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Variable Frequency

DrivesVFD ExploredVFD Control SchemesOver current and over load protection requirementsLoad Types Constant Torque vs. Variable TorqueMotor / VFD compatibility and requirements to be matched

Multi Motor ApplicationsDC Brake Resistors applied to Drives

What should an SCO be specifically looking for when inspecting a VFD installation

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VFD Explored

First, the Converter (usually a diode rectifier) converts three-phase AC

power to DC power. Next, the DC Bus stores and filters the DC power in a large bank of

capacitors.

Last, the Inverter (usually a set of six IGBTs) switches or inverts the DC

power in a Pulse Width Modulated (PWM) AC waveform to the motor.

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VFD Explored

EtherNet IP, DeviceNet, ControlNet,

Modbus, LonWorks, Profibus, Can-Open,

Interbus, Metasys, etc.

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Variable Frequency Drives

VFD Explored

VFD Control Schemes

Over current and over load protection requirements

Load Types Constant Torque vs. Variable TorqueMotor / VFD compatibility and requirements to be matched

Multi Motor Applications

DC Brake Resistors applied to Drives


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VFD / Motor Protection

Drive must be correct size for the motor and programmedaccordingly to ensure motor is protected Based on CEC 2-024, 28-300, 28-500

Motor OL protection on a VFD is provided electronically

Motor Data to be programmed during commissioning Motor Nameplate information

Power (HP/KW) FLA

SF

Volts

RPM

Frequency

Select Motor control type

V/Hz Control Motor control is calculated for performance and It protection

Vector Control Motor / drive auto-tuning to optimize performance & calculates It

protection

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VFD / Motor OL Protection

For single motor applications the drive shall be programmed to protectthe motor from overload conditions

An electronic thermal overload IT function emulates a thermaloverload relay

Models Class 10 OL

This operation is based on three parameters;[Motor NP FLA], [Motor OL Factor] and [Motor OL Hertz]

[Motor NP FLA] is multiplied by [Motor OL Factor] to allow the user todefine the continuous level of current allowed by the motor thermaloverload

[Motor OL Hertz] is used to allow the user to adjust the frequency belowwhich the motor overload is de-rated.

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Short Circuit Protection28-200, 28-202, 28-206, 28-208, 28-210

Short Circuit Protection Provided by Fuse or Breaker

Most VFDs rated at 200kA for short duration

Fuse preferred by most drive vendors

Semi-conductor fuse

Clear time to Cycle

Breaker

Thermal Type

1 to 2 cycles

VFD provides instantaneous over current

protection

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VFD Explored

VFD Control Schemes






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A constant amount of torque is required to turn the load regardless ofthe speed

Constant torque means constant current

Heat generated is same at all speeds

Motor heat is a function of heating losses in the motor

Power (heat) = I

2

R The cooling system of TEFC motors deteriorates at reduced speed and

is proportional to motor speed

Fan Cooling E Speed2

i.e. Cooling effect at 50% speed } 0.52} 25%

Motor heating becomes an issue and must be addressed with the typeof motor purchased (motor speed range).

Typical Ranges: 4:1 10:1 100:1 1000:1

Load Types

Constant Torque vs. Variable Torque

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0

10

20

30

40

50

60

70

80

90

100

0 6 12 18 24 30 36 42 48 54 60 66 72 78 84 90

TorqueTorque

TorqueTorque

Acceptable Region

for Continuous Operation)

Hz

%Torque

Speed Range - Constant Torque 4:1

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VFD Ratings

Constant Torque versus Variable Torque

Some VFD manufacturers rate their VFDs differently tooperate on both of the two predominant load types:

Constant Torque (CT) or Heavy Duty

Variable Torque (VT) or Normal Duty

The ratings are industry standards and basically state theamount of overload that the load can tolerate, and thatthe drive must supply, for a one minute period

Constant Torque (CT) = 150% current for one minute Variable Torque (VT) = 110 - 120% current for one

minute

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VFD Ratings

Constant Torque versus Variable Torque

These ratings basically qualify the drivechassis thermal rating

The thermal withstand of the chassis is afixed value and is based upon the productof the square of the current and time (I

2t)

Most VFD Manufacturers rate their drivesto meet the VT specification and thenhave to derate them to meet the CT rating

Some VFD Manufacturers do just the

opposite and rate their drives (usually230V & 480V Micro drives) for CT andthen can over rate them for VT.

(1.5 x lower current)2

x t } (1.15 x highercurrent)

2x t

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VFD Explored

VFD Control Schemes






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The Reflected Wave Phenomenon

First identified in 1900 with power distribution lines

Also known as Standing Wave or

Transmission Line Effect

Well documented in digital communications

Coming to the forefront in IGBT based drives

Can cause voltage peaks at the motor

Presents the possibility for insulation breakdown

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The Physics of it All

The cable between the drive and motor represents a

substantial impedance to the PWM voltage pulses of thedrive

Cable impedance is proportional to length

Inductance / unit lengthCapacitance / unit length

If the cable surge impedance does not matchthe motor surge impedance----

Voltage reflection WILL occur !!

Z0=

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What will the amplitude be?

The velocity of the reflection

(Dependent on cable capacitance & inductance)

PLUS

The rise time of the switching devicedetermine the cable distance at which the

reflected wave will reach maximum amplitude

The amplitude can be 2 - 3 times the DC Bus voltage

( 675VDC X 2 = 1350 Volts typical )

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Typical PWM VLL Output Pulse at the Motor

Terminal

0

-1

+1

+2

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Reflective Wave

Raw output voltage waveform

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Drive Motor Match

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Drive Motor Match

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Inverter Duty/Rated Motors

Drives are designed to work with all squirrel cage induction motors

Inverter duty motors allow longer distances with no degradation orbreakdown

Per NEMA MG1 Part 31:

Definite-purpose, inverter-fed motors are designed to withstand

maximum repetitive voltage peaks at the motor terminals equal to 3.1times the motors rated rms voltage with a rise time not less than 0.1 s.

These motors can be used on a control without additional filters or reactorsprovided the particular combination of control and cable does not generatepeak voltages which exceed this requirement at the motor terminals.

Only where motors are required to meet Temperature Code Ratings asper section 18, they must be Drive/Motor match, tested and labelledaccordingly.

Non inverter or standard motors can be used provided the necessaryprecautions are taken

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Protect the Motor

Installed solution between VFD and Motor

Output (Load) Reactor

Output dv/dt Filter

Sinewave Filter

Motor Terminating Device

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VFD Explored

VFD Control Schemes






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Multi Motor Application28-108, 28-500(3)(e)

All motors run at same speed

Drive size to be sum of all HP

No limit to number of motors

on one drive

Motors do not need to be

same HP rating

All OL Relays to be tied

together in series

Drive would run in V/Hz mode

Motor OL can be disabled in

drive

10HP 5HP 5HP

20HP

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VFD Explored

VFD Control Schemes






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VFD Explored

Low Voltage Protection as per CEC 28-400

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PowerStructure c/w DC Brake

Drive and brake are separatecomponents

Chopper Transistor Voltage Controlregulates the voltage of the DC busduring regeneration.

Drive with DC Brakeinstalled

Resistor terminatesat drive

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PowerStructure c/w DC Brake

Drive and brake are separatecomponents

Chopper Transistor Voltage Controlregulates the voltage of the DC busduring regeneration.Wire sized according to resistor size

Brake resistor wire size CEC 26-640 & 26-642

Drive with DC Brakeinstalled

Resistor terminatesat drive Terminate atBR1 & BR2

Caution should be take toensure proper

Wire Type

Wire size

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External Brake Resistor Circuitry

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VFD Explored

VFD Control Schemes






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Questions?

Documents

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