Vfds Flashcards
What does a Rectifier do?
AC to DC
What does an Inverter do?
DC to AC
What does a VFD do?
Can change the sync speed to enable an induction motor to run at any speed within a certain range
Other names for VFD
AFD- adjustable frequency
ASD-adjustable speed
Inverters
AC drives
What does a torque boost do?
Prevents a constant torque lid from stalling out
Can a VFD control torque output?
Yes, ramped soft start bud ramped deceleration
Can also provide variable or constant torque
Why are VFDs used?
Energy savings
Accurate process control
Extended equipment life
Variable torque loads require _of power and _of torque when ran at half speed?
Examples?
1/8 of power
1/4 torque
Ex. Fans, blowers , centrifugal pumps (HVAC)
Constant torque load require _ amount of power when at half speed?
Example of constant torque loads?
1/2 power if the speed is 1/2
Ex. (Piston pumps), compressors,mixers or conveyors
VFDs run with ‘open loop’
Preset values but no_?
Examples
Doesn’t receive feedback from the motor
Ex. Lights, sprinkler
VFDs run with ‘closed loop’
Preset values but _?
Examples?
They respond to feedback input from sensing devices
Ex. PLC (programmable logic controller)
PID ( proportional integral derivative controller)
Needs precise control
Examples of closed loop VFDs
Machine tools, textile manufacturing, paper or plastic film manufacturing
Impedance matching of equipment for VFD referral to?
Motor and cable
Go can you control excessive heat building up in VFD?
Separately fed blower motor (totally enclosed fan cooled motor)
Can a rectifier connect single and three phase ac input?
Yes
What is surge voltage amount that VFD could be placed further than recommended?
1000v
VFD to motor cables should be as short as possible
What is max length?
No greater than 15m
PWM- pulse width modulation
What does it do?
Reverses the polarity of DC voltage output by VFD which simulates an output of three AC voltages 120degrees apart
How a VFD changes frequency?
1.Single or three phase AC into rectifier converting it to Dc
2.Dc link - filters the dc voltage using capacitors and inductors to smooth the voltage
3. Inverter- converts dc to ac at desired speed from motor
Are the output Voltages of VFD true sinusoidal three phase AC voltages?
No- voltages r pulsed DC voltages . PMW method
What are examples of linear loads?
Resistor - current proportional to voltage
Impedance remains the same
Non-linear loads? What gets produced?
Impedance changes as voltage changes.
Harmonic currents are produced
What is RFI?
Radio frequency Interference
Can cause problems with monitoring, metering and sensing equipment
How to reduce RFI?
•VFD cables require shielding or continuous metal armour in construction
•RFI filter on input power cables to VFD
How to reduce harmonics?
Inductors - line reactors installed on line side
• harmonic filters
•built in harmonic reduction in VFD
What can harmonics due negatively?
•excessive neutral current
•hysteresis loss and overheating
•harm motor
What is the formula for the length of a time period?
Reciprocal of the carrier frequency
T= 1/f
Ex T=1/5khz = 200 useconds
