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870 CHAPTER 57 1965 Guide And Data Book Table 2.... Effect of Voltage and Frequency Variation on Induction Motor Characteristics* Voftoge and Frequency Vanaftoo Sfarfmg t Running Torque Syndirenovt SpMd FuO load Speed FwO toad 120% Voltage Increase 44% No Change Decrease , 30% Increase 1-5% Small Increase Voltage Variation 110% Voltage Function of voltage Increase 21% No Change (Voltage)1 Constant Decrease 17% -1 (Voltage)* Increase 1% (Synchronous Speed SUp) Increase M to 1% - 90% Voltage Decrease 19% No Change Increase 23% Decrease 1M% Decrease 2% 105% Frequency Decrease 10% Increase 5% Practically no change Increase 5% Slight Increase Frequency Variation Function frequency 1 (Frequency)* Frequency (Synchronous Speed SUp) 95% Frequency Increase 11% Decrease 5% Practically no change Decrease 5% Slight Decrease . * Them variation are ee&erml ead wiH difier anurrhit far cpeeifie ratings Efficiency Load Decrease M to 2% Practically no change - Practically no change Slight Increase Slight Decrease M toad Decrease 7 to 20% Decrease 1 to 2% Increase 1 to 2% Slight ' Increase Slight Decrease torque. The starting current, or locked-rotor current, is usually 400 to 600 percent of the current at rated full-load. Full-load torque is the torque developed to produce the rated hp at the rated speed. Full-load speed depends upon the design of the motor. For induction-motors, a speed of 1725 rpm is typical for 4-pole motors,-and a speed-of 3450 rpm for 2-pole motors (60 cycles). Motors have a maximum or breakdown torque which cannot be exceeded without causing an abrupt change in speed. The relation between breakdown torque and full-load torque varies widely depending upon motor design. Horsepower. The horsepower delivered by a motor is a product of its torque and speed. Since a given motor will deliver increasing horsepower up to maximum torque, a basis for horsepower rating is needed. The National Electrical Manufacturers Association (NEMA) bases horsepower rating upon breakdown torque limits. Full-load rating is re lated to the winding temperature rise. For open-type general purpose motors, temperature rise is limited to 40 C by ther mometer, and of 50 C by resistance for Class A. insulation. Higher temperature rises may be allowed for enclosed and special-purpose motors and for other insulations. The service factor of a general purpose motor is defined as "a multiplier which, applied to the normal horsepower rating, indicates a permissible loading which may be carried under the conditions specified for the service factor." Motors other than general purpose motors have a service factor of 1.0.'. MOTOR CONTROL In- general motor control equipment should provide (1) means of discommecting the motor and controller from the power supply, (2) means for starting and stopping the motor, (3) overheating protection for the motor, (4) protection for the operator, (5) control of motor speed, and (6) protection of motor branch circuit conductors and control apparatus. CvcuUrbreakers are used for disconnecting as well as circuit protection and are available in ratings for use with small household refrigerators as well as in very large commercial installations. Manual switches for disconnecting and fuses for short-circuit protection are also used. For single-phase motors up to 2 hp, 230-volt, a wall plug is an acceptable dis connecting device. Controllers must meet the requirements of power supply, size and type of motor, and of automatic control. They are available in a multiplicity of designs, ranging from manna! across-the-line starters to current-limiting acceleration con trollers for both alternating- and direct-current motors and variable-speed control devices. Three basic types of motor protection devices may be used: (1) devices sensing current only; (2) devices sensing both cur rent and temperature; (3) devices sensing temperature only. Devices sensing current only include magnetic and thermal overload relays. They may be used separately, or in conjunct tion with controllers. Devices sensing both motor current and temperature are inherent overheating protective devices, and are called thermal protectors (see the National Electrical Code). They are used in fractional and small integral hp single-phase and polyphase motors (usually under 10 hp). In open motors, the thermal protector is mounted near, on, or in the winding; in hermetically-sealed motors, it may be mounted either outside the compressor shell or inside the shell, on or near the stator winding. Sealed protectors must be used for the latter application. Temperature sensing devices are used with integral hp single-phase and three-phase motors. They are installed directly on or buried in the motor winding. Three types and their use are: 1. Bimetallic thermostats used in conjunction with thermal overload relays and connected in series with the control circuit of the magnetic starter which disconnects the motor. The thermos etat protects against excessive motor winding temperature for running overload. On locked rotor, the thermal relays protect for the initial heating cycle and the combined functioning of relays and thermostats protect for subsequent cycles. 2. Thermistors undergo a change in resistance with tem1 perature and the signal is used in a control cucuit-to operate a relay. This relay in turn interrupts the control circuit of magnetic motor starter which is used to disconnect the motor. Thermal overload relays in conjunction with thermistors are used when required to protect for initial hating cycle locked rotor. 3. Rate-of-rise thermostats have a differential expansion thermal element responsive to the rate of temperature change and respond faster as the rate of temperature ^nangn increases.' Motors and Motor Protection 871 Table 2 .... Effect of Voltage and Frequency Variation-on Induction Motor Characteristics* (Concluded) Voltage A Frequency Variatiaa Voltage Variation 120% Voltage 110% Voltage Function of voltage Power-Factor Full Load Load Decrease Decrease 5 to 15% 10 to 30% Decrease Decrease 3% 4% Load Fvtt food Current Decrease Decrease 15 to 40% 11% Decrease Decrease 5 to 6% , 7% Starting Current Tompurotaru IfruFuO load Increase 25% Decrease 5 to 6 C Increase Decrease 10 to 12% 3 to 4 C Maximum Overload Capacity Increase 44% ' Increase 21% Magnetic Noises, No Load m Particular Noticeable Increase Increase Slightly - - -- - Voltage ' - (Voltage)* - Frequency Variation 90% Voltage 105% Frequency Function of frequency . Increase . 3% Slight Increase - Increase 2 to 3% Slight Increase - Increase - Increase 4 to 5% 11% Slight Decrease Increase Slightly -- Decrease Increase 10 to 12% 6 to 7 C Decrease Decrease 5 to 6% Slightly 1 Frequency Decrease 19% Decrease Slightly. ' Decrease Slightly Decrease Slightly 95% Slight Slight Frequency Decrease Decrease Slight Decrease Increase Slightly Increase 5 to 6% Increase Slightly Increase Slightly Increase Slightly This.charecteristie provides protection for running overloads as well as stalled rotor which involves slow and fast rates of heating of the motor winding. The thermostat contacts are wired in series with the control circuit of a magnetic ,motor contactor which disconnects the motor. Hermetically sealed thermostats must be used for hermetic motors. .- Motor controllers for single-phase motors must provide means for disconnecting the starting winding of split-phase, or capacitor-start induction-run motors or the start capacitor of capacitor-start, capacitor-run motors. Open machines usually have a- centrifugal switch mounted on the motor shaft, which disconnects the starting winding at about 70 percent full-load speed. Hermetic motors usually have ex ternally mounted starting relays; current-type starting relays for small motors up to % hp, voltage-type starting relays for larger motors. Controllers and motor protection devices should be selected and installed in accordance with state and local electrical codes. Devices with Underwriters' Laboratories seal, are universally accepted in the United States as complying with the National Electrical Code and meeting minimum safety requirements. CODES AND STANDARDS Codes refer to requirements needed in the interest of public health or safety; standards present uniform methods of rating, giving and measuring the performance of electrical equipment in order to provide a yardstick for use and comparison. Codes and standards are sponsored at a national level, and usually Table 3 .... Motor. Types Type Horsepower . Typo Power Sandy Sttitatda far Hermetic Application Fractional Horsepower Sues: Spht-phase Capacitor-start Repulsion-start Permaoentapliticapacitor Shaded-pole Squirrel cage induction Direct-current t-1 i-l milli hp-J Hi V~l Integral Horsepower Sizes: Capacitor-start, Capac- itor-run 1-6 Repulsion-start 1-5 ' Squirrel cage induction (normal t/imue) 1-up Slip-ring 1-up 1-up Permanent-eplit-capao- itor 1-6 single-phase single-phase single-phase single-phase single-phase Polyphase *tn<rlp-phn(tn single-phase Polyphase Poljrphase single-phase Yes Yes No Yes No Yes No Yes No Yes No .No Yes Fig. 1 .... Performance Characteristics -of CapacitorStart Induction-Run 2-Pole Motor (1 hp). -