ecet 211 electric machines & controls lecture 8 motor...

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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


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

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• 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.

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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

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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

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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)

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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.



Required ampacity of the feeder conductor is

(1.25)(65) + 40 + 14 = 135.25 A

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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

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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

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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.

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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.

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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

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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

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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

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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.

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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

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Part 2. Motor StartingFull-Voltage Starting of AC Induction Motors: Manual Starters

Figure 8-12 Typical magnetic across-the-line starter

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Figure 8-13 Connection diagram for motor pushbutton stations

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Figure 8-14 Timed starting of two motors

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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

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Part 2. Motor StartingTable 8-1 Typical voltage, Current, and torque characteristics for NEMA Design B Motors

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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

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Reduced Voltage Starting of AC Induction Motors

Figure 8-22 Part-winding starting

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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

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Reversing of AC Induction Motors

• Reversing three-phase Induction Motor Starter

Figure 8-29 Magnetic full-voltage three-phase reversing


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• Figure 8-30 Mechanical interlocking of forward and reverse contactors

Figure 8-31 Magnetic reversing starter with electrical interlock in the motor starter

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• Figure 8-32 Reversing starter circuit implemented using IEC symbols


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• Figure 8-33 Pushbutton interlocking

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• Figure 8-34 Limit switches incorporated into a reversing starter circuit to limit travel


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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

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Figure 8-36 Reciprocating machine process

a repeated forward and reverse action


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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

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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


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Jogging

Figure 8-39 Jog circuit with control relay

Figure 8-40 Start/stop/selector jog control circuit

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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


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Plugging and Antiplugging

Figure 8-43 Antiplugging protection circuit

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Dynamic Braking

Figure 8-44 Dynamic braking applied to a DC motor


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DC Injection Braking

Figure 8-45 DC injection braking applied to an DC

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Electromechanical Friction Brake

Figure 8-46 Electromechanical drum and shoe-type friction brake used on DC series motor drives


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Electromechanical Friction Brake

Figure 8-47 AC electromagnetic brake

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Part 5. Motor Speed

• Multispeed Motors

Figure 8-48 Two-speed separate winding across-the-line motor starter


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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

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Summary & Conclusion

Questions?Contact Prof. Lin through:

Email: [email protected]

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ecet 211 electric machines & controls lecture 8 motor...

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