Most standard commercial induction motors are rated as National Electrical Manufacturers Association (NEMA) Design B, which provides a reasonable balance between LRT and LRC for most loads. For larger motors or specialty applications, however, finer performance requirements might be in place, which require a motor design with features like a custom rotor bar shape or specialized rotor alloy. Aside from starting the motor via a device such as an auto-transformer, special contactor arrangement, or variable frequency drive (VFD), the greatest effect that on-site conditions have on locked rotor performance is the customer’s voltage dip level.
LRC decreases in basically direct proportion with voltage, while LRT decreases as approximately voltage squared and can quickly cause starting issues for processes that start under load. Careful coordination between the end user and motor supplier is required to ensure that a proper motor design is selected for both onsite conditions and the load’s profile to provide reliable, safe locked rotor performance.
Determining Acceleration Time
Closely related to locked rotor performance is the acceleration time a motor takes to start its load. Many of the same cause and effect relationships between motor design, onsite conditions, and load characteristics hold true for effects on both locked rotor performance and acceleration time. In the case of acceleration time, however, the entire speed-torque characteristics of the motor and load must be considered with net accelerating torque and total inertia being the most vital characteristics.
There are several methods available for estimating acceleration time, but all methods must begin with the motor and load’s speed-torque curves and total inertia. The most basic method divides the speed torque characteristics up into equal intervals (typically somewhere around 10 as shown in Image 3) and finds the net accelerating torque available during that period to calculate its individual contribution to acceleration time. Net accelerating torque is defined as the load torque subtracted from the motor torque at a given point (generally the motor torque should be at least 10 percent greater than load torque). Once net accelerating torque and time have been found for all intervals from zero speed to full-load speed, the times in seconds are added to provide total estimated acceleration time. For every interval, the total inertia (motor and load reflected to motor shaft) must be accounted for, as high inertia loads (e.g. large centrifugal fans) can lengthen acceleration times.
Finally, similar to locked rotor performance, voltage dip and recovery must be considered, as they will reduce motor torque available and thus increase total acceleration time.
Other Related Areas
There are numerous questions surrounding motor starting that can be explored outside of this analysis, but some other points for consideration are:
- a motor’s rated KVA Code Letter (assigns a “code letter” defined by NEMA MG-1 that gives an idea of the motor’s locked rotor current level when compared to its rated hp)
- how to interpret a large motor’s acceleration nameplate information (for example, understanding what the number of hot and cold starts means and how to interpret the time required between them)
- the theory behind safe locked rotor times and how these values are derived (principally these are based on the amount of time anticipated for critical motor components like rotor bars, end rings and stator windings to reach damaging temperatures)
With this information, end users and those involved in specifying induction motors can confidently operate and select motors for various starting scenarios. Note: this article has been written with squirrel cage induction motors in mind, most of these concepts are applicable to other motor types, including wound field synchronous (with squirrel cage rotor windings) and wound rotor induction motors.