powerpoint presentation€¦ · ppt file · web view · 2017-01-02shaded-pole motor. the shaded...

Electric MotorsMECH1200

• AC current reverses direction• Two parts: stator and rotor

• Stator: stationary electrical component• Rotor: rotates the motor shaft

• Two types• Synchronous motor• Induction motor

AC Motors


AC Motors


• Most common motors in industry• Advantages:

• Simple design• Inexpensive• High power to weight ratio• Easy to maintain• Direct connection to AC power

source

AC – Induction motor



How induction motors work• Electricity supplied to stator• Magnetic field generated that moves

around rotor• Current induced in rotor

Electromagnetics

Stator

Rotor

• Rotor produces second magnetic field that opposes stator magnetic field

• Rotor begins to rotate


• Single-phase induction motor• One stator winding• Single-phase power supply• Squirrel cage rotor• Require device to start motor• Up to 3 to 4 HP• Household appliances: fans, washing

machines, dryers


Electric MotorsMECH1200Shaded-pole motor

• The shaded pole delays the creation of the magnetic field in that portion of the stator poles.

• This produces a magnetic field in the shaded portion that is approximately 90° apart from the magnetic field produced in the main portion of the pole.

• Considered a nonreversible motor.

Electric MotorsMECH1200Three Types of Capacitor

Start Motors

1. Capacitor Start (disconnects capacitor after motor speed picks up)

2. Capacitor Run (Keeps the capacitor connected during the operation of the motor, in order to keep the electric power consumption low)

3. Capacitor Start-Run (uses two capacitors, one for starting and one for running. This further improves Power Consumption)

Electric MotorsMECH1200Capacitor-start motor

• Start circuit has:– Centrifugal switch– Start winding– Start capacitor

• This produces higher starting torque.

• Run winding• Reverse direction of

rotation by interchanging run winding or start winding connections (preferred).

Electric MotorsMECH1200Capacitor-run motor

• The capacitor shifts the phase on one of the windings so that the voltage across the winding is at 90° from the other winding

• Run capacitor produces higher running torque.

• Start winding stays as part of the circuit

• Run winding• Reverse direction of

rotation by interchanging run winding or start winding connections (preferred).

Electric MotorsMECH1200Capacitor-start/capacitor-

run motor• Start circuit:

– Start winding– Centrifugal switch– Start capacitor

• Larger value produces higher starting torque.

• Run winding• Run capacitor

– Smaller value produces higher running torque.

• Reverse direction of rotation by interchanging run winding or start winding connections (preferred).

Electric MotorsMECH1200Capacitor start-run

reverse connections


•Winding(s) which produce the magnetic field(s) necessary to cause the rotor to turn.

3 Phase; 2 Pole Motor

What are Poles in a Motor?


• Requires DC voltage for starting excitation

• Has low starting torque • Suited for low load

applications• Rotor of the synchronous

motor travels at the same speed as the rotating magnetic field

AC - Synchronous motor

Electric MotorsMECH1200Synchronous motor

Three-Phase Motors• Parts

– Rotor with single winding

– Slip rings and brushes

– Three-phase stator windings


• Constant speed fixed by system frequency

• Used where there is a need to improve the power factor

• Synchronous speed (Ns):

AC - Synchronous motor

F = frequency of the voltage source supplied

P = number of polesPFNS *120

5252*)( )()( RPMlbsft SpeedTorqueHPPower

Electric MotorsMECH1200Synchronous speed

P 50 Hz 60 Hz

2 3000 3600

4 1500 1800

6 1000 1200

8 750 900

10 600 720

12 500 600

Electric MotorsMECH12004-Pole stator winding

• Each AC phase has 4 stator windings

• Each winding is in opposite direction from preceding winding, making a N-S-N-S field

• Field strength rotates with AC current of each phase


• Three-phase induction motor• Three-phase supply produces

magnetic field• Squirrel cage or wound rotor• Self-starting• High power capabilities• Fractional to 100’s of HP • Applications: pumps, compressors,

conveyor belts, grinders• 70% of motors in industry!



• Aka: Asynchronous motor• The induction ac motor is a common form of an asynchronous motor • Is basically an AC transformer with a rotating secondary



Components• Rotor

•Squirrel cage: conducting barsin parallel slots

•Wound rotor: 3-phase, double-layer, distributed winding


• Stator• Stampings with slots to carry 3-phase windings

• Wound for definite number of poles

Electric MotorsMECH12003-phase Induction Motor

Operation• Arrows shows

stator magnetic field vector

• Stator field precedes the rotor’s induction field

http://en.wikipedia.org/wiki/File:3phase-rmf-noadd-60f-airopt.gif


• Interchange any two of the three stator leads.– The industry standard is to switch T1

and T3 .• The wound-rotor induction motor is

considered to be a variable-speed motor.

• Initial cost is higher and maintenance costs are higher than for a squirrel-cage induction motor.

Reversing Direction of a 3 Phase Motor

Electric MotorsMECH1200Reversing Direction of a 3 Phase Motor

A

B

C

Electric MotorsMECH1200Changing AC Motor

Speed• Voltage – Hertz Ratio:

– Operating motor in a range different from rated frequency and voltage affects both torque and current .

HertzVoltageRatio

HzV AC

Electric MotorsMECH1200Changing AC Motor

Speed• Voltage – Hertz Ratio:

– Maintaining the ratio gives a constant torque range

HertzVoltageRatio

HzV AC

For a synchronous motor rated for 3 phase, 460 volts, 60 Hz and 3600 rpm, what will be the operating frequency and voltage if the motor controller commands the motor to run at 2750 rpm?

60460

RatioHzV 7.67

PfNS *120

3600

60*120P 2 poles

PfNS *120

2750*1202 2750rpmatf

45.83 Hz

83.4567.7 V

351.52 volts

Electric MotorsMECH1200Squirrel-cage induction

motor• Parts:

– Rotor– Stator

• The squirrel-cage induction motor is considered to be a fixed-speed motor.


Speed and slip• Motor never runs at synchronous

speed but lower actual rotor speed• Difference is “slip”• Install slip ring to avoid this• Calculate % slip:

Ns = synchronous speed in RPMNR = rotor speed in RPM


100*%S

RS

NNNSlip

Electric MotorsMECH1200Wound-rotor induction

motor• The rotor contains

windings.• Slip rings and brushes

provide an electrical connection to the rotor windings.

• The wound-rotor induction motor is considered to be a variable-speed motor.

• Initial cost is higher and maintenance costs are higher than for a squirrel-cage induction motor.


Relationship: load, speed and torque

Starting Torque (aka LRT): high torque and low speed

“Pull-up” torque: lower torque and increasing speed

“Breakdown” torque: 75% speed and highest torque = 178.6 ft-#’S

Full load torque: motor operates at rated voltage, frequency and load and stator current are zero30 HP

1765 RPM

Electric MotorsMECH1200Torque Curve

Calculate:Speed at 100% full load current% Slip

0 10 20 30 40 50 60 70 80 90 98 1000

100

200

300

400

500

600

Torque Curve For Asychronous Induction Motor with a synchronous speed of 1800 RPM

% Synchorous Speed

% F

ull L

oad

Curr

ent

Electric MotorsMECH1200AC Motor Data

Plate


Types of Motor Enclosures

• ODP – Open Drip Proof– Air flows through motor (fan blades help flow)– Used in environments free from contaminants

Electric MotorsMECH1200Types of Motor

Enclosures• TENV – Totally Enclosed Non-Ventilating

– Protect motor from corrosive and harmful elements

– Frame fins help to dissipate heat

Electric MotorsMECH1200Types of Motor

Enclosures• TEFC – Totally enclosed Fan Cooled

– Similar to TENV except has external fan for cooling

Electric MotorsMECH1200Types of Motor Enclosures

• XP – Explosion Proof – Similar to TEFC but enclosures are cast iron


Hazardous Locations

Division I – Hazardous material present in the air as a norm

Division II - Hazardous material present in the air as an abnormal event

Electric MotorsMECH1200Summary

• DC motors are:– permanent magnet– series-wound, – shunt-wound, – compound-wound

• AC single phase motors are:– the shaded-pole, – split-phase, – capacitor-start, – capacitor-run, – capacitor-start/ capacitor-run


Questions

• Name two motors that do not need brushes for their rotor windings.

• Which motor supplies the highest output torque to weight ratio?

• Why is it not recommended to use dc motors in artificial hearts?

• In an environment that contains explosive gases, such as in mines, which motor do you recommend using:

a) series dc motor b) shunt dc motor c) induction motor d) universal motor

powerpoint presentation€¦ · ppt file · web view · 2017-01-02shaded-pole motor. the shaded...

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