Alltec Corporation News & Articles
Protecting Variable Frequency Drives
Articles by Alltec Corporation
Thursday October 29, 2009
Surge Protection Devices (SPDs)
Sophisticated and highly susceptible microprocessor based electronics and data communication networks are integrated across every sector of today’s fast paced business world. Preserving these mission-critical systems from the damages of surges, spikes, and transients ensures that these systems are protected from equipment destruction, disruption in service, and from costly downtime. How to properly stage these SPDs can be as important as actually making the decision to purchase them.
Protection of Drives
The use of various types of drives to control motors is very common. The purpose of the drive is to increase the efficiency or to manage the speed of the motor being controlled. Through various processes and control mechanisms, the drive often reshapes the sinewave to provide a signal to the motor that allows for greater efficiency or varies the frequency of the signal to control the speed of the motor.
Due to the action of the drive, the power quality of the electrical environment can be compromised. That is, the drives can create voltage surges and harmonics on the system.
There are various technologies available that aid in correcting these issues. This application note focuses on applying surge protective devices (SPDs) to a drive system to mitigate the damage that can occur due to voltage surges while considering the effects of the harmonics on the surge protective device.
Application of SPDs
To aid in the description of the application of SPDs to a drive system, please refer to Figure 1.
This figure illustrates a typical drive layout. The incoming power is usually delta configured (3 phases and ground).
Often the incoming voltage is 480 V, but other voltages may be used. The incoming power is usually stepped down to a lower voltage (typically 120 Vac) that provides power to the control circuit. The control circuit contains sensitive electronics. Once the power is acted upon by the drive the output is fed to the motor.
As noted, there are five opportunities for protecting the typical drive system—each are labeled with a circled number and are described below.
Drive Input
Protecting the drive input is an essential step in protecting the drive system. Providing protection at this location prevents surge damage due to events propagated on the electrical system from upstream sources, external events such as lightning and switching surges created by the utility, and the interaction of multiple drives on the same system.
At this location, a parallel connected, voltage responsive circuitry device is appropriate (one without frequency responsive circuitry). Frequency responsive circuitry is not recommended for this location due to the fact that this location is typically more susceptible to impulse transients as opposed to ring wave transients.
Inverter Input
The inverter input is one of the most sensitive and critical areas of the drive itself. It is at this location that care must be taken and the proper survey conducted. You may install a parallel connected, frequency responsive circuitry device provided you have confirmation that within this drive that no additional capacitors have been installed to mitigate harmonic currents.
IF THEY HAVE, then at this location, a parallel connected, voltage responsive circuitry device is appropriate (one without frequency responsive circuitry). Frequency responsive circuitry would not be recommended for this location due to the high harmonic content that necessitated the installation of additional capacitors. Installation of frequency responsive circuitry devices at this location will lead to failure of the SPD.
Control Circuit
The control circuit contains sensitive electronics that can be damaged by the environment created by the drive or by surges from external sources. Protection at this location is essential.
Since this circuit is isolated by a step down transformer and it feeds sensitive electronics, a series connected SPD with frequency responsive circuitry is recommended for this location.
Drive Output
Protecting the immediate drive output is recommended when the length of the connection between the drive and the motor is longer than 50 ft (15 m) or if the connection is routed along an external wall or outdoors.
One reason for protecting at the immediate output when the length of the connection to the motor is long is due to reflected waves that can occur as the signal (often higher frequency) from the output of the drive reaches the motor and is then reflect back and forth between the drive and the motor. This action can create "voltage piling" – the reflected voltage adds to the nominal voltage and other reflected waves. The SPD will aid in reducing the voltage peaks of the reflected waves.
More importantly, if the connection between the drive and the motor extends outdoors, along a path that is exposed to the environment or close to the building’s steel structure, protection at this location is important to diminish the effects of direct lightning or induced voltage surges due to nearby lightning. These surges can cause damage to the drive, even if protection is provided at the motor input.
At this location, a parallel connected, voltage responsive circuitry device is appropriate (one without frequency responsive circuitry). Frequency responsive circuitry is not recommended for this location due to the high harmonic content of the signal due to the normal operation of the drive. Installation of frequency responsive circuitry devices at this location will lead to failure of the SPD. Utilizing a voltage responsive circuitry device at this location will eliminate this possibility.
Motor Input
Protecting the motor input is an essential step in protecting the drive system. Providing protection at this location prevents surge damage due to events propagated from the drive output to the motor input. Providing protection at this location aids in extending the life of the motor as the SPD helps to prevent damage to the windings and bearings of the motor due to surges.
Further, if the connection between the drive and the motor extends outdoors, along a path that is exposed to the environment or close to the building’s steel structure, protection at this location is important to diminish the effects of direct lightning or induced voltage surges due to nearby lightning. These surges can cause damage to the motor, even if protection is provided at the drive output.
At this location, a parallel connected, a voltage responsive circuitry device is appropriate (one without frequency responsive circuitry). Frequency responsive circuitry is not recommended for this location due to the high harmonic content of the signal due to the normal operation of the drive. Installation of frequency responsive circuitry devices at this location will lead to failure of the SPD. Utilizing a voltage responsive circuitry device at this location will eliminate this possibility.
Overall, properly installed surge protective devices reduce the magnitude of random, high energy, short duration electrical power anomalies. These occurrences are typically caused by atmospheric phenomena (such as lightning strikes), utility switching, inductive loads, and internally generated overvoltages. The ultimate goal of our approach is to keep sites and systems operating safely and reliably. PowerTrip® Surge Protection Devices incorporate "Frequency Responsive Circuitry" technology years ahead of any other devices on the market today. Utilizing proprietary electro-chemical encapsulation, PowerTrip® SPDs dissipate large amounts of surge energy to prolong service life.
Alltec Protection Pyramid
Lightning by Alltec Corporation
Friday May 2, 2008
Lightning is an unpredictable act of nature. A “bolt from the blue” can occur 10-20 miles from the cloud source, and power transmission lines can carry extreme voltage transients many miles. It is prudent that certain steps be taken to protect people, equipment, and buildings; the TerraEval™ Advanced Solution Assessment engineering service is specifically designed for this purpose. Alltec’s three-step Protection Pyramid™ implementation begins with the installation of a stable, low resistance and low impedance grounding system to bond all electrically conductive surfaces together. TerraDyne®, TerraFill®, TerraBar®, and TerraWeld® grounding product lines are utilized to safely direct lightning energy to the earth and away from equipment and structures. After providing a stable grounding system, it is important to properly install a transient voltage surge suppression (TVSS) system, such as our PowerTrip® line of TVSS products. Only then, is equipment truly protected from both open and short circuit transients traveling on incoming electric, rf, telephone, coax, and data lines, as well as other internal equipment, and from direct surges, secondary, and electromagnetic effects. Finally, a well-designed structural lightning protection system will be installed. TerraStat®, TerraStreamer®, and traditional lightning protection products, are offered to fulfill this role of ensuring safety from direct lightning strikes.
We ensure our customers the highest caliber of services and products through our continual investment, testing, and evaluation of lightning protection, grounding, and transient voltage surge suppression (TVSS) components and systems. Innovation and sound engineering practices combine in Alltec Corporation’s research efforts. We partner with and direct national and international university level experimentation, analysis, standardization, and certification efforts in the field of lightning protection and grounding systems. Furthermore, our products are independently tested by preeminent scientific laboratories, universities, and certification authorities. Unequivocal results back up our Alltec AdvantageSM.
View More: Lightning, Tech-Tip
Minimizing Materials & Costs with Exothermic Welding
Tech-Tip by Alltec Corporation
Friday May 26, 2006
Often in grounding grids it is common to see main grounding conductors sized at 250 mcm or larger. When wire sizes are at this level or larger, it may become difficult and expensive to make proper connections to ground rods (type GET). Since a single 3/4-inch or 5/8 inch ground rod would not be able to withstand several thousand amperes, a fault current must be distributed across an entire ground grid. Therefore an alternative to welding large gauge wires to ground rods would be to use an intermediary wire between the main grounding conductors and the ground rods. Typically 1/0 AWG wire can be used because this size wire has over 5 times the equivalent cross section of a 3/4-inch ground rod. This method not only simplifies the welding procedure, but also offers considerable savings of wire and weld metal.
| Grid Wire to Ground Rod Connection | |||
|---|---|---|---|
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| Mold Type | Weld Metal | Mold Type | Weld Metal |
| GET 18-250A | #200 | GEE 18-250A | #90 |
| WT 250A-1/0A | #90 | ||
Several options for achieving a connection as specified in the engineering drawings are available. All the examples below show electrically equivalent connections as those specified in the engineering drawing represented by the cross and circle symbol.
| Equivalent Connections to Ground Rods | |
|---|---|
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| 1) 2 molds 2 welds | 2.) 2 molds 2 welds |
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| 3.) 2 molds 2 welds | 4.) 1 mold 1 weld |
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| 5.) 1 mold 1 weld | 6.) 1 mold 1 weld |
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Specification connection |
# 1 is the most common method |
