Download 7154/7156 Variable Speed Drives

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
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 class
Motor 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
motor
NEMA
Motor
Curves
1336 picture
1336 – Description of L7E option
1336 Drive literature link
http://www.ab.com/drives/1336PlusII/literat
ure/index.html
PWM
inverter
Motor selection criteria
Synchronous 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 35% of its base speed, which is 1,800 rpm at full load. For
example,
Volts/Hertz
Drive 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 freq
Flux vector drives
http://www.mikrokontrol.co.yu/sysdrive/Wh
atInv.htm#FV