Motors and Drives Demystified -- Terms and Definitions (M-Z)

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M-Z

Macro A macro is a pre-programmed set of defaults for all of the parameters, which are typical for the specified application. You typically select the macro that most closely defines the drive functions necessary for your particular application. After selecting the macro, you can modify or customize the macro to specifically conform to your application.

Mean-Time-Between-Failures (MTBF)--The average time that a device will operate before failure.

Meggar Test--A measurement an insulation system's resistance. This is usually 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.

Metal Oxide Varistor (MOV)--A surge protection device that has low resistance to a voltage spike above operating level. The spike is routed back to the AC line.

Motor Model A motor model is an electronic circuit or software modeling of the internal circuits of the asynchronous motor. Examples of inputs to the model are motor current, DC-bus voltage, and switching positions of the inverter.

Examples of outputs are calculated flux and calculated torque. Examples of internal parameters in the model are stator resistance, mutual inductance, and saturation coefficients. These parameters can be tuned during an identification run as part of the drive commissioning.

Multimeter Measures electric component functions and values such as voltage (volts), resistance (ohms), and current (amperes). Some multimeters also test the condition of diodes.

Multispeed Motor An induction motor that can obtain two, three, or four fixed speeds by the selection of various stator winding configurations.

NEC (National Electric Code)

Recommendations of the National Fire Protection Association. It is revised every 3 years. City or state regulations may differ from code regulations and take precedence over NEC rules.

NEMA The National Electrical Manufactures Associates is a nonprofit organization organized and supported by manufacturers of electrical equipment and supplies. Some of the standards NEMA specifies are HP ratings, speeds, frame sizes and dimensions, torque, and enclosures.

Nonlinear Load Any type of electrical equipment that changes or modifies the voltage or current waveform to one that is somewhat distorted. Prime examples of this type of load include personal computers, magnetic ballasts, electronic ballasts, and variable-speed DC and AC drives. Basically, a nonlinear load

is one that uses a "switch mode power supply" (a circuit that changes AC to DC. This supply takes voltage in "surges" not in a linear fashion.) OEM Original Equipment Manufacturer or Machine manufacturer.

Offset Deviation of a controlled variable from a fixed setpoint.

Open Loop A control system that does not use a feedback element.

ODP or Open Drip-proof Open drip-proof motors have ventilation openings that allow an exchange of cooling air to flow through the interior of the motor from the surroundings. The air is forced through the motor by fins on the end of the motor's rotor. Since outside air comes into contact with the stator windings, rotor, and air gap, ODP motors are suitable only for installation in clean, dry environments.

Open Machine (Open Motor) A machine having openings that allow external cooling air over and around the windings of the machine.

Drip-proof Machine is an open-type machine in which the ventilating openings are so constructed that successful operation is not interfered with when drops of liquid or solid particles strike or enter the enclosure at any angle from 0 to 15 degrees downward from vertical.

Splash-proof is an open machine in which the ventilating openings are so constructed that successful operation is not interfered with when drops of liquid or solid particles strike or enter the enclosure at any angle not greater than 100 degrees downward from the vertical.

Semi-guarded is an open machine in which part of the ventilating openings in the machine, normally in the top half, are guarded as in the case of a "guarded machine" but the others are left open.

Guarded Machine (NEMA standard) is an open machine in which all openings giving direct access to live metal or rotating parts (except smooth rotating surfaces) are limited in size by the structural parts or by the screens, baffles, grills, expanded metal, or other means to prevent accidental contact with hazardous parts. Openings giving direct access to such live or rotating parts shall not permit the passage of a cylindrical rod 0.75 inch in diameter.

Drip-proof Guarded Machine is a drip-proof machine whose ventilating openings are guarded in accordance with the definition of a guarded machine.

Open Externally Ventilated Machine is one which is ventilated by means of a separate motor driven blower-mounted on the machine enclosure. This machine is sometimes known as a blower-ventilated or a force-ventilated machine.

Open Pipe Ventilated Machine is an open machine except that it allows ventilating air inlet ducts or pipes connected to them. Air may be circulated by means integral with the machine or by means external to the machine (separately or forced ventilated).

Weather-Protected Machine is an open enclosure divided into two types:

1. Type 1 enclosures have ventilating passages constructed to minimize the entrance of rain, snow, and airborne particles and pre vent passage of a 0.75-inch-diameter cylindrical rod.

2. Type 2 enclosures provide additional protection through the design of their intake and exhaust ventilating passages. The pas sages are so arranged that wind and airborne particles blown into the machine can be discharged without entering directly into the electrical parts of the machine. Additional baffling is provided to minimize the possibility of moisture or dirt being carried inside the machine.

Operating/Service Deviation A means of specifying the speed-regulating performance of a drive controller generally in percentage of base speed.

Operating deviation defines speed change due to load change and typically assumes a change from one steady-state load value to another (not transient) and a 95% maximum load change.

Service deviation defines speed change due to changes in ambient conditions greater than these typical variations:

Condition change AC line voltage, +10%, -5% AC line frequency, + 3%, -3% Ambient temperature, 15ºC Output Reactors Inductors, also called reactors or chokes, placed in series with the output terminals of an variable-frequency drive. Inverter output reactors are usually located near the VFD.

Overcurrent This circuit shuts down the drive when a safe current level is exceeded (e.g., 375% instantaneous or 150% nominal RMS for 1 minute).

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 specific time period. Overload capacity is defined by NEMA as 150% of rated full load current for 1 minute for Standard Industrial DC Motors.

Overshoot The amount that a controlled variable exceeds desired value after a change of input.

Overvoltage This circuit shuts down the drive when a safe voltage level is exceeded.

Parallel Communications A digital communication method that transmits the bits of a message several at a time (usually 8 to 17 bits at a time), usually only used over distances of a few feet with electrical cables as the transmission medium.

Parallel Interface A type of digital interface using multiple data lines, each line transmitting one bit of data at a time.

Parallel Operation The type of electronic information transfer that occurs when all bits, bytes, or words are handled simultaneously.

Parity Bit An additional bit added to a memory word to make the sum of the number of 1's in a word always even parity or odd parity.

PI (PID) Control PI is an acronym for "proportional integral" control. PID is an acronym for "proportional integral derivative" control. In this type of motor control, two signals are sent to the drive. One signal acts as a process reference and the other acts as an actual signal (feedback) brought back from the pro cess. The drive compares the two signals and adjusts the output up or down to reduce the difference between the signals (zero error). The "D" function dictates how much error correction should take place per unit of time. This type of control maximizes the efficiency of the process. PI control is very useful in maintaining a process variable such as speed, flow, fluid level, pressure of a system, etc.

PLC--See Programmable Logic Controller.

Plugging--Motor braking provided by reversing either line voltage polarity or phase sequence so that the motor develops a counter-torque that exerts a retarding force to brake the motor.

Position Transducer--An electronic device (e.g., encoder or resolver) that measures actual position and converts this measurement into a feedback signal convenient for transmission. This signal may then be used as an input to a programmable controller that controls the positioning system.

Power--Work done per unit of time. Measured in horsepower or watts: 1 HP = 33,000 ft-lb/min. = 746 W.

Power Conversion Section--Electronic power devices that convert a fixed AC voltage into DC voltage (i.e., AC drive converter).

Power Factor (Displacement)--A measurement of the time phase difference between the fundamental voltage and fundamental current in an AC circuit. It represents the cosine of the angle of the phase difference.

Power Factor (Distortion)--A measurement of the ratio of the real power (kW) to the apparent power (kVA). Distortion power factor takes into account harmonic voltage and current distortion as well as voltage to current displacement.

Power Loss Ride-Through--The ability of a VFD to continue to operate for a period of time without input power. Typical power loss ride-through is 2 to 5 cycles (each cycle is 16 ms). For some drives, a more extensive circuit is used. If the supply to a VFD is lost, the drive may continue to run without external power supply utilizing the kinetic energy of the rotating motor and driven equipment.

The power loss ride-through time depends on the relationship between the load and the inertia of the rotating masses.

Power Output--The section of the drive that delivers adjustable voltage/frequency to the motor (AC drives). The output terminals are typically referred to as U2, V2, and W2.

Power Output Control --The circuit that automatically adjusts the switching of the power output subsection.

Plugging--Motor braking provided by reversing either line voltage polarity or phase sequence so that the motor develops a counter-torque that exerts a retarding force to brake the motor.

Position Transducer --An electronic device (e.g., encoder or resolver) that measures actual position and converts this measurement into a feedback signal convenient for transmission. This signal may then be used as an input to a programmable controller that controls the positioning system.

Power --Work done per unit of time. Measured in horsepower or watts: 1 HP = 33,000 ft-lb/min. = 746 W.

Power Conversion Section -- Electronic power devices that convert a fixed AC voltage into DC voltage (i.e., AC drive converter).

Power Factor (Displacement) --A measurement of the time phase difference between the fundamental voltage and fundamental current in an AC circuit. It represents the cosine of the angle of the phase difference.

Power Factor (Distortion) --A measurement of the ratio of the real power (kW) to the apparent power (kVA). Distortion power factor takes into account harmonic voltage and current distortion as well as voltage to current displacement.

Power Loss Ride-Through --The ability of a VFD to continue to operate for a period of time without input power. Typical power loss ride-through is 2 to 5 cycles (each cycle is 16 ms). For some drives, a more extensive circuit is used. If the supply to a VFD is lost, the drive may continue to run without external power supply utilizing the kinetic energy of the rotating motor and driven equipment.

The power loss ride-through time depends on the relationship between the load and the inertia of the rotating masses.

Power Output --The section of the drive that delivers adjustable voltage/frequency to the motor (AC drives). The output terminals are typically referred to as U2, V2, and W2.

Power Output Control --The circuit that automatically adjusts the switching of the power output subsection.

Characteristics of Power Loss Ride-Through

Precharge --A circuit within the drive that supplies a limited amount of current to the DC bus capacitors. This allows the capacitors to gain an small initial charge prior to receiving a full charge during a "drive enable" function. This pro cess reduces the amount of inrush current to the bus Bus capacitors.

Preset Speed -- Preset speed refers to one or more fixed speeds at which the drive will operate.

Programmable Logic Controller (PLC) -- A stored program device intended to replace relay logic used in sequencing, timing, and counting of discrete events. Instead of physical wiring relay, pushbuttons, limit switches, etc., a PLC is programmed to test the state of input lines, to set output lines in accordance with input state, or to branch to another set of tests. The instruction sets of these machines generally exclude all arithmetic and Boolean operators, but do include vital decision instructions such as skip, transfer unconditional, transfer conditional, and even transfer and link.

Protection Devices --The subsection of the drive that works to reduce the electrical hazard within the drive.

Protocol --A set of standards governing the format and timing of data between different types of communicating devices. Essentially, a protocol is a communication language (i.e., Modbus, Profibus, DeviceNet®, etc.).

PRSQ --This notation abbreviates the rated variable torque output power rating of an AC drive.

Pull-In Torque --The maximum constant torque that a synchronous motor will accelerate into synchronism at rated voltage and frequency.

Pull-Out Torque --The maximum running torque of a synchronous motor.

Pull-Up Torque --The torque required to accelerate the load from standstill to full speed (where breakdown torque occurs), expressed in percentage of running torque. It is the torque required not only to overcome friction, windage, and product loading but also to overcome the inertia of the machine. The torque required by a machine may not be constant after the machine has started to turn. This load type is characteristic of fans, centrifugal pumps, and certain machine tools.

PWM --Abbreviation for pulse width modulation. An adjustable frequency AC drive that accomplishes frequency and voltage control at the output section (inverter) of the drive. The drive's output voltage is always a constant amplitude and by "chopping" (pulse width modulating) the average voltage is controlled.

Radio Frequency Interference (RFI)--This is an abbreviation for "radio frequency interference." This is also referred to as an electromagnetic noise that can cause operating problems in other electronic equipment. RFI is caused by switching circuits in electronic equipment. The effects are noticed more with equipment that is not properly grounded or in inductive devices like solenoids that do not have noise suppressors.

Random Winding -- A method of winding wire in a motor in which the wire is inserted into the stator slots randomly without controlling the positions of the individual turns. With random winding, it is possible for the first turn of a coil to be in contact with the last turn.

Reactance -- Measurement of the opposition of a circuit or component to an alternating current, expressed in ohms.

Rectifier -- A device that permits current flow in one direction and blocks the flow of current in the other direction. In today's technology, rectifiers are of the silicon diode type. A 6-pulse AC drive uses six rectifiers, configured into a three-phase bridge configuration, as the power converter section of the drive.

Regeneration -- This occurs when a motor acts as a generator. Regeneration also occurs when the CEMF is larger than the drive's applied voltage (DC drives) or when the rotor synchronous frequency is greater than the applied frequency (AC drives).

Regenerative Braking -- The motor becomes a generator by taking the mechanical power of the motor and converting it into electrical power. The generated power is dissipated in the power source through a regenerative bridge circuit in the drive. The power may also be dissipated as losses in the power conversion section of the drive (within its limitations). Also see Braking.

Regenerative Control -- A drive that has capability to control the flow of power to and from the motor.

Regulation -- The ability of a control system to hold a set speed. Regulation is given in percentages of either base speed or set speed. Regulation is rated upon two separate sets of conditions:

Load -- Regulation (speed regulation) is the percentage of speed change with a defined change in load, assuming all other parameters to be constant.

Speed regulation values of 2% are possible in drive utilizing armature voltage feedback, while regulation of 0.01% is possible using digital regulator schemes.

Line -- Regulation is the percentage of speed change with a given line voltage change, assuming all other parameters to be constant.

Resolution -- The smallest distinguishable increment into which a quantity can be divided (e.g., position or shaft speed). It is also the degree to which nearly equal values of a quantity can be discriminated. For encoders, it is the number of unique electrically identified positions occurring in 360 degrees of input shaft rotation.

Resolution-10 and 12 Bit -- A circuit that has 10-bit resolution means has an accuracy of 1.7 rpm or 0.06 Hz. This number is generated by the fact that a speed range of 0 to 60 Hz is divided in even increments that equal 1024. In other words, (2) 10 =1024. A circuit that has 12-bit resolution would have an accuracy of 0.4 rpm or 0.015 Hz. 212 = 4096.

Resolvers--A resolvers (delete "s" in Resolvers) is inherently an analog device, as opposed to a digital encoder. A resolver accepts an AC signal, then modifies the signal relative to the rotor/stator position inside the case. A disadvantage of resolvers is the need to convert sinewave signals to digital pulses. This is required for the newer digital drive technology in existence.

Reversing -- Changing direction of the motor shaft. An AC motor is reversed by reversing the connections of one leg on the three-phase power line. A DC motor is reversed by changes the armature polarity. The reversing function can be performed in one of the following ways:

(DC) Contactor Reversing is done by changing the polarity to a DC motor armature with switching contactors. The contactors are operated by momentary pushbuttons, and/or limit switches to stop the motor and change directions. A zero speed (antiplugging) circuit is associated with this system to protect the motor and control.

(AC or DC) Static -- Reversing is the act of reversing the DC polarity of the DC motor armature or phase rotation of an AC motor with no mechanical switching. This is accomplished electronically with solid-state devices.

Solid-state anti-plugging circuitry is generally a part of the design.

AC Static -- Reversing is the act of reversing the phase rotation of an AC motor with no mechanical switching. This is accomplished electronically with solid-state devices.

Rise Time -- The time required for a voltage pulse to rise from 10 to 90% of the peak voltage.

RS-232 -- An electrical connection standard that is used as an interface between data terminal equipment and communications equipment. One disadvantage of this type is usually a maximum cable length of about 15 feet and communication only between two devices at separate locations (point-to-point communication).

RS-422 -- An electrical connection standard that is used as an interface between data terminal equipment and communications equipment. Unlike the RS-232 connection, the RS-422 allows data transmission to be received by multiple locations.

RS-485 -- An electrical connection standard that is used as an interface between data terminal equipment and communications equipment. This type of connection allows faster data transmission rates (100 ms, 9600 baud) as com pared with an RS-232 connection. In addition, longer cable lengths may be used (up to 1200 feet) with very little additional amplification required.

RS-485 also allows multiple-point transmission and receiving of data on the same communication link.

RTD Module -- This optional monitor circuit accepts temperature inputs from an RTD mounted on a motor. (RTD is an abbreviation for resistive temperature device.

This device changes resistance with changes in temperature and is an accurate indicator of heat generated within the motor.) A monitor circuit can provide actual temperature monitoring during motor operation as opposed to a thermistor (bi-metallic switch) that opens only when a dangerous condition exists.

Scalar -- A type of drive (inverter) control that regulates the frequency to the motor to achieve a set speed without use of a tachometer. This is the simplest form of control and is considered "open loop" (no feedback device).

Scalar Control -- Scalar control adjusts the motor speed by varying the output frequency of a drive. The motor speed is then defined by the frequency and loading torque. The speed accuracy can be improved by speed feedback (tach generator) and this system calls for closed-loop scalar control.

Separately Ventilated -- Separately ventilated motors have provisions for connecting an air duct that supplies cooling air from an external source. Since the cooling air comes into contact with the stator windings, rotor, and air gap, a clean, dry air source is required.

Serial Communications -- A method of digital communication where transmission occurs one electronic bit at a time. This is the most common long-distance communication method such as from PLC in the control room to drive on the assembly floor.

Serial Interface -- A method of data transmission that permits transmitting of a single bit at a time through a single line. Used where high-speed input is not necessary. Requires only one wire.

Serial Port -- A connection point on a piece of electronic equipment that allows communication to another device. This port has fewer signal lines than a parallel port and passes information as a series of binary "on's" and "off's" (0's and 1's). An I/O configuration of only three lines will allow two-way communication (send and receive). This simple design is suitable for long-distance transfer of information but is at a slower rate compared to parallel communication.

Service Deviation -- See Operating/Service Deviation.

Service Factor -- A number that indicates how much above the nameplate rating a motor can be loaded without causing series degradation (e.g., a motor with 1.15 S-F can produce 15% greater torque than one with 1.0 S-F). When used in applying motors or gear motors, it is a figure of merit that is used to adjust measured loads in an attempt to compensate for conditions that are difficult to measure or define.

Set Speed -- The desired operating speed.

Silicon Controlled Rectifier (SCR) -- A solid-state switch, sometimes referred to as a thyristor. The SCR has an anode, cathode, and control element called the gate. The device provides controlled rectification since it can be turned on at will. The SCR can rap idly switch large currents at high voltages. They are small in size and low in weight.

Shield -- A wire barrier, sometimes a wire mesh braid, that reduces the effect of electrical and/or magnetic fields. If shield braid is used, it completely surrounds the wires inside the outer casing.

Shock Load -- The load seen by a clutch, brake, or motor in a system that transmits high peak loads. This type of load is present in crushers, grinders, conveyors, winches, and cranes.

Skew -- The slight angular pattern of laminations on a rotor or armature with respect to the shaft axis. This pattern helps to eliminate low-speed cogging in an armature and minimize induced vibration in a rotor, as well as reduce associated noise.

Skewing -- Refers to time delay or offset between any two signals in relation to each other.

Slip -- The difference between rotating magnetic field speed (synchronous speed) and rotor speed of AC induction motors. Usually expressed as a percentage of synchronous speed.

Slip Compensation -- Slip compensation is a technique for reducing the speed drop caused by the application of load in the asynchronous motor. The speed drop can be reduced to about 10% of the nominal slip. If very high-speed control accuracy is required, a speed controller with a tach generator is required.

Special-Purpose Motor -- A motor with special operating characteristics or special mechanical construction or both, designed for a particular application and not falling within the definition of a general purpose motor.

Speed Range -- Minimum and maximum speed at which a motor must operate under constant or variable torque load conditions. A 10:1 speed range for a motor with a top speed of 1800 rpm means the motor must operate as low as 180 rpm and still remain within regulation specifications. Controllers are capable of wider controllable speed ranges than motors because there is no thermal limitation, only electrical. Controllable speed range of a motor is limited by the ability to deliver 100% torque below base speed without additional cooling.

Speed Regulation -- A measurement, in percentage, of how accurately the motor speed can be maintained. It is the percentage of change in speed between full load and no load.

Stability -- Ability of a drive to operate a motor at constant speed (under varying load), without "hunting" (alternately speeding up and slowing down). It is related to both the characteristics of the load being driven and electrical time constants in the drive regulator circuits.

Stiffness -- The ability of a device to resist deviation due to load change.

Supply Voltage -- Normally refers to the input voltage that provides power to the drive. Sup ply voltage is connected to terminals L1, L2, and L3 (U1, V1, and W1).

Surge Protection -- Absorbing and clipping voltage transients on an incoming AC line or control circuit. MOVs (Metal Oxide Varistors) and specially designed R-C net works are usually used to accomplish this.

Switching Frequency -- Switching frequency is the internal operating frequency of and inverter.

Typical values are from 1 to 16 kHz. Increase of switching frequency reduces the motor noise but also reduces the efficiency of the drive.

Switching Frequency Range --This is the frequency of the PWM waveform for driving the output switches. The motor will make a noise that has its fundamental at twice the switching frequency. Most drives that use IGBT switches can go to a 16-kHz switching frequency. When setting up a drive system, adjust the switching frequency for the lowest value that gives an acceptable noise level. As the switching frequency goes up, the drive efficiency goes down and losses increase. It is best to set the switching frequency as low as possible.

Synchronous Motors -- A synchronous motor is a motor that operates at its synchronous speed without any slip. As long as the load does not exceed the limit for synchro nous operation, the average operating speed is maintained exactly at the synchronous speed. There are several types of synchronous motors. The types that are sometimes used in VFD applications are permanent magnet motors, wound rotor synchronous motors, and synchronous reluctance motors. Synchronous reluctance motors are also called synchronous induction motors. Wound rotor synchronous motors require special VFD controllers that are specifically designed for use only with synchronous motors.

Permanent magnet and synchronous reluctance motors are sometimes used with AF controllers that are designed for use with induction motors, but special modifications are usually required.

Synchronous Speed -- The speed of an AC induction motor's rotating magnetic field. It is determined by the frequency applied to the stator and the number of magnetic poles present in each phase of the stator windings. Mathematically, it is expressed as sync speed (rpm) = 120 × applied freq. (Hz)/number of poles per phase.

Tachometer Generator (Tach) -- There are two main types: AC and DC. The speed accuracy and motor type will dictate which type is required (e.g., PY, AN, C42, C46, etc.). The cost of these devices is related to the accuracy provided. An AC tachometer has lower maintenance and is generally less expensive than a DC tach. The DC tach, however, operates over a higher speed range and has a higher accuracy than an AC tachometer. Also, no rectification is needed of a DC tachometer signal as compared with an AC tachometer. A tach is basically a small generator normally used as a rotational speed sensing device.

Tachometers are typically coupled to the shaft of DC or AC motors requiring close speed regulation. The tach feeds a signal to a controller, which then adjusts the output voltage or frequency to the motor.

TEFC (Totally Enclosed Fan Cooled) -- TEFC motors are completely enclosed to prevent the entry of air or moisture into the interior of the motor. They are cooled by a small fan that is mounted on the motor shaft at one end of the motor. The fan forces air to flow over the outside surface of the motor. The motor surface may have fins or ribs to increase the outside surface area.

TENV (Totally Enclosed Non-ventilated) -- TENV motors are completely enclosed to prevent the entry of air or moisture into the interior of the motor. They are cooled only by the convection flow of air over the outside surface of the motor. The motor surface may have fins or ribs to increase the surface area. TENV motors are available in sizes of about 10 HP and smaller.

TEBC (Totally Enclosed Blower Cooled) or TEAO (Totally Enclosed Air Over) -- TEBC and TEAO motors are completely enclosed to prevent the entry of air or moisture into the interior of the motor. They are cooled by a separately powered fan or blower that is mounted on one end of the motor.

The fan or blower forces air to flow over the outside surface of the motor.

The motor surface may have fins or ribs to increase the surface area.

Termination Resistor -- This resistor (located on many AC drive boards) provides an ending point for data transmission when connected to a PLC. The transmission and receiving devices therefore see the network as a complete electrical circuit.

Thread Speed -- An adjustable, low fixed speed that provides a convenient method for loading and threading machines. May also be called a preset speed.

Thyristor -- A controllable silicon rectifier is a contactorless switching element (also known as an SCR). 100 to 4500 V (0.4 to 1500 A) To r q u e A turning force applied to a shaft, tending to cause rotation. Torque is normally measured in ounce-inches or pound-feet and is equal to the force applied, times the radius through which it acts.

Torque Boost -- The automatic increase of starting current for loads with high starting torque. It is possible for a short time to have a starting current higher than the normal current limit for the drive in the frequency range up to 20 Hz.

Operates simultaneously with IR-compensation.

Torque Constant -- This motor characteristic provides a relationship between input current and output torque. For each ampere of current applied to the rotor, a fixed amount of torque will result. This constant is listed in ft-lbs or in-lbs.

Torque Control -- With torque control, the motor torque is always controlled by the torque reference. The motor speed operating point is defined by the intersection of motor torque and load torque curves. Direct torque control or Sensor less flux vector control is used when a very fast or accurate torque control is required.

Torque Loop -- Basically, the same as a Current Loop. Because current is in direct relation to torque, the terms are sometimes used interchangeably. Current sensors within the drive monitor the current output. These sensors tell the drive control logic if the current output is within power device limitations.

Total Harmonic Distortion (THD) -- A condition that exists when one or more harmonic current or voltage waveforms are added to the fundamental waveform (i.e., 60 Hz). This harmonic alters the fundamental waveform. The value is expressed in a percentage.

Totally Enclosed Machine (Enclosed Motor) -- A totally enclosed machine is one so enclosed as to prevent the free exchange of air between the inside and the outside of the case but not sufficiently enclosed to be termed air-tight.

Totally Enclosed Fan-Cooled is a totally enclosed machine equipped for exterior cooling by means of a fan or fans integral with the machine but external to the enclosing parts.

Explosion-Proof -- Machine is a totally enclosed machine whose enclosure is designed and constructed to withstand an explosion of a specified gas or vapor, which may occur within and to prevent the ignition of the specified gas or vapor surrounding the machine by sparks, flashes, or explosions of the specified gas or vapor that may occur within the machine casing.

Dust-Ignition-Proof -- Machine is a totally enclosed machine whose enclosure is designed and constructed in a manner that will exclude ignitable amounts of dust or amounts that might affect performance or rating and that will not permit arcs, sparks, or heat otherwise generated or liberated inside of the enclosure of cause ignition of exterior accumulations or atmospheric suspensions of a specific dust on or in the vicinity of the enclosure.

Waterproof -- Machine is a totally enclosed machine so constructed that it will exclude water applied in the form of a stream from a hose, except that leakage may occur around the shaft, provided the water is prevented from entering the oil reservoir and provision is made for automatically draining the machine. The means for automatic draining may be a check value or a tapped hole at the lowest part of the frame, which will serve for application of a drain pipe.

Totally Enclosed Water-Cooled -- Machine is a totally enclosed machine that is cooled by circulating water, the water or water conductors coming in direct contact with the machine parts.

Totally Enclosed Water-Air-Cooled -- Machine is a totally enclosed machine that is cooled by circulating air which, in turn, is cooled by circulating water. It is provided with a water-cooled heat exchanger for cooling the internal air and a fan or fans, integral with the rotor shaft or separate, for circulating the internal air.

Totally Enclosed Air-to-Air Cooled -- Machine is a totally enclosed machine that is cooled by circulating the internal air through a heat exchanger that is cooled by circulating external air. It is provided with an air-to-air heat exchanger for cooling the internal air and a fan or fans, integral with the rotor shaft or separate, for circulating the internal air and a separate fan for circulating the external air.

Totally Enclosed Fan-Cooled Guarded -- Machine is a totally enclosed fan-cooled machine in which all openings giving direct access to the fan are limited.

They are limited in size, the design of the structural parts, or by screens, grills, expanded metal, etc. These parts prevent accidental contact with the fan. Such openings shall not permit the passage of a cylindrical rod 0.75 inch in diameter and a probe shall not contact the blades, spokes, or other irregular surfaces of the fan.

Totally Enclosed Air-Over -- Machine is a totally enclosed machine intended for exterior cooling by a ventilating means external to the machine.

TR -- This notation abbreviates the rated output torque of the motor.

Transducer -- A device that converts one form of energy (e.g., mechanical to electrical) to another. For example, a tach or encoder converts mechanical rotation or position into electrical signals. These signals are understood by the drive control logic circuits. A pressure transducer (PT) converts air pressure to an analog electric signal.

Transient -- A momentary deviation in an electrical or mechanical system (e.g., voltage spikes imposed on the AC supply line).

Transistor--An electronic, three-terminal device that allows amplification of signals and can be used for switching and control. The three terminals are called the Emitter, Base, and Collector.

Variable-Frequency Drive (VFD)--A drive system including the electric machine with its mechanical and electrical control equipment where the speed of the driven equipment is varied electrically. The output of this drive is variable frequency (Hz) which changes the speed of an AC motor. A VFD typically indicates an AC Drive.

Variable-Torque Load -- A load where torque varies directly to the square of speed and horsepower varies directly to the cube of speed. The load requires much lower torque at low speeds than at high speeds. Typical applications include centrifugal pumps, fans, and some mixers.

Vector -- A quantity that has magnitude, direction, and sense. This quantity is commonly represented by a directed line segment whose length represents the magnitude and whose orientation in space represents the direction.

Vector Control -- The technique of controlling a standard AC induction motor by electronically modeling the motor within the logic of the AC drive. The AC drive logic simulates what actually happens in a DC motor. This process allows the AC motor to operate much like a DC motor, with high speed response and without speed oscillations. The result is a fast, controlled torque response, which was previously obtained only by DC systems. In vector control, a feedback device (e.g., digital encoder) sends back a signal to the drive indicating actual rotor/stator position or relationship. The drive then calculates the voltage and current "vectors." This permits the accurate control of speed and motor torque at all speeds, even zero.

VIN -- This notation abbreviates the input voltage of the drive. Refer to Supply Voltage in this section.

VN -- This notation abbreviates the voltage for which the drive is programmed (e.g., nominal).

Voltage Reflection -- A phenomenon in which a voltage wave or pulse is transmitted through the cable to a motor and is reflected or transmitted back to the AC drive.

V/Hz (Volts per Hertz) -- This is the fixed relationship between voltage and frequency that exists in a motor. The motor will develop rated torque if this relationship is kept constant (linear). The drive can vary this relationship to minimize audible noise, motor losses, and maximize efficiency. If a "squared" V/Hz pattern is programmed, the voltage of the motor varies as the square of the frequency applied by the drive. This is useful in applications where the load torque is proportional to the square of speed (e.g., centrifugal pumps and fans). These loads are called variable torque loads. An "automatic" V/Hz pattern automatically controls the voltage to the motor, thereby control ling the torque. This is considered an energy saving feature of the drive and is useful in lightly loaded applications.

VR -- In an AC drive, this notation abbreviates the rated input voltage setting, in volts.

VVI -- This type of AC drive controls the voltage and frequency to the motor to produce variable-speed operation. A VVI type drive controls the voltage in a section other than the output section where frequency generation takes place. Frequency control is accomplished by an output bridge circuit which switches the variable voltage to the motor at the desired frequency.

Wound Rotor Motors -- Wound rotor motors are sometimes used with variable-frequency drives by shorting their slip rings and operating them like regular induction motors.