Upload
lamngoc
View
222
Download
0
Embed Size (px)
Citation preview
1
1
ECET 211 Electric Machines & Controls
Lecture 8 Motor Control Circuits
Text Book: Electric Motors and Control Systems, by Frank D. Petruzella, published by McGraw Hill, 2015.
Paul I-Hai Lin, Professor Electrical and Computer Engineering Technology
P.E. States of Indiana & California
Dept. of Computer, Electrical and Information Technology
Purdue University Fort Wayne Campus
Prof. Paul Lin
Lecture 8 Motor Control Circuits
Part 1. NEC Motor Installation Requirements
• Sixing Motor Branch Circuit Conductor
• Branch Circuit Motor Protection
• Selecting a Motor Controller
• Disconnecting Means for Motor ad Controller
• Providing a Control Circuit
Part 2. Motor Starting
• Full-Voltage Starting of AC Induction Motors
• Reduced-Voltage Starting of Induction Motors
• DC Motor Starting Prof. Paul Lin 2
Part 3. Motor Reversing and Jogging
• Reversing of AC Induction Motors
• Reversing of DC Motors
• Jogging
Part 4. Motor Stopping
• Plugging and Anti-plugging
• Dynamic Breaking
• DC Injection Breaking
• Electromechanical Friction Brakes
Part 5. Motor Speed
• Multispeed Motors
• Wound-Rotor Motors
2
Lecture 8 Motor Control Circuits
Part 1. NEC Motor Installation Requirements
• NEC Article 430 covers application and installation of motor circuits including conductors, short-circuit, and ground fault protection, starters, disconnects, and overload protection.
• Motor Brach Circuits – include the final overcurrent device (disconnect switch and fuses or circuit breaker), the motor starter and associated control circuits, circuit conductors, and the motor.
Prof. Paul Lin 3
Figure 8-1 Basic elements of a motor branch circuit that the NEC addresses
Motor Control Circuits – NEC Motor Installation Requirements
Sizing Motor Branch Circuit Conductor
NEC Article 430, Part II
Article 430.6
• Installation requirements for motor branch circuit conductor
• A single motor used in a continuous-duty application must have an ampacity of not less than 125 percent of the motor’s Full-Load Current (FLC)
Article 430.247 through 430.250
• Conductor ampacity must be determined by NEC Tables 430.247 through 430.250 and is based on the motor nameplate horsepower rating and voltageProf. Paul Lin 4
Full-Load Current (FLC) –indicates the use of NEC table rating
Full-Load Amperes (FLA) –indicates the actual nameplate rating
Article 430.247 through 430.250
3
Lecture 8 Motor Control CircuitsPart 1. NEC Motor Installation Requirements: Sizing Motor Branch Circuit Conductor
Example 8-1
Problem: using your edition of the NEC, determine the minimum branch circuit conductor ampacityrequired for each of the following motors:
(a) 2 hp, 230V single-phase motor
(b) 30 hp, 230V, three-phase motor with a nameplate FLA rating of 70A
Solution:
(a) NEC Table 430-248 shows the FLC as 12 A. Conductor ampacity required is 12 x 125% = 15A
(b) NEC Table 430.250 shows the FLC as 80A.
Conductor ampacity required is 80 x 125% = 100A
Prof. Paul Lin 5
http://www.automationdirect.com/adc/Shopping/Catalog/Motors
Lecture 8 Motor Control CircuitsPart 1. NEC Motor Installation Requirements: Sizing Motor Branch Circuit Conductor
Feeder Conductors supplying two or more motors must have:
An ampacity not less than 125 percent of the FLC rating of the highest-rated motor, plus,
The sum of the FLC ratings of the other motor supplies.
Ampacity of the conductor => NEC Table 310.15(B)(16) => American Wire Gauge (AWG)
Prof. Paul Lin 6
4
Lecture 8 Motor Control CircuitsExample 8-2. Problem: Three 460V, 3Φ motors rated at 50, 30, and 10 hpshare the same feeder (Figure 8-2). Using your edition of the NEC, determine the ampacity required for size the feeder conductors.
Solution:
50hp motor – NEC Table 430.250 shows the FLC as 65A.
30hp motor – NEC Table 430.250 shows the FLC as 40A.
10hp motor – NEC Table 430.250 shows the FLC as 14A.
Required ampacity of the feeder conductor is
(1.25)(65) + 40 + 14 = 135.25 A
Prof. Paul Lin 7
Part 1. NEC Motor Installation RequirementsBranch Circuit Motor Protection Nonmotor loads – use circuit breaker that combines overcurrent
protection with short-circuit and ground fault protection.
Motor loads
• Draws up to 6 times of normal FLC of the motor.
• Best method of protection for motors – separate the overload protection devices from the short circuit and ground fault protection
• Figure 8-3 Motor branch circuit protection
Prof. Paul Lin 8
5
Part 1. NEC Motor Installation RequirementsBranch Circuit Motor Protection NEC Article 430, Part IV
• Explains the requirements for branch circuit short-circuit and ground fault protection.
• The NEC requires that branch circuit protection for motor circuits must protect the circuit conductors, the control apparatus, and the motor against over current due to “short circuit” or “ground faults.”
• Table 430.52 – maximum values on the ratings or setting of these devices
• NEC Article 240.6 – lists the standard sizes of fuses and breakers
Instantaneous trip circuit breakers
Inverse time circuit breaker – the higher the overcurrent, the shorter the time required for the breaker to trip and open the circuit
Prof. Paul Lin 9
Part 1. NEC Motor Installation RequirementsBranch Circuit Motor Protection Example 8-3. Problem:
• Determine the size of inverse time circuit breaker permitted to be used to provide motor branch circuit short circuit and ground fault protection for a 10 hp, 208V, 3Φ squirrel-cage motor.
Solution:
• NEC Table 430.250 => the motor FLC = 30.8A.
• NEC Table 430.52 => maximum ratings for an inverse time breaker as 250 percent of the FLC.
30.8 x 2.5 = 77A
Use 80A inverse time circuit breaker if a 70A’s is not adequate.
Prof. Paul Lin 10
6
Part 1. NEC Motor Installation Requirements
Selecting a Motor Controller Motor controller
• Any device that is used to directly start and stop an electric motor by closing and opening the main power current to the motor.
• It can be a switch, starter, or other similar type of control device.
• Figure 8-4 Examples of motor controllers
• NEC Article 430, Part VII – details the requirements for motor controllers – see page 204 for some of the highlights.
Prof. Paul Lin 11
Part 1. NEC Motor Installation Requirements
Disconnecting Means for Motor and Controller
NEC Article, Part IX – covers the requirements for the motor disconnecting means.
The Code requires that a means (a motor circuit switch rated in horsepower or a circuit breaker) must be provided in each motor circuit to disconnect both the motor and its controller from all ungrounded supply conductors.
Separate disconnects and controllers may be mounted on the same panel or contained in the same enclosure, such as Figure 8-5 Combination fused-switch, magnetic starter unit
Prof. Paul Lin 12
7
Part 1. NEC Motor Installation RequirementsDisconnecting Means for Motor and Controller
If a person is working on the motor, the disconnect will be where he or she can see.
It protects the person from a motor accidentally starting.
The NEC defines “within sight” as being visible and not more than 50 ft (15 m) distant from the other.
Figure 8-6 The disconnecting means must be located within sight from the controller, and the driven machine location
Prof. Paul Lin 13
Part 1. NEC Motor Installation RequirementsDisconnecting Means for Motor and Controller
For stationary motors rated more than 40 hp DC or 100 hp AC, a general-use or isolating switch can be used but should be plainly marked “DO NOT OPERAE UNDER LOAD.”
An isolating switch
• Intended to isolate an electric circuit from its source of power
• No interrupting rating
• Intended to be operated only after the circuit has been opened by some other means.
Example 8-4 Problem: determine the current rating of the motor disconnect switch required for a 460V, three-phase, 125 hp motor.
Solution: NEC Table 430.250 => Motor FLC = 156 A
NEC 430.110 => motor disconnecting means to have an ampere rating of at least 115 percent of the FLC rating of the motor
156A x 1.15 = 179A
A 200 A disconnect switch is requiredProf. Paul Lin 14
8
Part 1. NEC Motor Installation Requirements
Providing a Control Circuit
Has its load devices: coils of magnetic contactor, magnetic starter, relay, etc
NEC Article 430 covers the requirements for motor control circuits
• The elements of control circuit all the equipment and devices concerned with the function of the circuit:
Conductors, Raceways, Contactor coils, Source of energy supply to the circuit, Overcurrent protection devices, and all switching devices that govern energization of the operating coil
• Control circuit voltages and control transformers: 120V, 460V, 600V
• Ground fault
NEC Article 430.75 requires that motor control circuits be arranged so that they will be disconnected from all source of supply when the disconnecting means in the open position.
Prof. Paul Lin 15
Part 1. NEC Motor Installation RequirementsProviding a Control Circuit
Figure 8-7 The design of the control circuit must prevent the motor from being started by a ground fault in the control circuit wiring
Figure 8-7a. A ground fault on the coil side of the start button can short-circuit the start circuit and start the motor
Prof. Paul Lin 16
9
Part 2. Motor StartingFull-Voltage Starting of AC Induction Motors: Manual Starters
Figure 8-12 Typical magnetic across-the-line starter
Prof. Paul Lin 17
Part 2. Motor StartingFull-Voltage Starting of AC Induction Motors: Manual Starters
Figure 8-13 Connection diagram for motor pushbutton stations
Prof. Paul Lin 18
10
Part 2. Motor StartingFull-Voltage Starting of AC Induction Motors: Manual Starters
Figure 8-14 Timed starting of two motors
Prof. Paul Lin 19
Part 2. Motor Starting
Reduced-Voltage Starting of AC Induction Motors:
Two reasons:
1) Limits line disturbances
2) Reduces excessive torque to the driven equipment
When a motor is started at full voltage, the current drawn from the power line is typically 600 percent of normal full-load current
The large starting inrush current of a big motor could cause line voltage dips and brown-out.
Higher than full-load torque can cause mechanical damage such as belt, chain, or coupling breakage.
Electric utility current restrictions, as well as in-plant bus capacity, may require motors above a certain horsepower to be started with reduced voltage.
Typical reduced voltage starters: Primary-resistance, Autotransformers, Wye-Delta, Part-winding, solid-state starters Prof. Paul Lin 20
11
Part 2. Motor StartingTable 8-1 Typical voltage, Current, and torque characteristics for NEMA Design B Motors
Prof. Paul Lin 21
Motor starting current as a percent of:
Line current as a percent of:
Motor starting torque as a percent
of:
StartingMethod
% voltage at
motor terminals
Locked-rotor
current
Full-load
current
Locked-rotor
current
Full-load
current
Locked-rotor
current
Full-load
current
Full voltage 100 100 600 100 600 100 180
Autotransformer80% tap65% tap50% tap
806550
806550
480390300
644225
644225
30716425
1157645
Part-winding
100 65 390 65 390 50 90
Wye-delta 100 33 198 33 198 33 60
Solid-state 0-100 0-100 0-600 0-100 0-600 0-100 0-180
Reduced Voltage Starting of AC Induction Motors
Figure 8-20 Wye and delta motor winding connections
Figure 8-21 Wye-delta starter
Prof. Paul Lin 22
12
Reduced Voltage Starting of AC Induction Motors
Figure 8-22 Part-winding starting
Prof. Paul Lin 23
Part 2. Motor StartingReduced Voltage Starting of AC Induction Motors
Figure 8-24 Soft start ramped-up voltage and current limiting
Figure 8-25 Typical soft start starter
Starting Modes
• Soft start
• Selectable kick start
• Current limit start
• Dual-ramp start
• Full-voltage start
• Liner speed acceleration
• Preset slow speed
• Soft stopProf. Paul Lin 24
13
Lecture 8 Motor Control Circuits
Prof. Paul Lin 25
Part 3. Motor Reversing and Jogging
Reversing of AC Induction Motors
• Reversing three-phase Induction Motor Starter
Figure 8-29 Magnetic full-voltage three-phase reversing
Part 3. Motor Reversing and Jogging
Prof. Paul Lin 26
Reversing of AC Induction Motors
• Figure 8-30 Mechanical interlocking of forward and reverse contactors
Figure 8-31 Magnetic reversing starter with electrical interlock in the motor starter
14
Part 3. Motor Reversing and Jogging
Prof. Paul Lin 27
Reversing of AC Induction Motors
• Figure 8-32 Reversing starter circuit implemented using IEC symbols
Part 3. Motor Reversing and Jogging
Prof. Paul Lin 28
Reversing of AC Induction Motors
• Figure 8-33 Pushbutton interlocking
15
Part 3. Motor Reversing and Jogging
Prof. Paul Lin 29
Reversing of AC Induction Motors
• Figure 8-34 Limit switches incorporated into a reversing starter circuit to limit travel
Part 3. Motor Reversing and Jogging
Prof. Paul Lin 30
Reversing of AC Induction Motors
Figure 8-35 Reversing a single-phase motor
The direction of rotation is changed by interchanging the start winding leads, while those of the run winding remain the same
16
Part 3. Motor Reversing and Jogging
Prof. Paul Lin 31
Reversing of AC Induction Motors
Figure 8-36 Reciprocating machine process
a repeated forward and reverse action
Part 3. Motor Reversing and Jogging
Prof. Paul Lin 32
Reversing of DC Motors
The reversal of a DC motor can be accomplished in two ways:
• Reversing the direction of the armature current (IA); leaving the field current the same
• Reversing the direction of the field current (IF) and leaving the armature current the same
Figure 8-37 DC motor reversing power circuits
17
Part 3. Motor Reversing and Jogging
Prof. Paul Lin 33
Jogging
Jogging (sometimes called Inching) is momentary operation of a motor for the purpose of accomplishing small movements of the driven machine.
Figure 8-38 Push button job circuit
Part 3. Motor Reversing and Jogging
Prof. Paul Lin 34
Jogging
Figure 8-39 Jog circuit with control relay
Figure 8-40 Start/stop/selector jog control circuit
18
Part 4. Motor Stopping
Prof. Paul Lin 35
Motor Stopping
Remove the power supply plus electric braking
Plugging and Antiplugging
Plugging – stops a polyphasemotor quickly by momentarily connecting the motor for revere rotation while the motor is still running in the forward direction.
Figure 8-41 Plugging switch
Figure 8-42 Plugging a motor to stop it
Part 4. Motor Stopping
Prof. Paul Lin 36
Plugging and Antiplugging
Figure 8-43 Antiplugging protection circuit
19
Part 4. Motor Stopping
Prof. Paul Lin 37
Dynamic Braking
Figure 8-44 Dynamic braking applied to a DC motor
Part 4. Motor Stopping
Prof. Paul Lin 38
DC Injection Braking
Figure 8-45 DC injection braking applied to an DC
20
Part 4. Motor Stopping
Prof. Paul Lin 39
Electromechanical Friction Brake
Figure 8-46 Electromechanical drum and shoe-type friction brake used on DC series motor drives
Part 4. Motor Stopping
Prof. Paul Lin 40
Electromechanical Friction Brake
Figure 8-47 AC electromagnetic brake
21
Lecture 8 Motor Control Circuits
Prof. Paul Lin 41
Part 5. Motor Speed
• Multispeed Motors
Figure 8-48 Two-speed separate winding across-the-line motor starter
Lecture 8 Motor Control Circuits
Prof. Paul Lin 42
Part 5. Motor Speed
• Wound-Rotor Motors
Figure 8-49 Wound-rotor magnetic motor controller
Low speed (full resistance) – both S & H are open
Medium speed – S closed
Maximum speed – H closed
22
Summary & Conclusion
Questions?Contact Prof. Lin through:
Email: [email protected]
Prof. Paul Lin 43