| The starting current of motor can become 3 to 6 | | | | latch that will openthe motor control circuit. This action |
| times the normalrunning current! That's normal for | | | | will disconnect the motorfrom the line. |
| motors. The motor canexperience this high current for | | | | For reliable operation, the overload relay must be |
| a period ranging from 20 to 50seconds. | | | | located at thesame temperature environment as the |
| A fuse will not be very useful in protecting a motor | | | | motor. The heating effect ofthe bimetallic strip |
| from thistype of overload condition. A selected fuse | | | | mechanism is supposed to represent theheating of the |
| large enough topermit passage of the necessary | | | | motor windings. |
| starting current would givelittle or no protection against | | | | Melting alloy overload relays calibrated by the |
| overheating of the windings undernormal running load | | | | manufacturer areconsidered the most reliable of all the |
| conditions. The same case happens for thecircuit | | | | thermal overloadprotective devices. However, more |
| breaker. | | | | commonly used is the bimetallicoverload relays |
| Overload relays offer protection of motors that is | | | | because the tripping current setting can beadjusted. |
| bothcompatible with the starting current of the circuit, | | | | All the overload relays have one major limitation - |
| and therequirement of protection in the event of | | | | because theyoperate on line current, they do not |
| overload runningcondition. | | | | directly sense the motortemperatures. For normal |
| These conditions need to be adjustable. A particular | | | | steady running conditions, this posesno problem at all. |
| motorrequiring 56 seconds starting time under normal | | | | However, when a motor starts and stops frequently, |
| load will sufferserious damage if the rotor locks and | | | | the relay maynot completely protect the motor. Why is |
| the motor is not tripped in | | | | that so? |
| 20 seconds. Another motor may be able to withstand | | | | During the motor running, the relay temperature follows |
| 25 percentoverload for 30 minutes. A hermetically | | | | the motortemperature closely. When the motor is off, |
| sealed compressor motormay burn in 3 minutes at 25 | | | | the relay tends tocool off at a faster rate because of |
| percent overload! | | | | its lower mass. After anumber of starts and stops, the |
| Good protection against overheating of the motor | | | | temperatures of the relay and themotor may drift |
| windings can beobtained from temperature monitoring | | | | further and further apart. Eventually the |
| protector devices that areembedded into the motor | | | | motorbecomes hot, and yet the relay does not trip |
| windings. These do not protect the motoritself, but act | | | | because it is stillcool. The motor burns. |
| as sensors to trigger a temperature controlcircuit to | | | | Frequent starting and stopping of motors is no good |
| stop the motor. | | | | bothelectrically and mechanically. |
| A thermal overload relay, however, uses a heating | | | | Until next time... |
| element to heatup a bimetallic strip so that it can trip a | | | | |