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!
Limit -- db(µv) EMC Requirements for Electric Drive Systems 110 100 90 80 70 CE102 CISPR 15 DO-160D level assumes 50-ohm LISN impedance DO-160D: Cat B FCC Part 15: Class B 60 FCC, Part 18 Ultrasonic 50 DO-160D: Cat L,M&H CISPR Class B & CISPR 14 household appliances 40 10 4 10 5 10 6 10 7 10 8 Frequency (Hz) FCC Part 15: Class A CISPR Class A 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.
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 that flows in the same direction on conductors. 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.
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 Unanticipated 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 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. Parasitic
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 wirewound 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 and the 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 ftpounds).
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.