RS485 & Raspberry Pi: Monitoring Power

Commercial data centers, lighting controls, utility rooms for buildings, and others need to keep a tab on their power consumption. The normal way to do this is by using electronic voltage meters and multi-branch current monitoring circuits. Vytas Sinkevicius wants to monitor power consumption using the ubiquitous single board computer, the Raspberry Pi (RBPi) as the main controller and the RS485 interface in a Branch Current Monitor (BCM) system.

The heart of the power monitoring system is an RBPi 3. Other parts the system uses are a Pi-SPi-RS485 Interface, a VP-EC-BCM Interface, a breakout PCB for an 18-Channel Current Sense Transformer, and a few Current Sense Transformers. Vytas will be writing the software in C, using the Geany compiler.

Electrical engineers use two types of current sense transformers for measuring current. The first type has a continuous hollow core, with the wire carrying the current passing through the hollow of the core. This type of current transformer is suitable for new constructions and requires the main power to be turned off for installations. The breaker wire has to be removed and re-connected after the current transformer is attached.

The second type of current transformer has a split hollow core, where one-half of the core may be separated from the other. Split cores are ideal for applications where the power wiring to the breakers cannot be switched off. By separating the top half of the core, the breaker wire can be placed in the hollow of the lower part, and the top half of the core replaced thereafter. Vytas is using a split-core current transformer, model type CR3110-3000, and CR Magnetics manufacture it.

The Pi-SPi-RS485 Interface provides power to the VP-EC-BCM Interface and communicates with the RBPi. As the RBPi and Pi-SPi-RS485 combination uses the Modbus RTU and RS485 protocols, they can be located as far as 4000 feet away from the actual area where power is being monitored.

The Pi-SPi-RS485 is a perfect fit for the RBPi3, as its ports match the GPIO port on the RBPi3. Moreover, as it duplicates the GPIO expansion port on the other sides of the Pi-SPi-RS485 module, additional modules are easy to add. You can fit the module directly on the back on an RBPi3, or use optional mounting hardware to connect and keep them alongside. All RS485 signals are duplicated on terminal blocks on the board, and on the RJ45 connectors as well.

Each RS485 module has its own power input (9-24 VDC) for powering remote transmitters, and its LDO regulator operating from the 5 VDC bus provides the 3.3 VDC. Therefore, this does not load the 3.3 VDC bus of the RBPi. There are on-board LED indicators for indicating the status of power and RS485 signals. Termination resistors can be selectively switched in using jumper settings provided. The module provides power to the VP-EC-BCM Interface over a CAT5e cable via the dual RJ45 connectors.

The VP-EC-BCM Interface made by VP Process Inc. does the actual power monitoring. This is a converter unit for current sense transformer with 36 channels. It has a 3-kVAC isolation between the primary circuits and the Power/RS485 Interface.