures. Open Loop: A control system that lacks feedback. Operating/Service Deviation: A means of specifying the speed regulating performance of a drive controller, generally in percent of base speed. Operating deviation defines speed change due to load change and typically assumes: 1. A change from one steady-state load value to another (not transient). 2. A 95% maximum load change. Service deviation defines speed change due to changes in ambi- ent conditions greater than typical variations (noted in basic adjustable frequency drive specifications). Overload Capacity: The ability of the drive to withstand currents beyond the system’s continuous rating. It is normally specified as a percentage of full-load current for a specified time period. Overload capacity is defined by NEMA as 150% of rated full-load current for one minute. Overshoot: The amount that a controlled variable exceeds desired value after a change of input. PCC (Point of Common Coupling): The electrical connecting point or interface between the utility distribution system and the user’s electrical distribution system. The selection of the PCC within the system is often done by the utility.

Hysteresis Loss: The resistance offered by materials to becoming magnetized results in energy being expended and corresponding loss. Hysteresis loss in a magnetic circuit is the energy expended to magnetize and demagnetize the core. IGBT (Insulated Gate Bipolar Transistor): Semiconductor devices used for power circuits that are capable of high power output that operate at carrier frequencies of 20 kHz or more. IGBTs have a faster rate of rise resulting in high di/dt that occurs when the device is turned off. Induction Motor: An alternating current motor in which the pri- mary winding on one member (usually the stator) is connected to the power source. A secondary winding on the other member (usually the rotor) carries the induced current. For a squirrel cage induction motor, there is no physical electrical connection to the secondary winding; its current is induced. Inertia: A measure of a body’s resistance to changes in velocity, whether the body is at rest or moving at a constant velocity. The velocity can be either linear or rotational. The moment of inertia (WK 2 ) is the product of the weight (W) of an object and the square of the radius of gyration (K 2 ). The radius of gyration is a measure of how the mass of the object is distributed about the axis of rotation. WK 2 is usually expressed in units of lb/ft 2 . Instability: A situation where the output of a system does not track the input. For closed loop systems, the control system error is either increasing or oscillating. Intermittent Duty (INT): A motor that never reaches equilibrium temperature (equilibrium), but is permitted to cool down between operations. For example, a crane, hoist, or machine tool motor is often rated for 15 or 30 minute duty. Interposing Relay: A relay that accepts control signals of one logic level in order to provide isolated contact signals in a circuit operating a different logic level. Inverter: A term commonly used for an ac adjustable frequency drive. An inverter is also used to describe a particular section of an ac drive. This section uses the dc voltage from a previous cir- cuit stage (intermediate dc circuit) to produce an ac current or voltage having the desired frequency. Isolation Transformer: A transformer that electrically separates the drive from the ac power line. An isolation transformer pro- vides the following advantages: 1. Guards against inadvertent grounding of plant power lines through grounds in the drive. 2. Enhances protection of semiconductors from line voltage transients. 3. Reduces disturbances from other solid-state control equipment, such as drives without isolation transformers, time clock systems, electronic counters, etc. Jogging: Means of accomplishing momentary motor movement by repetitive closure of a circuit using a single push button or contact element. Kinetic Energy: The energy of motion possessed by a body.

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