Learn what is really behind the horsepower rating on motors.
by Blake Bailey
December 13, 2019

A motor’s horsepower (hp) rating is commonly taken as a fixed characteristic. However, there is much to be gained through a better understanding of what goes into a motor’s rating. Often, hp comes up in discussions such as uprating, overloading, and sometimes from service centers that analyze the motor’s windings and theorize that the motor’s hp is different than rated. The hp is typically dictated by driven equipment suppliers to accommodate some speed range and torque requirements, and thus sets the foundation for the motor’s rating. According to motor design choices including temperature rise, insulation class, machine size and enclosure, it will show that the variability for a given motor’s hp rating is likely larger than commonly perceived. Rather than being a fixed value, hp should be seen as something on a “sliding scale” that can be varied based on associated motor parameters and application requirements.

Horsepower & Speed

For users in North America, motors and their loads are most often referred to by their hp. Loads require a certain torque within a speed range to perform their work, with many industrial loads having speeds that relate closely to typical motor speeds—i.e., 1,800 rpm. When designing a process, analysis determines how much power is required at the chosen speed, thereby supplying the load its required torque. However, upon inspection of operating characteristics for driven equipment, it is often found that motor hp exceeds the actual load requirements. This follows the logic that the motor should always be sized with greater power than its load requires to guarantee reliable operation. While reasonable, in many applications this leads to oversized motors that operate within an inefficient range. Additionally, overload conditions are almost universally accounted for with a motor’s service factor (sf), providing even more operating margin for the motor (although National Electrical Manufacturers Association [NEMA] MG1 states reduced life should be expected for continuous operation in this range). Therefore, even at the start of choosing motor ratings, the base hp and speed of a motor are values that must be balanced.

Temperature Rise & Insulation Class

Rated temperature rise and insulation class are critical in the sizing and rating of any industrial motor. As machines that convert electrical power into mechanical power with inherent inefficiencies, motors produce heat resulting from their losses. For induction motors, this heat results from four of the five inherent losses (stator resistance loss [I2R], rotor I2R, stray load and core losses), with the bulk of the losses being dependent on the motor’s current draw (and, correspondingly, load). Therefore, as motor load increases, the motor’s internal losses will also increase, thereby leading to an increase in heat generation.

Relevant to the temperatures motors reach internally are the properties of the electrical insulation, which ensures that voltage is isolated between phases, as well as from ground. Along with their dielectric properties, insulating materials have thermal limits that greatly affect their useful life, meaning that a motor’s safe operating temperature range is limited by its insulating materials. For modern motors, common insulation total temperature limits are 155 C (311 F, Class F) and 180 C (356 F, Class H), with vintage machines sometimes having Class B (85 C or 185 F) insulation systems. These limits must account for both the ambient temperature (typically 40 C or 104 F) and expected temperature rise of the motor for continuous operation. For example, a motor rated for an 85 C or 185 F temperature rise at its 1.15 sf with a 40 C (104 F) ambient temperature is expected to operate with a 125 C (247 F) total temperature. It would be inappropriate here to use Class B insulation, but either Class F or H materials would be suitable.

From the preceding points, it should be clear that if a motor can withstand higher temperatures, more internally generated heat can be tolerated. This allows for higher losses, thereby permitting an hp increase.

Motors rely on convective and conductive cooling paths for heat removalImage 1. Motors rely on convective and conductive cooling paths for heat removal (Images courtesy of designmotors)

Machine Size & Enclosure

The relationship between motor size, enclosure and rated power is related to temperature rise. For the removal of internal heat, motors rely on both convective and conductive cooling paths (see Image 1). As an example, a motor’s stator windings generally will produce the most heat and this heat relies on multiple paths to exit the motor. These paths through the motor and its enclosure include conduction from the stator copper to the stator core, conduction from the core to the motor’s frame or convection via cool air flow around the core and coil extensions, exiting the machine through paths dictated by its enclosure. By a designer increasing machine size, the area through which conductive and convective cooling takes place is also increased. For a given rating, this increased cooling capacity naturally results in lower temperatures. Thus, motor heating also depends directly on motor size.