New Generation of Relay Drivers Simplify Demand Reduction Function
The Smart Grid is creating opportunities to improve electrical load management by identifying times of peak usage of electrical power and putting a value on demand reduction. Adjusting the electric rates for time of use, the user’s cost is higher during times of high demand. This gives users an incentive to reduce their power consumption during these periods of high demand. An example is freezer refrigeration, if the freezer temperature is operated a couple of degrees lower than required during low demand periods then during a period of high demand the compressor could be disabled for a set time period without compromising the needed refrigeration level.
The Smart Grid is creating opportunities to improve electrical load management by identifying times of peak usage of electrical power and putting a value on demand reduction. Adjusting the electric rates for time of use, the user’s cost is higher during times of high demand. This gives users an incentive to reduce their power consumption during these periods of high demand. An example is freezer refrigeration, if the freezer temperature is operated a couple of degrees lower than required during low demand periods then during a period of high demand the compressor could be disabled for a set time period without compromising the needed refrigeration level. The value of this can be quite high as electric rates during shortages are considerably higher than during low demand. These Time Of Use (TOU) rates have defined a need for efficient means of connecting and disconnecting large loads from the grid. This type of load management has been used by larger commercial and industrial electric consumers for some time with dedicated phone lines and custom direct connections to the utility.
Now with smart meters providing communications within residential homes and smaller commercial establishments and utilities having developed and deployed Smart Grid management systems to co-ordinate and set TOU rates the need for an effective disconnect function becomes apparent.
Some important features of the connect/disconnect solutions are; tolerant of the noisy environment of inductive load switching, provide strong DC current capability to open even welded shut contacts, possess accurate timing functions filtering out noise, providing relay protection and compatibility with low voltage micro-controllers.
The industrial market has a long history with contactor function; this has been implemented with a variety of approaches. Motors are used to drive large contactors but suffer from slow speed during the opening operation with resulting arc damage. These larger systems use various forms of arc suppression including compressed gasses and spring assisted mechanical fast-switching. These approaches are not economical for the newer higher volume cost sensitive markets. The dominant consensus is to use a polarized bi-stable latching relay with two coils with one coil to move the relay from the open position to the closed position and the second coil to move the relay from the closed position to the open position.
An important function for contactors is being able to disconnect after power has been lost. Compressors and other inductive loads have high start-up surge currents, these types of devices are best not cycled without a minimum off time (compressors overheat if cycled over short intervals). When power is restored surges can be reduced with delayed turn on and sequencing of loads, thereby reducing peak surge currents and stresses on delivery equipment as well as loads. Contactors can be operated after power is lost using a hold up capacitor supplying sufficient power to reliably drive the disconnect.
For modern contactors the bi-stable, two coil latching relay is the most common configuration. These can be enhanced with built in springs and other mechanical assistance in the open direction for lower-power operation.
The common configuration is a bi-stable contactor device with contacts for two or more poles to disconnect. This contactor is usually wound with two windings; one winding to close the contactor and the other to open the contactor. A mechanical latch or permanent magnetic material is used to hold the contactor in place when it is not switching.
To facilitate the mechanical movement, the relay coils need to be energized for a specific time interval. Once the contact(s) have changed position, the voltage is removed from the winding of the relay. See Figure 1 for a typical circuit diagram and the switching waveforms.
Figure1: Simplified Diagram of the Relay Drive
In the diagram the two coils are connected to the supply rail at the center point of the relay winding. Each coil can be energized by the switches connected to the relay winding. It is important that two switches not be on at the same time, this would cause excessive currents to be drawn from the supply rail, result in improper operation and potentially damage the relay. The switching time of the contactor, the time required for the relay contact to travel between its opened and closed positions, is specified as a minimum duration in the relay specification. The switching pulse must be longer than this specification. Also the pulse duration should be limited to prevent potential saturation of the relay winding and to avoid over heating the coils and drive electronics. The control signals need to be compatible with the latest generation of micro-controllers supporting both CMOS and TTL input levels.
The relay specification also defines the minimum and maximum operating voltages for reliable operation of the contact(s). The voltage requirements of the contactor vary by application. Lower voltages are common in lower-power applications where the relay sizes are smaller. Higher voltages are common where higher currents are needed where the larger contacts require more power to switch. The drive circuit should monitor the relay bias voltage for sufficient voltage level and an under voltage lockout for the drive circuit to facilitate smooth startup while circuits initialize.
The Fan3240 and FAN3241 integrated solutions provide input signal qualification for the control signals, protection against simultaneous activation of the two relay coils, a maximum drive pulse duration limit, bias voltage monitoring, driver enable input, and thermal protection for the driver. The circuit implementation minimizes component count and board space, while increasing the reliability of the system and the noise immunity of the circuitry when driving the coils of the relay.
Figure 2: FAN3240/41 connected to a typical micro-controller and disconnect relay
The FAN3240 and FAN3241 provide a superior solution to driving the connect/disconnect contactors in a variety of applications. As the Smart Grid functionality continues to expand in complexity, new opportunities will emerge for advanced power management functions. Fairchild Semiconductor has tools, application notes and application pages accessible through the Fairchildsemi.com web page to help you in your design process.
