Understanding Short Time Intermittent Loads

Electrical motors are often classified based on the type of load they are expected to handle. One important classification is based on the duration and nature of the load cycle. Different types of loads include continuous loads, short-time loads, intermittent loads, and various types of fluctuating or intermittent periodic loads.

A short time intermittent load is characterized by a duty cycle where the motor operates for a short period at a constant load, followed by a relatively long period of rest or no-load operation. The idle period is long enough for the motor to cool down significantly, potentially even to the ambient temperature. The key is that the operating time is short, and the rest time is significant before the next operating cycle begins.

Let's examine the given options to see which one fits the description of a short time intermittent load:

• Cranes: Cranes are used for lifting and moving heavy objects. The motor operates during the lifting, traversing, and lowering actions. These operations are typically of relatively short duration. Between lifts or moves, the crane motor is often idle for significant periods. This pattern of short operating times followed by long rest periods aligns well with the definition of a short time intermittent load.
• Metal cutting lathes: Lathes are used for shaping metal by cutting. The cutting process often involves relatively continuous operation for several minutes, or even hours, depending on the workpiece. While there might be setup times, the motor operation during the cutting phase is not typically characterized by very short bursts followed by long rests sufficient for complete cooling. These often represent continuous or varying loads depending on the cut.
• Rolling mills: Rolling mills are used to reduce the thickness or change the cross-section of metal sheets or plates. This process involves heavy loads and the motor operates for extended periods during the rolling process. Rolling mill loads are usually considered heavy-duty, often continuous or long-duration intermittent loads with short rest periods between passes, not typical of short time intermittent operation where cooling to ambient occurs.
• Centrifugal pumps: Centrifugal pumps are commonly used to move fluids. Once started, they typically run continuously for extended periods, often for hours or even days, depending on the application. The motor load is relatively constant during operation. This is a classic example of a continuous load.

Comparing the characteristics of each option with the definition of a short time intermittent load, we see that cranes most closely match this description. Their operation involves short periods of significant load (lifting, moving) interspersed with longer idle periods where the motor cools down.