A manually operated press motor has been blowing at least one of its bi-metal thermal overload strips nearly every day for the past month. The plant's power monitoring system doesn't have sensors at the load level, so a tech grabbed a DMM with high/low memory and connected it to record voltage on one phase. The DMM recorded a high spike of about 700VAC and a low of about 410VAC. Of course, these are point measurements, and the DMM shows nothing about their duration. Nor does it show when these occurred or how many spikes there were.

What other information do you need?

The spikes and dips the DMM is seeing are probably due to across the line starting, which isn't particularly useful information in solving this problem. A power analyzer can provide the detailed picture of the relevant electrical goings on. While the power analyzer is collecting data for later trending and analysis, you can perform some troubleshooting.

Begin by talking to the press operator. The typical press motor, once started, stays on for the shift. If the operator is turning the motor off while loading new material and then on for pressing, the motor is probably overheating from short cycle restarts and requiring increasingly greater current to run. If the thermal overloads are opening only toward the end of the shift, suspect this problem.

In addition, check the motor pedestal and base for deflection during pressing. Also check bonding per Art. 250 and install a vibration monitor.