Process downtime is expensive—even more so when it’s unexpected. So, when an electric motor fails, we tend to pull, repair, or replace it, and move on as quickly as possible. In doing so, however, we may miss an opportunity to capture basic information that could help improve the reliability of the application. With a little planning, these data can be gathered with no delay in startup.

The most stressful time for electric motors is during starting. At starting, the motor current is the highest it will ever be. This is referred to as starting or locked rotor current. These different terms describe that when the shaft speed is zero, the current is maximum. Once the motor has been successfully started, the load current level is reached and the cooling circuit of the motor is able to dissipate the additional heat produced by the starting current. Restarting the motor before this additional heat has been dissipated means more heat in the form of kW•h will be added on top of that which is there. Each subsequent start before the additional heat has been dissipated will add more heat — raising the temperature until some component in the motor reaches its failure point. This article looks at safe starting practices for electric motors.

The most common method of starting squirrel cage three-phase motors is across the line (direct-on-line). Some applications require limiting the motor starting current and/or torque to reduce the stress on the electrical and mechanical systems. Although there are other methods such as autotransformer, reactor and using a variable frequency drive (VFD), the focus in this article will be on the reasons behind the selection of the three most common methods of achieving these objectives. Specifcally, these methods are part winding, wye-delta, and electronic soft-starting. The windings in all three of these methods usually have 6 leads.

It should not be assumed that because a motor can drive a running load, it also has the capability to accelerate the load up to rated speed. During starting, a motor must deliver the energy required to accelerate the load. To do this, the motor torque must exceed that needed to accelerate the load. The motor torque value in excess of the load torque requirement is termed the "torque available for acceleration."

One of the most misunderstood winding connections is the part-winding start. Many customers (and some members) tend to blur the differences between the part-winding-start (PWS) connection and wye-start, delta-run connections.