AC Electric Motors: Introduction and Work Capacity

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FOR SAFETY, EFFICIENCY, AND ECONOMY, the size and type of motor you choose should be based on the requirements of the job it will be performing. This section discusses the work capacity, power consumption, and installation of the types of motors commonly used in homes and on farms. Special attention is given to safety factors, including grounding and protecting with overload devices.


The capacity of a motor to do work is based on its horsepower, starting and overload capacity, and speed. Choose a motor size that is appropriate to the task.


A motor is rated in horsepower (hp). One horsepower is defined as the work required to lift 33,000 pounds one foot (33,000 foot-pounds) in one minute. One horsepower is equal to 746 watts. Typical horsepower ratings are included in the descriptions of motor types in this section.

Starting capacity

Motors can deliver far more power while starting than they can at full speed. The proportion varies with the type of motor; some types have starting torques four or five times greater than at full speed. Naturally the amperes consumed during the starting period are much higher than while running at full speed. In selecting a motor, it is important to consider a machine’s start-up load. The motor will heat up quickly if too heavy a starting load prevents it from reaching full speed.

Overload capacity

Almost any good motor will develop from 1 1/2 to 2 times its normal horsepower for short periods after coming to frill speed. Thus a 1-hp motor is usually able to deliver 1 1/2 hp for perhaps 15 minutes, 2 hp for a minute, and usually even 3 hp for a few seconds. No motor should be deliberately overloaded continuously because overloading leads to overheating, which shortens the life of the motor. But this ability of the motor to deliver more than its rated horsepower is very convenient. For example, 1/2 hp may be just right for sawing lumber, but when a tough knot is fed to the saw blade the motor can instantly deliver 1 1/2 hp and then drop back to its normal 1/2 hp after the knot has been sawed.

Replacing gasoline engines with electric motors

Unlike an electric motor, a gasoline engine has no overload capacity. That is why it is often possible to replace a 5-hp gasoline engine with a 3-hp electric motor. If the gasoline engine always runs smoothly and if it seldom labors and slows down, it can be replaced by an electric motor of a lower horsepower. But if the engine is always laboring at its maximum power, the motor it replaces it should be of the same horsepower as the engine because no motor will last long if it must continuously deliver more than its rated horsepower.

Speed of electric motors

The most common speed for a 60-Hz motor is theoretically 1,800 rpm. Actually the motor runs at a little over 1,750 rpm while idling and somewhere between 1,725 and 1,750 rpm while delivering its rated horsepower. When overloaded the speed drops still more. If overloaded too much, the motor finally stalls. The speed of ordinary ac motors cannot be regulated by rheostats or switches, but there are solid-state controllers for regulating the speed of some ac single-phase motors. Special variable-speed motors are obtainable, but they are expensive special-purpose motors and are not described here.

How temperature rise affects operation

A 10-hp motor made today isn’t much bigger than a 3-hp motor made 50 years ago. This is possible because of advances made in the heat-resisting properties of insulations on the wires used to wind the motor and insulations used to separate the windings from the steel in the motor. Other advances have led to reduced air gaps and to improved magnetic properties of the laminated steel used in the pole pieces. Today’s motors will run much hotter without being damaged, but motors should always be installed where they will have plenty of air for cooling.

Motors have stamped on their nameplates a “service factor” ranging from 1.00 to 1.35. Multiply the horsepower by the service factor. The answer tells you what horsepower the motor can safely deliver continuously in a location where the temperature is not over 40°C (104°F) while the motor is not running. That means the motor might develop a temperature of over 100°C (212°F), the boiling point of water, but it will not be harmed.

Most motors of 1 hp or larger have a service factor of 1.15; smaller motors have a higher service factor, some as high as 1.35. A motor works most efficiently and lasts longer if operated at its rated horsepower.

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