What you Should Know about SCR Power Controllers
Since the development of SCR power controllers in the late 1950's, the power handling capabilities of SCR's (silicon controlled rectifiers) have advanced from a few hundred watt's to many megawatt's.
As a result, the use of SCR power controllers in industrial applications has increased dramatically and they are now used in almost every major industry.
SCR power controllers provide a relatively economical means of power control. SCR power controllers cost less and are more efficient than saturable core reactors and variable transformers. Compared to contactors, SCR power controllers offer a much finer degree of control and do not suffer from the maintenance problems of mechanical devices.
Features and benefits of SCR power controllers over other forms of control include:
- High reliability:
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Because the SCR power controller is a solid-state device, there are no inherent wear-out modes. Thus, they provide virtually limitless and trouble free operation.
- Infinite resolution:
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Power, current or voltage can be controlled from zero to 100% with infinite resolution. This capability allows extremely accurate, step less control of the process.
- Extremely fast response:
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The SCR controller can switch load power on and off extremely fast providing the means to respond rapidly to command changes, load changes and power supply changes. This feature allows the control of fast responding loads and eliminates the negative effects of variations in load or supply voltages that can occur with other types of control.
- Selectable control parameters:
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The SCR power controller can control the average load voltage, the RMS value of the load voltage, the RMS or the average load current or load power. It can also provide useful features such as current and voltage limiting. The ability to control the desired parameter as a function of a command signal and to incorporate limiting features is not normally available with other types of control.
- Minimum Maintenance:
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Because they are solid state there are no moving parts to wear out or replace. Therefore, the routine replacement required in some forms of control is eliminated.
VL = LINE TO LINE VOLTAGE
VP = PHASE VOLTAGE
IL = LINE CURRENT
IP = PHASE CURRENT
3 PHASE LOAD CALCULATIONS
DELTA CONNECTED LOAD
WYE CONNECTED LOAD
To determine the line current (in Amps) for balanced 3-phase resistive loads (wye or delta connected) where P = total 3 phase power in kilowatts
VLine = VPhase
For balanced delta-connected resistive loads onlyILine = IPhase
For balanced wye-connected resistive loads onlyfor VL = 480 IL 1.2 * P (in KW)
for VL = 240 IL 2.4 * P (in KW)
for VL = 208 IL 2.8 * P (in KW)ILine = 1.732 * IPhase
P = 1.732 x VLine * ILine
P = 3 X VLine * IPhaseVLine = 1.732 * VPhase
P = 1.732 x VLine * Lline
P = 3 X VPhase * ILine2.0 Determine the type of control mode (zero-cross or phase-angle) and features to enhance performance. (Transformer coupled loads, fast responding loads and loads that change resistance with age or temperature, typically require phase-angle control.)
3.0 Choose a reliable and serviceable controller.
Expect more than just the obvious. The following, often overlooked features and requirements are critical to the reliability and serviceability of SCR power controllers.
Features to Look for in your Next SCR Power Controller
- Spring Loaded Connections:
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The use of spring washers on all electrical junctions always insures a good connection. Heat is generated in electrical connections because of the resistance between one conductor or bus bar to another. This heat causes expansion and deformation of the connection and in turn can cause the resistance to increase, generating even more heat. The spring applies a constant force to the junction, allowing for expansion as well as contraction. In addition, an anti-oxidant should be applied to the surfaces of all connections to improve both thermal and electrical conductivity.
- Long Life Fans:
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Fans are often required to cool the semiconductors in SCR power controllers. Fans with ball bearings or extended life sleeve bearings provide superior performance and long life over less expensive fans with conventional sleeve bearings.
- Power Connectors:
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Connectors used to connect the supply leads and the load leads should be designed to remain secure when field wiring is installed. (The typical installer of the controller will over-torque the connectors; if the connectors become loose under these conditions problems with the controller will develop.) The connectors should also be located such that it is convenient to use a wrench and apply the right torque during installation. Additionally, a design which allows the air from the semiconductor cooling fans to pass over the large current connectors cools the connection and improves reliability.
- Force Indicating Clamps:
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SCR's and diodes of the hockey puck design must be "squeezed" between the heat sinks by spring loaded clamps to assure electrical and thermal conductivity. To facilitate field replacement (serviceability) of the semiconductor, the spring clamps should have a force indicator to determine the proper clamping force.
- Status Indicators:
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Status indicators consist of small LEDs that provide information regarding the magnitude of the command signal, the magnitude of the load current, the status of features such as over-current trip and shorted SCR detection. Controllers so equipped can facilitate troubleshooting.
- Transient Protection:
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Rapid voltage changes (dv/dt) and high voltage transients are common in industrial applications. Under such conditions, the SCR's can turn on, causing undesired surge currents or outputs if proper protection is not provided. Unprotected SCR's can actually be destroyed by repeated transients. If an SCR is turned on due to a transient, it is possible for very high current densities to destroy a small portion of the SCR. If this is repeated even higher current densities occur until failure. To prevent damage or failure due to high voltage transients, SCR's should have a high blocking voltage (1400 volts for 480 Vac) and must be protected with fast acting MOV's (metal oxide varistors). Further, a series resistor and capacitor circuit should be connected across the SCR's to provide a shunt for high frequency transients (dv/dt).
- Single Plug-in Circuit Board:
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The use of a single circuit plug-in circuit board greatly improves serviceability. For example, circuit boards can be switched from one controller to another when diagnosing problems. Also, use of a plug-in board reduces the possibility of mis-wiring during service. In addition, if the circuit can be used on all controllers of a similar type, independent of current rating, then only one board is required for spare parts.
- Circuit Synchronization and Timing:
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To synchronize with line frequency, many controllers use the zero-cross (the start of the AC voltage cycle) as a reference to determine the conduction or "ON" time of the SCR. However, this may be difficult because of electrical noise in the industrial environment. Use of a filter in the zero-cross detection circuit helps to reject electrical noise and is acceptable for some applications. Controllers intended for transformer coupled loads require a very precise and noise immune technique. Here, request a phase-locked digital circuit to determine the zero-cross by a voltage integration technique. Transformer load applications require that all SCR's be turned "ON" for precisely the same amount of time to avoid applying DC. The phase-locked self adjusting digital clock technique provides the means to achieve extremely accurate timing and can be used on 50 or 60 Hertz applications.
- The Gate Drive:
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SCR's are turned "ON" by the injection of current into the gate. The input impedance of the gate changes drastically during turn on. Therefore, a gate drive circuit, particularly on large SCR's, is required to inject the desired current independent of the gate impedance. A technique using optical couplers to initiate a fast rising constant current source has proven to provide reliable operation and excellent electrical isolation. An SCR is turned on approximately 2 million times in eight hours and can be slowly destroyed due to voltage transients if the gate drive is inadequate.
- Cooling:
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A 10°C increase in semiconductor temperature will decrease reliability by 50%. It is difficult for the user to determine if adequate cooling is provided. However characteristics of adequate cooling include:
- The ability to operate continuously in a 55°C ambient environment with no de-rating.
- Extremely smooth surfaces on which the SCR's are mounted.
- The use of a thermal compound or a thermal pad under the SCR to aid thermal conduction.
- Proper containment of the cooling air such that all of the air provided by the fan passes over the heat sink.
- Surge Current:
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Certain loads, such as transformers or loads with a low cold resistance like infrared lamps, can have a large surge current. Therefore, the surge current rating of the controller must be adequate to protect the SCR's from damage in the event the SCR's are turned on by transients. Any surge current that may occur is dependent upon the load and the source resistance and is difficult to determine. However, a surge current rating of 12 times rated current has proven to provide very satisfactory reliability.
- Panel Space Requirements:
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Electrical enclosures are expensive. Therefore, a physically smaller controller saves enclosure costs. In addition a smaller and relatively light weight controller can be shipped faster and less expensively.
- Experience and Knowledge of the Vendor:
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There are a great many factors which affect the application of SCR power controllers. An experienced and knowledgeable vendor is best able to provide the assistance and recommend the best product for the application.
- Enclosure Selection:
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As is true of any solid-state device, the reliability of an SCR power controller is greatly increased if operated within its temperature range and if kept free of contaminates. Therefore, the selection of the enclosure and the means to cool it are very important.
- Heat Dissipation Considerations:
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The enclosure should have proper ventilation or cooling to dissipate the heat generated by the equipment within it. An SCR power controller dissipates about 1.5 watt's per ampere of current it controls. Therefore, a 100 amp single-phase controller dissipates about 150 watt's, a 2-leg 100 amp zero-cross controller dissipates about 300 watt's and a 100 amp 3-phase phase-angle controller dissipates about 450 watt's. The heat generated within the enclosure can be dissipated by one of the following:
- Convection:
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The internal temperature of a NEMA 1 enclosure increases above the external ambient temperature about 4°F per watt dissipated per square foot of exposed surface area. The temperature rise in degrees Fahrenheit can be approximated by dividing the total watt's dissipated inside the enclosure by the exposed surface area in square feet and multiplying this number by 4. Temp rise = 4.0 x W / Area
For example if a NEMA 1 enclosure with 100 square feet of exposed surface area contained equipment dissipating 900 watt's, then the watt's dissipated per square foot is 9 and the internal temperature can be estimated at 36°F above the outside ambient temperature. Temp. rise = 4.0 x 900 = 36°F / 100
- Forced Air:
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If the enclosure cannot be cooled by convection then it may be possible to ventilate the enclosure by forcing air through it with a fan. The size of the fan can be determined by the following equation relating air flow (V) in cubic feet per minute to the power (W) in watt's dissipated within the enclosure and the allowable temperature rise (DF) above ambient that can be tolerated within the enclosure. V = 3.16 x W / DF
For example, if the power to be dissipated by forced air cooling is 900 watt's (.9 Kilowatt) and the maximum allowable temperature rise within the enclosure is 15°F, then the air flow required would be: 3.16 x 900 / 15°F = 189.6cfm
- Cooling When a Sealed Enclosure is Required:
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If the enclosure must be totally sealed because of water tight requirements, or extremely dusty conditions, etc., then other means of cooling such as air to air exchangers, water to air exchangers or air conditioning units specifically designed for cooling electrical enclosures can be used. A = 4.0 * W / DF
The surface area of a sealed enclosure for 900 Watts (.9 Kilowatt) dissipated power limited to an internal temperature rise of 15°F would be: 4 * 900 / 15 = 240 feet(squared).
If air conditioning is available, up to 3100 BTU's could be required to cool the cabinet. (A kilowatt-hour equals 3413 BTUs).
- Why Preventative Maintenance is Important:
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The SCR controller should be inspected periodically to determine that the electrical connections are tight and that the heat sinks are clean.
The ability of the heat sink to dissipate heat is greatly reduced if dirt and grime have accumulated on it and it must be cleaned if dirty.
Normally the cause of dirty heat sinks is failure to provide filtered air to air cooled enclosures or failure to clean the filters.
The electrical connections on new installations should be periodically inspected and tightened per the manufacturers instructions.
Preventative maintenance will greatly increase the reliability of the controller.
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