Understand these concepts before starting squirrel cage induction motors.
by Blake Bailey
June 19, 2019

Questions about starting electric motors are common. Motor starting problems resulting from these questions can be costly and confusing, leading to production delays and sometimes catastrophic motor failure. This article will briefly explore some of the most common questions about the direct online (DOL) starting of squirrel cage induction motors. Through proper application, end users can have confidence that their motors and related equipment will not incur damage during the starting cycle.

Reading Speed-Torque-Current Curves

Speed-torque-current curves are plots of a motor’s torque and speed characteristics from zero speed up to no load speed (practically synchronous speed for an induction motor). All terms are commonly plotted as either percentage of full load or in their actual units (e.g. revolutions per minute [rpm], foot-pounds of torque and amperes). It is conventional for torque and current to both be plotted on the same graph, as well as being presented separately as shown in Image 1. Typically, plots of reduced voltage torque and current, as well as the load’s speed-torque characteristics, are also given. For the latter, it is key to obtain the load’s speed-torque curve and not simply the “running curves” for pumps and fans. Finally, these graphs can be plotted either horizontally (speed on the x-axis, torque and current on the y-axis) or vertically (speed on the y-axis, torque and current on the x-axis).

torque curveImage 1. Intersection of the torque curve at zero speed will show locked rotor torque (Images courtesy of designmotors)

Key performance information related to motor starting is shown on speed-torque-current plots. In Image 1, the intersection of the torque curve at zero speed will show locked rotor torque (LRT), while the maximum torque value shown represents breakdown torque (BDT). Both full load torque (FLT) and current levels (FLC/FLA) are found by drawing a vertical line from the rated speed point until it intersects both plots. The point on the current plot corresponding to zero speed will show locked rotor current (LRC/LRA), while the motor’s no load current (NLC/NLA) is shown by the point corresponding to the highest speed shown.

Reading Thermal Damage Curves

Thermal damage curves are plots that provide information about acceleration times, operational time limits at finite current levels and time limits on operation above full load. These curves are also known as “thermal limit curves,” “safe operating time curves” or “acceleration curves.” They are typically plotted with motor current as the x-axis and time in seconds as the y-axis on a logarithmic scale. As seen in Image 2, the plots found closest to the current scale show motor current during acceleration. These plots, typically at various voltage levels, intersect the speed scale at a point that represents LRC and intersect the time scale at the full-load current point to show anticipated acceleration time.

thermal damageImage 2. Thermal damage curve

There are usually one or two plots shown above the accelerating current curves, which show safe locked rotor times as well as a “running” or “overload” curve. The safe locked rotor time (“safe stall time”) plot(s) are intended to show the number of seconds a motor can safely withstand a given current level at various voltage levels.

If shown separately, the “cold” curve provides the safe locked rotor times for the motor when it is first started (the “cold” condition), while the “hot” curve provides the safe stall times after a motor has either been recently started or run under load. It should be noted that Image 2 only shows a single plot connecting the safe stall time plots in cold condition with the “overload” curve. The safe stall time plots should never overlap or come substantially close to the motor accelerating current plots, as this represents dangerous heating in the rotor bars and end rings.

The overload curve shows the time that a motor can safely operate while running at, or above, full-load current. Typically, the upper bounds of this curve correspond to a continuous time associated with the motor’s highest safely rated load level (often the service factor). If this plot is followed to higher levels of current (and therefore lower values of time), it will often be plotted to intersect the safe stall time curve(s), but most often is shown to just nearly intersect them.

Locked Rotor Performance

Locked rotor performance encompasses the motor characteristics related to when a motor is in a locked (stalled) condition, such as at initial energization. Locked rotor performance is affected by combinations of the motor design, driven-load characteristics and onsite conditions. Typically, end users will specify locked rotor performance requirements and provide information on the driven load in addition to installation details such as typical voltage dip levels or if the motor will be started via a method besides DOL. Typically, the motor design for locked rotor is centered around providing the required torque while minimizing LRC and potential ill effects on full-load operation.

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