Correspondence Lesson 7: Motor Controls

We use motors more than any other electrical device. They vary in size from specially designed medical motors that are less than 1 in. long to gigantic industrial units of several thousand horsepower. In between are hundreds of different types of motors for thousands of different applications.To better understand the rules regarding the application and wiring of motors, as well as the control details,

We use motors more than any other electrical device. They vary in size from specially designed medical motors that are less than 1 in. long to gigantic industrial units of several thousand horsepower. In between are hundreds of different types of motors for thousands of different applications.

To better understand the rules regarding the application and wiring of motors, as well as the control details, we have categorized concerns into four sections.

Mechanical safety. We ensure the motors themselves are not a source of danger. For instance, we would not want to install open motors in areas where children may go; it's too easy for them to stick their fingers into operating motors. It's also often necessary to put a clutch on a motor, to avoid possible injury to a machine operator.

Mechanical stability and operations. Motors have mechanical stresses placed upon them. One of the primary forces is vibration, which unfortunately loosens bolts and screws. This also affects the equipment the motor operates and surrounding items.

Electrical safety. Motors should not become the source of an electrical shock or fault. They should not cause problems to the electrical system.

Operational circuits. The circuits on which you install the motors should operate continually and correctly. Motors place unusual demands on electrical circuits. They can cause large starting currents. (Fully loaded motors can draw starting currents of four to eight times their normal full-load current; in some circumstances even higher.) They also put inductive reactance into electrical systems. Because of the high currents some motors draw, they overheat electrical circuits more commonly than many other types of loads.

The basics. The operation of electric motors involves not only current and voltage, but also magnetic fields and their associated characteristics. All electric motors operate by using electromagnetic induction, which is the interaction between conductors, currents, and magnetic fields. Any time an electrical current passes through a conductor, it causes a magnetic field to form around that conductor. This is a law of physics. Conversely, any time a magnetic field moves through a conductor, it induces (causes to flow) an electrical current in that conductor. By manipulation of these two laws, in combination with magnetic attraction and repulsion, you can operate a motor. The operation of electric motors is such that, by intelligent use of electromagnetic induction, electricity turns into physical force, causing the motor to turn.

Here is the step-by-step operation of an electric motor. An electrical current flows through the motor's windings, causing a strong magnetic field to form around the windings. This magnetic field attracts the rotor (the part in the center of the motor that turns; the shaft is at the center of the rotor) and moves it toward the magnetic field, causing the initial movement of the motor. Various means of rotating the magnetic field perpetuate this movement. Various types of motors do it differently, although the most common method is by using several different windings and sending current to them alternately, causing magnetic strength to be in one place one moment, and another place the next. The rotor then follows these fields, causing continuous motion.

Motor controllers. The simplest controller is the branch-circuit protective device. With this, you can safely control motors of 1/8 hp or less that you would normally leave running. A simple "controller" is a cord-and-plug connection for portable motors of 1/3 hp or less. Controllers must have horsepower ratings no lower than the horsepower rating of the motor they control, except in these two cases.

• Stationary motors 2 hp or less and 300V or less can use a general-use switch that has an ampere rating at least twice that of the motor it serves. You can use general-use AC snap switches on AC circuits to control a motor rated 2 hp or less and 300V or less; having an ampere rating of no more than 80% of the switch rating.

• You can use a branch-circuit inverse-time circuit breaker rated in amperes only (no horsepower rating). Unless a controller also functions as a disconnecting means, it does not have to open all conductors to the motor. If you supply power to a motor by a phase converter, you must control the power in such a way that, in the event of a power failure, power to the motor cuts off and cannot reconnect until you restart the phase converter. Each motor must have its own controller, except when a group of motors (600V or less) uses a single controller rated at no less than the sum of all motors connected to the controller. This applies only in the following three cases:

• If a number of motors drive several parts of a single machine;

• When a group of motors is protected by one overcurrent device, as specified elsewhere; and

• Where the group of motors is located in one room, within sight of the controller. A controller must be capable of stopping and starting the motor, and interrupting its locked-rotor current.

The disconnecting means must be within sight of the controller location and within sight of the motor, except in the following two situations:

• If the circuit is more than 600V, you can place the disconnecting means out of sight of the controller; as long as the controller has a warning label indicating location of the disconnecting means to be locked in open position.

• You may locate one disconnecting means next to a group of coordinated controllers on a multi-motor continuous process machine. The disconnecting means for motors 600V or less must be rated at least 115% of the full-load current of the motor being served. A controller operating motors more than 600V must have the control circuit voltage marked on the controller. You must provide fault-current protection for each motor operating at more than 600V. (See NEC Sec. 430-125(c).)

All exposed live parts must be protected. (See Sec. 430-132 and Sec. 430-133 if necessary.)

Motor control installation requirements. Motor control circuits tapped from the load side of a motor's branch-circuit device controlling its operation are not considered branch circuits. You can protect them with a supplementary or branch-circuit protective device. Control circuits not tapped this way are signaling circuits: You must protect these following Art. 725.

You should protect motor control conductors usually with an in-line fuse and in accordance with Column A of Table 430-72(b), except:

• If they extend no further than the motor controller enclosure, you may then follow Column B of Table 430-72(b).

• If they extend further than the motor controller enclosure, follow Column C of Table 430-72(b).

• The primary side of the transformer protects control circuit conductors, taken from a single-phase transformer that has only a 2-wire secondary. However, the primary protection ampacity should not be more than the ampacity shown in Table 430-72(b) multiplied by the secondary-to-primary voltage ratio (secondary voltage divided by primary voltage).

• When the opening of a control circuit would cause a hazardous situation (as would be the case with a fire pump, etc.), the control circuit can tap into the motor branch circuit with no further protection. Control transformers must be protected according to Article 450 or Article 725, except:

• Protect control transformers that are an integral part of a motor controller and rated less than 50VA by primary protective devices, impedance limiting means, or other means.

• If the primary rating of the transformer is less than 2A, you may use an overcurrent device rated at no more than 500% of the primary current in the primary circuit.

• By other approved means.

• When the opening of a control circuit would cause a hazardous situation (as would be the case with a fire pump, etc.), you may omit protection. When damage to a control circuit would create a hazard, you must protect the control circuit (by raceway or other suitable means) outside of the control enclosure. When one side of a motor control circuit is grounded, the circuit must be arranged so that an accidental ground will not start the motor. You must arrange motor control circuits so they will shut off from the current supply when the disconnecting means is in the open position.

Overload relays. The motor starter is similar to the contactor in design and operation. Both have one feature in common: contacts operate when the coil energizes. The important difference is the use of overload relays on the motor starter.

The magnetic motor starter has three main contacts in the form used most frequently. These contacts are normally open. You use this arrangement in the starting of 3-phase motors. Almost all industrial, institutional, or large commercial facilities in the U.S. use 3-phase power. A few areas (such as upstate New York and a few others) use 2-phase, 4-wire power. In such cases, you can order four normally open contacts for the starter. Like the power contactor, magnetic motor starters are available in many sizes. The smallest units are approximately the size of a relay, and the largest starters can take up most of a room.

Motor starters can close and open the contacts that connect the motor to the source of electrical power. Unfortunately, it is not always possible to control the amount of mechanical load applied to the motor. Therefore, the motor may be overloaded, resulting in damage. This is why we add overload relays to the motor starter.

The goal is to protect the motor from overheating. The current drawn by the motor is a reasonably accurate measure of the load on the motor, and thus of its heating. Thus we call this protective device an overload device.

Most overloads today use a thermally responsive element. That is, the same current that goes to the motor coils (causing the motor to heat) also passes through the thermal elements of the overload relays.

After mechanically connecting the thermal element to a normally closed contact, any excessive current flowing through the thermal element for a long enough time period will trip the contact trips open. This contact connects in series with the control coil of the starter. When the contact opens, the starter coil de-energizes. In turn, the starter power contacts disconnect the motor from the line.

A motor can operate on a slight overload for a long period or at a higher overload for a shorter period. Overheating of the motor will not result in either case. Therefore, the overload heater element should be designed to have heat-storage characteristics similar to those of the motor. However, they should be just enough faster so the relay will trip the normally closed relay contact before excessive heating occurs in the motor.

The ambient temperature in the location of the motor and starter also has some effect. It's necessary to specify the rating of a given temperature base plus the allowable temperature rise due to the load current. For example, an open-motor rating is generally based on 40DegrC (104DegrF). The motor's nameplate will specify the allowable temperature rise from this base.

Because the motor and starter are usually in the same ambient temperature, the same temperature conditions affect the overload-heater elements. The overloads will open the motor starter control circuit through excessive motor current, a high ambient temperature, or by a combination of both.

The ambient-compensated overload relay operates through a compensating bimetal relay. The relay maintains a constant travel-to-trip distance, independent of ambient conditions. Operation of this bimetal relay is responsive only to heat the motor overcurrent generates.

All overload relays should be trip free. Overload relays are not intended to protect against short-circuit currents. Short circuit protection is the function of fuses and circuit breakers.

Another feature of the motor starter is the auxiliary contact, which is normally used in the control circuit. In some small starters, an additional load contact can be used instead.

Auxiliary contacts may be normally open or closed and have 10A rating.

Reversing starters. Reversing and multispeed motor starters are specialized applications of across-the-line starters. In the reversing starter, there are two starters of equal size for a given horsepower application. You accomplish the reversing of a 3-phase, squirrel-cage induction motor by interchanging any two line connections. You must properly connect the two starters to the motor so the line feed from one starter is different from the other. We use mechanical and electrical interlocks to prevent both starters from closing their line contacts at the same time.

Multispeed starters. Many industrial applications require the use of more than one speed in their normal operation. While many new methods for adjustment of speed are available, the pole changing method is still used. For example, in a squirrel cage induction motor, the speed is dependent upon the number of poles. This is obtained by the design of the stator winding. At 60 Hz, which is standard in the United States, a two-pole motor operates at approximately 3600 revolutions per minute (r/min). A 4-pole operates at 1800 r/min, a 6-pole at 1200 r/min, an 8-pole at 900 r/min, and a 10-pole at 720 r/min. By having two or more sets of winding leads brought out into the terminal connection box, the number of effective poles can be changed. With a change in speed, the horsepower will also change. For example, in a two-speed motor for which the slower speed is one-half the higher speed, the horsepower will also be one-half the horsepower of the higher speed. This means that two sets of overload relays must provide adequate protection.

Two features common to complex motor starters are no-voltage release and no-voltage protection (sometimes seen as low-voltage release and low-voltage protection). No-voltage or low-voltage release means that if there is a voltage failure, the starter will open its contacts. However, the contacts will close again as soon as the voltage returns. No-voltage or low-voltage protection means that if there is a voltage failure, the starter contacts will open, but they will not reclose automatically when the voltage returns.

Each set of STOP/START push buttons may be located remote from any of the other sets to energize or de-energize a motor starter, so long as all the stop buttons remain in series with all circuit current flowing through them. For safety, the START push buttons may have individual locks to prevent unauthorized operation of any units. The use of a pilot light to indicate that a motor is energized is a safety factor along with a jogging feature. You wire a jogging switch like a start switch, but without the parallel holding contacts, so the motor runs only as long as you depress the jog button.

You can sequences a number of motors into operation using the auxiliary contact of the preceding motor starter. This gives a short, fixed-time delay of only the starter closing time. The opening of any of the motor starters through overload will de-energize all the motors in sequence. All the motors will be de-energized through operation of the one common STOP push-button switch.

Reduced-voltage starters. You can divide reduced-voltage motor starters into several different designs: Autotransformer (or compensator), primary resistor, wye (or star) delta, and part winding.

Connecting a motor directly across the line, the resulting current at start condition is frequently 4 to 12 times the full-load rating of the motor, and occasionally higher. This high starting current often causes voltage sags if the power supply is inadequate or the motor is large.

The basic principle of the reduced-voltage starter is to apply a percentage of the total voltage to start. After the motor starts to rotate, switching is provided to apply full line voltage. For example, in the autotransformer type, starting steps may be 50%, 65%, or 80% of full voltage.

At reduced voltage, the torque available from the motor reduces. This may cause concern when the motor is starting under load. Torque varies as the square of the impressed voltage.

For example, starting on the 50% voltage tap, torque will be 25% of normal. On the 65% voltage tap, torque will be 42% of normal. On the 80% voltage tap, torque will be 64% of normal.

Solid-state starters. Many applications require the lower starting torque and smooth acceleration that solid-state systems offer. Typical of such applications are conveyor systems, pumps, and compressors. Solid-state reduced-voltage controllers provide smooth, stepless acceleration of a motor through the use of silicon-controlled rectifiers. By controlling the conduction of the silicon-controlled rectifiers, voltage is gradually applied to the motor. We sometimes call this a soft start to the motor. An adjustable current-limit feature limits current to 25% to 70% and starting torque to 6% to 49% of full voltage values.

Solid-state reduced-voltage starters are available in voltage ratings of 200V, 230V, 460V, and 575V to 600V. Such starters are available from 1 hp through 1,000 hp at 480V or 600V, and from 10 hp through 300 hp at 208V or 240V.

Variable-frequency drives. One thing that never changes in traditional electrical work is the standard 60 Hz frequency of our AC power. But the rotational speed of an AC motor is a direct function of the frequency of the current it uses. That frequency (as well as phasing and number of poles) determines the speed of the motor's rotating magnetic field.

Equipment, such as boiler fans, large pumps, or conveyors require speed control. Traditionally this was accomplished with wound-rotor motors and large, variable rotor-resistors, or other complex arrangements. These methods were inefficient and expensive, but they did allow for speed control.

Varying the frequency of the current supplied to an AC motor is a much more elegant method than any of the others that have been tried, but it was not something that could easily be done. It was expensive and difficult.

Affordable variable-speed drives are possible because of advances in solid-state electronics. Insulated gate bipolar transistors (IGBT) made it possible to switch large current levels at high-kHz rates, leading to pulse-width-modulated (PWM) inverter drives. Distortion levels are reasonable, units are compact, and operating efficiencies are very high.

PWM inverters do the job by controlling the kHz-chopping rate of a wave; varying its envelope in voltage and fundamental frequency; to control the acceleration and running of the familiar induction motor. The pulse rate can be set high enough to make audible noise of vibration, stimulated in the motor by the VFD, inaudible. You can arrange the range of fundamental frequencies through which the drive accelerates and operates the motor to skip critical frequencies where resonances in the mechanical equipment and structural supports might be excited to damaging levels.

The circuitry of a variable frequency is similar to other system applications of solid-state motor controllers and uninterruptible power supply systems, as used for large computer systems and data centers.

There are two general categories of VFD designs: Voltage source inverter (VSI) and Current source inverter (CSI.) Manufacturers of VFDs seemed to use one style or the other. The two types differ in the manner in which power is passed to the motor after being processed by the drive. The VSI drive treats the motor as a parallel-connected load and controls the overall performance envelope by adjustments to the output voltage of the drive. The CSI drive centers on motor impedance (inductive reactance) since it is driving current through the motor as part of the performance envelope. You must match CSI drives with their motors.

This motor matching is less important for VSI drives. Some use VFDs with motors of unknown origin, particularly in retrofits. However, one source should specify, purchase, and install VFDs with their associated motors whenever possible. Keep track of the thermal budget for the operating motor, and even for rooms housing large groups of drives.

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