VARIABLE FREQUENCY DRIVES

Need for variable frequency drives

• Match the Torque of a drive to the process requirements• Match the Speed of a drive to the process requirements • Save Energy and improve efficiency

Smooth acceleration/deceleration to ..... Reduce mechanical wear and water hammer Reduce current surges in the power supply system

Energy savings are possible ..... Most significant with centrifugal pumps and fans because Power/energy consumption changes with Speed3

Speed controlled to match the process requirements ....

e.g. ... flow or pressure controlled to match demand Automatic Control of the Process Variable is

possible Closed loop control from a Process Controller

Need for variable frequency drives

Variable speed – Energy consumption

Principles applied to centrifugal Pumps and Fans

Compare two methods of speed control in a Motor Car .... Speed controlled using Drive Control (AB) Speed controlled by using Load Control (AC)

Variable speed – Energy consumption

Common example of VS control

The Motor Car is a common example of VS control Control Torque to provide Acceleration and

Braking Controls Speed to match the traffic conditions Controls the use of Fuel

Main controls in a Motor Car are : Accelerator, which controls the Driving torque Brake, which adjusts the Load torque Control System .... the driver

4 – Quadrant drive

1st QUADRANT ..... Torque is +ve and Speed is +ve Therefore ..... Power is +ve Energy transferred from Drive to Load

2nd QUADRANT ..... Torque is -ve and Speed is +ve Therefore ..... Power is -ve Energy transferred from Load to Drive .... Braking

3rd QUADRANT ..... Torque is -ve and Speed is -ve Therefore ..... Power is +ve Energy transferred from Drive to Load

4th QUADRANT ..... Torque is +ve and Speed is -ve. Therefore ..... Power is -ve Energy transferred from Load to Drive .... Braking

4 – Quadrant drive

Fundamental principles Power is Rate at which Work is being done by a machine

Power is measured in Watts, or usually kW or MW Power is product of Torque x Speed At standstill .... Output Power = Zero

Energy represents the work done over a period of time Energy is the product of Power x Time Energy is measured as kiloWatt-hours .... kWh

9550)(rev/ Speedx (Nm) Torque = (kW) Power min

)(rev/ Speed(kW) Power x 9550 = (Nm) Torquemin

Load TorqueMachine Load Characteristic Curve Formulae

ConveyorsScrew ConveyorsPos. Displ. PumpsCompressors

(Constant) k = TT . n . k = P

Constant = k

Centrifugal PumpsCentrifugal Fans

n x k = T 2

n x k = P 3

ExtrudersSlurry Pumps

Breakaway = T B

WindersLathes

k = P

nP . k = T

Machine Load Characteristic Curve Formulae

ReciprocatingMachines

Presses

CrushersMillsWood Chippers

CranesSawmillsPresses

Load Torque

Torque – Speed curves Torque, Power & Speed are the most important

parameters Torque-Speed curves illustrate the performance of the

VSD shows the rotational force at various speeds

Power-Speed curves illustrate the performance of the VSD shows the rate of energy consumption at various speeds

These parameters are all related ... for example the Motor Car

Pressing the accelerator produces more torque .... which provides acceleration and gives more speed .... which requires more power (torque x speed) .... which requires more energy (fuel) (power x time)

Types of variable speed drives Mechanical Variable Speed Drives

Belt and chain drives with adjustable diameter sheaves Metallic friction drives

Hydraulic Variable Speed Drives Hydrodynamic types Hydrostatic types

Electrical Variable Speed Drives DC Drive with DC motor VVVF Converter with AC motor Slip Control with Slip ring Induction Motor Cyclo-converter with AC motor Electromagnetic Coupling or "Eddy Current" Coupling Servo Drives and Stepper Drives

Common types of variable frequency drives

Migration from DC to AC drives

Principles of AC variable drives

Speed controlled by adjusting the Power Frequency (f) Synchronous Speed

Actual speed is slower due to the Slip Actual Speed

Stator field flux () is derived from the supply voltage Air-gap Flux

Output Torque is product of flux density and rotor current IR

Output Torque

minrev/ p

f 120 = ns

minrev/ slip)- n( = n s

Nm I T R

fV

AC Variable speed driveFrom these equations, the following deductions can be made Speed is controlled by Frequency AND Stator Voltage Speed reaches Base Speed when VS = maximum,

Further speed increase reduces the Field Flux This is known as the Field Weakening range

Torque is dependent on VS

Full torque possible at ALL speeds in normal speed range But Torque falls to zero at standstill

In the Normal Speed range Output power increases in proportion to the speed

In the Field Weakening range, Torque falls in proportion to the speed Output power of the AC Motor remains constant

AC Variable speed drive

AC Variable speed drive Main Features of the AC Variable Speed Drive

Good control and performance characteristics AC converter relatively complex and expensive AC Motor needs no maintenance ... high reliability Efficiency : Converter ± 97% ... overall AC drive >90%

Basic definitions Rectifier ... AC to DC converter

Inverter … DC to AC converter

AC Converter converts one AC voltage and frequency to

another AC voltage and frequency .... often variable

Usually requires an intermediary DC link with smoothing

Basic definitions

DC Converter ... Converts one DC voltage to another DC voltage

Usually requires an intermediary AC link, such as a transformer

Basic definitions

Electronic Switch ....... Electronically connects or disconnects an AC or DC circuit Can often be switched ON or OFF from a gate terminal

Basic definitions

Bistable switching

Electronic Switch usually operated in the bistable mode Blocking Mode : fully switched OFF

• Voltage across switch is High• Current through switch is Low (only leakage current)

Conducting Mode : fully switched ON• Voltage across the component is Low• Current through the component is High

Diodes, Thyristors & GTOs are inherently bistable Transistors are NOT inherently bistable

Must be biased fully ON or OFF to behave like a bistable device

Power diodes

IDEAL : Forward Conduction : Resistanceless

Reverse Blocking : Lossless

Switch on/off Time : Instantaneous

Main terminals are the Anode (A) and the Cathode (K) Names come from the days when Valves were common

When the anode is positive relative to the cathode it is said to be forward biased and the diode conducts

When the anode is negative relative to the cathode is said to be reverse biased and current is blocked

Many different mechanical designs are used

Rated from a few amps … to thousands of amps Most common is for several diodes to be encapsulated into an

Insulated Module ... 6-pulse bridge, half bridge, etc The base of the module electrically isolated ... Can be mounted

directly onto heatsink

Power diodes

Bipolar junction transistor Main advantage of Bipolar Junction Transistors (BJT) ....

Turned on and off from the base terminal Suitable for Self commutated inverter circuits

Disadvantage is low base amplification factor .... 5 to 10 base circuit must be driven by an auxiliary transistor called the Darlington connection

Field effect transistor FET is a special type of transistor ...

particularly suitable for high speed switching applications Gate is voltage controlled .... not current controlled behaves like a HF voltage controlled resistance

MOSFET is a three terminal device Source (S), Drain (D) and the Gate (G) correspond to Emitter (E), Collector (C) and Gate (G) of BJT

Field effect transistor MOSFET is a majority carrier device .... short switching time

so ... switching losses are low best suited to high frequency switching applications

With development of Pulse Width Modulated (PWM) inverter high frequency switching has become a desirable feature to provide a smooth output current waveform

MOSFETs are used for Small PWM frequency converters MOS stands for Metal Oxide Silicon. Ratings from 100Amp @ 50Volt to 5Amp @ 1000Volt

Insulated gate bipolar transistor Insulated Gate Bipolar Transistor (IGBT) .....

unites best features of BJT and MOSFET technologies Construction similar to a MOSFET with additional layer to provide conductivity modulation, similar to BJT low conduction voltage drop

IGBT is a three terminal device .... Power terminals are called Emitter (E) and Collector (C) Control terminal is called the Gate (G)

IGBT has ...... good forward blocking ability very limited reverse blocking ability Operates at higher current densities than BJT or MOSFET

Electrical equivalent circuit of the IGBT .... hybrid device MOSFET driver integrated with a Bipolar PNP transistor

Insulated gate bipolar transistor

Gate driver requirements similar to those of power MOSFET Turn-on : 10V - 15V takes 1s .... Threshold typically 4V Turn-off : zero volts takes 2s ... accelerated by -ve volts

IGBT devices can be produced with faster switching times at the expense of increased forward voltage drop

Main advantages of IGBT are : Good power handling capabilities .... 500A at 1,500V Low forward conduction voltage drop of 2V to 3V … higher

than BJT but lower than MOSFET of similar size Gate is voltage controlled with low gate current Relatively simple voltage controlled gate driver High speed switching capability .... up to about 20kHz VF increases with temperature .... making device suitable

for parallel operation ... without thermal instability

Insulated gate bipolar transistor

Comparison of PE switches

Overall control system Overall Control System divided into 4 main areas :

Inverter Control System Speed Control System and Speed feedback Current (Torque) Control System and Current feedback External System Control Interface

Overall control system Inverter Control System

Controls the Switching Sequence of Inverter Switches Provides Component Protection

Speed feedback and Speed Control System Controls the Speed output relative to Setpoint

Current Control System and Current feedback Controls the Current output relative to Limits Provides Short-circuit and Earth-Fault Protection Motor Modelling and Thermal Overload Protection

External System Control Interface User Settings and Programming Digital and Analog interface to Control System (PLCs) Fault Diagnostics

Power supply requirements Simplest Method for Power Supply ... Mains Transformer

Major problem ... interruption of the Mains Power VSD Stops ... even for short dips in the supply

Commonly use Switched Mode Power Supplies (SMPS) Control power maintained until motor stops Mains failure ... power initially from large DC Capacitors Thereafter ... motor behaves as AC induction generator

Usually have several Power Supplies to modules such as ... Device Driver Power Supplies need to be isolated Cooling fans for the converter heatsinks DC Link Bus Charging Circuits Control Cards .... Microprocessor circuits

Two main approaches to DC Bus Charging .... Charging resistors with Contactor Bypass (most common) Phase-controlled bridge rectifier instead of diode bridge

DC Bus charging

DC Bus charging - Resistors

Many variations on Charging Resistor theme ... Resistors can be in DC link or on 3-phase supply lines Single large resistor or multiple sets of smaller resistors Electronic Switch instead of Relay ... smaller VSDs

Main Advantages of Charging Resistors are .... Simplicity of the control circuit Cheap and easy to implement

Main Disadvantages are ..... Losses due to relay contacts and coils Physical size of these components Reliability of electromechanical devices ... Can be a problem with numerous starts and stops

Controlled thyristor bridge

Phase-controlled rectifier bridge ... Used mainly on larger sizes ... above 22kW

Phase-controlled rectifier bridge .... Capacitor voltage increased gradually

Main Advantages of Controlled Thyristor Bridge are .... Conduction losses are lower Physical size reduced by not having the relay

Main Disadvantages are ..... Thyristors more expensive than Diodes More complex control circuit Reactive power requirements are slightly higher

Some VSDs with PWM Rectifier ... other advantages

Controlled thyristor bridge

PWM Rectifier bridge Controlled PWM Rectifier Bridge ... also called Active Front End

Capacitor DC Voltage increased gradually Also has other advantages … Also called ... Active Front End Drive

Main Advantages of PWM Rectifier Bridge are : Reduces the level of harmonic currents in mains … AC Line

current waveform is much smoother Makes full 4-quadrant operation possible Can control power factor angle ... power factor correction

Main Disadvantages of PWM Rectifier are ..... IGBT Bridge is more expensive than Diode Bridge Control Circuit is more complex and expensive Require line chokes to limit rate of current rise

PWM Rectifier bridge

AC Waveform-Ideal

Synthesized AC Waveform-Square

Synthesized AC Waveform-Part Square

Synthesized AC Waveform-Trapezoidal

PWM Inverter Output Frequency controlled ... by changing switching speed Output Voltage controlled ... by changing the Pulse Width Output Current waveform … depends on load impedance

Synthesized AC Waveform-PWM

PWM Inverter

Modulation Technique for sine-coded PWM using the Sine-Triangle intersection method - digital implementation

PWM Inverter

3-Phase PWM Inverter

3-Phase PWM Inverter

AC Variable speed drives

In general, AC Variable Speed Drives are designed to ... Transform Electrical Energy into rotational Mechanical

Energy

In most applications ... Control Speed with reasonable accuracy

In special Applications ... Need accurate and fast dynamic control of speed and

torque

There are 3 basic types of AC Variable Speed Drive available today

“Standard” Fixed V/f Drive (also known as a VVVF Drive) … OK for Pumps & Fans Less expensive than the devices below

Sensorless Vector Control Drive … Better Speed Regulation Better Starting Torque & Acceleration

Field Oriented Flux Vector Control Drive ... Full implementation of Vector Control Strategy Excellent Speed and Torque control characteristics

AC Variable speed drives

Variable speed drive control loopsThe Level of Control can be ... Simple Open-Loop Control ...

This is the strategy used for Fixed V/f drives No internal feedback from the motor (except for protection)

Closed-Loop Control ... This is the strategy used for Vector Control drives Feedback from the motor used to adjust PWM output Achieves enhanced performance

Cascade Closed-Loop Control ... Strategy used for Field Oriented Flux Vector Control drives Feedback of Torque and Speed used to improve dynamic

performance Achieves better than DC Drive performance

Open loop control

Open loop speed control is suitable for .... Applications where Speed Accuracy not important Consequences of changes in the process not severe

Standard fixed V/f drives ... are essentially Open Loop type

Speed Reference fed to Ramp Circuit to convert step change in the speed request to a slowly changing signal

V/f Regulator sets magnitude of Voltage and Frequency Finally, PWM Switching logic section controls the switches

according to a PWM algorithm (sine-coded, etc) No speed feedback from the motor .... open-loop control Current feedback is for protection, indication & current limit

Open loop control

Closed Loop Control used for more difficult drive applications ... Torque, Speed or Position accurately controlled Accuracy of control ..... very important Errors .... have a large influence on the process

For these applications, Closed Loop Control is necessary Generally applies to high performance VSDs .... such as DC

Drives and Vector controlled AC VSDs Standard fixed V/f AC Drives can be used in closed loop

systems ... but they are not capable of high performance

Closed loop control

Closed loop control

Accurate Feedback from motor transducer..... Speed transducer … tacho or encoder Position transducer … position encoder Torque transducer … current transducer

Power Converter .... controls the Motor response Controller .... which controls the Converter

Closed Loop Control System operates as follows .... Measurement of Process Variable (PV) .... eg an encoder Comparison of PV with Set Point (SP) gives an error

value ....

error value = SP - PV Error value processed by Controller to adjust the Output,

which in turn Controls the AC converter and motor In industrial applications, the controller is a Microprocessor

Closed loop control

Motor Car ..... example of a closed-loop feedback control Speed assessed by the driver looking at speedometer Measured speed (PV) is compared to desired speed (SP) Depending on the error, the driver may decide to

• increase speed by pressing the accelerator• decrease speed by pressing the brake

Driver continually measures PV, calculates the error and gives the appropriate Output

At the same time, Driver might be simultaneously engaged in several other tasks of closed-loop feedback control, such as steering, controlling cabin temperature, etc.

Closed loop control

Cascaded loop control

If each variable was proportional to the variable before it Simple Open-loop control, without feedback would be OK In a VSD .... some time delays not simply proportional ...

motor current responds to new frequency with a rise The time is dependent on its leakage inductance Motor speed follows the torque with a rise time ... dependent

on its inertia Inaccuracies acceptable in simple applications ....

Speed control for pumps, conveyors, etc Some applications require Close Speed and Torque control

Speed and Torque control of paper machine VSDs

Design techniques have evolved from DC Drives ... and deals with control problem in two smaller stages

Speed Loop compares speed ... calculates current setpoint Current Loop compares current ... calculates frequency

setpoint

Cascaded loop control

Speed Loop allows for one of time delays in system delay between the torque and the measured speed

Current Loop allows for other time delay .... delay between the output frequency and the current rate of change of current is faster than change of speed

The two control loops required for accurate speed control are An outer Speed Control Loop, which compensates for the

mechanical transients, mainly load inertia An inner Torque Control Loop, which compensates for the

electrical transients, mainly winding X and R

Cascaded loop control

Vector control Vector Control ...

Been available since the mid-1980s Promoted as an AC equivalent to DC Drives Only become possible as a result of the large strides in the

fields of power electronics and digital control

Gets its name from the fact that .... System can separately measure and control the two Vector

components of the stator current Vectors represent magnitude and direction of the current Specifically ....Flux current and Torque-producing current

Vector Control is a “generic” name applied to all AC Drives that provide performance that is higher than “standard” AC Drives

Vector Control is one of the more abused terms used to promote the use of modern AC Variable Speed Drives ...

performance said to be equivalent to high performance DC Drives

Dynamic Performance that is equivalent to a DC Drive is only possible if Vector Control is fully implemented

There are many Vector Control Drives on the market that only partially implement the strategy

Vector control

Simplified equivalent circuit of an AC Induction Motor Using a Hall-effect current transducer, the drive can measure

the Stator Current IS flowing to the motor ... but NOT IR and IM

Vector control

IR and IM are the Vector components of Current IS

Vector components can be calculated from measured values

Vector control

Therefore, the main purpose of Vector controller is to ... Continuously calculate value of Flux Current IM Continuously calculate value of Torque Producing Current IR Continuously calculate other variables such as Slip, Shaft

Speed, etc Central part of Vector control is the Active Motor Model ...

Uses motor constants stored in memory as part of calculation to continuously model the connected motor

Measures stator current in each phase and uses this to calculate the torque current (IR) and flux current (IM)

Measures actual speed and calculates slip For adequate dynamic response of the drive, these calculations

need to be done at a rate of more than 2,000 times per sec This only became commercially viable within last 10 years with

development of 16-bit microprocessors

Vector control

These are essentially Fixed V/f “open loop” drives with some performance enhancements due to a digital “Motor Model”

The Motor Model is used to calculate 2 main variables Flux Current (IM) ... to automatically regulate output V/f

ratio. This results in improved Torque performance, particularly at low speeds

Percentage Slip ... to automatically regulate output frequency. This results in improved Speed holding performance ... without the need for a shaft mounted encoder (hence the name Sensorless Vector Control)

For Sensorless Vector Control to be effective, the drive needs to be “tuned” to the connected motor

Auto-tuning feature normally available ... used to measure the required motor parameters and store them in memory

Sensorless vector control

Field oriented flux vector drives These are ”Closed loop” drives with an Active Motor Model

and cascaded Speed and Torque control loops The high performance microprocessor runs a Motor Model that

can calculate numerous drive variables For the Vector Control to be effective, the drive needs to be

“tuned” to the connected motor Auto-tuning feature normally available ... used to measure

the required motor parameters and store them in memory Field Oriented Flux-Vector Control necessary on drives where ...

Accurate speed and/or torque control is necessary High dynamic performance is required ... speeds and/or load

torque change rapidly Full torque is required at zero speed ... for example on hoists

In a Flux Vector drive ... The Power circuit is identical to fixed V/f drive Main difference ...... is in the control system

Field oriented flux vector drives

Flux-Vector control of a PWM Converter .... Control is essentially Cascaded Closed-loop type

Field oriented flux vector drives

Vector control performance

Some interesting Figures …

DC Drivewith

encoder

V/f VectorSensorless

V/f Vectorwith

encoder

FieldOriented

Sensorless

FieldOriented

withencoder

SpeedAccuracy 0.01% 1.0% 0.1% 0.5% 0.001%

TorqueResponse 10-20msec 100msec 10-20msec 1-10msec 1-10msec

Basic setting parameters

Remaining Parameters settings can be selected as follows : Maximum speed ... usually set to 50Hz or higher Minimum speed ... usually 0Hz for a pump or fan drive and

higher for constant torque applications Rated Motor Current ... size of motor may be small Current Limit ... determines Starting Torque Acceleration Time ... determines the Ramp-up Time Deceleration Time ... determines the Ramp-down Time Braking Method ... 3 Options usually available Starting Torque Boost ... cover Breakaway Torque

Note : Avoid over-fluxing the motor !!!

Other Settings ... possibly adjust "default" settings

Synthesizing an AC Wave-Examples

Simple square wave Square wave with smaller conduction width Trapezoidal waveform A series of pulses of fixed amplitude but varying width within a

cycle (known as pulse width modulation or PWM)

Protection of AC variable speed drives

The protection of AC Variable Speed Drives includes ... Protection of the AC Converter Protection of the Electric Motor

AC Converter protection

Protection for front-end rectifier usually NOT provided … Usually require external upstream short-circuit protection HRC fuses or fast Circuit Breaker

Following protection usually included ... Protection systems for the PWM Inverter Protection for DC Busbar Protection for Output … Motor and Motor Cable

• Motor thermal overload protection• Motor short circuit protection• Motor earth fault protection

Summary of overall protection

Input phase imbalance

One input phase voltage low .... Other 2 phases will conduct majority of supply current Possible failure of rectifier diodes

DC Current ... miss every 3rd pulse, other 2 pulses higher can lead to failure of the rectifier diodes or capacitors

Input Phase Imbalance Protection implemented by ..... Measuring the Input Currents .... Costly Monitoring the DC bus .... analyse waveform distortion

DC bus under voltage

Power Circuit can operate at any voltage 0 - VMax

Under-voltage protection required for Power Supplies Typically set to 15% below lowest rated voltage

When Power Supply output voltage regulation is lost ... Microprocessor could switch to an indeterminate state Driver circuits lose control of the Power Switches Power Switch may attempt to operate in the linear region Power Switch may be slow to switch off Power Electronic Switches will fail

DC bus over voltage All electrical components fail if exposed to high Over-voltage

In AC drives, high DC over-voltages can occur due to ... High voltages in the mains power supply ... Very Rare Motor behaving as Induction Generator .... dynamic

braking of a high inertia load

The following components have the lowest tolerances to High Voltages ....

DC bus Capacitor Bank DC bus connected Power Supply Modules Power Electronic Switching Devices

DC capacitor bank ... series and parallel capacitors Sharing will not be perfect

Peak voltage on DC bus is approx 1.4 x supply voltage With a maximum capacitor voltage of VMax = 750VDC ... the

practical limit for input voltage is 480VAC + 10% For AC drives with >500Volt input, special capacitors are

required

Maximum voltage of Semiconductor switches is 1,400 VDC Seems well above Capacitor rating But, voltage across a device during turn-off can be 400V higher, due to stray circuit inductances Bus voltage must be limited to 800 VDC Maximum

DC bus over voltage

In Modern Digital AC drives ... Over-voltage Protection provided by microprocessor

because DC bus voltage changes relatively slowly

Digital controller can also provide over-voltage control During deceleration of load ... override ramp-down setting to

prevent the over-voltage trip DC bus voltage allowed to rise to safe 750 VDC Trip level typically at 800 VDC

DC bus over voltage

Operation interface & diagnostics

Human Interface Module (HIM) .... LCD or LED Display

3 Main levels of operator information and fault diagnostics :1. Parameters ... Settings, Status and Metering2. Diagnostic information ... Status of Protection

Circuits3. Diagnostic information ... Status of Internal Circuits

... internal diagnostics only found in special VSDs

Internal parameters

Typical Internal Parameters and Fault Diagnostics ...

Module Parameters and Fault DiagnosticsPower Supply Power Supply Voltage, Current and Frequency

DC Bus DC Link Voltage and Current

Motor Output Voltage, Current, Frequency, Speed, Torque, Temperature

Control Signals Setpoint, Process Variable, Error, Ramp times

Status Protection circuits, module failures, internal temps, fans running,switching frequency, current limit, motor protection, etc

Fault Conditions Power device fault, power supply failed, driver circuit failed, currentfeedback failed, voltage feedback failed, main controller failed

Fault diagnostics - parameters

Common Faults ... possible Internal/External Problems

Protection Internal Fault External FaultOver-voltage Deceleration rate set too fast Mains voltage too high

Transient over-voltage spike

Under-voltage Internal power supply failed Mains voltage too lowVoltage sag present

Over-current Power electronic switch failedDriver circuit failed

Short circuit in motor or cable

Thermal Overload Control circuit failed Motor over-loaded or stalled

Earth Fault Internal Earth fault Earth fault in motor or cable

Phase Imbalance Power diode in rectifier failed Mains phase voltage imbalancePhase connection loose

Over-temperature Cooling fan failedHeatsink blocked

Ambient too highEnclosure cooling blocked

Thermistor Trip Motor thermistor protection

Motor side filter

Connection Example of Line Filter and Motor Filter

Natural ventilation

Enclosure can be Smaller ... additional Ventilation required Exchange air between Inside and Outside of enclosure

Natural Ventilation Convectional cooling airflow through air vents Vents at Bottom and Top ..... the "chimney" effect

Forced ventilation

Cooling airflow assisted by fan at Top or Bottom of cubicle

General safety requirements

Requirements for Safety .... should be carefully followed Australian Standard AS 3000 : SAA Wiring Rules apply Safety earths must be installed before power connected

AC Converters have Large Capacitors on the DC link After VSD switched off ..... wait several minutes Allow internal capacitors to fully discharge Visual Indication ... shows when capacitors are charged

Hazardous areas

AC Converters should NOT be mounted in Hazardous Areas ..... even when connected to an Ex rated motor

When necessary ..... AC converters may be mounted in Approved Enclosure Certification should be obtained for entire VSD System ....

including both Converter and Motor

Any questions ?