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3 pole DC motors

When alternating current is applied to the field coil the magnetic field will also alternating. Therefore the permanent magnet will rotate

When alternating current is applied to the field coil the magnetic field will also alternating. Therefore the permanent magnet will rotate

When alternating current is applied to the field coil the magnetic field will also alternating. Therefore the permanent magnet will rotate

FINAL CONTROL ELEMENT

- The final control element adjust the amount of energy/mass goes into or out from process as commanded by the controller
- The common energy source of final control elements are:
- Electric
- Pneumatic
- Hydraulic

ELECTRIC FINAL CONTROL ELEMENT

- Electric current/voltage
- Solenoid
- Stepping Motor
- DC Motor
- AC Motor

CHANGING CURRENT/VOLTAGE

- Current or voltage can be easily changed to adjust the flow of energy goes into the process e.g. in heating process or in speed control
- Heater elements are often used as device to keep the temperature above the ambient temperature. Energy supplied by the heater element is
W = i2rt (i=current, r=resistance, t=time)

- Motor is often used as device to control the speed

CHANGING CURRENT/VOLTAGE

- Using
- Potentiometer
- Amplifier
- Ward Leonard system
- Switch (on-off action)

Changing Current/VoltageUsing Rheostat

I = V/(R1+R2)

Power at rheostat

P1 =I2R1

Power at heater

P2 =I2R2

Disadvantage loss of power at rheostat

Rheostat

Heater

R1

V

R2

I

Changing Current/VoltageUsing Amplifier

Potentiometer

V+

Heater

amplifier

R1

V

R2

V−

Disadvantage loss of power at potentiometer (very small) and at Amplifier

Changing Current/VoltageUsing Ward Leonard System

- Introduced by Harry Ward Leonard in 1891
- Use a motor to rotate a generator at constant speed
- The output of generator voltage is adjusted by changing the excitation voltage
- Small change in excitation voltage cause large change in generator voltage
- Able to produce wide range of voltage (0 to 3000V)
- Ward Leonard system is popular system to control the speed of big DC motor until 1980’s
- Now a days semi conductors switches replaces this system

Changing Current/VoltageUsing Switch

- The switch is closed and opened repeatedly
- No power loss at switch

Switch closed

VL

Switch

V

LOAD

V

VL

t

Switch opened

DUTY CYCLE

VL

V

- T is period time typical in millisecond order (fix)
- Ton is switch on time (adjustable)
- Toff is switch off time
Duty Cycle is:

(Ton/T) 100%

t

Ton

Toff

T

- Of course we can not use mechanical switches to carry on this task, electronic switches to be used instead.
- E.g. Transistor, Thyristor, or IGBT
- This methods is often called as Pulse Width Modulation (PWM)

Solenoid Usage

- pushing buttons,
- hitting keys on a piano,
- Open closed Valve,
- Heavy duty contactor
- jumping robots
- etc

STEPPING MOTOR

The top electromagnet (1) is turned off, and the right electromagnet (2) is energized, pulling the nearest teeth slightly to the right. This results in a rotation of 3.6° in this example.

The top electromagnet (1) is turned on,

attracting the nearest teeth of

a gear-shaped iron rotor. With the teeth aligned to electromagnet 1, they will be slightly offset from electromagnet

STEPPING MOTOR

The left electromagnet (4) is enabled, rotating again by 3.6°.

The bottom electromagnet (3) is energized; another 3.6° rotation occurs.

When the top electromagnet (1) is again enabled, the teeth in the sprocket will have rotated by one tooth position; since there are 25 teeth, it will take 100 steps to make a full rotation in this example.

STEPPING MOTOR

- Practical stepping motor can be controlled for full step and half step.
- Common typical step size is 1.8o for full step and 0.90 for half step
- Full step is accomplished by energizing 2 adjacent electromagnet simultaneously.
- Half step is accomplished by energizing 1 electromagnet at a time.

DC Motor

The brush

Practical DC Motors

Every DC motor has six basic parts –

axle,

rotor (a.k.a., armature),

stator,

commutator,

field magnet(s),

and brushes.

For a small motor the magnets is made from permanent magnet

2 pole motor

Animate

2 pole motor

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2 pole motor

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2 pole motor

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2 pole motor

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2 pole motor

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2 pole motor

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2 pole motor

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3 pole DC motors

1

The coil for each poles are connected serially.

The commutator consist of 3 sector, consequently one coil will be fully energized and the others will be partially energized.

2

3

−

+

3 pole DC motors

The commutator and the coil is arranged in such a way that the polarity of each pole is as shown

animate

next

3 pole DC motors

The commutator and the coil is arranged in such a way that the polarity of each pole is as shown

animate

next

3 pole DC motors

The commutator and the coil is arranged in such a way that the polarity of each pole is as shown

animate

next

The commutator and the coil is arranged in such a way that the polarity of each pole is as shown

animate

next

DC motors

- As the rotor is rotating, back emf (Ea) will be produced, the faster the rotor turn the higher Ea and the smaller Ia.
- The starting current of motors will be much higher then the rating current.

motor

Ia

Ea

V

DC motors

For big motors the magnet is made from coil and core. The current flowing in the coil is called If and the current flowing in the armature is called Ia.

The armature winding and the field winding are connected to a common power supply

The armature winding and the field winding are often connected in series, parallel, or compound. The torque characteristic will be different for each connection.

The figure shows a parallel connection

Field winding

Armature winding

SERIES DC MOTOR

Field and armature winding are series connected, this type of motor is called series DC motor

DC motors

Field and armature winding are parallel connected, this type of motor is called shunt DC motor

DC MOTOR

Compound DC motor is DC motor having 2 field winding the first one is connected parallel to the armature winding and the other is connected series

DC MOTOR Magnetic flux is constant if it is from permanent magnet It is depend on the If if it is produced by current

Torque: T = KΦIa

- K is a constant
- Φ magnetic flux
- Ia is armature current

DC MOTOR TORQUE-SPEED CURVE

Torque: T = KΦIa

SHUNT DC MOTOR TORQUE-SPEED CURVE

Torque: T = KΦIa

S

SYNCHRONOUS AC MOTORThe rotating field.

When alternating current is applied to the field coil the magnetic field will also alternating. Therefore the permanent magnet will rotate

o

311

-311

~

S

SYNCHRONOUS AC MOTORThe rotating field.

When alternating current is applied to the field coil the magnetic field will also alternating. Therefore the permanent magnet will rotate

o

311

-311

~

S

SYNCHRONOUS AC MOTORThe rotating field.

When alternating current is applied to the field coil the magnetic field will also alternating. Therefore the permanent magnet will rotate

o

311

-311

~

S

SYNCHRONOUS AC MOTORThe rotating field.

o

311

-311

~

S

SYNCHRONOUS AC MOTORThe rotating field.

o

311

-311

~

S

SYNCHRONOUS AC MOTORThe rotating field.

o

311

-311

~

S

SYNCHRONOUS AC MOTORThis motor has 2 poles

If the frequency of the current is f hertz (cycle/s) then the rpm

n = f rps

n = (120f)/p rpm

Where p is the number of the poles

o

311

-311

~

S

T

SYNCHRONOUS AC MOTOR USING EXTERNAL EXITERThe magnetic flux of permanent magnet is low for a bigger motor we have to use externally exited magnetic field

ASYNCHRONOUS AC MOTOR

- When instead of exited, the rotor coil is shorted an induced current will be generated and the rotor will be magnetized and start to turn.
- The faster the speed the smaller the induced current and finally the current will cease at synchronous speed and so does the rotation
- This motor will turn at speed less the its synchronous rotation that is why it called asynchronous motor
- This motor is also called induction motor

Iinduced

Calculating Motor Speed

- A squirrel cage induction motor is a constant speed device. It cannot operate for any length of time at speeds below those shown on the nameplate without danger of burning out.
- To Calculate the speed of a induction motor, apply this formula:
Srpm = 120 x F P

Srpm = synchronous revolutions per minute.120 = constantF = supply frequency (in cycles/sec)P = number of motor winding poles

- Example: What is the synchronous of a motor having 4 poles connected to a 60 hz power supply?
Srpm = 120 x F PSrpm = 120 x 60 4Srpm = 7200 4Srpm = 1800 rpm

Calculating Braking Torque

- Full-load motor torque is calculated to determine the required braking torque of a motor.To Determine braking torque of a motor, apply this formula:
T = 5252 x HP rpm

T = full-load motor torque (in lb-ft)5252 = constant (33,000 divided by 3.14 x 2 = 5252)HP = motor horsepowerrpm = speed of motor shaft

- Example: What is the braking torque of a 60 HP, 240V motor rotating at 1725 rpm?
T = 5252 x HP rpmT = 5252 x 60 1725T = 315,120 1725T = 182.7 lb-ft

Calculating Work

- Work is applying a force over a distance. Force is any cause that changes the position, motion, direction, or shape of an object. Work is done when a force overcomes a resistance. Resistance is any force that tends to hinder the movement of an object.If an applied force does not cause motion the no work is produced.
- To calculate the amount of work produced, apply this formula:
- W = F x D
- W = work (in lb-ft)F = force (in lb)D = distance (in ft)
- Example: How much work is required to carry a 25 lb bag of groceries vertically from street level to the 4th floor of a building 30' above street level?
- W = F x DW = 25 x 30W = 750 -lb

Pneumatic Actuator

Reverse-Acting Actuator

I/P Converter

- A "current to pressure" converter (I/P) converts an analog signal (4-20 mA) to a proportional linear pneumatic output (3-15 psig).
- Its purpose is to translate the analog output from a control system into a precise, repeatable pressure value to control pneumatic actuators/operators, pneumatic valves, dampers, vanes, etc.

I/P

Air supply

30 psi

Pneumatic 3 to 15 psi

Supplied to actuator

Current

4 to 20 mA

Generation and distribution of pneumatic pressure

- Compressor is needed for pneumatic system

PC

PS

Regulator valve

Tank

compressor

To I/P

100 psi

30 psi

Disadvantage

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