emc issues in electric drives - wayne state universityapandya/seminars/index_files... · emc issues...

EMC Issues in Electric Drives


• EMC Due to:

– Control systems requirements.

– Motor operation.

– Physical constraints.

• “Scaling” of EMC Methods to Electric Drive

Analysis.

• New Developments / Future Methods


Why Use Electric Drives?

• Advances in power electronics as well as motor

design and manufacturing have made electric

drives very attractive.

• Benefits of electric drives include high efficiency

with lower mass as a result of implementation of

adjustable/variable speed or frequency drives

(ASD/VSD/VFD).

• Applications include transportation and

manufacturing.


System Engineering Aspects of

Electric Drives

• EMC (primarily emissions).

• Safety:– Safety aspects TAKE PRIORITY in the design of any

high voltage / high current electric drive system.

– Requirements for system integration (such as grounding for safety) MUST BE ADHERED TO.

– If a “Best Practice” for EMC conflicts with a safety requirements – the safety requirements take precedence!


Electric Drive EMC Case

Study

• Variable frequency electric drives (VFD) were

installed on a HVAC system in a renovation of an

older medical facility. During the renovation –

MRI machines were also installed.

• False reading from the MRI machines were then

taken by medical personnel – who were then sued

for malpractice.

• Quickest solution was to install a separate

electrical service for the MRI machines!


EMC Requirements for

Electric Drive Systems

• May or may not be

exempt from legislated

requirements (FCC

Section 15.103).

• Typical requirements

are for the control of

CE – with levels

shown at left.104

105

106

107

108

40

50

60

70

80

90

100

110

Frequency (Hz)

Lim

it -

- d

B(µ

V) CE102

CISPR 15

CISPR Class A

FCC Part 15: Class A

FCC, Part 18

Ultrasonic

FCC Part 15: Class B

CISPR Class B &CISPR 14 householdappliances

DO-160D: Cat B

DO-160D: Cat L,M&H

DO-160D level assumes 50-ohm LISN impedance


Importance of CE from

Electric Drives

• CE from electric drives are a concern for the

following reasons:

– May exceed legislated requirements.

– May exceed customer requirements.

• May impact functionality of other

components/systems that have not been designed

to be immune to the CE levels that are developed.


Mechanism of CE Coupling

• CE from electric drives exist because the system components must be connected by wiring.

• Many of the wiring practices in use today where developed before VFD’s were developed and other sensitive electronic devices utilized.


Wiring “Caused” EMC Issues

•The process of wire routing is an important contributor to electric drive EMC issues.

– Need to comprehend sources and receivers in systems.

– Wire routing affects EMC “Path”.

– Critical to recognize that due to parasitic inductance and/or capacitance effects exist.


“Small Scale” EMC Issues

Due to Electric Drives

• Electrical energy consists of conducted emissions (CE) traveling along wiring into receiver.

• Wiring acts as very efficient coupling mechanism.

• In addition to CE conditions - this may also result in radiated emissions (RE).


Causes of Electric Drive Radiated

Emissions - DM

• Differential Mode Current – Current that flows in

opposite direction on conductors in a system.

• Differential mode current is the motor drive

current that can be identified on schematics.


RE Due to DM Current

• Radiated emission levels can be determined from the following equation:

E (v/m) = 131.6 x 10 -16 (f 2 A I ) / D

• Where:

– E is the radiated field strength (in the “far field”).

– f is the frequency of the current.

– A is the “loop area” of the DM current.

– I is the noise current.

– D is the distance from the loop.


Causes of Electric Drive

Emissions - CM

• Common mode current is an “unintended” current.

– May be caused by “grounding” of the

components in the drive system.

– Parasitic capacitance from motor winding can

also cause CM current to occur.

• Common Mode –

Current that flows in

the same direction on

conductors.


RE Due to CM Current

• For CM current, the RE level is determined by:

E (v/m) = 12.6 x 10 -7 (f L I) / D

• Where:

– E is the radiated field strength (in the “far field”).

– f is the frequency of the current.

– L is the length of the current path.

– I is the noise current.

– D is the distance from the current path.


Example of CE Due to Un-

anticipated CM Current

• CE may also be due to conductive chassis as

electrical paths (sometimes un-intentional).

• Example – “Grounding” (or even parasitic

capacitance) creates current path.


Important Observations About

CM and DM RE

• With DM current, there can be some cancellation of the fields due to each part of the “loop” of the source current.

• For CM current – VERY LITTLE cancellation takes place – and the resulting electric field is ORDERS OF MAGNITUDE greater than the electric field caused by similar levels of DM current.

• Consequently – CM current effects can be very significant even with low levels of current.


Combination of DM and CM

• Circuits may have both types of current

flow.

• Important to understand methods to address

each.


Effect of Wiring “Inductive

Parasitic”

• Coupling of energy from the wiring of system 1 to the wiring of system 2.

• Noise is induced in system 2 by “di/dt” of system 1.

• Can be due to “common grounds”.


Effect of Wiring “Capacitive

Parasitic”

• Capacitive coupling

from system 1 to

system 2 due to close

proximity of wires in a

harness.

• Noise is induced in

system 2 by “dV/dt”

of system 1.


Electric Drive Control Systems

• Control systems for electric drives typically consist of active switching of the primary current for the motor (similar to basic switching power supply).

• Output voltage is determined by switching speed and “on” duration of the drive transistor's).

• Multiple phases can be obtained by utilizing multiple driver transistors with appropriate timing.


Electric Drive Control Systems

• The benefits are:– This results in small size compared to previous methods

of “linear” control.

– Uses components and circuits similar to switching power supplies (shown below).

– High efficiency control methods.


EMC Due to Control System

Operation

• Unanticipated CE may occur due to:

– The intentional creation of a fast rise / fall time motor

current (intended to minimize hardware power

dissipation requirements).

– The subsequent current that is developed as a result of

very efficient capacitive parasitic coupling of the

harmonic energy in the motor power current.


Control Current Switching

• The switching process can create significant CE

issues.

• The frequency content of the CE may extend far in

frequency before significant “roll-off”.


Controller Design Goals

• The advantages are the following:

– Changing the frequency and/or the duty cycle (pulse width) will change the motor speed.

– High efficiency due to minimal time that the switching devices are in their linear operation condition.

• The trade-offs in order to achieve these advantages need to be understood to minimize resulting CE issues.


Balancing EMC and

Performance Requirements

• Important to understand the impact of fast “slew

rate” operation with power drive devices such as

Insulated Gate Bipolar Transistors (IGBT).

• The switching operation results in low power

dissipation along with:

– Possible of operation at an order of magnitude faster

than the response time of electromechanical devices.

– Causing radiated/conducted emission issues.


Examples of Electric Drive

Controller

• Figures (a) and (b) show the control electronics.

• Figure (c) shows an EMC shield over the IGBT’s

to prevent noise from affecting low-level signals.

• Figure (d) shows the driver IGBT’s.


Electric Drive Motors

• Advances in driver electronics have made now made it practical to use a.c. induction motors.

• Can attach to directly to output shaft without mechanical speed adjustment methods required with fixed speed motors.

• Allows for less mechanical complex system.

• Speed can be adjusted by:

– Frequency (and pulse width) of the motor current.

– Amplitude of the current.


Drive Motor Construction

• “Reverse” of small d.c. motors (those with a wire-wound rotor contained within a magnetic field generated by permanent magnets).

• Motor winding (“Stator”) surrounds rotor constructed with permanent magnets.

• Stator generates changing magnetic field due to a.c. current.

• Rotor turns as it “follows” the magnetic field that circles around the internal of the stator.


Steps in the Construction of A

Drive Motor

• A stator is produced

that contains a number

of “poles” that are

used to hold the

windings.

• Application of drive

current for each phase

generates magnetic

field.


Stator Characteristics

• Results in a three-phase induction motor

that operates on the principle of the rotating

magnetic field.

• Windings for each phase are located 120

degrees apart around the stator.

• Phase windings are connected in a “Wye”

configuration.


Schematic of Three Phase

Controller and Motor Circuit

• IGBT’s generate three-phase motor drive current

which is supplied to “Wye” stator windings.


Motor Drive Waveform

• Three-phase waveform used for motor operation.

• Each vertical division represents 60 degrees.

• A voltage (current) maximum occurs at each 60

degree increment.


Actual Stator Construction

• Figure at right shows a

typical stator from a

variable speed drive

motor.

• Significant portion of

the stator (and it’s

mass) is due to the

large number of

windings required.


Motor Speed Control

• Speed of induction motor is dependant on the motor design.

• The motor operates at “synchronous” speed –which is the speed that the stator magnetic field rotates.

• Determined by the frequency of the a.c. input andthe number of poles in the stator.

– As the poles increase – the speed decreases.

– As the frequency increases – the speed increases.


Induction Motor Speed

Determination

• The speed of a induction motor is known as the

synchronous speed and is determined by the

following relationship:

RPM = 120 f / NP

• The speed is directly related to the applied

frequency (f) in Hertz and inversely related to the

number of poles (NP).


Actual Rotor Mechanical

Speed

• Rotor mechanical rotation does not achieve same

speed as magnetic field rotation.

• Rotor “lags” behind the magnetic field due to

“slip” – which must occur in order for the motor to

operate.

• Slip is typically a few percent of the field rotation.

• For synchronous speed of 3600 rpm – the rotor

speed would be approximately 3400 to 3500 rpm.


Permanent Magnet Rotor

Construction

• Rotor contains high-strength permanent magnets

arranged around the perimeter.

• “Movement” of field in stator causes magnets to

try to track the field – resulting in rotation.


Example of Rotor Positioning

• Magnetic field polarity

from poles causes

rotor to move in an

attempt to align

permanent magnets

with field from stator.

• As field moves from

pole to pole – rotor

turns.


Typical Electric Drive Motor

Specifications

• The motor shown at

left has an output

capability at 1500

RPM of:

– 50 kW (approximately

67 hp)

– 400 NM

(approximately 300 ft-

pounds).


“Scaling” of EMC Analysis

Techniques to Electric Drives

• EMC analysis methods have been developed to analyze circuit boards, component placement, and layouts.

• These methods can easily be “scaled” to provide insight into electric drive EMC issues.

• One method is to determine the level of radiated emissions (RE) from current flow.

• Cause of RE can be due to both common and differential mode currents.


Minimizing EMC Issues in

Future Electric Drive Systems

• Determine the optimum “slew rate” of the motor current taking in consideration the desired response time of the motor and the power dissipation capability of the controller.

• Understand the actual path of any noise current that exists.– Minimize loop areas for DM noise current.

– Reduce levels of CM current or convert the current to DM.


Minimizing EMC Issues in

Future Electric Drive Systems

(continued)• Incorporate devices with high impedance to

the noise current and that do not affect the functional current (such as ferrite clamps).

• Understand the implications of required safety grounding and how that may contribute to EMC issues.– Make the return ground wire the shield of the

cable?

– Place PRIORITY on Safety first – then EMC!


Summary

• Electric variable speed drives are becoming more common and have advantages over previous systems and may need to meet EMC requirements.

• Can have EMC issues due to:

– CE caused by common mode and differential mode current.

– RE issues can also occur due to wire lengths, loop areas, and noise current levels.

• Understanding the effect of wiring and component parameters can minimize EMC issues.

emc issues in electric drives - wayne state universityapandya/seminars/index_files... · emc issues...

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