Motors and Generators:NEMA Classification of Motors and Generators

NEMA Classification of Motors and Generators

Under NEMA MG1 standards, all machines are classified according to size, application, electrical type, environmental protection and method of cool- ing, and variability of speed. A machine is an electrical apparatus which depends on electromagnetic induction for its operation and which has one or more component members capable of rotary movement. These machines are generally referred to as motors and generators.

According to Size

• Small (fractional) machine:

A machine built with frame size having two-digit numbers or a machine built in a frame smaller than that of an integral-horsepower motor.

• Medium (integeral) machine:

is an alternating-current (AC) or direct current (DC) medium machine, (1) built in a three-or four-digit frame number series. The AC medium machines have synchronous speed from 451 to 3600 RPM, continuous rating from 125 HP up to 500 HP for motors, and 100 kW up to 400 kW for generators. The DC medium machines have continuous rating up to and including 1.25 HP per rpm for motors or 1.0 kW per rpm for generators.

• Large machine:

An AC large machine is a machine having a continuous power rating greater than that of medium machine for synchronous speed ratings above 450 rpm; or having a continuous power rating greater than that of small machine for synchronous speed ratings equal to or below 450 rpm.

A direct-current large machine is a machine having a continuous rating greater than 1.25 horsepower per rpm for motors or 1.0 kilowatt per rpm for generators.

According to Application

• General-purpose AC motor:

An induction motor, rated at 200 hp or less, and of open construction; it is continuously rated, has a service factor of 1.15 for integral-horsepower motors, and has class B insulation system.

• General-purpose DC motor:

An integral-horsepower motor of mechanical construction suitable for industrial applications under usual service conditions and has ratings and constructional and performance characteristics applying to direct-current small motors.

• General-purpose generator:

is a synchronous generator of mechanical construction suitable for general use and has ratings and constructional characteristics for performance under usual service conditions.

• Industrial DC generator:

A generator of mechanical construction suitable for industrial application under usual conditions.

• Definite-purpose motor:

A motor designed and constructed in standard ratings for service conditions other than usual or for use on a particular type of application.

• Part-winding-start motor:

A motor arranged to start with part of the winding, subsequently energizing the remainder of the winding in one or more steps.

• Special-purpose motor:

A motor with special characteristics and/or mechanical construction and not falling under the definition of a general-purpose and definite-purpose motor.

• General DC industrial motor:

Motors designed for all general industrial service with speed operation (when specified) above base speed by field weakening.

• Metal rolling mill motor:

Motors designed for metal rolling mill service and known as class N or S types.

• Reversing hot mill motor:

hot mills.

Motors designed for application to reversing According to Electrical Type

AC Motors

AC motors are of three types: induction motors, synchronous motors, and series motors. They can be defined as follows:

• Induction motor:

An AC motor in which the primary winding (the stator) is connected to the electric power source and the secondary winding (the rotor) carries induced current. Induction motors are of two types: squirrel-cage induction motors and wound-rotor motors. In the squirrel-cage induction motor, the secondary circuit consists of a squirrel-cage winding suitably dispersed in slots in the secondary core. In the wound-rotor induction motor, the secondary circuit consists of polyphase windings or coils whose terminals are either short circuited or closed through suitable external circuits.

• Synchronous motor:

An induction motor that is equipped with field windings in the secondary circuit and excited with DC voltage. The synchronous motor is started as an induction motor and synchronized with the rotating magnetic field after the rotor circuit reaches excitation at the appropriate time. It is operated at synchronous speed (i.e., at the speed of the rotating magnetic field).

• Series-wound motor:

A motor in which the field circuit and armature circuit are connected in series.

Both induction and synchronous motors are built as polyphase or single- phase motors.

Polyphase Motor

Polyphase motors are constructed with multiphase stator windings and rotor. The rotor is constructed in two types: the cage rotor and the form-wound rotor. Both types of rotor have a laminated cylindrical core with parallel slots in the outside circumference to hold the windings in place. The cage rotor has an uninsulated bar winding, whereas the form-wound rotor has a two-layer distributed winding with preformed coils. In the polyphase motor, the rotor currents are supplied by electromagnetic induction. The stator windings contain two or more out-of-time-phase currents, which produce corresponding magnetomotive forces (mmfs). These mmfs establish a rotating magnetic field across the air gap. This magnetic field rotates continuously at constant speed regardless of the load on the motors. The revolving magnetic field produced by the stator cuts across the rotor conductors, inducing a voltage in the conductors. This induced voltage causes rotor currents to flow. This action is known as mutual induction (similar to transformer action), which takes place between the stator and the rotor under operating conditions.

Polyphase motors range in horsepower rating from fractional- to integral- horsepower to large-apparatus motors. The fractional- and integral-horse- power motors are generally cage-rotor type. Large-apparatus induction motors are of cage-and would-rotor types, where the synchronous motors are of the salient pole and cylindrical rotor type.

In accordance with NEMA standards, polyphase squirrel-cage integral- horsepower induction motors are designated by design letters:

• Design A:

A squirrel-cage motor designed to withstand full-voltage starting and develop a starting torque of 110%–120%, starting locked rotor current of 6–10 times rated, and having a slip at rated load of less than 5%.

• Design B:

Similar to design A motor with the same starting torque, however, the locked rotor current is limited to five times.

• Design C:

A squirrel-cage motor designed to withstand full-voltage starting, developing a high starting torque of 200% and locked rotor current less than the standard type of motor, and having a slip at rated load of less than 5%.

• Design D:

A squirrel-cage motor designed to withstand full-voltage starting, developing a very high locked rotor torque of 300%, low lock rotor current, and having a slip at rated load of 5% or more.

• Design F:

A squirrel-cage motor built to withstand full-voltage starting, developing a low starting torque, very low locked rotor current, and a slip at rated load of less than 5%.

Single-Phase Motor

Single-phase motors are not self-starting because they have only one primary (stator) winding and cage rotor. The single primary winding when excited from a single source produces a pulsating magnetic field in the motor air gap, and with the rotor at standstill no breakaway torque is produced. However, if the rotor is brought up to speed by external means, the induced currents in the rotor will combine with the stator currents to produce a revolving field. The revolving field in turn causes the rotor to continue to run in the direction in which it was started. Several methods are used to provide the single-phase induction motor with starting torque. These methods identify the motor as a particular type of single-phase motor. Some of the important single-phase motors are split-phase, capacitor start and run, repulsion, and shaded pole. Single-phase motors are constructed as induction, wound rotor, and synchronous motor types.

Alternating single-phase motors designated by design letters similar to polyphase motors. These design letters are the following:

• Design N:

A single-phase fractional-horsepower motor designed to withstand full-voltage starting and with a locked rotor current not to exceed the values shown in NEMA standard MG1.

• Design O:

A single-phase fractional-horsepower motor designed to withstand full-voltage starting and with a locked rotor current not to exceed the values shown in MG1.

• Design L:

A single-phase integral-horsepower motor designed to withstand full-voltage starting and to develop a breakdown torque as shown in NEMA standards MG1 and locked rotor current not to exceed values shown in MG1.

• Design M:

Universal Motor

A universal motor is a series-wound motor designed to operate at approxi- mately the same speed and output on either DC or single-phase AC of fre- quency not to exceed 60 Hz. There are two types of universal motors:

• Series-wound motor:

A commutator motor in which the field circuit and armature circuit are connected in series.

• Compensated series motor:

A motor with compensating field winding.

DC Motors

DC motors are of three types:

• Shunt–wound motor:

A shunt-wound motor is either a straight shunt-wound motor or a stabilized shunt-wound motor. The difference between the two shunt fields is that the stabilized shunt-wound motor has a light series winding to prevent a rise in speed or to obtain a slight reduction in speed from no-load to full-load conditions.

• Series-wound motor:

A series-wound motor has the field circuit and armature circuit in series. The torque and speed are load dependent.

Generally, a series-wound motor should not be operated at full volt-age while uncoupled from its load.

• Compound-wound motor:

A compound-wound motor has two separate field windings, one connected in the shunt field and the other connected in series in the armature.

DC Generators

DC generators are of two general types:

• Shunt–wound generator:

A generator in which the field is connected in parallel with the armature or to a separate source of excitation.

• Compound-wound generator:

A generator that has two separate field windings, one usually the predominating field, connected in parallel with the armature and the other connected in series with the armature.

According to Physical Protection (Enclosure) and Methods of Cooling

The machine is provided with an enclosure to give physical protection from external sources of motor damage. The following standard enclosures have been adopted by NEMA:

• Open enclosure:

An enclosure with ventilating openings that permit passage of external cooling air over and around the windings of the machine.

• Drip-proof enclosure:

An open enclosure in which ventilating openings are so constructed that successful operating is not interfered with when drops of liquid or solid particles strike or enter the enclosure at any angle from 0° to 15° downward from the vertical.

• Splash-proof enclosure:

An open enclosure in which ventilating openings are constructed so that successful operation is not interfered with when drops of liquid or solid particles strike or enter the enclosure at any angle not greater than 100° downward from the vertical.

• Guarded enclosure:

An open enclosure in which all openings giving direct access to live metal or rotating parts are limited in size by structural parts or by screens, baffles, grilles, or other means to prevent accidental contact with hazardous parts.

• Externally ventilated enclosure:

An open closure that is ventilated by a separate motor-driven blower mounted on the enclosure.

• Pipe–ventilated enclosure:

An open enclosure with provision for connecting inlet ducts or pipes. It is called force-ventilated when the air through the enclosure is driven by an external blower.

• Weather–protected type 1 enclosure:

An open enclosure with ventilating passages constructed and arranged to minimize the entrance of rain, snow, and airborne particles to the live and rotating parts.

• Weather-protected type 2 enclosure:

An open enclosure with ventilating passages at both intake and discharge constructed and arranged to permit high-velocity air and airborne particles to be discharged without entering the internal ventilating passages of the enclosure.

• Totally enclosed enclosure:

This enclosure prevents free exchange of air between the inside and outside of the enclosure. This enclosure is not airtight.

• Totally enclosed nonventilated enclosure:

An enclosure that is not

equipped for cooling by means external to the enclosing parts.

• Totally enclosed fan-cooled enclosure:

An enclosure that is equipped for exterior cooling by means of a fan or fans integral with the enclosure but external to the enclosing parts.

• Explosion-proof enclosure:

A totally enclosed enclosure designed and constructed to withstand an explosion of a specified gas or vapor that may occur within it and to prevent the ignition of gas or vapor surrounding the machine by sparks.

• Dust-ignition-proof enclosure:

A totally enclosed enclosure constructed in a manner to exclude ignitable amounts of dust or amounts that might affect the performance or rating, and which will not permit heat, arcs, or sparks liberated inside the enclosure to cause ignition of exterior accumulations or atmospheric suspensions of a specific dust on or in the vicinity of the enclosure.

• Waterproof enclosure:

A totally enclosed enclosure so constructed that it will exclude water coming externally from a hose. Leakage around the shaft is allowed provided it does not enter the oil reservoir. A check valve or drain is provided at the lowest part of the enclosure for drainage.

• Totally enclosed pipe-ventilated enclosure:

A totally enclosed enclosure except for openings arranged for inlet and out-ducts or pipes for connection to the enclosed for admission and discharge of ventilating air.

• Totally enclosed water-cooled enclosure:

A totally enclosed enclosure cooled by circulating water or water pipes coming in direct contact with the motor parts.

• Totally enclosed water–air-cooled enclosure:

A totally enclosed enclosure cooled by circulating air, which in turn is cooled by circulating water, the heat exchanger medium.

• Totally enclosed air–air-cooled enclosure:

A totally enclosed enclosure cooled by circulating internal air through heat exchangers, which, in turn, are cooled by circulating external air.

According to Variability of Speed

• Constant-speed motor:

A motor that operates at constant or near constant speed, from no load to full load.

• Varying-speed motor:

A motor whose speed varies with the load, ordinarily decreasing as the load increases.

• Adjustable-speed motor:

A motor whose speed is adjustable over a considerable range and is not affected by load.

• Adjustable varying-speed motor:

An adjustable-speed motor asdescribed previously, but whose speed will vary as a function of load.

• Multispeed motor:

A motor that can operate at one or two or more definite speeds, each speed being practically independent of the load.

Terminal Marking of Machines

The terminal markings of machines are made in accordance with NEMA standards. Terminal markings use a combination of capital letters and Arabic numerals. The letters and symbols as shown in Table 10.1 are used for AC motors and generators in accordance with NEMA standards. (Refer to NEMA MG1-2006 for A.C. Machines Terminal Markings.)

The terminal markings and connection procedure for single-phase motors is based upon three general principles:

• Principle 1:

The main winding of a single-phase motor is designated by

T1, T2, T3, and T4. The auxiliary winding is designated by T5, T6, T7, and T8. This is done to distinguish it from a quarter-phase motor, which uses odd numbers for one phase and even numbers for the other phase.

• Principle 2:

When odd-numbered terminals of each winding are connected together, they will provide a lower voltage, that is, a parallel connection. When odd-to-even-numbered terminals of each winding are connected to odd-numbered, they will provide a higher voltage, that is, a series connection.

• Principle 3:

The rotor of a single-phase motor is represented as a circle, even though there are no external connections to it. It also

serves to distinguish it from a quarter-phase motor in which the rotor is never represented.

Based upon these three principles, the single- and dual-voltage single-phase motors can be represented as in the following sections.

Single-Voltage Motors

T1 and T4 are assigned to the main winding and T5 and T8 to the auxiliary winding. The standard direction is obtained when T4 and T5 are joined to one line and T1 and T8 to the second line. This is shown in Figure 10.1.

Dual-Voltage Motors

For the purposes of terminal markings, the main winding is considered to be divided in two halves. One half is assigned T1 and T2, and the other half is assigned T3 and T4. Similarly, the auxiliary winding is divided into two halves with one half assigned terminal markings T5 and T6, and the other half T7 and T8. The standard direction of rotation is obtained when the main winding terminal T4 and auxiliary winding terminal T5 are connected or when an equivalent circuit connection is made between the main and the auxiliary windings. The terminal marking is shown in Figure 10.2.

Polyphase Motors

The marking of polyphase motors is based on the principle that they show the electrical relationship between the various circuits inside the motor.

Therefore, the NEMA standards employ a system that uses a clockwise rotating spiral with T1 and the outer end and finishes with the highest number at its inner end as a means of determining the sequence of numerals. Such a numbering system does not imply standardization of the direction of rotation of the motor shaft. This principle will now be used to show the terminal marking of a three-phase single-speed induction motor in Figure 10.3a through f.

Step 1: Draw a schematic vector diagram showing an inverted wye connection with two individual circuits in each phase arranged for series connection with correct polarities.

Step 2: Starting with T1 at the outside and top of the diagram, number the ends of the circuits consecutively in a clockwise direction, proceeding on a spiral toward the center of the diagram. This is shown in Figure 10.3b.

Step 3: Show the schematic vector diagram of the particular interconnection of the circuits for the (two circuits per phase) motor and terminal markings as determined in Steps 1 and 2. Arrange the vector diagram to give the correct polarity relation of the circuits. For example, connect the two circuits in parallel per phase; the vector diagram is shown in Figure 10.3c.

Step 4: When two (or more) terminals are permanently connected together, the highest terminal number is dropped and only the lowest number is retained. In our example, suppose that it is desired to have three line leads and three neutral leads brought out; the terminal markings are as shown in Figure 10.3d. If it was desired to have the windings in series or multiple connections with the neutral point brought out, the vector diagram and terminal markings are as shown in Figure 10.3e.

Step 5: Where the ends of three coils are connected together to form a permanent neutral, the terminal markings of the three leads so connected are dropped. If the neutral point is brought out, it is always marked as T0.

Step 6: Where the windings are to the connected delta, the inverted-wye diagram shown in Figure 10.3a is rotated 30° counterclockwise. The outer end of the top leg is assigned the terminal marking T1 and the remaining windings are numbered in accordance with Step 2. The vector schematic is then constructed in which the T1 leg of the rotated delta becomes the right side of the delta, the T2 becomes the bottom (horizontal side), and T3 the left side of the delta. This is shown in Figure 10.3f.

Many polyphase motors have dual voltage and are connected as wye or delta connections. The same principles as discussed can be applied to achieve the terminal markings for these types of motors. The wye and delta connections for dual-voltage motors are shown in Figure 10.4a and b.