PWM

Elevator Drives - Discussion• History• Requirements• Motor and Control Types• Industry Trends• Future Drives

History• 236 BC – First Passenger Lift,

Archimedes• 1853 – Safe Elevator Demo,

Elisha Otis• 1857 – First Safe Elevator

Installation, Cooper Union, NYC

• 1861 – Otis Elevator Patent

Otis Patent 1861

History• 1873 – First Modern DC Motor

• 1874 – J. W. Meaker Door Opener Patent

• 1880 – First Electric Motor Controlled Elevator Siemens / Sprague

• 1882-1889 – Tesla AC Induction Motor 3-Phase Squirrel Cage Design

• 1889 – Otis Elevator Uses DC Motor

Otis DC

Elevator Motor

Circa 1889

History• 1891 – Ward Leonard Variable

Speed Control– AC Induction Motor Turning DC Dynamo– Rheostat to Control Generated Voltage– DC Voltage Controls DC Motor Speed

• 1900-1970’s – Ward-Leonard M-G Sets and DC Motors Used for Variable Speed Elevators

• AC Motors Used 1 and 2 Speed Starters

Otis No. 1 Geared DC Machine with DC Motor

Circa 1915

Otis Gearless DC Machine

Circa 1919

M-G Set Controls (Otis Elevator, 1920’s)

Otis Type 84 26

Broadway,NYC

Circa 1930’s

History• 1975-Present

– Thyristor (SCR) DC Drives Control Elevators

– All Analog Components in the 70’s

– Replaces Aging M-G Sets

• 1980’s – Microprocessors Improve– Car Dispatch and Motor Drive Controllers

Otis type 84,NYC with Encoder

Westinghouse #205 with Encoder

History• Late 1980’s –

– Variable Frequency Inverters AC Induction Motors, Geared Applications Only

• Early 1990’s – – More AC Inverters and Motors Begin to Displace Small

DC, 3-15 HP

• Mid-1990’s – – Vector Control AC Inverters 10-40 HP Almost as Good

as SCR-DC. – KONE Introduces PM EcoDisc AC Machine

History

– Custom Gearless AC Induction Machines

– First Fully Regenerative AC Elevator Drives

– Much Discussion on PM-AC and MRL

– SCR-DC Still Used for Medium and Large Building Mods

Late 1990’s –

History

– More PM-AC Motor Manufacturers. PM Gearless Begins to Replace AC Geared

– EU Focus on Efficiency and Harmonics/EMC

– Lower Cost IGBT Inverter Components

– North America Begins to Focus on Energy Reduction

– New Construction Leaning toward AC

– SCR-DC Still Used on Medium-Large Building Mods

2000-Present –

Four Quadrant Operation

Linear power stage

advantages– simple, low priced controller– low electromagnetic noise level– no minimum inductance needed

disadvantages– high power losses at the final stage at

high currents or low motor voltages (PV = R I2)

– for small nominal power up to 100 W

M

R

controllerUT

Vcc

Gnd

LSC

Umot

time

Umot, Imot

Pulsed power stage (PWM)advantages– low power losses– high efficiency– for higher nominal

power

disadvantages– electromagnetic noise in

the radio frequency range

– high power losses in the motor at standstill

– minimum inductance necessary

M

power stage

Umot

Vcc

Gnd

pulse

generator

ADS,DEC, AECS, DES, MIP, PCU, EPOS

time

cycle time: 20 - 50 s

Umot, Imot

Pulsed power stage: current ripple)LL(f2

VIchokemotS

ccmax

general measures: reduce motor voltage enhance total inductance

- motor choke in controller- additional motor choke

enhance PWM frequency

50% 50% 30% 70%low motor inductance

additional motor choke Umot, Imot

Time scales in control loops

0.02 0.05 0.1 0.2 0.5 1 2 5 10 20 ms

50 20 10 5 2 1 0.5 0.2 0.1 0.05

PWM cycle time

"slow" position controller

position controller MIP

current controller

mechanical time constants

speed controllerspeed controller as "link" between fast current controller and a slow position control (PLC)

frequency kHz

cycle time

PWM• PWM(Pulse Width Modulation• Cambiando il duty cycle, la velocità cambierà

Figure : PWM Control Signal

Duty Cycle 20%Lowest Speed

Duty Cycle 50%Middle Speed

Duty Cycle 80%High Speed

Cycle

Duty Cycle

Duty Cycle = DT / T (%)

(DT)

Lo scopo è :Lo scopo è : 1. Ridurre la dissipazione di potenza.1. Ridurre la dissipazione di potenza. 2. Ridurre I problemi di raffreddamento dei transistors)2. Ridurre I problemi di raffreddamento dei transistors)

Duty cycle

• si definisce duty cycle d il rapporto tra la durata del segnale "alto" ed il periodo totaleT del segnale, e serve ad esprimere per quanta porzione di periodo il segnale è a livello alto:

t

Td

• PWMUn segnale PWM (Pulse Width Modulation ovvero modulazione a variazione della larghezza d'impulso) è un' onda quadra di duty cycle variabile che permette di controllare l'assorbimento (la potenza assorbita) di un carico elettrico(nel nostro caso il motore DC), variando modulando) il duty cycle.

• Un segnale PWM è caratterizzato dalla frequenza (fissa) e dal duty cycle (variabile);

• si deduce dalla Figura, il duty cycle è il rapporto tra il tempo in cui l'onda assume valore alto e il periodo T (l'inverso della frequenza: T=1/f)

• Es. un duty cycle dell'80% corrisponde ad un'onda quadra che assume valore alto per l'80% del tempo e basso per il restante 20%,

DC Motor DrivesDC motor speed control using Switching Control or PWM

PWM

Full-bridgeDC-DC

converterDiode

rectifierFilter

capacitor

Pluse-width-modulation

MVoltage Source VoutVs

Pulse Width%Duty cycle = x 100 Cycle

out s V = % Duty cycle x V

Power Electronic converter

Vcc

Q1

Q4

Q3

Q2G

G

G

GA B

C D

ON

OFF

OFF

ON

M

D1

D4

D3

D2

H-bridge converters circuit