Energy Management

ems

Working with Energy management Systems

I have worked with EMS systems since the mid 80's. The school where I worked had purchased an EMS system made by Detection Systems and it was installed by Hershey energy management. It was based on SART devices that are commonly used for addressable fire alarm systems. Each device has 2 digital inputs and outputs. That leads to the first problem, how do you get analog data? One of the digital outputs turned on the analog output that then sent the analog value back to the controller. That was a major issue as lightning liked to fry the analog circuitry. Another issue was that the power supply was centrally located. Large wires had to be used to go to the other buildings so that too much voltage was not lost.

Detection systems kept the system running for years, they even put some upgrades into it. But eventually it fell into repair and I had the job of replacing it with a more modern system. We needed something with 8 analog inputs, 8 digital inputs and 8 digital outputs preferably in one IC. After studying all of the alternatives I settled on using Motorola 68HC11's and RS-485 to communicate with them. The 68HC11 requires virtually no support chips, except a relay driver, and RS232 or 485 drivers.

Here is a list of our objectives.

Central computer tied into remote converters.

Central Computer will be Programmed in Basic.

Addressed with dip switches for 16 devices on a line.

Use serial with RS-485 for distances up to 4000 feet.

Report 8 temperatures via analog to digital converters.

Turn on and off 8 devices, with an override switch.

Report the status of 8 devices via digital inputs.

What does a good energy management system do? It has the ability to turn down the heat at night mostly in empty rooms like classrooms. It can turn down the temperature of the boiler on warmer days. It can also turn off the heating systems when the outdoor temperature reaches 68 degrees. Finally it allows the remote monitoring, logging and control of the heating systems.

Special communication devices are needed for the long distances between buildings. Rs 485 works with Rs 232, or a standard serial port. The only difference is the voltage levels involved and driver methods. Rs 232 uses positive and negative 12 volts. That gives it a potential difference of 24 volts. According to the books, that is good for a only few hundred feet. Rs-485 uses only 5 volts but uses a differential reciever giving it much higher noise immunity. There is a signal and its inversion. The receiver only looks for what is different between the two wires. Since noise pickup will be the same on both of the wires, the noise is canceled out. The wires must be identical and twisted together.

The next problem is that there are two ways to use Rs-485. One method is to use one wire pair to both send and receive, using only one telephone line. The drawback is that it is hard to add optical isolation or buffers. The other option is to have two pairs one to send data and one to receive data. The drawbacks are that it takes two telephone lines. We own our phone lines where I work, so using two lines is not a problem. Using two lines also makes it easy to check on the 68HC11 with Windows terminal.

LM34 temperature sensors are used. They have a range of 0 degrees to 225 degrees Fahrenheit corresponding to 0 to 2.55 volts. They can go lower but only with a negative bias supply. Converting the results to degrees Fahrenheit is made easy by first setting the analog to digital converter’s reference voltage to 2.55 volts. This is done with a 24K resistor that trims the 2.5 volt reference. You can test your modified regulator and change the resistor if it is needed.

B&B Electronics

My controllers were recently upgraded so that they are now compatable with the controllers made by B&B Electronics. They make a serial controller called a 232SDA10 and a RS 485 controller called a 485SDA10. Each has three digital inputs, three digital outputs and eleven ten bit analog inputs if I remember right. The limit of three digital inputs and outputs still gives enough control for most applications. One of them is pictured below with a lightning protector added on the analog inputs. These devices are easily damaged by lightning.

Power monitoring

For this project I was approached by an electrician who wanted to know if it was possible to monitor and record power consumption on my energy management system. The problems with recording electrical power are many. You cannot just tap into high power circuits, they must be first reduced to lower power levels. This is done with a pickup transformer. We did not have an easy source for them, however I have since discovered that Jameco electronics sells one. We set out to make our own pickup transformers. First we tried the commonly available ferrite torroidal transformers, but all they track is the amount of noise. Next we tried taking a steel pipe and cutting off 1/2 inch slices. Then we covered it with 100 turns of 24 gage insulated telephone type wire. This worked much better. We tried cutting them and clamping them back together on the wire but that led to a loss of about 30% of its efficiency. So we settled on disconnecting the power and routing the feeds through the home made pick up transformers.

The next problem is that the analog to digital converter requires a five volt DC input and the output of the transformers is a very small ac voltage. To solve this problem more than just an ordinary AC bridge rectifier was needed. An OP amplifier IC based amplifier and rectifier was needed. A gain of 10 was settled upon, this gives a range of up to 100 amps. The rectifier design was taken from my audio digital VU meter design. I added a zener diode protection for the input to the analog to digital converter. That was also needed to protect the delicate electronics when it was used as part of an energy management circuit.

The schematic shows 1 of up to 8 circuits to monitor as many electrical loads. The question we wanted to answer was if a building was really drawing as much power as the power meter had indicated, and if so when. we discovered that all kinds of devices were running 24 hours a day even though the building was only in use less than 12 hours a day. For home use you might not need to monitor as many electrical circuits.