Chapter 11 - Motor Controls
Chapter 11 - Motor Controls
11-1. Objectives
Upon completion of this chapter, you will know
”why a motor starter is needed to control large AC motors.
”the components that make up a motor starter and how it operates.
”why motor overload protection on AC motors is needed and how a eutectic metallic alloy motor overload operates.
”why, until recently, ac motors were used for constant speed applications, and dc motors were used for variable speed applications.
”how a pulse width modulated (PWM) dc motor speed control controls dc motor speed.
”how a variable frequency motor drive (VFD) operates to control the speed of an ac induction motor.
11-2. Introduction
Since most heavy machinery is mechanically powered by electric motors, the system designer must be familiar with techniques for controlling electric motors. Motor controls cover a broad range from simple on-off motor starters to sophisticated phase angle controlled dc motor controls and variable frequency ac motor drive systems. In this chapter we will investigate some of the more common and popular ways of controlling motors. Since most heavy machinery requires large horsepower ac motors, and since large single phase motors are not economical, our coverage of ac motor controls will be restricted to 3-phase systems only.
11-3. AC Motor Starter
In its simplest form, a motor starter performs two basic functions. First, it allows the machine control circuitry (which is low voltage, either dc or single phase ac) to control a high current, high voltage, multi-phase motor. This isolates the dangerous high voltage portions of the machine circuits from the safer low voltage control circuits. Second, it prevents the motor from automatically starting (or resuming) when power is applied to the machine, even if power is removed for a very short interval. Motor starters are commercially available devices. As a minimum, they include a relay (in this case it is called a contactor) with three heavy-duty N/O main contacts to control the motor, one light duty N/O auxiliary contact that is used in the control circuitry, and one light duty N/C overload contact which opens if a current overload condition occurs. This is shown in Figure 11-1.
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Chapter 11 - Motor Controls
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Figure 11-1 - Simple 3-Phase Motor Starter
with Overloads
Most starters have terminal labels (letters or numbers) etched or molded into the body of the starter next to each screw terminal which the designer may reference on schematic diagrams as shown in Figure 11-1. The coil of the contactor actuates all of the main contacts and the auxiliary contact at the same time. The overload contact is independent of the contactor coil and only operates under an overload condition. More complex starters may have four or more main contacts (instead of three) to accommodate other motor wiring configurations, and extra auxiliary and overload contacts of either N/O or N/C type. In most cases extra auxiliary and overload contacts may be added later as needed by “piggybacking” them onto existing contacts.
Figure 11-2 illustrates one method of connecting the starter into the machine control circuitry. The terminal numbers on this schematic correspond to those on the motor starter schematic in Figure 11-1. Power from the 3-phase line (sometimes called the mains) is applied to terminals L1, L2, and L3 of the starter, while the motor to be controlled is connected to terminals T1, T2, and T3.
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Chapter 11 - Motor Controls
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Figure 11-2 - Typical Motor Starter Application
In operation, when the rails are powered, pressing the Start switch provides power to the motor starter coil M. As long as the overload contacts OL are closed, the motor starter will actuate and three phase power will be provided to the motor. When the starter actuates, auxiliary contact M closes, which bypasses the Start switch. At this point the Start switch no longer needs to be pressed in order to keep the motor running. The motor will continue to run until (1) power fails, (2) the Emergency Stop button us pressed, (3) the Stop switch is pressed, or (4) the overload contact OL opens. When any one of these four events occurs, the motor starter coil M de-energizes, the three phase line to the motor is interrupted, and the auxiliary contact M opens.
11-4. AC Motor Overload Protection
For most applications, ac induction motors are overload protected at their rated current. Rated current is the current in each phase of the supplying line when operating at full rated load, and is always listed on the motor nameplate. Overload protection is
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Chapter 11 - Motor Controls
required to prevent damage to the motor and feed circuits in the event a fault condition occurs, which includes a blocked rotor, rotor stall, and internal electrical faults. In general, simple single phase fuses are not used for motor overload protection. When a motor is started, the starting currents can range from 5 to 15 times the rated full load current.
Therefore a fuse that is sized for rated current would blow when the motor is started. Even worse, since we would need to fuse each of the three phases powering a motor, if only one of the fuses were to blow, the motor would go into what is termed a single phasing condition. In this case the motor shaft may continue to rotate (depending on the mechanical load), but the motor will operate at a drastically reduced efficiency causing it to overheat and eventually fail. Therefore, the motor overload protection must (1) ignore short term excessive currents that occur during motor starting, and (2) simultaneously interrupt all three phases when an overload condition occurs.
The solution to the potential single phasing problem is to connect a current sensing device (called an overload) in series with each of the three phases and to mechanically link them such that when any one of the three overloads senses an over-current condition, it opens a contact (called an overload contact). The overload contact is connected into the motor starter circuit so that when the overload contact opens, the entire starter circuit is disabled, which in turn, opens the three phase motor contactor interrupting all three phases powering the motor. The nuisance tripping problem is overcome by designing the overloads such that they react slowly and therefore will not trip when a short term overload occurs, such as the normal starting of the motor.
The most popular type of overload is the thermal overload. Although some thermal overloads use a bimetallic temperature switch (the bimetallic switch is covered earlier in this text), the more popular type of thermal overload is the eutectic metallic alloy overload.
This device, shown in Figure 11-3, consists of a eutectic alloy1 which is heated by an electrical coil (called a heater) through which the phase current passes. If the phase current through the overload heater is excessive, the eutectic metal will eventually melt. This releases a ratchet, which cams the normally closed overload contact into the open position. In order to make the overload reusable, the eutectic alloy in the overload is sealed in a tube so that it will not leak out when it melts. The sealed tube, eutectic metallic alloy, and spindle are commonly called the overload spindle, which is illustrated in Figure 11-4.
When the overload cools such that the eutectic alloy solidifies, the ratchet again will be prevented from rotating and the overload can then be reset by manually pressing a reset lever. For clarity, only one phase of the overload system is shown in Figure 11-3. In
1 A eutectic alloy is a combination of metals that has a very low melting temperature and changes quickly from the solid to liquid state, much like solder, instead of going through a “mushy” condition during the state change.
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Chapter 11 - Motor Controls
practice, for 3-phase systems there are three overloads operating the same overload contact.
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Figure 11-3 - Eutectic Metallic Alloy Thermal Overload (One Phase)
Figure 11-4 - Overload Spindle
with Ratchet
(Allen Bradley)
One major advantage in using the thermal overload is that, as long as the overload is properly sized for the motor, the overload heats in much the same manner as the motor itself. Therefore, the temperature of the overload is a good indicator of the temperature of the motor windings. Of course, this principle is what makes the overload a good protection device for the motor. However, for this reason, the designer must consider any ambient temperature difference between the motor and the overload. If the motor and overload cannot be located in the same area, the overload size must be readjusted using a
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