7154/7156 Variable Speed Drives

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7154/7156 Variable Speed Drives. Paul Weingartner 569-1776. Overview. Variable Frequency drives (VFD) Application of VFDs Power quality issues Human Machine Interface (HMI). Standards organizations. NEMA IEEE IEC. NEMA. Enclosures Motor characteristic curves.
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7154/7156 Variable Speed DrivesPaul Weingartner569-1776Overview
  • Variable Frequency drives (VFD)
  • Application of VFDs
  • Power quality issues
  • Human Machine Interface (HMI)
  • Standards organizations
  • NEMA
  • IEEE
  • IEC
  • NEMA
  • Enclosures
  • Motor characteristic curves
  • History of adjustable speed systems
  • Variable pitch pulley
  • Motor-Generator (MG) set
  • Eddy current clutch
  • Solid state drives
  • Problems
  • Expensive
  • Electrical (utility) issues
  • Motor wear/tear
  • Solid State drives
  • DC drives
  • AC soft start
  • AC Variable frequency drives
  • AC vector drives
  • DC drives
  • High torque
  • Large speed ratios
  • Regenerative braking
  • DC motors – high maintenance
  • Basics
  • Speed
  • Torque
  • Horsepower
  • Efficiency
  • Power factor
  • Real power
  • Apparent power
  • Leading power factor
  • Inductive reactance
  • Capacitive reactance
  • Electric utilities
  • Commerical customers are defined as users above 15KVA
  • Electric charge
  • Demand charge
  • Power factor penalties
  • Braking
  • None – let load coast to stop
  • Dynamic breaking – resistive load, uses generator effect
  • Plugging – reverse polarity across motor
  • DC injection – DC voltage is applied across two phases of an AC induction motor. Current must be limited and timing is critical for proper use
  • Regenerative
  • Mechanical brake
  • Goals
  • Ability to vary speed
  • Limit power factor issues
  • Sensitive to electric demand issues
  • Often need “soft start”
  • Cost savings
  • Ways to start a motor
  • Full voltage – Across the line starting
  • Reduced voltage starting
  • Soft start – limit current and rate of startup
  • VFD – great latitude over motor control
  • Relative cost difference for 1 HP motor
  • Full voltage - $120
  • Reduced V - $200
  • Soft state - $250
  • VFD - $400
  • Motors
  • 3 phase squirrel cage induction motor
  • Principle of operation
  • Synchronous speed
  • Slip
  • Starting characteristics
  • NEMA classifications
  • Motor Insulation classMotor VFD issues
  • Volts/Hertz ratio
  • Constant volts range
  • VFD principle of operation
  • 3 phase rectifier
  • DC bus
  • 3 phase inverter
  • VFDs – 1st Generation
  • VVI – Variable Voltage Inverters
  • 6 step drive
  • Uses SCRs on rectifier front end
  • Variable voltage DC bus
  • Problems with VVI drives
  • Motor signal – not very sinusoidal, causes problems
  • Sensitive to source voltage flucuations – 5-10% change will fault the drive
  • At low speed the drive will “cog” creating stresses on shafts, etc – freq should be above 15 Hz
  • Drive will reflect harmonics back to the line
  • Short power loss is bad
  • CSI – Current Source Inverter
  • Similar to VVI, but adds a line reactor on the DC bus
  • Supports regenerative braking without needing extra hardware
  • Creates harmonics
  • PWM
  • Operating frequency – carrier frequency
  • Increasing the carrier frequency decreases the efficiency of the drive electronics
  • Duty cycle
  • t-on
  • t-off
  • Transistor example
  • Linear operation vs. PWM
  • Power dissipation
  • PWM drives
  • Uses diodes for the rectifer, creating a Constant voltage DC bus
  • Constant power factor – due to diode front end
  • Full operating torque at near zero speed
  • No cogging
  • Can ride thru a power loss from 2 Hz to 20 seconds
  • VFD drives
  • Scalar
  • Vector
  • 3 phase motorNEMA Motor Curves1336 picture1336 – Description of L7E option1336 Drive literature link
  • http://www.ab.com/drives/1336PlusII/literature/index.html
  • PWM inverterMotor selection criteriaSynchronous speed
  • AC motors have a sync design speed that is a function of the number of poles and the line frequency
  • At sync speed ZERO torque is generated
  • Therefore, motors cannot run at sync speed
  • Motor slip
  • Since motors cannot run at sync speed, the will run at slightly less than this speed.
  • “Slip” is the term used to describe the difference between the sync speed and the maximum rated speed at full load
  • Motor slip calc
  • This formula includes a characteristic called slip. In a motor, slip is the difference between the rotating magnetic field in the stator and the actual rotor speed. When a magnetic field passes through the rotor's conductors, the rotor takes on magnetic fields of its own. These induced rotor magnetic fields will try to catch up to the rotating fields of the stator. However, there is always a slight speed lag, or slip. For a NEMA-B motor, slip is 3-5% of its base speed, which is 1,800 rpm at full load. For example,
  • Volts/HertzDrive frequency
  • The speed at which IGBTs are switched on and off is called the carrier frequency or switch frequency. The higher the switch frequency, the more resolution each PWM pulse contains. Typical switch frequencies are 3,000 to 4,000 times per second (3-4 kHz). As you can imagine, the higher the switch frequency, the smoother (higher resolution) the output waveform. However, there is a disadvantage: Higher switch frequencies cause decreased drive efficiency. The faster the switching rate, the faster the IGBTs turn on and off. This causes increased heat in the IGBTs.
  • High motor voltages
  • http://www.mtecorp.com/solving.html
  • High peak voltages
  • Fast rise times
  • Standard Motor Capabilities established by the National Electrical Manufacturers Association (NEMA)and expressed in the MG- I standard (part 30), indicate that standard NEMA type B motors can withstand 1000 volts peak at a minimum rise time of 2 u-sec (microseconds). Therefore to protect standard NEMA Design B motors, one should limit peak voltage to 1KV and reduce the voltage rise to less than 500 volts per micro-second.
  • Constant torque loads
  • Conveyor systems
  • Constant horsepower loads
  • grinders, winders, and lathes
  • Variable torque loads
  • fans and pumps
  • Motor ventilation
  • TENV
  • TEFC
  • ODP
  • High Altitude considerations
  • Motor soft start
  • Limit inrush current
  • Linear ramp
  • S-curve
  • Skip freqFlux vector drives
  • http://www.mikrokontrol.co.yu/sysdrive/WhatInv.htm#FV
  • Search Related
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