STARTING GEAR

Make certain that the type of starting gear is the same as that informed to the company when ordering the motor and that it is correctly rated. The usual methods of starting cage induction motors are direct-on-line, start-delta and auto-transformer.

METHOD OF STARTING

Move the starter handle from the OFF position to the RUN or ON position. If there is a START position, move the handle first from OFF to START position, and after the motor attains full speed, move the handle smoothly to RUN position. In direct-on-line and star-delta starters where only START and STOP push button are provided, the starting is much simplified.

Cage Motors

Move the starter handle from the OFF position to the RUN or ON position. If there is a START position, move the handle first from OFF to START position, and after the motor attains full speed, move the handle smoothly to RUN position. In direct-on-line and start-delta starters where only START and STOP push buttons are provided, the starting procedure is much simplified.

REDUCED VOLTAGE METHODS OF STARTING

Star-delta and auto-transformer starters belong to this category. The object is to reduce the RMS line current during starting. But at the same time the motor torque also gets reduced in the same ratio with respect to the rated voltage value at any speed, namely 1/3 for star-delta starting and (per unit voltage tap)2 for auto-transformers.

The reduction in motor torque must be considered when starting a load with a given torque speed characteristic. The load torque at any speed during starting MUST be comfortably LESS than the effective motor torque at that speed to enable acceleration to the final rated speed. The effective motor torque be calculated from the rated voltage motor torque as obtained from the data or curve supplied by the Company for the concerned machine, by multiplying by the ratio stated above, and taking into account any voltage drop in the supply system.

When a motor is started by any reduced voltage method, its torque may get reduced to such an extent that, if the motor is driving an equipment that is starting on load, the full speed may not be reached and the motor cannot accelerate beyond some fraction of the final rated speed. At this lower speed therefore, when the motor and load torque are equal and acceleration becomes zero, the starter will have to be changed over to its RUN position in order to impress the full line voltage on the motor whereupon the motor torque will increase to effect acceleration to full speed. But along with the torque the line current will also increase to its rated voltage value, and if the speed at change-over is less than about 75% of the full speed, the magnitude of the current will be quite considerable. This somewhat defeats the purpose of using reduced voltage starters; only the duration of high starting current is reduced. The advantage of such starters is realized fully only when the motor is started on reduced loads.

FAILURE TO START

If a motor fails to start from the rest as soon as it is energized or the rotor controller is operated, or it fails to attain full speed within a reasonable time not exceeding half a minute, switch off the supply and first INVESTIGATE the reasons for this failure. Also refer to FAULT FINDING CHART.