Drive a heavy load relay/contactor using a micro controller

[RohanDey]

Hello,

I am trying to design a system where I need to drive a relay using a wireless signal. On one end there is a MCU board with a switch, when pressed will transmit a RF signal and the receiver will drive a relay upon receiving this signal.

I have developed the wireless Tx Rx part. Also, I have worked with 12VDC relays using relay drivers. Now my requirement is to drive a relay/contactor of heavy load, about 63A, 440V. This relay/contactor will be connected to a 3 phase motor.

The actual application is, when the mains are OFF (in case of a power cut), the motor will run on a generator. Currently the system works using manual change over switches placed near the motor, which are pushed after the generator is turned ON. The generator is 200 meter far from the motor, hence the RF signal.

Since I do not have any experience in driving heavy load relays, I am looking for some help, firstly in deciding the type of relay available in the market, to be used in driving such 3-phase loads. Since it is a 3-phase line I believe the relay will be 4PDT or 4xSPDT (for 3 phase and 1 neutral). Secondly, the process how such relays are to be driven using a micro controller. Thirdly, what are the necessary precautions to be taken in the circuit, while working in such loads.

Any kind of help is greatly appreciated. Thanks..

 

[cl10greg]

Since it is a 3-phase line I believe the relay will be 4PDT or 4xSPDT

Agreed

Secondly, the process how such relays are to be driven using a micro controller

You need isolation between your microcontroller and your relay since it will most likely take a lot of power to close the switch. Depending on your relay selection, observe the voltage and current to drive the relay. From there you can look into optoisolators or transistors to drive your relay. They can be either high side or low side drivers. Usually low side drivers are better in this case for power dissipation. Be sure to use a flyback diode across your relay coil for EMF.

Thirdly, what are the necessary precautions to be taken in the circuit, while working in such loads.

Well with that high of current and voltage I would hope you have some kind of fusing or safety features but it depends on your system setup. Keep in mind that motors have very high on transients when sizing components and safety parts. Inrush limitation would be good if possible (soft start).

 

[chuckey]

A three phase 440V 63 A contactor will need a inrush drive of 100W, with a holding power of 20W or so. You can drive this via a solid state relay which will contain a opto isolator and a triac and will require something like 5 V at 20 mA to operate it.
The real problem is that the contactor coil will require a supply when the mains is not available. They are available in all sorts of ratings, but I have never seen one at less then 50V. So you could have a bank of batteries to power the coil. Another way would be to have a relay with a mains powered coil that provides the "maintained mains" supply, and flips over to the generator supply in the absence of real mains.
These sort of set ups require a lot of thought to make them operate safely under all conditions, please take care.
Frank

 

[cl10greg]

They are available in all sorts of ratings, but I have never seen one at less then 50V. So you could have a bank of batteries to power the coil

Maybe be able to use a boost circuit to reduce the amount of batteries?