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must be cooled (generally by convection) to a lower sta- tor and rotor temperature prior to another attempted start. Motors that comply with API Std 541 or Std 546 usually have greater starting capabilities. This time between starts must be coordinated with the manufacturer for automatic-restart or...

must be cooled (generally by convection) to a lower sta- tor and rotor temperature prior to another attempted start. Motors that comply with API Std 541 or Std 546 usually have greater starting capabilities. This time between starts must be coordinated with the manufacturer for automatic-restart or frequentstart duty conditions. Note: Time between hot starts may exceed 1 hour. Motors driving high inertia loads, or operating under high power system voltage drops should receive’special consideration. 6.17 TEMPERATURE,VIBRATION, AND CURRENT INDICATORS Motors larger than 1,000 HP and special-purpose motors frequently require temperature, current, vibration, air flow, water flow, or differential pressure monitoring. (See API Std 541 and 546 for proper application.) 6.1 8 CONDUIT OR TERMINAL BOX Attention should be given to the size and direction of con- duit entrances to motor terminal boxes. Sizing requirements of the local electrical code should be observed. Medium and high voltage main terminal boxes may also require special construction if ANSINEMA Type II design, space for stress cone-type cable termination, or auxiliary protection devices are used. 6.19 SPACE HEATERS 6.19.1 Application In locations where motor windings are likely to be sub- jected to accumulations of excessive moisture during extended periods of idleness, consideration should be given to the installation of space heaters or direct winding heating -" control modules (which apply low power directly to the stator winding) to maintain the winding above the dew point. This applies, especially, to motors operating at greater than or equal to 2,300 V. Space heaters are particularly applicable to large totally enclosed motors installed outdoors and operated intermittently, and to vertical weather-protected motors, such as those used for water well service. Space heaters are also used in many large motors located indoors, particularly those that operate intermittently.. Some designs of totally enclosed fan-cooled motors are adaptable to space heater installations while others are not. Space heaters are also recommended for terminal boxes that enclose surge protection components or instrument transformers. 6.19.2 Installation Precautions Space heaters should be selectedmd applied in a manner that prevents unsafe surface temperatures, and they should possess the correct heater rating and element temperature as well as materials that are necessary for obtaining satisfac- tory operation and long life. Generally, sheaths made of Monel or other normally corrosion-resistant .materialp should be used. The maximum sheath temperature of space heaters must be limited to 80% of the ignition temperature of the gases or vapors expected within the area unless there are special reasons for a lower limit. It is common practice to operate space heaters at half the rated voltage (or other reduced voltages), or to specify low-surface temperature [e.g.. 200°C (392’F)I to prevent excessive temperatures and to increase heater life. Space heater leads are often wired out to a separate terminal box. 6.19.3 Ratings Space heaters usually have an operating voltage rating of 115 V or 230 V, single phase. Heating capacity should be sized to maintain the winding temperature 5°C to 10°C (10°F to 20°F) above ambient temperature. 6.1 9.4 Operation When auxilia7 contacts are used in the motor starter, the supply circuit to the motor heater is normally arranged to be de-energized automatically when the motor is started, and energized when the motor is stopped. If used, terminal box heaters are normally continuously energized or controlled by differential temperature thermostats. A local nameplate at or near the space heater auxiliary terminal box or connection point should indicate when a separately derived power source is employed. 6.19.5 Low-Voltage Winding Heating Low-voltage winding heating is a method for heating a motor winding while the motor is shut down. This heating is accomplished by applying low voltage directly to one phase of the motor winding. The amount of heating voltage neces- sary to circulate the proper current in the winding and keep the internal temperature 5°C to 10°C (10°F to 20°F) above ambient must be selected. Approximately 5% voltage is nor- mally sufficient to maintain this temperature. A low-voltage contactor must be interlocked with the main contactor to keep the two sources of power electrically separated. Low-voltage winding heating is normally used for small motors because it is usually more economical to use space heaters for motors over 100 HP. 6.20 BEARINGS AND LUBRICATION 6.20.1 Horizontal Motors Motors are available with either antifriction (ball or roller) or hydrodynamic radial (sleeve) bearings. The type of bearing lubrication, whether oil, oil mist, or grease, should be chosen when the bearings are selected. Most NEMA-kame and IEEE-841 motors will have grease-lubricated antifriction bearings. Motors above NEMA standard sizes should be designed according to API Stds 541 and 546. Most sleeve bearings for horizontal motors are oil-lubri- cated using oil rings. Except where a forced-oil lubrication system is used, the bearings should be equipped with con- stant-visible-level automatic oilers. Wick or yarn oilers are motor sizes. An opening should be provided to permit observation of the oil rings if the motor is in operation. Suitable slingers, pressure equalizers, and vents are required to prevent loss of lubricant and to maintain the proper level. For large (1,000 HF’), sleeve-bearing motors, particularly those used to drive equipment that requires forced-oil lubrica- tion, consideration should also be given to using forced-oil lubrication for the motors. API Std 614 covers lubrication systems for special drive trains. Sleeve-bearing motors, usually in the larger sizes, require the use of limited-end-float couplings to keep the motor rotors centered. When the couplings are properly installed, the motors will operate at or near their magnetic center. Ball bearings for horizontal motors are usually grease- lubricated, except in the larger sizes and in horizontal motors that operate at higher speeds. Horizontal motors operating at higher speeds often use oil-lubricated ball or roller bearings. Some manufacturers provide grease-lubricated ball-bear- ing motors with sealed bearings that permit several years of operation without regreasing. At the end of these periods, the bearings are either repacked or replaced. Because many bear- ing failures are the result of too-frequent greasing, overgreas- ing, or mixing of incompatible greases, motors which permit long periods between regreasings are the most desirable, par- ticularly in plants that lack suitable maintenance personnel and control over their regreasing programs. When oil mist lubrication is used, internal and noncontact- ing external shaft seals should be used. The seal and main lead insulation material shall be compatible with the oil. 6.20.2 Vertical Motors The thrust bearings in vertical motors include antifriction (ball or roller) and plate-type thrust bearings. When oil is used as the lubricant for either thrust or guide bearings, the oil reservoir should be deep enough to serve as a settling cham- ber for foreign matter; should be provided with drain plugs accessible from outside the motor housings; and, except where a forced-oil type of lubrication system is used, should be equipped with constant-visible-level automatic oilers. In vertical motors, it is generally preferred that all bearings use the same type of lubricant. The magnitude and direction of external thrust, operating speed, and required bearing life will determine the type of bearing used. Where required, it is common practice to supply motors that are subject to high thrust, equipped with bearings that are capable of carrying thrusts from driven equipment. The motors on vertical pumps are examples of motors equipped with bear- ings capable of carrying the high thrusts from the pump. When high-thrust driven equipment is being used, it is essential to specify the maximu thrust loads in both directions. (For ver- tical motor bearing requirements, see API Std 610.) 6.21 TORQUE REQUIREMENTS 6.21.1 Torque Considerations Most motors used in petroleum processing and associated operations drive centrifugal or rotary pumps, centrifugal blowers, centrifugal compressors, and other equipment that do not impose unusually difficult torque requirements. Nor- mal-torque motors are well-adapted to such equipment and usually will have sufficient torque to meet the normal condi- tions of service, provided the supply voltages are satisfac- tory. The net torque delivered by the motors to the driven equipment is less than the rated torque of the motors when the voltages at the terminals of the motors are less than the rated voltages of the motors. Table 2 shows characteristic torque variations of large squirrel-cage induction motors and synchronous motors, with respect to applied voltage. For example, a motor capable of exerting a locked-rotor (or starting) torque of 100% (with respect to full-load running torque) with its nameplate voltage at its terminals may be found to have only 90% of its nameplate voltage at its termi- nals at the instant it is started across the line, due to a 10% voltage drop during this period of high current in-rush. The output torque developed by the motor is proportional to the terminal voltage squared times the full voltage locked-rotor torque; or, under a 10% voltage-drop condition, this is calcu- .lated to be: 0.9 x 0.9 x 100, or 81%. Similarly, the entire starting torque curve is reduced by the same value. From NEMA MG 1 paragraph 20.41, a medium voltage induction motor minimum torque curve is 60% locked rotor; 60% pull-up; and 175% breakdown torque (under full voltage condition). Under a 20% voltage drop, this curve then becomes 38% / 38% / 112%. (See Figure 14.) Additionally, any further reductions in voltage due to line loss or auto-transformers will be added to the system drop. This figure also includes the typical “square-of-speed” type curve for centrifugal loads. The top line is for open valve or damper- type starting, while the lower line is for throttled- type starting (this example is for a 50% closed valve/damper start). It can be seen from this example that the effect of reducing the voltage at the motor terminals may prevent start- up unless the load-starting curve can be reduced. 6.21.2 Torque Analysis The maximum torque that can be developed by a motor is proportional to the square of the voltage, resulting in acceler- ation torque reduction for reduced-system voltage. Power system, motor, and load characteristics should be evaluated to assure adequate torque during starting and acceleration. It should also be evaluated during re-acceleration and re-syn- chronizing following voltage sags and disturbances. However, if the inspection of the available data does not yield a clear result, it is recommended that a detailed engi- neering analysis be performed to resolve marginal cases and Table 2-Characteristic Torques Squirrel-Cage Induction Motors Synchronous Motors ~ Locked-rotor torquea 60% Locked-rotor torquea 40% Pull-up torquea 60% Pull-in torquea 30% Breakdown torquea 175% Pull-out torquebJ 150% aThe output torque varies approximately as the square of applied voltage. bThe output torque varies directly as the applied voltage. cWith excitation constant. to avoid any delays or inconveniences that may be caused by the failure of motor-driven equipment to start satisfactorily. If reduced-voltage reactor or resistor starting is used, a sub- stantial amount of impedance is introduced into the supply circuit of the motor when the controller is in the starting posi- ti,on. This impedance as well as the other impedance between the motor and its supply source must be taken into account. All reduced-voltage starting applications should evaluate the starting torque available versus that required by the load (acceleration torque) to determine that adequate torque mar- gin is available for starting the load. A torsional analysis should be undertaken for high-speed synchronous motors to determine the effects of torsional pul- sations during across-the-line start acceleration of the motor and driven equipment. 6.21.3 Low-Voltage Considerations Voltage that is lower than normal may exist, particularly dur- ing starting, because of the system’s design or characteristics. Some causes for this lower-than-normal voltage are as follows: a. The motor being started is large in relation to the capacity of the electrical supply system. b. The supply circuit’s length and design cause an unduly high voltage drop between the power source and motor. Where it is questionable whether the voltage received at the terminals of the motor will be satisfactory, the voltage at that point should be calculated under the most unfavorable conditions likely to exist in actual service. In most cases, this will be at the instant of starting, when the current inrush is several times the rated full-load current and the power factor is low, usually in the 0.2 to 0.4 range. If the circuit under con- sideration will be used to carry other loads, the effect of these other loads on voltage should be taken into account at the same time. When a synchronous motor is to be used, voltage condi- tions at the instant of pull-in should be checked. It must be determined that correct torque will be developed at the pull-in point with net voltage available at the motor terminals. I 6.21 -4 Minimum Torque Specifications Occasionally, the normal starting torque characteristic of the motor will not be sufficient to accelerate the driven equip- minimum motor torque characteristic that is acceptable. For borderline cases, the motor torque characteristic (including any voltage drop considerations) should be specified to be 10% greater than the driven equipment starting-speed-torque curve throughout its entire range. If the application requires more than normal torque, it will be appropriate to determine which will be more economical: to obtain a motor with higher than normal torque, within available limits, or to improve voltage at the point of utiliza- tion. In an extreme case, it may be correct to do both. In most instances, satisfactory results can be obtained most economi- cally by determining torque requirements and specifying these requirements to suit the predetermined voltage condi- tions at the terminals of the motor. In some cases, increased torque designs require higher in- rush current, increasing the voltage drop, which in turn, low- ers the net output torque. 6.21.5 High Torque I, ment. In these marginal cases, it is advisable to establish the For motors used to drive machines that require extra-high starting torque (e.g., most conventional pulverizers, shred- ders, cmshers, and some air blowers or fans), it is advisable to predetermine the voltage conditions and to stipulate the torque requirements on the basis of anticipated voltage condi- tions, as net torque requirements may be high, even when the machine is started unloaded. High-torque motors are avail- able for a variety of applications requiring higher-than-nor- mal torque. i 6.21.6 High-Inertia Loads For high-inertia loads and other loads where the motor is subjected to heavy loading during acceleration (0% to 100% speed), calculations should be made to ensure it will have suf- ficient torque and thermal capacity to bring the driven equip- ment up to rated speed under actual operating conditions within the allowable length of time. A motor that drives equipment that may be subject to occasional sudden, heavy loads while running at rated speed should be checked to determine if it will have sufficient breakdown (induction motor) or pull-out (synchronous motor) torque under this condition to keep it from stalling or from abruptly losing speed. The use of high-slip motors, as well as the possible need for additional flywheel effect, should be considered for such conditions of service. This type of problem is not encountered often, but does call for detailed consideration of equipment and load characteristics. Following a shutdown of a motor driving a high-inertia load, the restarting of the motor should be delayed suffi- ciently until the motor-generated voltage has decayed to a value of 25% or less of the rated voltage. Otherwise, high transient torques can be produced that exceed the mechanical limit of the motor shaft, coupling, or driven equipment. 6.21.7 Additional Torque Requirements. Recognition should be given to the requirement for greater torque under certain conditions of operation. For example, in the case of a centrifugal blower or centrifugal pump, more torque is required to bring the machine up to rated speed with the discharge valve open than with it closed. If, for some rea- son, it is not practical to follow the customary practice of starting a centrifugal blower or pump with the discharge valve closed, sufficient torque should be made available to start it with the discharge valve open. 6.22 METHOD OF STARTING 6.22.1 Starting Control Starting control for all motors is essentially the same. In the larger motor sizes, which represent a considerable invest- ment of capital and upon which a higher degree of depend- ability is placed, the complexity of starting control increases. Larger motors can require power distribution systems with high system capacity to prevent undesirable voltage drops when the motors are started at full voltage under load. If these undesirable effects are produced, reduced-voltage. starting should be considered. With reduced-voltage starting, the motor characteristics must be checked to ensure that there is sufficient torque to accelerate the load at the reduced voltage. It is also important to consider the current in-rush to vari- ous motors following a voltage dip. The in-rush during reac- celeration often will nearly equal the starting in-rush; so if motors are to operate satisfactorily through a voltage dip, the system must be stiff enough to handle the subsequent in-rush. Control circuits which provide for the reacceleration of motors are complex and require additional considerations. Reduced-voltage controllers reduce the net torque exerted by the motors and, in some cases, may complicate the starting problem, especially for synchronous motors. 6.22.2 Full-Voltage Starting In general, the full-voltage magnetic controllers supplied with air-break, vacuum-break, or oil-immersed contactors offer the simplest and most economical method for starting induction motors. See Figure 15 for an example of a simpli- fied, full-voltage nonreversing starter using an air-break con- tactor. This method is based on acceptable motor-loading conditions and the ability of the power distribution system to function without undue voltage disturbance during motor start-up. Most motors, particularly the small and medium horsepower motors, we designed for full-voltage starting. Synchronous and large induction motors, usually at the higher voltages, require more control selectivity because their size may represent an exceptionally large portion of the avail- able power system capacity. In this connection, circuit break- ers operating at a normal breaker duty cycle may provide the dual service of controller and disconnecting means. 6.22.3 Reduced-Voltage Starting The autotransformer, reactor, and resistor types of reduced- voltage controllers provide methods for decreasing the start- ing in-rush current of squin-el-cage and synchronous motors. See Figure 16 for an example of reduced-voltage starting using an autotransformer. Though more costly than the full- voltage controller method, these reduced-voltage controller methods may be required where specific high-inertia loads or system limitations are encountered 6.22.4 Wye-Delta Starting A motor that normally has its windings connected in delta may be started by connecting its windings in wye. This reduces the current in-rush and starting torque to one-third of the full-voltage starting values. This method should only be used if moderate starting torque is satisfactory. This starting method usually requires an “open transition” where the motor is disconnected for a couple of seconds when changing from the wye to the delta configuration. When the motor is recon- nected to the delta (run) configuration, the power system will be subjected to a severe current in-lush (approaching the full- voltage, locked rotor current) unless the transition time is made very short (less than 0.1 second). A short transition time is not recommended for most applications because of the risk of mechanical coupling or motor winding damage that could result from out-of-phase closure between the power system voltage and the residual motor voltage. 6.22.5 Solid-state Control Soft-start controllers (reduced starting-current in-rush) using solid-state devices may also be used with or without a standard contactor to bypass the solid-state starter. Starting times for high inertia or high torque loads should be reviewed with the soft-start supplier. 6.22.6 Capacitor Assisted Starting Motors to be started on “weak” power systems can use a technique where a relatively large capacitor bank is switched onto the same bus as the motor an instant before the motor is connected. The capacitors provide most of the reactive requirements of the motor during the motor acceleration, minimizing the system voltage drop. As the motor accelerates to rated speed and the bus voltage recovers, the capacitor bank is disconnected. Surge arresters and surge capacitors, applied at the motor terminals, are recommended for this application to protect the motor from switching surges. 6.22.7 Wound Rotor Control The typical wiring diagram for a wound rotor motor (Fig- ure 17) begins with the basic control system for a full-voltage type motor (see Figure 15). Rpm adjustment is obtained through the addition of a speed control rheostat external to the motor enclosure, and near the motor control center, in a safe area. This variable resistance is typically implemented through sets of fixed contactors and resistors, or a stepless liq- uid rheostat. See 6.10.4.3 for concerns regarding the installa- tion of wound rotor-type motors. 6.23 MOTOR CONTROLLERS Motor controllers provide the means to start, regulate speed, and stop electric motors. In addition, controllers afford protection against abnormal operating conditions that may result in production losses, equipment damage, and exposure of personnel to unsafe conditions. 6.23.1 Select