Industrial Alternating Current Motors
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Study Unit Industrial Alternating Current Motors By Robert L. Cecci Technical Writer This study unit will cover the construction and operation of single- and three-phase or polyphase alternating current (AC) motors. In the first section, you’ll learn about AC motor basics through examples of how coils create magnetic fields P r e v i e w when they’re supplied with AC electricity. Also, you’ll see how P r e v i e w electromagnetic induction allows the rotor of a motor to have a current flow and, therefore, its own magnetic field. Next, you’ll see how single-phase and split-phase motors operate. This information is then followed by a description of the capacitor motor and a brief presentation of the repulsion- induction motor. The next two sections cover polyphase motors and AC control systems. When you complete this study unit, you’ll be able to • Explain how AC electricity creates a changing magnetic field in and around a coil • Discuss the principles of electromagnetic induction • Explain why a motor needs a system for starting the rotor and how this is performed with a shaded-pole, split-phase capacitor, and repulsion-induction motor • List the possible problems with single-phase motors and the steps taken to troubleshoot these problems • Identify the components of a polyphase motor and describe its operation • Explain how to troubleshoot polyphase motor systems • Identify the basic motor starter systems used in single- phase and three-phase AC motors iii BASIC AC MOTOR THEORY 1 Alternating Current and Coils 1 Magnetic Induction 3 C o n t e n t s C o n t e n t s A Basic AC Motor 4 Getting the Motor to Turn 5 SINGLE-PHASE AC MOTORS 7 Uses of Single-phase Motors 7 The Rotor 8 The Shaded-pole Motor 9 The Split-phase Motor 12 The Capacitor Motor 17 The Repulsion-type Motor 19 POLYPHASE MOTORS 21 Polyphase Motor Basics 21 The Delta-connected Motor 22 The Wye-connected Motor 24 Multiple-speed Operation 25 Motor Nameplate Information 29 Polyphase Motor Troubleshooting and Repair 32 AC MOTOR RELAY CONTROLS 36 The Manual Control Circuit 36 The Basic Electric Control Circuit 38 A Reversing Circuit 40 Two-speed Magnetic Starters 41 SELF-CHECK ANSWERS 47 APPENDIX 49 EXAMINATION 53 v Industrial Alternating Current Motors BASIC AC MOTOR THEORY Alternating Current and Coils Alternating current (AC) motors are one of the most popular forms of motors used in industry. Visit any industrial plant and you’ll most likely find AC motors in sizes from a fraction of a horsepower to 1,000 horsepower or more. AC motors are very popular because the normal current supplied to industry is in the form of alternating current. Where direct current (DC) motors require special rectifiers or controllers, AC motors can be connected directly to the AC line power or controlled by simple contactors. Up until a few years ago, when industry needed adjustable or variable-speed motors, a DC motor was most likely used. However, with the use of a special control called an inverter, AC motors can now easily be used for these purposes. When direct current is applied to a coil of wire, it creates a magnetic field around each wire in the coil. The magnetic fields from the coils can run together and get quite strong in magnetic force or flux. A metal core or pole that’s surrounded by the magnetic field will concentrate that field and exhibit a north and south magnetic pole. When alternating current is applied to that same coil of wire, a magnetic field is also formed. This field will be different from the magnetic field that’s created by direct current in that the field will alternate in two ways. 1 Figure 1A displays a single cycle of AC. Note that the voltage and current rise in a positive direction until a positive maximum (A) voltage is reached. The voltage and current then reverse and head towards zero. After crossing zero, the voltage and current head for a negative peak where the voltage and current reverse once again and head towards zero. This positive and negative (B) cycle occurs sixty times per second (60 Hertz or 60 Hz). FIGURE 1—At each peak in the AC cycle, the magnetic flux in the motor’s coil or pole also Figure 1B displays the magnetic flux peaks. density, or the strength of the magnetic force produced by the coil as the applied AC goes through one cycle. The field strength isn’t constant but rather follows the applied voltage and current through the cycle. Figure 2 displays the other property of a coil that’s supplied by AC. In Figure 2A, the AC signal has created a north pole (N) on the left of the core and a south pole (S) on the right of the core. In Figure 2B, the AC signal is now going through its negative transition and the poles have reversed with the south to the left and the north to the right. FIGURE 2—As this AC cycle changes from a positive half cycle to a negative half cycle, the N S poles will switch from north to south and back (A) to north in a continuous cycle. S N (B) 2 Industrial Alternating Current Motors So as you can see, AC voltage and current applied to a coil create a magnetic field with constantly changing properties: 1. The magnetic field strength constantly varies. 2. The magnetic pole polarities constantly alternate from the north to the south pole and back. Magnetic Induction There’s one other property of AC and coils that should be discussed. That property is magnetic induction. Figure 3 displays two DC-powered circuits. In Figure 3A, the switch has just been closed. The change in voltage from zero to some value of DC voltage creates a magnetic field around the primary coil P. If the primary and secondary coils are very close together, this change of magnetic field from zero to a higher level will magnetically induce a field on the secondary coil S. For a brief instant, the meter would measure a pulse of voltage. After the pulse arrived, there would be no further voltage generated in the secondary coil or viewed at the meter. When the switch is opened as in Figure 3B, the collapsing field will also induce a pulse of voltage in the secondary coil that can be viewed on the meter. Again, after this pulse arrives, there will be no further voltage changes in the secondary. FIGURE 3—If DC is SWITCH applied to a coil, a mag- CLOSED netic field is created around that coil. The DC field is induced on the P SOURCE S RL secondary coil S when the V switch is closed (A) or opened (B). V (A) COM SWITCH OPENED DC SOURCE P S RL V V (B) COM Industrial Alternating Current Motors 3 Figure 4 displays the same circuit as Figure 3, except that the voltage source is an AC source. The primary coil P will, of course, develop a changing magnetic field that follows the two properties listed previously. Its strength and magnetic polarity will change in step with the changing AC cycle. Also, this coil will induce a magnetic field in the secondary coil S. This coil’s magnetic field will follow the same properties of the primary field. At the secondary coil, both a magnetic field and an AC voltage will be created. FIGURE 4—When AC is supplied to the same circuit as in Figure 3, the AC constantly changing pri- POWER mary field at P is induced P SOURCE S RL on the secondary coil S. V An AC voltage will be measured across RL by the meter. V COM Most rotors of AC motors will operate on magnetic induction. The magnetic fields created by the field windings or stators will induce a voltage on the conductors of the rotor. These induced voltages will create magnetic fields around the motor conductors that are attracted to or repelled by the stator fields to rotate the rotor. A Basic AC Motor It might appear that we have everything needed to build a motor. We have a changing magnetic field in the stator or field windings and an induced current and voltage in the rotor that creates the second magnetic field. There’s one problem, however. Figure 5 displays a simplified view of a motor using two stator poles and a permanent-magnet rotor. The coils are supplied with a source of AC voltage. Assume that on the first AC half-cycle, the left pole near the permanent magnet will become a north pole and the right pole will become a south pole. The permanent magnet will quickly try to turn until it’s horizontally aligned with the field 4 Industrial Alternating Current Motors 1/120 poles. However, of a second later, the STATOR STATOR field poles will switch magnetic polarity and POLE 1 ROTOR POLE 2 attempt to repel the permanent magnet. At N 1/120th of a second later, the magnetic poles again reverse and again attempt to attract S the permanent magnet. What’s happening here is that the perma- AC POWER nent magnet rotor will move a certain small SOURCE amount at 60 cycles per second creating a hum rather than rotating motion. This motor would only work if the rotor was FIGURE 5—This illustration shows a basic spun at high speed and then the AC power two-pole motor with a permanent magnet rotor.