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624_____________________ , CHAPTER 35 . 1948.Guide Adjustable. Speed Motors are normally shunt wound and are operated at.-various speeds by varying a resistance, connected in series with the motor field. A maximum range of speed of.about 5 to 1 can be obtained by this, means. Rated speed regulation is 22 per cent for motors.of this type from 2 to. 5 hip; 15 per cent is standard in.larger sizes. The NEMA practice on rating adjustable speed drc motors is; defined in the Glossary ..at .the end of the chapter. The control for the adjustable speed motor consists of the addition of a field rheostat for speed control to the equipment specified for . the constant speed motor. .. k HP & LARGER [rj] SAFETY 11 SWITCH RHEOSTAT ADJUSTABLE SPEED MOTOR CONSTANT SPEED MANUAL STARTING CONSTANT. SPEED PUSH BUTTON STARTING STARTER & SPEED REGULATOR AUTOMATIC STARTER MOTOR Fig. 2. Recommended Controls foe d-c Motors t.'Adjustable Varying Speed ` Motors are d-c motors in which the speed is- varied by the addition of resistance in series with the armature..:.'The speed of the motor by this means is always less than the rated full field speed and varies widely with a change in load, especially with high series resistance. Fig. 3 illustrates typical speed characteristics of this type of motor for different 'values: of. armature 'resistance. ' The addition of series resistance in the armature circuit reduces the motor speed by lowering the voltage on the armature. At one-half speed theWbitage is approximately one-half of line voltage. Consequently, with rated fullload current'the power delivered by the motor will be only one-half of the maximum, e.g., it will.be 5 hp from a 10 hp motor, because the other 5 hp will be lost in the resistance. It is, therefore,'evident that the efficiency of the motor is reduced at reduced speeds due to the loss in the resistor. - '' Control for the adjustable; varying speed motor-is "similar to that- for the constant speed motpij .with, the exception that, the .starting resistor- Refer to Glossary at end of chapter ; Motors aOd-Motor Controls- 62S Typical Specifications for d-c Motors 'Constant j 'Shunt 1 1151 ' ____l___ -Hp, __ ____-RPM, Adjustable V Speed,. Compound } Wound, 230} Volts Varying j . Series J 600J i d-c Motors For Driving------(Application) Motor Shall Be Arranged.For Mounting And Shall Be Provided With A_iLi:___________--:____ _ \Vertical J - (Open, Splashproof, etc.): Type Of Enclosure, NEMA' Insulation, And |si^ve| Bearings. Typical Specifications for d-c Motor Control Control For 1 Adjustable j Speed d-c Motors Shall Be {Maonetirl And Shall (Adjustable VaryjngJ (Magnetic! Consist of a And an Enclosed Controller Providing Overload Pro tection [And Low Voltage ^^,on}]. Typical Specifications for Squirrel-Cage Motors ------------ --------------Hp. ...--------- ---------- ---RPM, (2081 Cycles, "01 Volts, /|\ Phase. Squirrel-Cage'Induction Motors of the NEMA (550J (Class A--Normal Starting Torque, Normal Starting Current] Class B--Nornial Starting Torque, Low Starting Current 1 T Class C--High Starting Torque, Low Starting Current J Class D--High Slip Motor Stall Be Arrange Fj or Mounting Driving --. (Application) \: ; -Type of Enclosure, NEMA (Open, Splashproof, etc.). And Shall Be Provided With A, Cm2 b} Insulat;on, {laT6} B<*rinp.- must be designed for speed regulating duty which means that it must be capable of carrying the motor current continuously: Speed controllers are available both for constant torque applications and varying torque drives, such as required by fans, in which-the torque is reduced con siderably at reduced speed. The Adjustable Voltage type of speed control is also often known as the variable voltage or Ward-Leonarb system. . For machines requiring a wide range in speed control and a large number of steps of control this type of system is used most extensively. The drive consists' of.one or more d-c motors, the armatures of which are supplied with power from a d-c generator and the fields of all machines are excited from a constant voltage.exciter. A schematic diagram of connections for an adjustable voltage drive is shown in Fig. 4. In most cases the d-c generator is a part of a three-unit set including a constant speed a-c driving motor, and a constant voltage exciter. As the voltage on the generator and con sequently on the d-c motor or motors is adjusted by a rheostat in the