Line and Load Reactor Buyer's Guide
Why Choose a Line Reactor?
Utilizing variable speed drives to control motor speed has impacted industry both in energy savings and increased efficiencies. The challenge for today's designers is dealing with non-linear wave shapes generated by solid state devices.
By choosing a line reactor, many line problems can be eliminated. Additionally, performance, life expectancy and efficiency of both the motor and the drive itself are significantly enhanced.
Eliminate Nuisance Tripping
Transients due to switching on the utility line and harmonics from the drive system can cause intermittent tripping of circuit breakers. Furthermore, modern switchgear, equipped with solid state trip sensing devices is designed to react to peak current rather than RMS current. As switching transients can peak over 1000 volts, the resulting overvoltage will cause undesirable interruptions. A reactor added to your circuit restricts the surge current by utilizing its inductive characteristics, and therefore eliminates nuisance tripping.
Extend the Life of Switching Components
Due to the attenuation of line disturbances, the life of your solid state devices are extended when protected by the use of a line reactor.
Due to the care in the selection of the core material with its optimum flux density, Line reactors will not saturate under the most adverse line conditions. Since the inductance is linear over a broader current range, equipment is protected even in extreme overcurrent circumstances.
Extend the Life of Motor
Line reactors, when selected for the output of your drive, will enhance the waveform and virtually eliminate failures due to output circuit faults. Subsequently, motor operating temperatures are reduced by 10 to 20 degrees and motor noise is reduced due to the removal of some of the high frequency harmonic currents.
Low Heat Dissipation
Particular attention has been focused on the design and field testing of this product line. The result is reactors with ideal operating features including low temperature rises and reduced losses. Reactors will operate efficiently and heat dissipation in your equipment will be of minimal concern.
Minimize Harmonic Distortion
Non-linear current waveforms contain harmonic distortion. By using a line reactor you can limit the inrush current to the rectifier in your drive. The peak current is reduced, the waveform is rounded and harmonic distortion is minimized. Current distortion typically is reduced to 30%.
Severe harmonic current distortion can also cause the system voltage to distort. Often, high peak harmonic current drawn by the drive, causes "flat-topping" of the voltage waveform. Adding a reactor controls the current component, and voltage harmonic distortion is therefore reduced.
Short Circuit Capability
Line reactors can withstand current under short circuit conditions, reducing the potential of severe damage to electronic equipment. In a short circuit, the inductance of the coil is necessary to limit overcurrent after the core has saturated. We have extensive experience in designing and testing dry-type transformers to withstand short circuits for the most demanding applications, and this experience has been applied to line reactor design.
Reduce Line Notching
Whenever AC power is converted to DC by a rectifier using a non-linear device, such as an SCR, the process of commutation occurs. The result is a notch in the voltage waveform. The number of notches is a function of both the number of pulses and the number of SCR’s in the rectifier.
Line reactors are used to provide the inductive reactance needed to reduce notching, which can adversely effect equipment operation.
Selection - 3% OR 5% Impedance Reactor
Choose 3% impedance reactors to satisfy most solid state applications in North America. Reactors rated for 3% impedance are ideal for absorbing normal line spikes and motor current surges, and will prevent most nuisance line tripping of circuit protection devices or equipment.
Where considerably higher line disturbances are present, a 5% impedance reactor may be required. Additionally, if the application is overseas, or when it is necessary to comply to IEEE 519, the higher impedance reactor is recommended. These units may also be selected to further reduce harmonic current and frequencies if desirable or to both extend motor life or diminish motor noise.
Line Reactors or Drive Isolation Transformers?
When true line isolation is required, such as limiting short circuit current, or where it is necessary to step up or step down voltage, use a drive isolation transformer. We carry an extensive line of drive isolation transformers in stock.
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Understanding Percent Impedance
Drives use semiconductor devices for electrical power conversion. These devices are sensitive to power surges, voltage spikes, current surges, line distortion and power anomalies, all of which may have detrimental effects on semiconductor device operation. Line inductance reduces power surges. Inductive power circuit components such as reactors, inductors, chokes and transformers reduce rate of current change in the circuit and are used to "condition" power circuit. Inductance is often expressed in value of "percent impedance".
Percent Impedance or Percent IZ (%IZ) is the voltage drop due to impedance, at rated current, expressed in a percent of the rated voltage.
Drives require certain line impedance for three important reasons:
- Minimum inductance is necessary for proper commutation of semiconductor devices.
- Line inductance reduces power sub-transient and transient surges.
- Impedance reduces available short circuit current in case of malfunction.
Drives Installation and Operation Manuals list necessary minimum impedance and short circuit ratings. What do these impedance percentages really mean? As stated in the above definition, percent impedance is always expressed at rated base current. It is very important to understand that recommended percent impedance is based on drive full load current rating and not the reactor, transformer, or other device current rating.
Calculating line impedance
The purpose of these examples is to illustrate the importance that percent impedance in drives application must be evaluated on drive current rating base. Lets us further examine the above examples. Let us assume that the 10HP, 460V, 14A drive above is the only load on this 100kVA transformer. How much will voltage drop on the transformer when transformer is loaded only with 14A of drive's current? In order to calculate this drop we use the simple ratio formula. If 125A drops 23V, then 14A will drop 14A/125A*23V or 2.6V. This value 2.6V is less then recommended drive input line impedance voltage drop of 4.6V. The conclusion is that 5% impedance 100kVA transformer does not meet the requirement of 1% impedance for 10HP drive.
Definition: %Z = ((VD x 100) / VS ) x √3
Z = Impedance (three phase)
VD = Voltage drop across reactor
VS = Voltage supply for rated current to flow through reactor
Evaluating short circuit impedance
Power source impedance is also and easy way to evaluate available short circuit rating. In the above example, we have discussed H2, 10HP, 460V drive which has listed short circuit rating of 5,000A symmetrical RMS (root means square) current. Let us conduct and a simple short circuit study. Let us assume that 100kVA transformer has unlimited power available from the utility on the primary side (which is mostly a case in short circuit studies) and let us assume that there are no rotating motors on this power system to contribute to short circuit level (which is not a case in most short studies). In this circuit with the assumptions made, the phase to phase of short circuit in the drive, at the simplest calculation will be full load current divided by percent impedance or 125A/.05=2500A. The available short circuit current is less then drive listed short circuit rating. There is no reason for concern. Now let us look at 200kVA, 250A, transformer with 4% impedance. Short circuit current on the drive feed from this transformer would be 6.3kA, more then drive short circuit rating. Installing this 10HP drive on 200kVA transformer possibly compromises drive short circuit rating and increases the possibility of drive failure.
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Applying AC Line Reactors
Input to Inverter/Drive
AC Line Reactors protect your sensitive equipment from noise generated by the drive or inverter. They protect the controller from power surges, spikes and harmonic distortion.
Output of Inverter/Drive
Motors run cooler and quieter with an AC Line Reactor placed between the inverter and motor. This application also reduces dv/dt and protects the controller from short circuits and surges.
Multiple Controllers on a Single Power Line
Each drive or inverter on a single power line requires its own AC Line Reactor in order to provide adequate surge protection, prevent crosstalk and reduce harmonic distortion.
Multiple Motors Controlled by a Single Drive
Multiple motors controlled by a single drive require only one AC Line Reactor between the controller and motors.
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