variable frequency drives
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variable frequency drives - 1

[Product training engineer 1 [Paul Avery [Senior producttraining engineer* Variable frequency drives (or VFDs) are widely used vide adjustable speed control of ac motors. Although modern versions are relatively simple to install and operate, VFDs are quite complex, containing a myriad of advanced Learn the terminology used to describe VFD internal operations and external interfaces — and you'll understand their application more thoroughly. often be improved by understand- that end, we present the terminol- ogy used to describe VFD internal operations and external interfaces. This lesson contains 40 of the most important VFD terms for todays design engineers. the three primary sectio^^ff a VFD's main power circuit, and first in terms of power flow. Incoming ac line voltage is recti- fied to dc voltage in the converter section, which consists of diodes, silicon-controlled rectifiers (SCRs), or insulated gate bipolar transistors Variable frequency drive sections A VFD's rectifier/converter Then its dc bus stores Finally, its inverter accepts ac line voltage. converted power on capacitors. creates sinusoidal output.

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variable frequency drives - 2

second prima- VFD's main power that store power recti- primary section of a VFD's main power circuit. prised of IGBTs that create sinusoi- dal output current using pulsed dc bus voltage, or pulse width modu- lation (PWM). VFDs themselves are sometimes called inverters, as the presence of an inverter section is the primary difference between IGBT: Very fast semiconduc- tor switches that are actuated By applying a small positive volt- age between the gate and emitter points of the IGBT, current is al- lowed to flow from a collector point to an emitter point. IGBT switch- ing rates in VFDs...

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variable frequency drives - 3

Line reactor the current drawn by induction mo- tors, a motor's power capability is not just volts x amps, but hp. JL mode voltages over time (dv/dt) generated by PWM pulses interacting with parasitic capaci- tances found between motor cables tween the motor's stator and rotor. Leakage current produced in this way is found on the ground circuit and can present problems for sensi- tive equipment attached to the same properties; synonymous with flux or voltage losses. Voltage loss results from voltage that is dropped across the motor conduc- tors, yet produces no flux to link stator and...

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variable frequency drives - 4

Four motor poles torted input current draw) reactor, but provides less protection against voltage transients. Unlike a line reactor, a dc link choke doesn't have a current-de- pendent voltage drop. choke in that it reduces current harmonics created by a VFD, as well as bus ripple inside loaded, it creates a higher harmonic content than at full load. A swing choke increases its inductance when the current passing through it is lower, usually due to partial load- ing. This increased inductance re- duces the harmonics as a percent of load current. / ac-to-ac VFD that doesn't available drives....

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variable frequency drives - 5

V/F mode or volts-per-hertz mode provides VFD-based control for ac induction motors. The ratio of voltage to frequency is the flux level in the machine, which in turn dictates the amount of torque <^ ^ Single phase: A typi- mrt %J electrical system uses transmit power. Such a system is mainly used in residences where three-phase power isn't needed, or in remote areas where three-phase power transmission is too costly. <^ A Three phase: Mainly three-phase electrical system uses a neutral or ground, and three hot wires that each transmit one ac Each phase is a sine wave offset third of the...

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variable frequency drives - 6

Acting as the VFD's brain, the PCB accepts real-world commands executes the target function. The control PCB generally interfaces to the VFD's main circuit via the gate drive board. ^ ^VGate drive board: operating (gating) the output tran- sistors of the VFD. The gate drive board can also monitor main circuit temperature, current, and voltage. Typically, smaller VFDs have no separate gate drive, but combine gating with logic power supplies to form a power board. some VFDs. IPMs include IGBTs, gate circuitry, thermal sensors, and easier to include in a VFD package and takes up less space...

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variable frequency drives - 7

slight loss of motor performance, as compared to closed-loop vector \J fective method for motor control that allows VFDs to realize dc drive control benefits without the physical limitations of dc motors. Feedback devices such as encod- ers or resolvers supply necessary motor slip information to close the loop between VFD output frequen- cy and actual motor shaft speed. S tive control algorithm is widely used throughout industrial When a process loop is created by adding feedback (from a variable such as airflow, pressure, or level) the variable is possible via PID loop uses mathematical...

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