Electric motors come in many shapes and sizes. Some are standardized electric motors for general-purpose applications. Other electric motors are intended for specific tasks. In any case, electric motors should be selected to satisfy the dynamic requirements of the machines on which they are applied without exceeding rated electric motor temperature. The first and most important step in electric motor selection is determining load characteristics - torque and speed versus time. Electric motor selection is also based on mission goals, power available, and cost.
Motor Categories:
Motors are usually grouped into three main categories:
•General Purpose Motors feature standard operating characteristics designed around industry standards.
•Definite Purpose Motors are designed to meet specific application requirements with specific standards. A good example of this motor class is a pump motor.
•Special purpose Motors are designed around one specific application and must meet specific requirements for that application.
Motor Types:
Capacitor-start motor- An alternating current split-phase induction motor that has a capacitor connected in series with an auxiliary winding for starting. The auxiliary circuit disconnects when the motor is up to speed. This motor requires an internal starting switch and governor.
Permanent split capacitor motor- A single-phase electric motor that uses a phase winding in conjunction with the main winding. The phase winding is controlled by a capacitor that stays in the circuit at all times and is rated for continuous running. The capacitor improves starting and running power factors. This motor does not require either an internal starting switch or a governor.
Split-phase motor- A single-phase induction motor that has an auxiliary winding connected in parallel with the main winding. The auxiliary winding's magnetic position is not the same as the main winding, so it can produce the required rotating magnetic field needed for starting. This motor requires an internal starting switch and a governor.
Three-phase electric motor- A motor that operates from a three-phase power source. In three-phase power, three voltages are produced that are 120 electrical degrees apart in time. This motor has no internal starting switch.
Two-capacitor motor- An induction motor that uses one capacitor for starting and one for running. The starting capacitor is in parallel with the running capacitor as the motor is starting; at 75 percent of speed, the starting capacitor is cut out of the circuit. This type of motor is sometimes called capacitor start/capacitor run and requires an internal starting switch and governor.
Motor Electrical Characteristics:
•Horsepower (as applied to motors) — as applied to an electric motor, the horsepower is an index of the amount of work the motor can produce in a period of time. For example: One horsepower equals 33,000 foot pounds of work per minute, that is a one horsepower motor, with suitable gearing
and neglecting all losses, can lift 33,000 pounds one foot in a minute, or one pound 33,000 feet in a minute. Assuming 100% efficiency, 746 watts of electrical power will produce one horsepower.
•Phase - This concept is fairly simple in the United States. You either have a single-phase or 3-phase motor.
•Voltage - Measurement for units of electrical potential or pressure.
The installer should try to get a match as close as possible to the exsisting motor. It’s okay for the replacement motor to have a lower nameplate amperage value. If the replacement motor has a higher value make sure to check the ability of the electrical system to handle the added load.
Motor Performance:
Some considerations for motor performance include motor type, horsepower, speed, and service factor. Motor types include shaded pole, split phase, three-phase, permanent split capacitor, or capacitor start. In many applications more than one type of motor may work and in many more applications an exact replacement will not be found. Using a similar motor with slight differences in the electrical and mechanical features should still provide reliable operation.
Motor Enclosures:
Drip-proof (ODP) or Open Drip-proof - Ventilation openings in bearing housings and some yokes placed so drops of liquid falling within an angle of 15° from vertical will not affect performance. Normally used indoors in fairly clean, dry locations.
Totally Enclosed Fan Cooled (TEFC) - Has an external fan to move cooling air over the motor. Suitable for outdoor and dirty locations.
Totally Enclosed Non-Ventilated (TENV) - Does not have external cooling fan but is dependent on radiation and convection for cooling.
Totally Enclosed Air Over (TEAO) - Special motor used to drive a fan blade. Has no external fan and is dependent on air stream of driven fan for cooling.
Explosion-proof - Motor designed to withstand an internal explosion of gas or vapor and not allow flame or explosion to escape. Generally TEFC but also built TENV in smaller horsepower ratings. Motors are labeled to meet UL and NEC requirements.
