About Electric Motors

Motor Construction

Both the inner and outer parts of electric motors play an important role in motors turning power into motion.

How Windings and Poles Determine a Motor’s Speed

When the coils have two centers, they form a two-pole motor; when they have four centers, they form a four-pole motor. In short, the number of coil centers determines the number of poles a motor has.

A motor’s number of poles determines its speed. 60 Hz power “cycles” 60 times each second, or 3,600 times each minute. When the stator is wound in the form of two poles, they change their polarity 3,600 times each minute. The rotating magnetic field induces the rotor conductors to follow.

All alternating current induction motors “slip.” That is, they cannot quite keep up with the speed of the pole changes within the stator. The speed of the magnetic field within the stator is called the synchronous speed of the motor. A two-pole motor has a synchronous speed of 3,600 RPM on 60 Hz alternating current. However, slip accounts for an actual speed of 3,450 RPM. To determine the synchronous speed (or RPM) of an electric motor, divide the cycles per minute by the pairs of poles.

Other Electric Motor Components

Motor Bearings

Bearings are an important part of any electric motor setup. In short, bearings support rotation of the motor’s shaft. They reduce friction to smooth rotation. Bearings also maintain the correct position for the shaft as it rotates. Two bearing types used in electric motors are the sleeve bearing and the ball bearing.

Sleeve Bearings

Sleeve bearings, sometimes called bushings, are usually thin-walled tubes made from steel-backed babbitt or bronze. Oil grooves are cut on the inside surface to insure proper lubrication of the bearing.

Sleeve bearings are used in the smaller motors for light and medium duty service. When correctly maintained by periodic lubrication, these bearings give years of trouble-free service. Most sleeve bearings are designed for all position mounting, and may be used in the vertical position.

Ball Bearings

Ball bearings are formed by enclosing a number of hardened steel balls between grooved inner and outer rings of hardened steel called races. Sometimes a light metal spacer called a cage is used to space the balls evenly around the races. To insure long, maintenance-free life, these bearings can be packed with lubricant and sealed by mounting these metal discs or rings on each side of the opening between the inner and outer race.

Electric Motor Enclosures

Enclosures are the housing, or outer part of a motor. Enclosures protect a motor from the ingress of liquids and substances that can harm a motor while also allowing air to circulate. Certain types of locations will require a specific motor enclosure.

There are many types of motor enclosures, with the most common being the open enclosure. Other enclosure types include drip-proof, open drip proof, totally enclosed, and totally-enclosed fan cooled (TEFC).

Open Motor

An open motor is one having ventilating openings which permit passage of external cooling air over and around the windings of the motor. The term open designates a motor having no restriction to ventilation other than that necessitated by mechanical construction.

Drip-Proof Motor

Like an open motor, a drip-proof motor has ventilating openings. However, these openings in drip-proof motors are made so that successful operation is not interfered with when drops of liquid or solid particles strike or enter the enclosure at any angle from 0 to 15 degrees downward from the vertical.

Open Drip-Proof Motor

Open drip proof motors have venting in the end frame and/or main frame that prevents drops of liquid from falling into the motor within a 15 degree angle from the vertical. ODP motors are designed for use in areas that are reasonably dry, clean, well-ventilated, and usually indoors. If installed outdoors, ODP motors should be protected with a cover that does not restrict air flow.

Totally-Enclosed Motor

A totally-enclosed motor is one constructed so the enclosure prevents the free exchange of air between the inside and outside of the case but is sufficiently enclosed to be termed air-tight.

Totally-Enclosed Fan Cooled Motor

A totally-enclosed fan-cooled motor (or TEFC motor) is equipped for exterior cooling by means of a fan (or fans) integral to the motor but external to the enclosing parts.

Other motor enclosure types include:

• Explosion-Proof
• Totally-Enclosed Nonventilated
• Dust-Ignition-Proof
• Totally-Enclosed Fan-Cooled Guarded
• Totally-Enclosed Air-Over (TEAO)
   

Electric Motor Mounts

Mountings hold electric motors in place. The four most frequently mounting methods are:

• Rigid Mounts
• Resilient Mounts
• Stud Mounts
• Flange Mounts

Rigid Motor Mounts

Rigid mounts are the most common type of motor mounting, a rectangular steel plate conforming to the shape of the bottom of the case or frame of the motor. Rigid mounts get their name because the frame of the motor is welded to the base.

Resilient Motor Mounts

Resilient mounts are another very common type of motor mounting. Resilient mounts feature rubber rings fitting over cylindrical notched parts of the end bells. The cylindrical parts support the weight of the motor while the notches keep it from turning within the rubber rings. These rings are bonded to a metal or molded plastic band surrounding the outside diameter of the rubber. In turn, the metal or molded plastic bands are supported by a U-shaped steel support base and are firmly held in place with a simple screw- operated clamp arrangement.

Resilient mounting is one method of isolating the vibration or noise caused by the motor drives. The rubber rings quiet and isolate the motor from its surroundings. They also afford a torque cushion for the motor, lessening the shock caused by starting or stopping.

Stud Motor Mounts

Stud mounts are typically found on small fan motor applications. The end bells of these motors are held together by two or more long bolts that project approximately 1 inch from the end bell at the shaft end of the motor. These bolt ends are secured to the cross bar of the fan housing with a nut and lock-washer.

Flange Motor Mounts

Flange mounts usually require an end bell with a special configuration, most commonly a machined flat surface perpendicular to the axis of the motor shaft and a machined pilot fit concentric with the shaft. It may have two or more holes (plain or tapped) through which bolts are secured. The most typical flange mountings are the oil burner and NEMA flanges.

Motor End Bells

Motor end bells are known by several names, including end shields, cover plates, end brackets, and covers. No matter the name, they are a part of the housing that support the bearings and protect other internal components.

End bells must fulfill several very important physical requirements. They must be strong enough to support the motor shaft bearings under the most severe load conditions, and they must be rigid to maintain alignment of the bearing bores throughout the life of the motor.

Weights mounted on the shaft or hanging from the shaft, such as chain and sprocket or belt and sheave drives, must be supported by the output shaft end bell. This type of overhung loading usually determines the sizes of the new shaft bearing and the shape of the end bell.

The end bells also center the rotor (or armature) accurately within the stator to maintain a constant air gap between the stationary and moving cores.

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Single Phase Vs. Three Phase Motors

Single phase motors receive their power from one AC line. Three phase motors receive their power from three single-phase lines. Each of these lines is one third of a cycle out of phase from the one before it. This produces a more consistent power level.

The main advantages of three phase motors are in power rates and motor construction. Three phase motors offer improved efficiency and power factor with lower power rates. Additionally, the lack of capacitors and internal mechanical switches makes them more reliable and lighter. Three phase motors are self-starting and require less starting current. Three phase motors are also more easily reversed by switching any two of the three power supply leads.

Permanent Magnet Motors (PMAC Motors)

Permanent magnet motors use powerful permanent magnets in place of rotor windings and brushes traditionally found within standard induction electric motors. This feature gives PMAC motors a simplified design. The powerful permanent magnets create a constant flux in the air gap (making the rotor windings brushes unnecessary). Replacing the windings and brushes with permanent magnets results in the accurate and controllable performance of a synchronous motor with the robust design of a standard AC induction motor.

This internal design gives permanent magnet motors benefits in motor life, energy efficiency, and speed control.

Permanent magnet motors are ideal in applications where variable speed and torque requirements are essential. You can find permanent magnet motors in industrial manufacturing, renewable energy and HVAC systems.

2. What Horsepower and Speed are Required?

Horsepower is the capability to do a given amount of work. Motors are rated in horsepower (or watts), either fractional or integral. 746 Watts = 1 HP. Horsepower rating depends upon speed, so both of these factors must be determined. Most small motors are either 2 pole (3450 rpm), 4 pole (1725 rpm), 6 pole (1140 rpm), or 8 pole (850 rpm).

The first thing to determine is the desired full load speed, and then the desired horsepower at that speed.

Matching these specifications to your needs is critical for optimal performance. For example, a motor with insufficient horsepower may overheat and fail, while one with too much horsepower can be inefficient and costly.

3. What type of motor will best do the job?

Different applications require different types of motors. Here are some common types:

• Shaded Pole
• Split Phase
• Capacitor Start
• Two-Capacitor
• Permanent Split Capacitor
• Three Phase
• Electronically Commutated Motor (ECM or EC motors)
• Permanent Magnet AC (PMAC)

In most cases, the best type of motor for an application depends on the application. Each type of motor has its advantages and is suited for specific tasks.

For many applications, especially for HVAC motors, users have been turning to EC motors, which have computerized control technology. EC motors are versatile and can provide a range of speeds depending on the application. These motors excel at meeting energy efficiency requirements which require motors to operate at certain levels.

Applications in the food and beverage industry will often require the motor be able to withstand washdown procedures. So you’ll want to look for a motor with a specific ingress protection rating.

Other applications may require the motor be placed in a high-temperature environment. In that case, choose a motor with a temperature rating that suits the specific application.

Motor Type and Torque

The two factors that are probably most important in determining which type of motor to use are the type of torque requirements of the load and the starting current limitations.

Many applications like pumps and compressors may require more torque to start and accelerate the load than to run it. Others, such as direct-connected propeller fans, may require more torque to run the load than to start it.

Motor torque characteristics must match those of the load. Consider load requirements in terms of starting torque and breakdown torque.

4. What Frame Size Is Needed?

The motor frame size determines its physical dimensions and mounting configuration. Ensure that the motor you select fits within the space available and can be mounted securely. The frame size also affects the motor’s cooling and overall performance, so it’s essential to choose the correct size for your application.

The most common small motor frame sizes are the 42, 48 and 56 frames. NEMA has set certain standards for the dimensions of these frame sizes and most manufacturers build their motors to conform to these standards.

6. What type of Bearing is Required?

Bearings are crucial for the smooth operation of the motor. The type of bearing needed depends on the load and speed requirements. Choosing the right bearing is crucial. Bearings will fail due to excess heat or if contaminants are able to get into the bearings. If a bearing fails, they can be costly to replace. 

7. In What Direction Is the Motor Going to Rotate?

Even though the rotation of many small motors can be reversed, it’s still an important question. Determine the rotation by looking at the end of the motor opposite the shaft extension end. Motors that rotate clockwise will have the notation “CW” and motors that rotate counterclockwise will be notated “CCW.”

If the motor will be mounted in the vertical position, then care must be taken to see that the motor has either a ball bearing or sleeve bearing designed to operate in the vertical position. Some motors, such as the Flex-in-1 motor from LEESON Motors, offer multiple mounting options, allowing the motor to be mounted in a variety of ways. Another option is a direct drive motor, which can be directly connected to the load without requiring additional transmission components. If the motor does not have such flexibility, users can purchase accessories to ensure proper installation. Some accessories include:

Pedestals: Often found in farm and barn applications.

Belly Bands: Used in HVAC applications, belly bands allow a motor to be placed within the airflow of HVAC units.

9. Will Thermal Protection Be Needed?

In many applications, heat is the greatest danger to a motor. Heat can wear down a motor’s grease and bearings and destroy insulation. All of this can cause a motor to overheat, requiring costly repairs or replacement. Thermal protection is essential to prevent damage in the event of overheating.

For a motor, ambient temperature (also known as room temperature) is the temperature of the air which, when coming into contact with the heated parts of a motor, carries off its heat. When selecting a motor, this will tell you the type of environment in which a motor can operate.

If your application environment will have high heat, you will need to consider thermal protection. Some motors come with built-in thermal protection, while others may require external protection devices.

Motors with built-in thermal protection feature a temperature sensitive element which activates a switch. This switch will stop the motor if the motor reaches the pre-set temperature limit.

Thermal Protection Switches

Two major types of thermal protection switches are available: automatic reset and manual reset.

Automatic reset, which automatically resets the switch, will re-start the motor when the temperature has been reduced.

Manual Reset is usually is in the form of a small pushbutton on the end of the motor opposite the shaft. When the motor has cooled sufficiently, the operator pushes the button, restarting the motor.

The type of reset chosen will often depend on the application. Automatic reset protection would be very dangerous on a drill press, for example, because the motor may have cooled to the point where it will automatically re-start, just as the operator is loosening the chuck with a chuck key. For applications that involve drives of power tools which are exposed, or could be harmful if started without warning, always use manual restart. However, by the same token, a fan or furnace blower motor would be well to have automatic protection.

Insulation Ratings

Motors with built-in insulation systems are grouped into temperature tolerance classes.

Class A: Uses materials with a preferred temperature index of 105˚Celsius.

Class B: Uses materials with a preferred temperature index of 130˚Celsius.

Class F: Uses materials with a preferred temperature index of 155˚Celsius.

Class H: Uses materials with a preferred temperature index of 180˚Celsius.

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