Current Monitor

There are many devices that operate as you describe, please Google "current monitors".

In my opinion is please find the EATON Current Culter Hammer. In this current moniter can set to trip at under and over current. In this moniter 4-20mA will get as output can use this in PLC as a graph format. As details are given in manual.

http://www.eaton.com plz visit the link.

Although current monitoring is one way to do it, there is an inherent problem if you are trying to relate current consumption to motor shaft load. The current will also vary with the applied voltage, so if your line supply is subject to variations throughout the day, the current changes will not reflect a consistent picture of the mechanical load.

For that reason, it is better to use kW monitoring relays. They are very similar to current monitors, they just have an additional input for line voltage and the ability to measure and calculate Power Factor (on AC units) to determine the actual mechanical shaft loading, regardless of what is happening with the line supply. I highly recommend using that type over current monitor alone if you are primarily interested in protecting a piece od equipment from mechanical issues.

The current drawn by the motor is only an indirect indication. What makes the motor burnout is heat, and of course the major reason for that is an overload current. However, many motors are cooled by the air passing through the rotor. If this passage is blocked by some fluff, cooling is affected, and the motor may burn out even at rated load. So, a thermistor protector, embedded in the stator windings, senses the heat directly and trips the starter/contactor before the motor is harmed.

For underload, there are relays which sense this condition and trip the starter.

There are current transducers in the market that put out 4-20mA signals proportional to any Ampere band. If you have a PLC handy, it can be input into an analogue module, scale it > or <some figure xxx can be made to alarm, trip, etc.

If no PLC, some current sensing relays that will do the job. But I found them very coarse and I had difficulty setting them up at lower end below which they should trip on UNDER CURRENT.

I think you should ask a trained electrical engineer or technician to do your this job for you. There are many parameters that play a role in protecting a motor in adverse conditions. There is this start up time, start up high current, then run time dynamic current for dynamic load, and finally current into defective motor. This all requires a device that is programmable for parameters for a type of motor or a device just has been made for the motor you are using.

If you are learning and have reason to do so then it is different thing as your half knowledge will also yield not so perfect results you are looking for and it is going to be a big project for you.

You can read about motor protection devices in details and if you can understand these fully then you can buy one from market and use it.

As well as over/under current situations,which have been well covered here, it is important to also install phase monitoring,which will indicate a loss of phase as well.Phase monitoring relays are readily available.

Use two protection

1. KW monitering Relay

KW monitering relays gives you the accurate power consumption of motor. In PLC

set the Lower and higher ranges of KW consumption.

2. Speed moniter:

Install a Speed moniter toward the Load side. It provides you protection in case where transmission between gear box and motor breaks

Same problem we have faced on the Belt Weigh feeder. This Belt weigh feeder is driven by the two motor. Once transmission between gearbox and motor of one side fails, the whole load was diverted to the other motor which trip on overload.To avoid such kind of condition we place two current transducer one for each motor. These current transducer measure the current drawn by both the motor, send a corrosponding signal of 4-20 mA to the PLC. PLC moniter these currents of both motors and if the difference in current is more than 10 %, it stop the weigh feeder.Further we add a proximity sensor to sense the Stall postition.