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I am about to buy a PLC system SlimLine to manage the electrical system of the house to do home automation and above all, at the same time, to learn something: I am a professional computer technician with a distant electrical engineering past and I am building a house;) As for PLCs, I consider myself a neophyte.
For the load control function (i.e. giving different priority to the various devices and "disconnecting" the less important ones in the event of exceeding the contractual power to avoid forced disconnection by the supplier) I had thought of detecting the current absorbed on the monitored lines with analogue inputs and CTs or the like and through logic outputs and external relays to drive the loads.
Is this a sensible approach? Is there anything simpler or cheaper? Especially for the measurement part: do you have any suggestions? Do you have anything in the catalog?
Your request is very common, in the forum you will already find answers for example in this topic you use a very cheap energy meter connected in Modbus (You can use many all connected in daisy chain with RS485 connection to the CPU module).
In this topic we treat the DMX bus used to manage the dimming of the lamps.
In this topic we treat the acquisition of a true RMS value that could be the value in AC output from a CT.
By acquiring the pulse of the Enel meter with an optical transducer, you can calculate the instantaneous and total power (Topic, Topic). The meter can also be read via the IEC 62056-21 protocol but there are access passwords to activate the reading we use the protocol only on industrial meters (Topic).
In this topic find other ideas.
For the management of air conditioning from a look a this topic. I remember we have wall temperature sensors.
Since we are talking about home automation, I wanted to highlight the combination of home automation, cloud, Internet of thinks. Just using a system SlimLine with the REST service management FB (Topic) you can send all data to a server in the cloud.
We have a series of PHP classes that allow you to historicize data in a MySQL database and view it with web pages (Here is the screenshot of the historical display of the grid voltage). For these solutions we can provide a tranining at our headquarters.
The same SlimLine inside it has a Web server that allows you to manage dynamic HTML pages (Topic) even with automatic update via AJAX (Topic).
Good afternoon, I am stepping up to the request, I would like to know if it is possible to connect one or more CT sensors to detect the amperes in alternating current, and possibly which models ...
As you saw the example of this topic deals with the acquisition in volts using the A / D converter of the compact CPU module (There are 2 inputs) so you can connect a maximum of 2 CTs.
But to read the current nothing is cheaper and more practical than the SDM120 energy meter discussed in this topic, versions with only impulse output are on eBay less than 30 euro.
Thank you very much, I was thinking of using a CT to carry out a stress control on a three-phase motor to eventually turn off the previous motor, in short an amperometric relay directly on the PLC.
Yes, of course you can also use only one CT and acquire a single phase assuming that, however, being a motor, the load on the phases is balanced. In this case the example of reading the RMS value of this topic it can be a starting point.
In the hope of not being off topic, regarding load control, I would like to carry out a load control linked to the production of a photovoltaic system. In practice I would like to divert the excess power of a photovoltaic system towards the resistances of a puffer for heating and / or towards a boiler for domestic hot water: in this case the power on the resistances should be variable.
The PLC should measure the power of the photovoltaic system (in three-phase) and the power used in the house (possibly with meters with RS485 output) and divert any excess towards the resistances, rather than towards the Enel network. I saw the output of the I / O module with zero crossing static outputs. Would it be convenient to use this module to dim the resistances? Would it be possible to have some basic scheme for this use?
You're on the subject, now as regards the power reading you can use the three-phase meters with RS485 interface, we have specific libraries for management.
Surely you can use the PWM outputs to modulate the electrical resistances, the FB exists SysSetPWMOut for the management of the fast PWM outputs, and the FB PWMOUT which allows to use any digital output for a PWM command. The latter is the ideal solution for the control of resistors in fact it is used a lot in that context. Combining it with a zero crossing output is perfect.
But my perplexity in your case is that you create consumption peaks at the activation of the resistance and this will produce energy consumption on the Enel side. Because it is true that by modulating in PWM the thermal and electrical energy is an average between the On / Off times but the instantaneous electrical energy depends on the load.
For your application I think you should modulate the resistances in phase partialization, or have many separate resistors and turn them on according to the photovoltaic energy.
Thank you so much for your reply, always very punctual. In fact I had some suspicions about controlling the resistances with the PWM method, instead of the phase control. Do some research on the subject. Believe that with PLCs SlimLine Is it possible to check the resistance with the phase control method?
Even if it is more convenient to use an ad hoc throttle driven with a voltage reference, theoretically a phase control is feasible, I see how to do it.
Using a program in 1 mS Fast task it is possible to sample a digital input that reads the line voltage (With a transformer). This way I can detect when it passes through zero (Input off). In reality the input will turn off a little before the zero crossing but the time is determinable.
Now knowing the network half-wave start time, I can delay the activation of the load in steps of 1 mS by turning it on sooner or later according to the partialization I want. If I use a static card (Non SSR) as an output card and activate a power triac after activation, I can immediately deactivate the output. Due to its characteristic, the triac will remain in conduction until the next zero crossing of the half wave.
As I / O I have to use those of the CPU module, the extensions have a management time of 200-300 uS. Since the operations to be performed in the Fast task are very few, I can also speed up its execution (Function SysSetTaskLpTime). Now since at 50 Hz the half wave lasts 20 mS at 1 mS of execution I can have at most 20 levels of partialization, but if I reduce the time to 500 uS I will have 40 levels and so on.
Apart from the technical side, consider well because if the primary generator that heats the storage is a heat pump, the latter works with a system COP of about 3 or greater, so even paying the KWh at 30 cents / € you find that You don't want to divert the surplus energy from the photovoltaic to a resistance with COP equal to 1, as ENEL remunerates you around 9-10 cents / €.
So by calculating the costs you have to incur to build the system: work, PLC, SCR dimmer, etc ... evaluate well. I had thought about it too but I got stuck at the moment for this reason.
I am not able to go into the details of the calculation considering the coefficient of performance as regards the convenience in the context of domestic heating. But it is certainly possible to use the surplus energy for heating domestic water and for "heavy" uses such as washing machines and dishwashers ...
Some of our customers in professional installations use the surplus energy to increase the production speed of the plants or customers who have cold rooms for the storage of frozen foods use it to further lower the temperature. In this way the cold room becomes a thermal accumulator.
I see the subject arouses some interest. I agree with what has been said about the heat pump. In my case the main generator for heating the puffer is a pellet boiler, while for the domestic hot water I have a thermodynamic system that uses a heat pump that heats a 52-liter tank to 250 ° C. The production of DHW in a year consumed me about 1300 kWh, partly produced by photovoltaic panels. But the greatest consumption is in the evening when they are all at home and the photovoltaic panels do not produce.
I thought it might be interesting to increase the DHW storage with an additional boiler that works only with the surplus energy, and, therefore, with a low cost, represented by the non-payment of the energy poured into the network because it is used by the boiler. If I still have something left, I thought I would help the heating with a resistance in the puffer, saving a little on the consumption of pellets.
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