DOL Circuit for Motor Control, Can We Use PLC to Trip Functions?

A1) The primary protection against overcurrent ideally needs to be a properly-sized and set overload device in the starter circuit for the motor, wired directly. If some control logic is required then it will be of the form of "Motor start has been sent and after n₁ seconds the contactor has not closed, therefore raise alarm₁" and "motor stop has been sent and after n₂, seconds the contactor has not opened therefore raise alarm₂", which can be better done perhaps with a PLC/HMI set-up rather than in the starter.

A2) Some motors have a thermistor embedded in the windings that is connected to a device that is wired within the motor starter to trip the main contactor off should overtemperature occur. Its use is secondary to the overcurrent device mentioned above and the same logic above can be used to detect and report a fault. Thermistors are offered as options by most motor manufacturers, who can offer application notes to designers that encourage the correct selection and use of the motor during commercial enquiry.

Connection to the PLC, if any, is a matter of local design and its compliance to local Client standards, which cannot be seen from here. If the <...we...> is unsure how to proceed, then the answer to the question <...can we...> is probably "no, on the basis of competence demonstrated".

good advice from you as usual, I might suggest adding a time delay on any trip before restarting is attempted

I think PLC work to be limited to LOGICAL Sensing and Activating Pre-planned operations. If PLC is dedicated for some operation then analog monitoring and control by PLC becomes part of the local circuit and not global general PLC operations. Low cost small PLCs are used as local controllers.

If overheating is the only problem which is a slow process, PLC with analog input with ADC can provide dynamic range of sensing and then it can either trip or work in delayed restart mode. It still needs a proper interface to the motor starter which is a tricky part as motor wiring to the starters and control mechanisms differ considerably.

I am trying to interface water pump motor starters with PLC or LOGIC controllers and some starters simply can't be interfaced as they have mechanical position switches, other starters require different amount of ON Time as they have extra winding for starting energy and their start time and peak charge through relay differs.

I think it is a good idea to analyze few standard design that are specially designed for automation and can be easily interfaced to PLC. For this purpose, I thought of manufacturing few generalized type starters also but then they have to handle various types of motors of different power ratings so it is good to have starter matched to the motor rather than to have a generalized starter. The idea is given up for time being.

Other way is to use current transducer analog output wire to analog input on your PLC. You can use scaling, limit or other instruction at your preference. To be wired in series with the supply voltage and PLC analog input

You can do it, but my question is why? There are perfectly good overloads that will protect the motor.

...In addition to overloads that protect the motor AND interface to PLCs for monitoring and control purposes. ABB have a range for example.

Just to share idea and information I raised this question.

I think using RTD's or Thermocouple to measure the winding condition and send the measured temp. to a trip system is more effective, because overload heaters may be slow in action or more susceptible to failure.

Using RTD's or Thermocouples is for very high power motors or processes where the motor playing critical role!

for the contactor, yes we can terminate the toggling-switches (start, stop) bottons to the PLC input card, as well as auxiliary switch to keep the motor running when the operator releases his hand from the start switch, while the PLC output is terminated to the contator, so the PLC will apply the logic function only.

if you are looking for something to react faster, then you need a certain type of motor controller that will monitor over and underload, voltage imbalance, Hz and a lot of other stuff.

much better than a PLC type controller.. and there are loads out there in the market place

In MOST jurisdictions, there are regulations with regard to the MINIMUM qualifications of what a motor overload protection scheme must accomplish, with the primary basis being the prevention of a fire. In that endeavor, whomever is building one must prove, often to a regulatory authority, that the scheme they have devised will accomplish that basic function. So here in the US, that would fall upon a third party listing agency, referred to as an "NRTL", Nationally Recognized Testing Lab, the most widely known being UL. I cannot speak for other countries, but I'm sure there are similar standards that must be met, and probably must be demonstrated to have been met.

The design of a current based thermal overload relay is to approximate the thermal damage curve of the motor windings by sensing the amount of current passing through them, in such as way as to prevent them from failing. To accomplish that, the OL relay manufacturers have amassed a lot of data on motor winding designs and effects, including cooling time constants, starting heat profiles etc. etc. This is not a flippant undertaking, there is a lot of science that goes into it.

If you choose to do it via direct temperature measurement of the motor, the sensors, either RTDs or Thermistors, must be DEEPLY embedded into the windings of the motor, in other words the motor must have been MANUFACTURED with the sensors inside, you cannot effectively add them to the outside. Then you must account for the possibility of sensor failure, and since they cannot be replaced, that is usually accomplished by embedding twice as many as you really need, in case you must switch a failed one to a good one. And just FYI, RTD and TC inputs for PLCs are the most expensive ones you can buy.

If you are going to take on the R&D to do this right in a PLC, and assuming you will be adding these expensive analog inputs to get the information INTO the PLC, I cannot for the life of me imagine that this is going to cost less than just buying one off of the shelf somewhere.