Vvvf Drive Block Diagram

8/3/2019 Vvvf Drive Block Diagram

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Vector Drive BasicsKen DekenReliance Electric CompanyCleveland, Ohio

'1 ' RocIIwell Automat ionRe lia n ce E le c tr ic


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Modem, high-peiformanceinduction drives can be usedwhere D-C or brushless driveswere previously needed.

The power conversion section iscommon to virtually all PWMdrives, whether they use a VVVFor vector regulator.

Vector Drive BasicsK en D ekenRe liance E lectric CompanyC leve land , Oh io

Several drive manufacturers are touting the vector drive - the newest type ofA-C variable-frequency drive (VFD) - as the A-C solution for tough variable-speed applications.In many ways, vector drives offer users the best features of both A-C drivesand D-C drives in a single, problem-solving package. New electronicstechnology has dramatically reduced the complexity and cost of vectordrives to make them practical for new uses.This article 1) explains what a vector drive is, 2) shows how it is differentfrom a standard A-C drive, and 3) discusses some application and pricingcomparisons with D-C and brushless drives.

Vector Drives Prov ide Independent ControlWhat is a vector drive and where does it get that mysterious name? Youmay recall from math classes that a "vector" quantity has both magnitudeand direction. A vector drive borrows that name, since it uses a more so-phisticated electronic regulator to control both the magnitude and direction(or strength and speed) of the magnetic flux in an A-C motor through inde-pendent control loops.Conventional "variable-voltage, variable-frequency" (VVVF) A-C drives locktogether the excitation control of the magnitude and direction of magneticflux in the motor.The control strategy of VVVF drives is fine for steady-state conditions, or forloads encountered in applications such as fans and pumps that allow lots oftime for a speed change. But in many real-world applications, loads, speed,or position are likely to change abruptly. Vector drives are much better suitedto handle these conditions since they provide direct torque control, as wellas a dynamic response capability that is ten times that of WVF drives.

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The block diagrams in Figures 1, 2, and 3 provide more details about the sim-ilarities and differences between vector drives and conventional VWF drives.

D-CtoA-CInverterBipolar or IGBT(High Speedtransistors)

D- C FilterCapacitor

A-CtoD-CConverterPWM Pow er C on ve rs io n)-------IIIIIIII

IIIII: I_______ J

- - - - - - - - - - - - - - - - - 1IIIIIIII

IIIIIII_______ J

Figure 1 - Pulse Width Modulation (PWM) TechnologyNote first the power conversion section shown in Figure 1. That portion of thedrive is common to virtually all pulse-width-modulated (PWM) drives, whetherthey use a WVF or vector regulator. Three-phase A-C power flows throughthe six input diodes which rectify it into D-C power of a fixed voltage. The D-Cpower is smoothed by the filter capacitor. A set of six transistors with diodesin the switching section are controlled to turn the D-C power back into A-Cpower of variable voltage and frequency. That "synthesized A-e" power is fedto the motor.

VoltsHertzReferenceShaping

Cummt _ t o PUfor lETFigure 2 - Voltz/Hertz Block Diagram

Figure 2 shows a simplified block diagram of the regulator of a conventionalVWF drive. The speed reference command from the user is fed to a rampblock to convert step-function speed changes to slower-changing ramps thatlimit current flow and save machine wear and tear. The signal then moves toa section that sets both the rate of change and strength of the magnetic fieldin the motor. It is important to recognize that the single-speed control inputcommand controls both of these variables in a VWF drive.

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Calculated Rux OutputSpeed Fdbk toMotor--

Figure 3 - Vector Block DiagramA vector drive regulator block diagram is shown in Figure 3. Two separatecontrol loops have been added, making it more complex. The control loopsallow independent control of the speed and strength of the motor's magneticfield. They also allow the regulator to measure the actual speed of the motorand to estimate closely the amount of torque being produced.

T he "M o to r M o del B lo ck " S ecretThe "Motor Model Block," shown in Figure 4, is the secret of the vector drive.It uses high-speed electronics technology (often a micro-processor or digital-signal processor) to compute torque calculations at 2,000 times per second ormore.The Motor Model Block makes use of a very important principle for A-C induc-tion motors: Torque, Current, and Slip are related.

Slip, Current and Torque are Related

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10.3 1514.1 30119.1 45325.07

SLIP = STATOR RPM - ROTOR RPM(Data for Reliance Electric Model P25G312, 20HP,360460V AC, Std. Efficiency)

Figure 4 - Motor Model Block

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The "Motor Model Block" makesuse of a very irri.portant principle ofA-C induction motors: Torque,Current, and Slip are related.

Look at the Table in Figure 4. It compares the slip, current, and torque versusload for a single motor. (Slip is defined as the difference between the synchro-nous electrical excitation speed of the stator and the motor's actual shaftspeed. Current is the value of A-C current that is measured by using a clamp-on ammeter on one of the motor leads. Torque is the output torque of the mo-tor at the shaft.) It's easy to gain an intuitive understanding of the data in thistable. We expect that an induction motor will slow slightly and draw more cur-rent as the load connected to it is increased.Torque and slip are directly related. An increase in load torque requires a pro-portional increase in Slip. However, even though current increases withtorque, the change is not linear.Fortunately, the change of current with torque varies in a predictable way. Thetotal current drawn by the motor has two components. The first is no-load ormagnetizing current. This portion is simply reactive current that does not rep-resent mechanical power. The second is torque-producing current that iselectrically in-phase with the voltage applied to the motor. This is the portionthat delivers power to the motor and varies directly with output torque.

Magnetiz ingCurrentB . 5Amp s25.0Amp sF u J ILoad

To rq ue- P ro d u ci ng Cu rr en t2 3 .5 Amp s

Vector calculates Torque-Producing Current byknowing actual amps and magnetizing current.

(Magnetizing Amps = = No Load)Figure 5 - Current Diagram

Figure 5 may help explain the last concept. It shows a "current diagram."Magnetizing current of 8.5 amps is represented by the Y axis. The X axis rep-resents torque-producing current of 23.5 amps. The actual full load currentmeasured at the motor (25.0A) is represented by the hypotenuse of the righttriangle formed by the X /Y components. Once again recalling the use of "vec-tor" from math classes, you will see that the actual current drawn by the motoris the vector sum of the magnetizing and torque-producing components ofcurrent.In real applications, we can obtain the magnetizing current from the motormanufacturer (or approximate it by measurement). We can certainly measurethe total current drawn by the motor by using high-speed hall-effect currentsensors. The regulator must then solve the following equation to determinethe torque-producing component of current that is the last part of the MotorModel Block puzzle:

I TORQUE V I ~EASURED - I ~AGNETIZINGThat's enough theory! What will vector drives do for you and what are theiradvantages?

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Start with all advantages ofstandard. WVFA-C drives.

Unlike "btushless" drives, whichhave been sold as replacements forD-C drives, vector drives do notneed special design motors withpermanent-magnet rotors.

Vector Drive AdvantagesVector drives provide all the benefits and advantages of VWF drives andmore.Start with all the advantages of VVVF drives:

1. Use "standard", low-cost, induction motors. Explosion-proof motorsand other special construction is widely available. Brushes, commu-tators, or special "permanent-magnef' rotors are not required.

2. High-input displacement power factor (about 95%), for lower-costpower.3. Some inherent ability to "hold back" loads through regeneration with-

out extra hardware.4. High-speed capability (6,000 RPM is easily attainable).

Add to those the following advantages not achievable with standard VWFdrives:1. Closed-loop speed regulation to 0.01 %.2. High-dynamic response of greater than 50 radians/second.3. Smooth low-speed operation, even under changing loads.4. High breakaway torque (150% or even 200% is common).5. Linear torque control for positioning or tension.

These advantages make vector drives the best answer for many applica-tions.Compared with D-C drives, a vector drive's high power-factor, high-speedcapability, and the ability to use induction motors are important advantages.Unlike "brushless" drives, which have been sold as replacements for D-Cdrives, vector drives do not need specially-designed motors with permanent-magnet rotors.

1 0 0 : 1 1 0 0 :1 1 0 :1 1 0 0 :1 2 0 0 0 : 1Y E S N O Y E S N O Y E S Y E S

4: 1 2 :1 2 :1 4: 1 N O N E1 5 0 % 1 5 0 % 1 0 0 % 1 5 0 "1 0 2 0 0 %

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Applications which requiredynamic braking will requiredifferent solutions when vectordrives are used.

Special consideration should begiven to the motors used withvector drives to obtain optimizedperformance.

Pricing for A-C vector drives is competitive with D-C drives in applicationsbelow 50 HP. Vector drives are often more expensive than D-C drives in larg-er ratings, primarily due to the more expensive power semi-conductors foundin A-C drives. The lower cost of A-C motors offsets some of this difference,though. Application considerations may also justify a premium for vectordrives in those cases where the initial cost is higher.

S p e c i a l C o n s i d e ra t io n sWhile vector drives offer many advantages, they have a few limitations, too.First, virtually all of today's vector drives require tachometer feedback fromthe motor. The tach increases the price of the A-C motor, and there is costassociated with mounting, wiring, and maintaining it. However, brushlessdrives and many D-C drives also require tachometer feedback when speedregulation better than 1% is required.Second, regeneration is more difficult with vector drives and with brushlessdrives than with conventional thyristor D-C drives. Snubbers, add-on regen-eration kits, and common-bus configurations have all been used for regenera-tion with vector drives, but D-C drives have the advantage for low-cost,single-section regeneration.Third, applications which need dynamic braking (slow-down braking withoutthe drive operating) will require different solutions when vector drives areused. D-C and brushless drives make use of the fact that they can operate asgenerators without an active regulator to provide a dynamic braking function.Vector (and other A-C) drives may require D-C injection braking or mechani-cal brakes to provide the functional equivalent of dynamic braking. This isnecessary because the regulator on A-C drives must be active to brake theload.Finally, a word about motors is appropriate. Throughout this article it hasbeen noted that "standard" induction motors may be used with vector drives.However, the high-performance capabilities of these drives can place heavierdemands on the motors they power. Special consideration should be given tousing motors optimized for constant-torque, high-overload duty over a widespeed range. Several manufacturers now offer "vector-duty" motors withtachometer-mounting provisions and blower-cooling. Some companies havegone a step further to design vector-duty motors with lower inertia and specialconstruction features designed exclusively for variable-speed duty.Vector drives represent the step forward in performance that many applica-tions have needed to enjoy the advantages of A-C technology. It is clear wewill see them widely used in new applications. They can also be an alterna-tive to existing mechanical and electrical variable-speed drives.

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Need more information?Let the broad capabilities of Reliance Electricwork for you. For more information, contact yourauthorized Reliance Electric sales representativeor distributor. Or, call 1 800 245-4501.

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Vvvf Drive Block Diagram