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  Alignment of Directly Coupled Motor

Checking the Alignment of a Directly Coupled Motor

The alignment is simply checked through laying the edge of a steel foot rule against the sides of the two flanges and testing whether the steel edge sits completely against the sides of the two flanges or if there is any gap. Any differences in levels is corrected through an appropriate steel shims. The alignment should not just be correct in the vertical and horizontal planes although the axis of both the shafts should be in similar line and not make an angle with each other. This can be tested through calculating the gap among the flanges faces at four points that are top, bottom, front and back. The below diagram displayed the two types of misalignment, in an exaggerated way for clarity.

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Diagram - Misalignment - 2 types

Balancing

Though thoroughly constructed, a motor armature shaft will comprise some not equal distribution of weights in its body that results in its axis of gravity being a little off centre and out of line along with the axis of rotation. So, while the armature rotates, centrifugal forces are made that set on the bearings. This causes the complete machine to vibrate. Its intensity changes at dissimilar speeds and becomes maximum at some critical speeds because of the influences of resonance. The quantity of unbalance ascertains the degree of vibration. For smooth running and long helpful life, the rotating parts should be correctly balanced. Balancing contains re- adjusting the distribution of masses in the body in such type of a way as to bring the axis of gravity to coincide along with the axis of rotation. This is made by placing a counter-weight on or eliminating some weight from some part of the armature in such type of a way that the unbalanced centrifugal force is cancelled out. To do this it is essential to ascertain accurately where the counter-weight is to be placed or removed, and as well the weight of material to be removed. Balancing contains two types. First is Static Balancing and second is dynamic balancing.

Static balancing:

In this type of balancing, the rotor is supported on a pair of completely horizontal knife edges. If the armature is in perfect balance, the rotor has to be rest in any position. If, alternatively, the rotor is not well-balanced and has not even distribution of weight, the rotor will turn round and come to rest along with the very heavy portion in the lowest position, and the hollow portion will engage the top position. Small counter-weights shall be fixed on the top part (lesser weight portion) of the armature. The larger the distance from the centre, the smaller should be the weight. A substitute method is to take out some weight from the heavier portion of the rotor through drilling a hole in the end supports or through chipping, when found convenient. The operation should be repeated until the rotor can come to rest uniformly well in every positions.

Dynamic balancing:

This means performing the balancing operation while the rotor is actually rotating. Even though a body may appear well-balanced through the static test, any slight unbalance becomes prominent at high speeds. Unique dynamic balancing machines are presented by means of that the exact amount of weight to be added (or removed) also its location can be precisely ascertained. The rotor is situated on a pair of pedestals carrying spring suspension systems, and driven at any speed needed. The machine contains a visual indicator that amplifies the vibration felt on the bearings because of the unbalance of the rotor. An appropriate device is also incorporated by means of that a counter-weight of the right value might be temporarily inserted at the right place on similar shaft to which the armature is coupled, until all oscillations are neutralized. After the weight of the counter-weight, the accurate radial length and angle of location are ascertained, the armature is eliminated and a permanent counter-weight fixed at the correct point, and a checking conducted one time again to verify the correction. While balanced in this way, the armature will run extremely smooth at all speeds without any vibration. Care has to be taken in fixing the counter-weights correctly so that they do not fly off at high speeds.

For balancing large armatures that may not be feasible to be moved easily, particular electronic equipment is available, so that the balancing operation may be carried out 'on site' on the machine itself.

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