Connect Motor Terminals Correctly to Avoid Downtime

There's something about the terminal block arrangement in figure 1 that I don't like at all. What you're looking at is a mock up of the terminal block of an AC electric motor. You may have hundreds of these on your site spinning away happily, making you money. But sometimes they suddenly stop, the machines aren't running, everyone's in a panic, and you're losing money.

A high resistance joint, which can lead to overheating of the connection and subsequent separation of the motor lead cable from the crimp, is often the cause. The term' loose connection' is bounded about. That may be true; the connection may have been loose, and localised vibration could have caused the overheating, but the cause is not always a simple loose connection. It might have been a poor connection at the crimp that caused this. Or, the reason could be as simple as how these terminals are arranged. I see this frequently when an electric motor comes to us for repair. Often, people are simply not aware of how the terminal posts should be arranged.

In my example, the first three brass-coloured posts are where the three-phase power is introduced to the motor. Ideally, a face-to-face connection is needed between the lug on the incoming supply and the lug on the motor lead.

I've shown an arrangement that is wrong. It might look ok, but if you think of the current path, it has to flow from the top incoming lead lug, down through the securing nut, down the threaded post, down through the delta link, and down through the other securing nut to the motor lead lug. Now I'm unsure of the current carrying capacity of a nut, washer or threaded post. What I do know is that there are many contact faces in this arrangement. Just one of these could be the reason for a high resistance joint or loose connection. Then there are six of these per motor, so six times the possibility of a fault.

Now take a look at the correct arrangement in figure 2. The lugs and delta connection are all faceto- face. The washers, nuts and threaded posts are there to secure that connection, not to be a part of it.

I urge users of electric motors to check inside the terminal box periodically, safely, when the power is off. Very often, when a motor is running, and vibration levels are ok, there is the assumption that everything is ok. If in doubt, a resistance comparison test can identify a high-resistance joint. Sometimes just looking at the connection is enough too.

I'd offer a word of caution here. Simply breaking out the socket set to tighten or rearrange the terminals is not adequate; there are torque guidelines for motor terminal connections which should be followed. I only know a few people who actually take notice of these for their electric motors. Most fitters just make sure 'it's tight enough'.

If, after rearranging and tightening the connections, the resistance comparison test shows more than a 2% difference in the winding resistances that could point to a poor crimp connection. Now we're getting into some invasive work. The crimp could be remade, or the motor could be swapped out.