Variable Frequency Drive Flashcards
A motor operated with a VFD is subject to voltages higher than the peak of the line. T or F?
True
What can high frequency from a VFD do to a motor?
Bearings can become pitted and may prematurely wear out
Create induced current in the rotor core causing overheating
What are the three induction motors that can be used with VFDs?
Inverter Ready
Inverter Duty
Vector Duty
A reluctance motor is what type of motor?
Non-induction
Name 4 advantages of VFD’s
Energy Savings
Reduced Noise
Reduced Maintenance Costs
Control Flexibility
VFDs are often controlled by _______________ like the type used in a PC.
Microprocessors
What distorts the voltage and current waveforms?
Harmonics
Why does a VFD produce harmonic distortion?
Its a result of high frequency solid state switching in the drive.
What is the limit of THD allowed and what must be done if the THD is too high on the line side?
3%
If too high, filtering on the line side of the VFD can be accomplised with an input line reactor.
VFD’s are best operated in what temperature range?
10°C to 40°C
Control wiring of a VFD is, at minimum a Class _ wiring method.
Class 1 wiring method
Skip t
How many pulses does 3 Three Phase, Full-Wave rectifiers have? And how many degrees out of phase are their supply voltages?
18-Pulses
20 degrees out of phase, provided by a phase shifting transformer
A dc Link section in a VFD is compromised of what?
An L-Section Filter
(An inductor in series with the output, and a capacitor in parallel with the output)
An IGBT is a semiconductor consisting of an _______, _________ and a ____________.
Emitter
Gate
Collector
What motor speed control technique is the lowest cost and requires no feedback?
V/Hz Control
What is Flux Vector technology sometimes called instead?
Field Oriented Control
What is an autotune in regards to a VFD?
A process where the VFD learns about the motor it is operating.
Manual speed control of the motor is usually accomplished by keypad settings or locally or remotely by a ______________ connected to the VFD.
Potentiometer
The type of control system that may be used if direct feedback was desired would be?
Closed Loop Control
To control the amount of braking from a VFD, the current through the resistor is controlled by a _____________.
Transistor
What is THD?
Total Harmonic Distortion
What are the 3 main components or sections of a VFD?
- Recifier
- DC Link
- Inverter
What section of the VFD filters the pulsating dc to smooth dc?
DC Link
What section of the VFD converts an AC supply voltage to a DC voltage for the VFD?
Rectifier
What are the 2 most common control techniques for producing a PWM output of a VFD?
- V/Hz Control
- Flux Vector Control
What is the volts per Hz ratio for a 600V motor with a 60Hz frequency rating?
10V/Hz
(600V divided by 60Hz)
What type of control does a system have if a heater is energized by a contactor controlled by a start stop station?
Open-Loop
What type of braking is done by putting a DC voltage on the motor stator windings?
DC Injection Braking
What type of braking uses a resistor to dissipate the regenerative voltage as heat?
Dynamic Braking
What type of braking uses a resistor to dissipate the regenerative voltage as heat?
Dynamic Braking
What type of braking recycles the energy produced by a decelerating motor back to the electrical system?
Line Regenerative Braking
(Regenerative Braking)
What is the Frequency of the 3rd harmonic in a 60Hz system?
180Hz
(60Hz x 3)
What is the Frequency of the 7th harmonic in a 60Hz system?
420Hz
(60Hz x 7)
What is the major concern if a standard motor is operated by a VFD at slow speeds?
Overheating of the Motor
Know these for Roy’s Exam
What are the three types of non-induction motors suitable for VFD use?
- Permanent Magnet
- Reluctance
- Wound Field Synchronous
(Fancy term for Synchronous Motor)
How does a VFD change the speed of a motor?
Changing the Frequency
What 3 things could be on a motor nameplate if it is VFD rated? only one would be present
“VT” (Variable Torque) for Inverter Ready
“CT” (Constant Torque) for Inverter Duty
“Precise Control” for Vector Duty
What is the speed at which a rotating field moves around a stator called?
Synchronous Speed
What is the difference between synchronous speed and rotor speed called?
Slip Speed - This is the actual speed in rpm
Slip is represented as a percent the slip speed divided by synchronous speed
n=1750 RPM nsync=1800rpm
1800-1750=50RPM
50rpm/1800rpm= 2.78% slip
What does slip affect in a rotor?
E-Voltage
Z-Impedance
F-Frequency
I-Current
X-Inductive Reactance
What two parameters change synchronous speed?
Frequency and Poles
If Frequency Increases synchronous speed __________
Increases
What is the basic formula for VFDs?
What is Low Speed Boost and when is it used?
Increases starting Torque.
Only used at Startup
What are the disadvantages of using VFDs?
- Harmonics
- Special Cabling
Which Harmonics are the bad Harmonics?
Even or Odd?
Odd
What is a Reactor?
It’s another name for Harmonic filter.
Filters out the harmonics.
