FIGURE 1: Sine wave model for three-phase alternating current.

Considering this prevalence and the corresponding energy usage, facility designers, maintenance personnel, and others should have a basic understanding of motors and how they work. We're here to cover basic operating concepts, construction, performance characteristics, and motor circuit design considerations.

Because three-phase, alternating-current induction motors are most commonly used for building systems applications such as fans and pumps, this article will focus on the fundamental principles for operation of these types of motors.

Concepts And Construction

These magnetic forces, generated by current-carrying conductors, are the building blocks for the operation of electric motors. A simple analogy for electric motor operation would be to take a magnet and move it around a compass, causing the compass needle to rotate by following its attraction to the moving magnet.

Three-phase induction motors are available in sizes of 0.5 hp and above. Their relatively low-cost, low-maintenance, and straightforward simplicity are factors that combine to make them a preferred choice for an increasing number of applications.

Three-phase induction motors are built with two basic components: the stationary stator and the rotating rotor. The stator consists of a laminated iron core in the shape of a hollow cylinder, with internal slots so that insulated conductor windings from each of the three phases may be inserted into them. The rotor, which turns inside the stator, consists of another iron core, with copper or aluminum bars arranged in slots parallel to the center shaft and connected at the ends by rings, similar to a squirrel cage. The rotor can also be wound like the stator; however, this is not as prevalent.

Fan blades are typically mounted on the shaft to draw cooling air through the motor housing. The magnetic field produced by the stator windings operates like a transformer to induce current into the rotor, causing it to produce its own magnetic field, which results in torque to turn the rotor.

It should be emphasized that this applies to three-phase motors only. For single-phase motors, a slightly different principle is used to produce starting torque.

Three-phase current refers to a typical configuration of power distribution within buildings where three wires conduct three separate electrical phases. These three supply conductors connect to three sets of windings in the stator. The current flowing in each supply conductor alternates direction along the conductor with a frequency of 60 cycles/sec (Hz) in the United States, versus 50 Hz for many other countries. This is what is referred to as alternating current (ac).

Ac flow is typically modeled by a sine wave graph as shown in Figure 1. The diagram also shows that current flow in each of the three-phase wires (A, B, and C) is offset from each other by 120 degrees of each 360-degree cycle. Since the stator windings connect to these separate phases, the magnetic field they produce rotates by virtue of their common 120-degree displacement, even though the stator itself is stationary. Figure 2 shows the schematic representation of the construction of a three-phase induction motor.

Each of the three phases of stator winding is arranged in pairs called poles. The number of poles in the stator determines the speed at which the rotor rotates: The greater the number of poles, the slower the rotation. This is because the circumferential distance between poles is shorter with a greater number of poles. The rotor, therefore, can move slower to "keep up" with the rotating magnetic field. The speed of the motor's magnetic field (referred to as the synchronous speed), in revolutions per minute (rpm), is calculated using the following formula:
rpm = (f x 120) ? p
where f = the frequency in Hz, and
p = the number of poles in any multiple of two.

Therefore, in a 60-Hz system, the synchronous speed of a two-pole motor is 3,600 rpm, that of a four-pole motor is 1,800 rpm, etc.

In order to produce torque, the rotor in an induction motor must rotate slower than the magnetic field. The difference between rotor speed and synchronous speed is called slip, usually expressed as a percentage of synchronous speed. Hence, the actual speed of a motor with a synchronous speed of 1,800 rpm is generally 1,750 rpm.

Rated torque depends on the rated speed and hp of the motor, as expressed by the following equation:
T = (hp x 5,252) ? S
where T = torque in ft-lb, hp =
horsepower, and S = actual full-load speed in rpm.
The full-load current drawn by a motor is dependent on its hp rating, efficiency, power factor (pf), and the voltage supplied. This formula is:
Full-load current = (746 x hp) ?
(1.73 x efficiency x pf x voltage).
Efficiency expresses the energy losses inherent in the construction of the motor, and the ratio of power delivered at the shaft to power input. Power factor is a form of electrical efficiency due to voltage and current waveforms being out of phase with each other. It is the ratio of real power input (watts) to the product of the actual current and voltage (volt-amperes). For the purposes of the previous equation, both are expressed in decimal form.

Performance Characteristics

The National Electrical Manufacturers Association (NEMA) classifies types of motors based on these characteristics. NEMA publishes a voluntary standard called "MG-1, Motors and Generators," which defines these characteristics and is generally followed by the industry. The two classifications of motors most commonly used in building systems are Design B (standard) and Design E (high-efficiency) motors. Their distinctive torque curves are very similar to those shown in Figure 3.

As illustrated in Figure 3, the torque developed by a typical Design B motor at standstill (locked rotor) is a minimum of 150% of its full-load torque rating. As speed increases, torque may actually decrease a little to its "pull-up" torque, on its way to its maximum "break-down" torque, before reaching 100% rated speed. This type of curve is appropriate for starting a load such as a fan or pump, whose torque increases with speed. In contrast, a Design D motor produces a high starting torque that steadily decreases to its full-load value. This design is appropriate for a conveyor belt or other loads with a high starting torque.

During the starting process, the motor draws a large inrush current until the rotor reaches its rated speed. In Figure 3, the locked-rotor current is about 600% of the current at full-load speed. Current then steadily decreases as the motor reaches its rated speed. For this reason, the National Electrical Code (NEC) allows the circuit breaker protecting the motor circuit to be sized large enough so that the momentary starting current can pass through without tripping.

NEMA MG-1 requires each motor to include a nameplate with information characteristic to the motor such as horsepower, rpm, voltage, full load, and locked rotor amperes.

High-Efficiency Motors

Although Design B motors have become more energy efficient as a result, NEMA also added the category of Design E motors in 1994. This category allows higher inrush currents than the Design B requirements; this generally enables greater efficiencies as well. However, circuit breakers protecting the motor circuit need to be sized to take that inrush into account.

Another important detail to consider is that Design E motors may have less slip, and therefore would drive a fan or pump impeller at a slightly higher speed. This results in a flow slightly higher than the design flow, as well as a correspondingly higher torque requirement for the motor. This has the effect of overloading the motor, which shortens its life.

FIGURE 3: Typical current and torque curves for a NEMA Design B motor.

Overloads

If this torque requirement is greater than the rated torque of the motor at its rated speed, but is still less than the breakdown torque, the motor must increase its slip - that is, it must slow down in order to provide it. The current will exceed the rated value, creating an overload condition. The stator and rotor windings will incur thermal damage if this condition persists.

For this reason, the NEC requires overload protection to interrupt current flow, typically at approximately 125% of the motor's rated full-load current. This overload protection is usually provided within the motor control device upstream of the motor, either a motor starter or variable-speed drive (vsd).

Often, motor manufacturers will design motors with a service factor, which is an additional margin of hp and torque that can be required of the motor with a corresponding effect on its service life, within acceptable limits. This factor is typically 1.15, which, when applied to a 20-hp motor, for instance, means that the pump could require 23 hp from the motor on a continuous basis.

However, if the motor is operated continuously within the service factor, the motor insulation will age quicker, so it is standard to regard service factor only as a safety margin.

Motor Circuit Sizing

When designing the components of an electrical distribution system, it is easier to go down in motor size than up. So if 7 brake-hp (bhp) is required by an air handler, for instance, it may be appropriate to design for a 10-hp motor until the system characteristics can be firmly established, selecting a smaller motor later if appropriate. ES