During commissioning. Measure the time the assembly takes to come up to speed. Set the trip time a bit longer than this. The use of this device is to tell the control system that the motor has not started in response to its start command.
The above external trip is not to be confused with the motor overload device within the starter, which needs to be set to the full load current of the motor for the supply voltage as stated on the manufacturer's nameplate attached to it. The use of this device is to trip the motor if the mechanical load is beyond the motor's ability to sustain it without damage.
A higher starting current than normal indicates a circuit wiring fault and is the domain of the curcuit protection device(s), which are usually a set of fuses. Once these blow, the trip in the first paragraph will operate, which can be used to raise a "failed to start" alarm.
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As told time and again in this forum, the starting time of a motor is decided by the total GD2 (Motor GD2 + Load GD2). Various types of loads require various acceleration times. A centrifugal pump might accelerate in about 10 seconds whereas a centrifugal blower may need 25 to 30 seconds to accelerate. And, remember, the starting current magnitude in both the above cases will be the same, if the HP of the motor is same.
To address such issues, IEC 60947-4 has brought in what is called "Trip Class" for overload relays. There are 4 classes (viz.) Class 10A, Class 10, Class 20 & Class 30. It indicates the trip time band when the current thro the relay is 7.2 times the setting made in the relay. Thus, when 7.2 times current is flowing thro the relay, a Class 10A relay would trip between 2 & 10 seconds, a Class 10 Relay between 4 & 10 seconds, a Class 20 Relay between 6 & 20 seconds and a class 30 relay between 9 & 30 seconds.
Hence, if the acceleration time of the load is below 10 seconds, you go for either a Class 10A or a Class 10 Relay; if the starting time is above 10 seconds but below 20 seconds, go for a Class 20 Relay; if the starting time is above 20 seconds but below 30 seconds, go for a Class 30 relay.
Motor Starting Time.
Re: motor starting time.
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