Creeping in energy meters is phenomena due to which the disc of energy meter rotates even in absence of any load current in the current coil (CC) and only pressure coil (PC) is energized. The necessary condition for creeping is that PC must be energized. This is required as the friction compensating device is voltage actuated and therefore the friction compensating torque is independent of load current flowing in CC. Why PC must be energized for Creeping?
The major cause of creep is over compensation for friction. Generally the friction compensation device is so adjusted to give a driving torque required to compensate for the starting friction of the disc. But if the friction compensation is more than the required starting friction then there is a tendency for the disc to run even when there is no current through the current coil as friction compensating device is voltage actuated. This is the reason, PC must be energized.
Other causes of creeping are excessive voltage across the PC (which causes excessive friction compensating torque is developed), vibrations and stray magnetic fields.
Method of Prevention of Creeping
To prevent creeping, two diametrically opposite holes are drilled in the disc of energy meters. Due to this hole, the disc will come to rest when the hole comes under the edge of pole of shunt magnet. Thus creeping is limited to a maximum of half of the rotation. Now we will discuss how the disc comes to rest when hole comes under the edge of pole of shunt magnet. Let us consider the figure below.
In the figure only one hole is shown for the sake of simplicity and to ease better understanding. Edge of pole is shown by red dotted line. Let us assume this pole to be South Pole. When the hole comes under this pole, because of changing magnetic field through the disc, eddy current will start flowing in such a way to oppose the cause as per Lenz’s Law. Therefore the path of eddy current will be in clockwise direction as shown in figure. The center of eddy current will be at point A. Due to this eddy current, a magnetic pole will be induced in the disc whose South Pole will face the pole of shunt magnet. As similar poles repel each other, therefore South Pole of Shunt magnet and South Pole of induced magnet due to eddy current will repel each other which will balance the over friction compensating torque. In this way, creeping is restricted to only half the revolution of disc. It shall be understood that in absence of hole, the center of eddy current will be at point B which coincided the pole center of shunt magnet. Due to this the center of induced magnet due to eddy current and pole of shunt magnet of energy meter will be in line and therefore no repulsion force will take place. This is the reason, hole is drilled to make the two pole centers apart. It shall also be mind that this repulsion force or torque is not sufficient to impact the action of meter on load.
In some cases, a small piece of iron is attached to the edge of disc. The force of attraction exerted by the Brake Magnet on the iron piece prevents the Creeping of disc.