Service Application Manual SAM Chapter 620-130 Section 6A ADJUSTABLE SPEED DRIVES By: Richard D. Beard P.E. Consultant, RSES Manufacturers’ Service Advisory Council INTRODUCTION Many commercial and industrial machines and processes require adjustable speed. Adjustable speed usually makes a machine more universally compatible and increases its versatility. Adjustable-speed drives also are being used in residential equipment, including air conditioners, refrigerators, heat pumps, furnaces, and other devices driven by motors. These drives optimize speed and torque, making them generally more efficient than non-adjustable-speed drives. An adjustable-speed motor is one in which the speed can be varied gradually over a wide range—but, once adjusted, it remains nearly unaffected by the load. A variable-speed motor is one in which the speed varies with the load, usually decreasing when the load increases. The term "adjustable speed" implies that some external adjustment, which is independent of load, will cause the speed to change. A variable-frequency inverter drive is an example. The term "variable speed" describes a drive in which load changes inherently cause significant changes in speed. A direct current series motor, for example, exhibits this characteristic. An adjustable variable-speed motor is one in which the speed can be adjusted gradually. However, once adjusted for a given load, the speed will vary with changes in the load. A multispeed motor is one that can be operated at any one of two or more definite speeds, each being practically independent of the load. The multispeed motor is neither an adjustable-speed nor a variable-speed drive. Multispeed motors usually have two, three, or four definite operating speeds. DIRECT CURRENT MOTORS In general, direct current (DC) motors are classified by how the field windings are connected to the armature. The field windings, sometimes referred to simply as "fields," are the stationary coils attached to the frame of a DC motor. The armature is the rotating part. There are two basic types of DC motors, called shunt motors and series motors. A compound motor combines characteristics of both types. Other types of DC motors include universal motors and permanent-magnet motors. SHUNT MOTORS The shunt-wound motor is the most widely used type of DC motor built with armature and field windings. The name originates from the fact that the field windings are connected in parallel (shunt) across the armature, as shown in Figure 1. 1 Service Application Manual SAM Chapter 620-130 Section 6A ADJUSTABLE SPEED DRIVES By: Richard D. Beard P.E. Consultant, RSES Manufacturers’ Service Advisory Council DC shunt-wound motor. In the separately excited shunt motor, the field circuit is energized from a separate source of DC power, as shown in Figure 2. Separately excited DC shunt-wound motor. This field circuit power supply is independent from the armature circuit power supply. Once the desired speed has been obtained through variations of the voltage applied to the armature or field, a shunt motor provides relatively small changes in speed under changing load conditions. Shunt motors are frequently used on adjustable-speed DC drives because of this characteristic of excellent speed regulation. 2 Service Application Manual SAM Chapter 620-130 Section 6A ADJUSTABLE SPEED DRIVES By: Richard D. Beard P.E. Consultant, RSES Manufacturers’ Service Advisory Council SERIES MOTORS As the name implies, the field windings in a series-wound motor are connected in series with the armature, as shown in Figure 3. Series DC motor. Both the field and the armature carry full motor current. In series motors, motor speed is a function of load, once the speed has been adjusted by the voltage applied from the DC power supply. Series motors are commonly used as traction motors for transportation equipment drives, cranes, and hoists. COMPOUND MOTORS A compound motor is one in which there are two field windings, as shown in Figure 4. Compound DC motor. One is a shunt field connected in parallel with the armature, and the other is a series field connected in series with the armature. By properly selecting the shunt and series field windings, the designer can make the motor more nearly like a shunt or a series motor. 3 Service Application Manual SAM Chapter 620-130 Section 6A ADJUSTABLE SPEED DRIVES By: Richard D. Beard P.E. Consultant, RSES Manufacturers’ Service Advisory Council OTHER TYPES OF DC MOTORS Universal motors are series-wound motors that can be operated on either DC or AC power. Performance is the same, regardless of which power supply is used. The operating characteristics of universal motors are similar to those of DC series motors. However, universal motors are small in size, with ratings usually less than one (1) hp. Typical applications of universal motors include power hand tools. In some smaller DC motors, permanent magnets are used in place of field windings. Permanent-magnet DC motors provide shunt motor characteristics with speed adjustment obtained by changing the power supply voltage to the armature. A relatively new type of permanent-magnet DC motor is the electronically commutated motor, or ECM™ , from General Electric. This motor operates on AC power—however, it is a brushless DC permanent- magnet motor. Control of the ECM™ output speed is accomplished with solid-state switches. The solid- state switches eliminate the need for the traditional mechanical commutator and brushes. The permanent magnets are in the rotor, while the windings in the stator create electronically controlled rotating electromagnets. POLYPHASE ALTERNATING CURRENT MOTORS The speed of an alternating current (AC) induction motor is related to the frequency of the power supply. The equation that shows this relationship is: The speed determined by this equation is actually the synchronous speed, which is the speed of the rotating magnetic flux in the stator of the motor. The stator is the stationary part of an induction motor. The rotor is the rotating part. The rotating magnetic flux is created as current flows through the three- phase stator windings. This rotating magnetic flux induces voltage into the rotor bars, which causes current to flow in the rotor. As a result, electromagnets are created in the rotor. These electromagnets follow the electromagnetic field of the stator and cause the rotor to rotate. The amount by which the rotor speed lags behind the speed of the rotating magnetic flux in the stator is called the slip. This determines the rotor voltage. Without slip, no voltage would be induced into the rotor. Because of the slip, there is a difference between the speed of the rotating magnetic flux in the stator and the speed of the rotor. Thus, there are actually three speeds to consider: 1. the synchronous speed (of the rotating stator flux) 2. the speed of the rotor 3. the slip speed, which is the difference between the synchronous speed and the rotor speed, and is usually expressed as a percentage. 4 Service Application Manual SAM Chapter 620-130 Section 6A ADJUSTABLE SPEED DRIVES By: Richard D. Beard P.E. Consultant, RSES Manufacturers’ Service Advisory Council SYNCHRONOUS MOTORS A brief description of synchronous motor operation is included here, even though synchronous motors are not adjustable-speed motors. Polyphase synchronous motors have stators and stator windings very similar to those of induction motors. The primary difference between synchronous motors and induction motors is in the rotor construction. The rotor of the synchronous motor has distinct (salient) poles wound with insulated magnet wire and connected in series. In addition, bar windings are placed in the upper part of each pole. These bar windings are similar to the windings in the rotor of a squirrel-cage induction motor. The synchronous motor starts and accelerates as an induction motor. Because of the slip, the rotor reaches a speed that is somewhat slower than the speed of the rotating magnetic flux in the stator. Direct current then is applied to the rotor circuit, which includes each of the poles. This DC power is connected to the rotor circuit through two insulated slip rings mounted on the rotor shaft. Carbon brushes make contact with the slip rings. When the rotor circuit is energized, each pole becomes an electromagnet. These magnets lock into step with the stator flux electromagnets. The rotor then turns at exactly the same speed as the rotating magnetic flux in the stator. When this happens, there is no slip and the motor runs at synchronous speed. WOUND-ROTOR MOTORS Wound-rotor motor construction differs from that of the induction motor only in the rotor. Rather than having a rotor with bar windings whose ends are connected together, as in a squirrel-cage induction motor, the wound-rotor motor has insulated coils of magnet wire inserted in the rotor core iron. These coils are similar to the stator winding coils. One end of each phase winding is connected to one of the three slip rings mounted on the shaft and insulated from it. Connections to the slip rings are made through carbon brushes, so that any value of secondary resistance may be added to the rotor circuit. With the three slip rings connected together, the wound-rotor motor runs exactly like the squirrel-cage induction motor. Reduced speed is obtained by connecting resistance into the rotor circuit. Thus, the wound-rotor motor is an adjustable secondary resistance motor that provides adjustable and variable speed, by means of changing the value of the external resistance in the rotor circuit. SQUIRREL-CAGE INDUCTION MOTORS Since the speed of a polyphase induction motor is equal to the synchronous speed minus the slip, electrical speed control must adjust one of these two speeds. Slip speed control is utilized in wound-rotor adjustable-speed motors.