We recently addressed a major problem at an OEM customer in regards to connecting motor leads to the power source.
This company had utilized a “box” connector that is essentially a mating device that uses a screw to press a metal plate onto the wire strands. The problem with this type of connector is that one or more of the wire strands can migrate up into the screw threads. When the screw is tightened, it pulls on those strands only and gives a false-positive indication that the connection is tight. As the motor starts, stops and experiences vibration over time, the already loose connection fails to make a proper electrical transfer and the motor fails.
Here’s a summary of the documentation we sent to our customer:
- Strands of wires can be easily yet mistakenly threaded onto the screw that is meant to tighten the connection – resulting in a connection that initially feels tight but is not fully tightened.
- Heating, cooling and variation in current flow causes expansion and contraction of the connection, resulting in a loose connection that eventually causes a failure.
- The bulkiness of these connectors promotes the “stuffing” of the connections into the terminal box. This often bends or crimps a connection, causing a weak spot in the insulation. As the motor heats, cools and vibrates, this weak spot eventually becomes exposed and causes a failure.
- This type of connector does not adequately protect the connection from moisture. Thus, any condensation or egress of moisture into the terminal box can cause a failure.
To avoid future failures, we recommend the following procedure for connecting electric motors:
- Provide the proper connectors, crimping devices, bolts, washers, lock washers, nuts and tape in ample quantities to the technicians responsible for connecting Electric Motors.
- If OEM provides an improperly sized lug, remove. Select properly sized lug. Strip wire casing back to accommodate lug.
- Insert wire into lug, ensuring that all wire strands are inside the lug.
- Using properly sized crimping device for lug, crimp lug onto wire.
- Connect using properly sized bolt, washer on each side, lock washer and nut.
(DO NOT USE LOC-TITE).
- Insulate with Cambric Tape.
- Insulate with 130 ºF rubber tape.
- Insulate with final layer of 33 Plus Electrical Tape.
- Ensure that no wire is exposed on any of the leads.
We utilize Thomas & Betts lugs and crimping devices. The products are color coded, easy to use and are designed for the proper wire size, current flow, and voltage and temperature range. A reputable supplier such as Fastenal can provide an easy bin-stock protocol for these products. Please note that tape should be applied as to cover half the previous winding of tape. This “half lap” method ensures that moisture does not infiltrate the lead wires.
It is also important that technicians are properly trained and are provided with the time and facilities necessary to complete this important procedure.
What problems with power source connections have you had? How did you overcome them?