Serial Number Perspective Rectifier Tubes. National building code of india 2005 national building code of india 2005 bureau of indian standards sp 7:2005 first published first revision. Radio Boulevard Western Historic Radio Museum the hallicrafters inc. SX-28 'a pre-war masterpiece' (includes SX-28,.

Chassis of above VFD (cover removed) A variable-frequency drive ( VFD; also termed adjustable-frequency drive, variable speed drive, AC drive, micro drive or drive) is a type of used in drive systems to control and by varying motor input and. VFDs are used in applications ranging from small appliances to large compressors. About 25% of the world's electrical energy is consumed by electric motors in industrial applications, which can be more efficient when using VFDs in centrifugal load service; however, VFDs' global for all applications is relatively small.

Over the last four decades, technology has reduced VFD cost and size and has improved performance through advances in semiconductor switching devices, drive topologies, simulation and control techniques, and control hardware and software. VFDs are made in a number of different low- and medium-voltage and DC-AC topologies. VFD system A variable-frequency drive is a device used in a drive system consisting of the following three main sub-systems: AC motor, main drive assembly, and drive/operator interface.: 210–211 AC motor [ ] The AC electric motor used in a VFD system is usually. Some types of motors or advantageous in some situations can be used, but three-phase induction motors are generally preferred as the most economical motor choice. Motors that are designed for fixed-speed operation are often used.

Elevated-voltage stresses imposed on induction motors that are supplied by VFDs require that such motors be designed for definite-purpose inverter-fed duty in accordance with such requirements as Part 31 of Standard MG-1. Controller [ ] The VFD controller is a power electronics conversion system consisting of three distinct sub-systems: a bridge converter, a (DC) link, and an inverter. Inverter (VSI) drives (see 'Generic topologies' sub-section below) are by far the most common type of drives. Most drives are drives in that they convert AC line input to AC inverter output. However, in some applications such as common DC bus or applications, drives are configured as DC-AC drives. The most basic rectifier converter for the VSI drive is configured as a three-phase, six-pulse,.

In a VSI drive, the DC link consists of a which smooths out the converter's DC output and provides a stiff input to the inverter. This filtered DC voltage is converted to quasi- AC voltage output using the inverter's active switching elements. VSI drives provide higher and lower than inverter (CSI) and load-commutated inverter (LCI) drives (see 'Generic topologies' sub-section below). The drive controller can also be configured as a having single-phase converter input and three-phase inverter output. Controller advances have exploited dramatic increases in the voltage and current ratings and switching frequency of solid-state power devices over the past six decades. Introduced in 1983, the (IGBT) has in the past two decades come to dominate VFDs as an inverter switching device. In variable- applications suited for Volts-per-Hertz (V/Hz) drive control, AC motor characteristics require that the voltage magnitude of the inverter's output to the motor be adjusted to match the required load torque in a V/Hz relationship.

For example, for 460 V, 60 Hz motors, this linear V/Hz relationship is 460/60 = 7.67 V/Hz. While suitable in wide-ranging applications, V/Hz control is sub-optimal in high-performance applications involving low speed or demanding, dynamic speed regulation, positioning, and reversing load requirements. Some V/Hz control drives can also operate in V/Hz mode or can even be programmed to suit special multi-point V/Hz paths. The two other drive control platforms, and (DTC), adjust the motor voltage magnitude, angle from reference, and frequency so as to precisely control the motor's magnetic flux and mechanical torque. Although (SVPWM) is becoming increasingly popular, sinusoidal PWM (SPWM) is the most straightforward method used to vary drives' motor voltage (or current) and frequency. With SPWM control (see Fig. 1), quasi-sinusoidal, variable-pulse-width output is constructed from intersections of a saw-toothed with a modulating sinusoidal signal which is variable in operating frequency as well as in voltage (or current).

Operation of the motors above rated nameplate speed (base speed) is possible, but is limited to conditions that do not require more power than the nameplate rating of the motor. This is sometimes called 'field weakening' and, for AC motors, means operating at less than rated V/Hz and above rated nameplate speed.

Synchronous motors have quite limited field-weakening speed range due to the constant magnet. Wound-rotor synchronous motors and induction motors have much wider speed range. For example, a 100 HP, 460 V, 60 Hz, 1775 (4-pole) induction motor supplied with 460 V, 75 Hz (6.134 V/Hz), would be limited to 60/75 = 80% torque at 125% speed (2218.75 RPM) = 100% power. At higher speeds, the induction motor torque has to be limited further due to the lowering of the breakaway torque of the motor. Thus, rated power can be typically produced only up to 130-150% of the rated nameplate speed. Wound-rotor synchronous motors can be run at even higher speeds.

In rolling mill drives, often 200-300% of the base speed is used. The mechanical strength of the rotor limits the maximum speed of the motor. 1: SPWM carrier-sine input & 2-level PWM output An governs the overall operation of the VFD controller. Basic of the microprocessor is provided as user-inaccessible. User programming of, variable, and function block parameters is provided to control, protect, and monitor the VFD, motor, and driven equipment. The basic drive controller can be configured to selectively include such optional power components and accessories as follows: • Connected upstream of converter -- or, isolation, filter, line, passive filter • Connected to DC link --, braking • Connected downstream of inverter—output reactor, sine wave filter, dV/dt filter. Operator interface [ ] The operator interface provides a means for an operator to start and stop the motor and adjust the operating speed.

Additional operator control functions might include reversing, and switching between manual speed adjustment and automatic control from an external signal. The operator interface often includes an display or indication lights and meters to provide information about the operation of the drive. An operator interface keypad and display unit is often provided on the front of the VFD controller as shown in the photograph above. The keypad display can often be cable-connected and mounted a short distance from the VFD controller.

Most are also provided with (I/O) terminals for connecting push buttons, switches, and other operator interface devices or control signals. A is also often available to allow the VFD to be configured, adjusted, monitored, and controlled using a computer. Drive operation [ ].

Topology of direct matrix converter AC drives can be classified according to the following generic topologies: • Voltage-source inverter (VSI) drive topologies (see image): In a VSI drive, the DC output of the -bridge converter stores energy in the capacitor bus to supply stiff voltage input to the inverter. The vast majority of drives are VSI type with PWM voltage output. • Current-source inverter (CSI) drive topologies (see image): In a CSI drive, the DC output of the -bridge converter stores energy in series- connection to supply stiff current input to the inverter. CSI drives can be operated with either PWM or six-step waveform output. • Six-step inverter drive topologies (see image): Now largely obsolete, six-step drives can be either VSI or CSI type and are also referred to as variable-voltage inverter drives, (PAM) drives, drives or inverter drives. In a six-step drive, the DC output of the SCR-bridge converter is smoothed via capacitor bus and series-reactor connection to supply via or inverter quasi-sinusoidal, six-step voltage or current input to an induction motor.

• Load commutated inverter (LCI) drive topologies: In an LCI drive (a special CSI case), the DC output of the SCR-bridge converter stores energy via DC link inductor circuit to supply stiff quasi-sinusoidal six-step current output of a second SCR-bridge's inverter and an over-excited synchronous machine. • Cycloconverter or matrix converter (MC) topologies (see image): and MCs are that have no intermediate DC link for energy storage. A cycloconverter operates as a three-phase current source via three anti-parallel-connected SCR-bridges in six-pulse configuration, each cycloconverter phase acting selectively to convert fixed line frequency AC voltage to an alternating voltage at a variable load frequency. MC drives are IGBT-based. • Doubly fed slip recovery system topologies: A recovery system feeds rectified slip power to a smoothing reactor to supply power to the AC supply network via an inverter, the speed of the motor being controlled by adjusting the DC current. Control platforms [ ].

See also: and Most drives use one or more of the following control platforms: • PWM V/Hz control • PWM (FOC) or vector control • (DTC). Load torque and power characteristics [ ] Variable-frequency drives are also categorized by the following load torque and power characteristics: • Variable torque, such as in centrifugal fan, pump, and blower applications • Constant torque, such as in conveyor and positive-displacement pump applications • Constant power, such as in machine tool and traction applications. Available power ratings [ ] VFDs are available with voltage and current ratings covering a wide range of single-phase and multi-phase AC motors. Low-voltage (LV) drives are designed to operate at output voltages equal to or less than 690 V. While motor-application LV drives are available in ratings of up to the order of 5 or 6 MW, economic considerations typically favor medium-voltage (MV) drives with much lower power ratings.

Different MV drive topologies (see Table 2) are configured in accordance with the voltage/current-combination ratings used in different drive controllers' switching devices such that any given voltage rating is greater than or equal to one to the following standard nominal motor voltage ratings: generally either 2.3/4.16 kV (60 Hz) or 3.3/6.6 kV (50 Hz), with one thyristor manufacturer rated for up to 12 kV switching. In some applications a step-up is placed between a LV drive and a MV motor load. MV drives are typically rated for motor applications greater than between about 375 kW (500 HP) and 750 kW (1000 hp). MV drives have historically required considerably more application design effort than required for LV drive applications. The power rating of MV drives can reach 100 MW, a range of different drive topologies being involved for different rating, performance, power quality, and reliability requirements. Drives by machines and detailed topologies [ ] It is lastly useful to relate VFDs in terms of the following two classifications: • In terms of various AC machines as shown in Table 1 below • In terms of various detailed topologies shown in Tables 2 and 3 below. ^ Inverter switching device (with std.

Main article: Carrier frequencies above 5 kHz are likely to cause bearing damage unless protective measures are taken. PWM drives are inherently associated with high-frequency common-mode voltages and currents which may cause trouble with motor bearings.

When these high-frequency voltages find a path to earth through a bearing, transfer of metal or (EDM) sparking occurs between the bearing's ball and the bearing's race. Over time, EDM-based sparking causes erosion in the bearing race that can be seen as a fluting pattern. In large motors, the of the windings provides paths for high-frequency currents that pass through the motor shaft ends, leading to a circulating type of bearing current. Poor of motor stators can lead to shaft-to-ground bearing currents. Small motors with poorly grounded driven equipment are susceptible to high-frequency bearing currents. Prevention of high-frequency bearing current damage uses three approaches: good cabling and grounding practices, interruption of bearing currents, and filtering or damping of common-mode currents for example through soft magnetic cores, the so-called inductive absorbers. Good cabling and grounding practices can include use of shielded, symmetrical-geometry power cable to supply the motor, installation of shaft grounding brushes, and conductive bearing grease.

Bearing currents can be interrupted by installation of insulated bearings and specially designed electrostatic-shielded induction motors. Filtering and damping high-frequency bearing can be done though inserting soft magnetic cores over the three phases giving a high frequency impedance against the common mode or motor bearing currents. Another approach is to use instead of standard 2-level inverter drives, using either 3-level inverter drives or matrix converters. Since inverter-fed motor cables' high-frequency current spikes can interfere with other cabling in facilities, such inverter-fed motor cables should not only be of shielded, symmetrical-geometry design but should also be routed at least 50 cm away from signal cables. Dynamic braking [ ]. See also: and Torque generated by the drive causes the induction motor to run at speed less the slip.

If the load drives the motor faster than synchronous speed, the motor acts as a, converting mechanical power back to electrical power. This power is returned to the drive's DC link element (capacitor or reactor). A DC-link-connected electronic power switch or controls dissipation of this power as heat in a set of resistors. Cooling fans may be used to prevent resistor overheating. Dynamic braking wastes braking energy by transforming it to heat. By contrast, regenerative drives recover braking energy by injecting this energy into the AC line.

The capital cost of regenerative drives is, however, relatively high. Regenerative drives [ ]. Simplified Drive Schematic for a Popular EHV Regenerative AC drives have the capacity to recover the braking energy of a load moving faster than the designated motor speed (an overhauling load) and return it to the power system. Cycloconverter, Scherbius, matrix, CSI, and LCI drives inherently allow return of energy from the load to the line, while voltage-source inverters require an additional converter to return energy to the supply.

Regeneration is useful in VFDs only where the value of the recovered energy is large compared to the extra cost of a regenerative system, and if the system requires frequent braking and starting. Regenerative VFDs are widely used where speed control of overhauling loads is required. Some examples: • Conveyor belt drives for manufacturing, which stop every few minutes. While stopped, parts are assembled correctly; once that is done, the belt moves on.

• A crane, where the hoist motor stops and reverses frequently, and braking is required to slow the load during lowering. • Plug-in and hybrid electric vehicles of all types (see image and ).

Historical systems [ ] Before solid-state devices became available, variable-frequency drives used rotary machines and the obtained several patents for these in the early 20th century. An example is US patent 949320 of 1910 which states: 'Such a generator finds a useful application in supplying current to induction motors for driving cars, locomotives, or other mechanism which are to be driven at variable speeds'. See also [ ] • • • • • Notes [ ]. • NEMA Guide defines a motor's breakaway torque as 'The torque that a motor produces at zero speed when operating on a control', and a motor's breakdown torque as 'The maximum torque that it will develop with rated voltage applied at rated frequency on sinewave power, without an abrupt drop in speed.' • The mathematical symbol dV/dt, defined as the of voltage V with time t, provides a measure of rate of voltage rise, the maximum admissible value of which expresses the capability of capacitors, motors, and other affected circuit elements to withstand high current or voltage spikes due to fast voltage changes; dV/dt is usually expressed in V/microsecond. • A topology is defined in power electronics parlance as the relationship between AC drives' various elements. • The term PWM is often used to mean VSI-PWM, which is misleading as not only VSI drives are with PWM output.

• The term six-step refers strictly speaking to an inverter waveform output alternative to PWM, some drives being configured as combined six-step and PWM options. • The harmonics treatment that follows is limited for simplication reasons to LV VSI-PWM drives. References [ ].

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