AC&DC Drive Basics variable speed drives .ppt
- 3. AC DRIVE BASICS All AC Drives convert “fixed” voltage and frequency into “variable” voltage and frequency, to run 3-phase induction motors. LINE INPUT MOTOR OUTPUT
- 4. Types of AC Drives In today’s marketplace, there are 3 basic AC Drive categories: • Open loop “Volts / Hz” Drives • Open loop “Sensorless Vector” Drives • Closed loop “Flux Vector” Drives All are Pulse-Width-Modulated (PWM) Some manufacturers offer 2-in-1 & 3-in-1 Drives, combining these attributes. V/Hz SENSOR- LESS VECTOR FLUX VECTOR
- 5. Open loop “Volts / Hz” Drives V o l t s 230 460 30 60 Hz RPM* 900 1800 (Base) 0 • Motor voltage is varied linearly with frequency • No compensation for motor & load dynamics • Poor shock load response characteristics *( 4-pole motor) Motor Nameplate V/Hz Torque Boost
- 6. Sensorless & Flux Vector Drives V o l t s 230 460 30 60 Hz RPM* 900 1800 (Base) 0 • Motor voltage is varied linearly with frequency, with dynamic self-adjustments • V/Hz compensation for motor & load dynamics • Excellent shock load response characteristics & high starting torque *( 4-pole motor) Motor Nameplate V/Hz
- 7. AC Motor Torque & HP vs. Speed 50 100 30 60 900 1800 0 T & HP % Torque HP Hz RPM • Motor Torque is constant to base speed • HP varies proportionally to speed
- 8. AC Input DC Bus Caps AC to DC Rectifier Pulse-Width-Modulated Inverter Basic Power Circuit DC Filter DC to AC Inverter IGBTs AC Output All PWM inverters (V/Hz, Vector & Sensorless Vector) share similar power circuit topologies. AC is converted to DC, filtered, and inverted to variable frequency, variable voltage AC. M
- 9. PWM Power Circuit: AC to DC Converter Section AC Input DC Bus Caps AC to DC Rectifier DC Filter + - Input Reactor (option) DC Reactor The AC input is rectified and filtered into fixed-voltage DC • Certain manufacturer’s units contain an integral DC reactor (choke) as part of the DC filter. • Adding an external AC input reactor will yield similar benefits. • Both reduce harmonics, smooth and lower peak current.
- 10. Power Switches The IGBT: (Insulated Gate Bipolar Transistor) An IGBT is a hybrid between a MOSFET and a Bi-polar Darlington Transistor. = GATE COLLECTOR EMITTER SWITCH • An IGBT can switch from “OFF” to “ON” in less than a microsecond. • Amplified logic signals drive the high-impedance GATE. Application Issues: • A 1 microsecond state-change will generate a 1 MHz RF pulse. • Dv/dt (rapid voltage changes) can stress motor insulation systems.
- 11. PWM Power Circuit: DC to AC Inverter Section DC Filter DC to AC Inverter IGBTs AC Output M An IGBT (Insulated Gate Bipolar Transistor) is a high-speed power semiconductor switch. IGBTs are pulse-width modulated with a specific firing pattern, chopping the DC voltage into 3- phase AC voltage of the proper frequency and voltage. The resulting motor current is near-sinusoidal, due to motor inductance. Imotor Vu-v U V W IGBT Firing Signals + -
- 12. IGBT Switching Issues Controller-to-motor lead length > 125’ Reflected (standing) wave phenomena Carrier frequency in 2 to10Khz range High dV/dT from fast switching R.F. & Electromagnetic interference CONDITION SOLUTION RESULT Output reactor installed near controller RFI/EMI input filter; shielded motor cable; separate ground conductor Nuisance trips from capacitive coupling to ground Nuisance trips; Motor insulation damage from voltage doubling Output reactor; Improved motor insulation Higher carrier or “quiet” algorithm Interference with other equipment; telecommunications Motor acoustic noise Motor insulation damage from voltage doubling Improved motor insulation
- 13. IGBT Firing Signals PWM microprocessor controller Operator Interface S E Q REF LO CA L AC MOTOR DRIVE 0.75 KW 200 V v 1.3 HEALTH L R PROG E M RUN F W D RE V JOG RESET STOP RESET V f Basic V/HZ Control Circuit: Input, Feedback and Control Signals Motor current & voltage feedback DC Bus current & voltage feedback Speed reference
- 14. IGBT Gating Signals PWM microprocessor controller with Vector algorithm Man- machine Interface S E Q REF LO CA L AC MOTOR DRIVE 0.75 KW 200 V v 1.3 HEALTH L R PROG E M RUN F W D RE V JOG RESET STOP RESET Flux Vector Control Elements Input, Feedback and Control Signals Encoder Feedback Motor current & voltage feedback DC Bus voltage feedback Speed and / or Torque reference
- 15. AC VECTOR CONTROL LOOPS Speed Regulator Torque Regulator PWM Firing Frequency Feedback Speed Feedback Speed Loop Torque Loop Actual Torque Speed Error Torque Ref. Encoder Freq. & Voltage Reference AC Vector Drive Torque Calculator Speed Reference Torque Reference
- 16. Typical AC Induction Motor Speed / Torque Curve “Across-the-line” operation @ 60 Hz, NEMA ‘B’ motor Full load operating point (100% current & torque) 1750 RPM (nameplate) Breakdown point: Maximum torque motor can produce before locking rotor Synchronous “no-load” speed 1800 RPM (50 rpm) 100 175 225 Starting Torque Pull-Up Torque 150 %T Speed LO AD SLIP
- 17. Typical AC Induction Motor Current & Torque Curves “Across-the-line” operation @ 60 Hz, NEMA ‘B’ motor 100 175 225 150 % T Speed % I 650 400 Linear range: 40-150% load (operating range in which current is proportional to torque) Starting (inrush) current Breakdown current: maximum level when motor locks rotor (stalls)
- 18. Speed AC Motor Speed / Torque Curve family on Inverter Power Slip (50 rpm) 100 175 225 150 %T Slip (50 rpm) 100% load torque operating line Motor base speed: 1750 RPM At any applied Frequency, an induction motor will slip a fixed RPM at rated load. Peak Inverter Torque (150 -200%)
- 19. AC MOTOR FORMULA 120 x Frequency # of Poles SYNC RPM = Example: 4-pole motor SYNC RPM = 120 x 60 / 4poles = 1800 RPM %SLIP = SYNC RPM - FULL LOAD RPM SYNC RPM X 100 Example: 1750 RPM motor % Slip = (1800 - 1750) / 1800 x 100 = 3% Slip SYNCHRONOUS SPEED MOTOR SLIP VOLTS / HERTZ V/Hz = Motor Line Volts Motor Frequency Example: 460 V, 60 Hz motor V/Hz = 460/60 = 7.66 V/Hz VOLTS FREQUENCY V/Hz 460 60 7.66 345 45 7.66 230 30 7.66 115 15 7.66 7.66 1 7.66
- 20. AC MOTOR SIZE Frame size is directly related to base RPM, for a given Horsepower Example: 15 HP motors of different base speeds Base RPM Frame Size Torque Amps 3600 (2-pole) 215 22.5 lb-ft 18.5 1800 (4-pole) 254 45 lb-ft 18.7 1200 (6-pole) 284 67.5 lb-ft 19.3
- 21. How Slip Compensation improves speed regulation Full load 30 Hz operating point (100% current & torque) 850 RPM Sync. or “no-load” 30 Hz speed 900 RPM Slip (50 rpm) 100 175 150 %T Speed Slip (50 rpm) 100 175 150 %T Speed Example: Motor under load at 30 Hz A motor will lose 50 rpm under full load with 30 Hz applied frequency, slipping from 900 to 850 RPM. By sensing current and other variables, SLIP COMP will apply 31.7 Hz to the motor, restoring the speed to 900 RPM. BEFORE AFTER New 31.7 Hz curve 900 RPM 950 RPM 30 Hz curve
- 23. Induction Motor Advantages • Low cost (compared with DC) • Wide availability • Low maintenance - no brushes or commutator • Rugged design - can be used in harsh environments • Low inertia rotor designs • High electrical efficiency • Wide speed ranges • No separately-powered field windings • Good open-loop performance
- 24. Elements of an Induction Motor: The Rotor Laminations of high-silicon content steel Cast aluminum rotor bars Cast aluminum end rings Low-eddy current loss magnetic medium Electrically joins rotor bars at both motor ends Carry induced current (skewed bars shown) No direct electrical connections are made to the rotor. All forces are magnetically induced by the stator, via the air gap. Rotor Bar Current
- 25. Elements of an Induction Motor: The Stator Stator Core Lamination stack of notched steel plates
- 26. Elements of an Induction Motor: Stator Windings (4-pole) Steel Laminations Stator Windings Slots wye or delta connection types
- 27. Elements of an Induction Motor: The Stator (4-pole) t The stator induces magnetic lines of flux across the air gap, into the rotor Rotating magnetic field
- 28. stator rotor Induction Motor Slip SLIP = (s - r ) / s • Motor slip is proportional to load torque. • Stator speed is known by frequency • Rotor speed is measured with an encoder (Vector). • Rotor speed can be approximated, knowing motor and bus current (Sensorless Vector algorithm)
- 29. Magnetic Flux Lines Rotor Magnetic Field Dynamics: SLIP creates TORQUE As the rotor slips, rotor bar current slip frequency increases, resulting in greater rotor field strength (more torque). When rotor speed is near stator speed (light load), few stator flux lines are cut . Rotor bar current and slip frequency are low. Magnetic Flux Lines R o t o r B a r C u r r e n t Magnetic Flux Lines Light Load Heavy Load