The Case of the Unreliable Conveyors

Jul 1, 2008 12:00 PM, Edited by John DeDad, Senior Director, Editorial and EC&M Development

The next step was to verify that the isolation transformers were indeed providing noise rejection and a stable ground reference. Using a true rms multimeter, a voltage measurement was taken between the case of one drive control system and the power system neutral. You would expect a measurement of zero volts at this point but, instead, 35V was measured. Measurements at the other two drives were the same.

Fig. 1. Proper wiring of a shielded isolation transformer has incoming ground, shield, core, secondary neutral and secondary ground all bonded at one point (single-point ground), which is then bonded to the building ground system.

Figure 1 shows the proper wiring of these isolation transformers. Notice how the incoming ground, shield, core, and secondary neutral are all bonded at one point. This is called “single point grounding.” This point is then bonded to the building ground system.

When the covers of the transformers were removed to verify their wiring connections, a different grounding scheme was found, as shown in Fig. 2. The neutral was not bonded to the single point ground, and the transformer was not locally grounded to the building system ground. When these missing connections were made, the neutral-to-case ground voltage returned to zero, and the unexplained conveyor upsets ceased to occur.

Fig. 2. Improper wiring of shielded isolation transformer has neutral not bonded to the single-point ground. The transformer is also not locally bonded to the building ground system.

The above case history shows the difference between a power-related anomaly, which repeats in a cyclical manner, and a disturbance that exhibits random or transitory behavior. Here, the end-user did not think the problem was wiring-related, what with the installation of dedicated circuits and shielded isolation transformers. Mistakenly, the transitory disturbances were attributed to harmonics.

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