The motor nameplate (Figure 2-38) contains important information about the connection and use of the motor. An important part of making motors interchangeable is ensuring that nameplate information is common among manufacturers
NEC Required Nameplate Information
MOTOR MANUFACTURE
This will include the name and logo of the manufacturer along with catalog numbers, parts numbers, and model numbers used to identify a motor. Each manufacturer uses a unique coding system.
VOLTAGE RATING
Voltage rating is abbreviated V on the nameplate of a motor. It indicates the voltage at which the motor is designed to operate. The voltage of a motor is usually determined by the supply to which it is being attached. NEMA requires that the motor be able to carry its rated horsepower at nameplate voltage ±10 percent although not necessarily at the rated temperature rise. Thus, a motor with a rated nameplate voltage of 460 V should be expected to operate successfully between 414 V and 506 V.
The voltage may be a single rating such as 115 V or, for dual-voltage motors, a dual rating such as 115 V/230 V. Most 115/230 V motors are shipped from the factory connected for 230 V. A motor connected for 115 V that has 230 V applied will burn up immediately. A motor connected for 230 V that has 115 V applied will be a slow-running motor that overheats and trips out.
NEMA standard motor voltages are:
- Single-phase motors 115, 230, 115/230, 277, 460, and 230/460 V
- Three-phase motors up to 125 hp—208, 230, 460, 230/460, 575, 2,300, and 4,000 V
- Three-phase motors above 125 Hp—460, 575, 2,300, and 4,000 V
When dealing with motors, it is important to distinguish between nominal system and nameplate voltages. Examples of the differences between the two are as follows:
CURRENT RATING
The nameplate current rating of a motor is abbreviated A or AMPS. The nameplate current rating is the full-load current (also known as FLA) at rated load, rated voltage, and rated frequency. Motors that are not fully loaded draw less than the rated nameplate current. Similarly, motors that are overloaded draw more than the rated nameplate current.
Motors that have dual voltage ratings also have dual current ratings. A dual-voltage motor operated at the higher voltage rating will have the lower current rating. For example, a ½ hp motor rated 115/230 V and 7.4/3.7 A will have a rated current of 3.7 A when operating from a 230 V supply.
LINE FREQUENCY
The line frequency rating of a motor is abbreviated on the nameplate as CY or CYC (cycle), or Hz (hertz). A cycle is one complete wave of alternating voltage or current. Hertz is the unit of frequency and equals the number of cycles per second. In the United States, 60 cycles/second (Hz) is the standard, while in other countries 50 Hz (cycles) is more common.
PHASE RATING
The phase rating of a motor is abbreviated on the nameplate as PH. The phase rating is listed as direct current (DC), single-phase alternating current (1ϕ AC), or three-phase alternating current (3ϕ AC).
MOTOR SPEED
The rated speed of a motor is indicated on the nameplate in revolutions per minute (rpm). This rated motor speed is not the exact operating speed, but the approximate speed at which a motor rotates when delivering rated horse-power to a load.
The number of poles in the motor and the frequency of the supply voltage determine the speed of an AC motor. The speed of a DC motor is determined by the amount of supply voltage and/or the amount of field current.
AMBIENT TEMPERATURE
The ambient temperature rating of a motor is abbreviated AMD or DEG on the nameplate of a motor. Ambient temperature is the temperature of the air surrounding the motor. In general, maximum ambient temperature for motors is 40°C or 104°F unless the motor is specifically designed for a different temperature and indicates so on its nameplate.
Motors operating at or near rated full load will have reduced life if operated at ambient temperatures above their ratings. If the ambient temperature is over 104°F, a higher-horsepower motor or a special motor designed for operation at higher ambient temperatures must be used.
TEMPERATURE RISE
A motor’s permissible temperature rise is abbreviated Deg.C/Rise on the nameplate of the motor. This indicates the amount the motor winding temperature will increase above the ambient temperature because of the heat from the current drawn by the motor at full load. It can also be thought of as the amount by which a motor operating under rated conditions is hotter than its surrounding temperature.
Thermal imagers (also known as infrared cameras or infrared imagers) capture images of infrared energy or temperature. Thermal images of electric motors (Figure 2-39) reveal their temperature operating conditions as reflected by their surface temperature. While the infrared camera cannot see the inside of the motor, the exterior surface temperature is an indicator of the internal temperature. As the motor gets hotter inside, it also gets hotter outside. Such condition monitoring is one way to avert many unexpected motor malfunctions in systems.
INSULATION CLASS
Motor insulation prevents windings from shorting to each other or to the frame of the motor. The type of insulation used in a motor depends on the operating temperature the motor will experience. As the heat in a motor increases beyond the temperature rating of the insulation, the life of the insulation and of the motor is shortened.
Standard NEMA insulation classes are given by alphabetic classifications according to their maximum temperature rating. A replacement motor must have the same insulation class or a higher temperature rating than the motor it is replacing. The four major NEMA classifications of motor insulation are as follows:
DUTY CYCLE
The duty cycle is listed on the motor nameplate as DUTY, DUTY CYCLE, or TIME RATING. Motors are classified according to the length of time they are expected to operate under full load as either continuous duty or intermittent duty. Continuous duty cycle–rated motors are identified as CONT on the nameplate, while intermittent-duty cycle motors are identified as INTER on the nameplate.
Continuous-duty motors are rated to operate continuously without any damage or reduction in the life of the motor. General-purpose motors will normally be rated for continuous duty. Intermittent-duty motors are rated to operate continuously only for short time periods and then must be allowed to stop and cool before restarting.
HORSEPOWER RATING
The horsepower rating of the motor is abbreviated on the nameplate as HP. Motors below 1 horsepower are referred to as fractional-horsepower motors and motors
horsepower and above are called integral-horsepower motors. The HP rating is a measure of the full load out-put power the shaft of the motor can produce without reducing the motor’s operating life. NEMA has established standard motor horsepower ratings from 1 hp to 450 hp.
Some small fractional-horsepower motors are rated in watts (1 hp = 746 W). Motors rated by the International Electrotechnical Commission (IEC) are rated in kilowatts (kW). When an application calls for a horsepower falling between two sizes, the larger size is chosen to provide the appropriate power to operate the load.
CODE LETTER
An alphabetic letter is used to indicate the National Elec-tric Code Design Code letter for the motor. When AC motors are started with full voltage applied, they draw a starting or “locked-rotor” line current substantially greater than their full-load running current rating. The value of this high current is used to determine circuit breaker and fuse sizes in accordance with NEC requirements. In addition, the starting current can be important on some installations where high starting currents can cause a voltage dip that might affect other equipment.
Motors are furnished with a code letter on the nameplate that designates the locked-rotor rating of the motor in kilovolt-amperes (kVA) per nameplate horsepower. Code letters from A to V are listed in Article 430 of the National Electrical Code. As an example, an M rating allows for 10.0 to 11.19 kVA per horsepower.
DESIGN LETTER
Design B is the standard industrial-duty motor, which has reasonable starting torque with moderate starting current and good overall performance for most industrial applications.
The design letter is an indication of the shape of the motor’s torque–speed curve. The most common design letters are A, B, C, D, and E.
Optional Nameplate Information
SERVICE FACTOR
Service factor (abbreviated SF on the nameplate) is a multiplier that is applied to the motor’s normal horse-power rating to indicate an increase in power output (or overload capacity) that the motor is capable of providing under certain conditions. For example, a 10 hp motor with a service factor of 1.25 safely develops 125 percent of rated power, or 12.5 hp. Generally, electric motor service factors indicate that a motor can:
- Handle a known overload that is occasional.
- Provide a factor of safety where the environment or service condition is not well defined, especially for general-purpose electric motors.
- Operate at a cooler-than-normal temperature at rated load, thus lengthening insulation life.
Common values of service factor are 1.0, 1.15, and 1.25. When the nameplate does not list a service factor, a service factor of 1.00 is assumed. In some cases, the running current at service factor loading is also indicated on the nameplate as service factor amperes (SFA).
MOTOR ENCLOSURE
The selection of a motor enclosure depends on the ambient temperature and surrounding conditions. The two general classifications of motor enclosures are open and totally enclosed. An open motor has ventilating openings, which permit passage of external air over and around the motor windings. A totally enclosed motor is constructed to prevent the free exchange of air between the inside and outside of the frame, but not sufficiently enclosed to be termed airtight (Figure 2-40).
FRAME SIZE
Refers to a set of physical dimensions of motors as established by NEMA and IEC. Frame sizes include physical size, construction, dimensions, and certain other physical characteristics of a motor. When you are changing a motor, selecting the same frame size regardless of manufacturer ensures the mounting mechanism and hole positions will match.
Dimensionally, NEMA standards are expressed in English units (Figure 2-41) and IEC standards are expressed in metric units. NEMA and IEC standards both use letter codes to indicate specific mechanical dimensions, plus number codes for general frame size.
EFFICIENCY
Efficiency is included on the nameplate of many motors. The efficiency of a motor is a measure of the effectiveness with which the motor converts electrical energy into mechanical energy. Motor efficiency varies from the nameplate value depending on the percentage of the rated load applied to the motor. Most motors operate near their maximum efficiency at rated load.
Energy-efficient motors, also called premium or high-efficiency motors, are 2 to 8 percent more efficient than standard motors. Motors qualify as “energy efficient” if they meet or exceed the efficiency levels listed in the NEMA’s MG1 publication. Energy-efficient motors owe their higher performance to key design improvements and more accurate manufacturing tolerances.
POWER FACTOR
The letters P.F. when marked on the nameplate of motors stand for power factor. The power factor rating of a motor represents the motor’s power factor at rated load and voltage. Motors are inductive loads and have power factors less than 1.0, usually between 0.5 and 0.95, depending on their rated size. A motor with a low power factor will draw more current for the same horsepower than a motor with a high power factor. The power factor of induction motors varies with load and drops significantly when the motor is operated at below 75 percent of full load.
THERMAL PROTECTION
Thermal protection, when marked on the motor nameplate, indicates that the motor was designed and manufactured with its own built-in thermal protection device. There are several types of protective devices that can be built into the motor and used to sense excessive (overload) temperature rise and/or current flow. These devices disconnect the motor from its power source if they sense the overload to prevent damage to the insulation of the motor windings.
The primary types of thermal overload protectors include automatic and manual reset devices that sense either current or temperature. With automatic-reset devices, after the motor cools, this electrical circuit-interrupting device automatically restores power to the motor. With manual reset devices, the electrical circuit–interrupting device has an external button located on the motor enclosure that must be manually pressed to restore power to the motor. Manual reset protection should be provided where automatic restart of the motor after it cools down could cause personal injury should the motor start unexpectedly. Some low-cost motors have no internal thermal protection and rely on external protection between the motor and the electrical power supply for safety.
CONNECTION DIAGRAMS
Connection diagrams can be found on the nameplate of some motors, or the diagram may be located inside the motor conduit box or on a special connection plate. The diagram will indicate the specific connections for dual-voltage motors (Figure 2-42). Some motors can operate in either direction, depending on how the connections to the motor are made, and this information may also be given on the nameplate.
Guide to Motor Terminology
Terminology is of the utmost importance in understanding electrical motor control. Common motor control terms are listed below. Each of these terms will be discussed in detail as they are encountered in the text.
- Across-the-line A method of motor starting. Connects the motor directly to the supply line on starting or running. (Also called full voltage.)
- Automatic starter A self-acting starter. Completely controlled by the master or pilot switch or some other sensing device.
- Auxiliary contact The contact of a switching device in addition to the main circuit contacts (Figure 2-43). Operated by the contactor or starter.
- Contactor A type of relay used for power switching. Jog Momentary operation. Small movement of a driven machine.
- Locked-rotor current Measured current with the rotor locked and with rated voltage and frequency applied to the motor.
- Low-voltage protection (LVP) Magnetic control only; not automatic restarting. A three-wire control. A power failure disconnects service; when power is restored, manual restarting is required.
- Low-voltage release (LVR) Magnetic control only; automatic restarting. A two-wire control. A power failure disconnects service; when power is restored, the controller automatically restarts.
- Magnetic contactor A contactor that is operated electromechanically.
- Multispeed starter An electric controller with two or more speeds (reversing or nonreversing) and full or reduced voltage starting.
- Overload relay Running overcurrent protection. Operates on excessive current. It does not necessarily provide protection against a short circuit. It causes and maintains interruption of the motor from a power supply.
- Plugging Braking by reverse rotation. The motor develops retarding force.
- Push button A switch, Figure 2-44, that is a manually operable plunger or button for actuating a device, assembled into pushbutton stations.
- Reduced voltage starter Applies a reduced supply voltage to the motor during starting.
- Relay Used in control circuits and operated by a change in one electrical circuit to control a device in the same circuit or another circuit. Ampere rated.
- Remote control Controls the function initiation or change of electrical device from some remote point. Selector switch A manually operated switch that has the same construction as push buttons, except that rotating a handle actuates the contacts. The rotating cam may be arranged with incremental indices so that multiple positions can be used to select exclusive operations.
- Slip The difference between the actual speed (motor rpm) and the synchronous speed (rotation of the magnetic field).
- Starter An electric controller used to start, stop, and protect a connected motor.
- Timer A pilot device, also considered a timing relay, that provides an adjustable time period to perform its function. It can be motor driven, solenoid actuated, or electronically operated.
- Torque The twisting or turning force that causes an object to rotate. There are two types of torque that are considered for looking at motors: starting torque and running torque.