How well do you know variable frequency drives and drive systems?
Take a short quiz to test your knowledge about variable frequency drives (VFD) and VFD based drive systems.
1. Nominal current of asynchronous motor with pf = 0.88 is 500 A (FLC).
How much is the current at 50% loading?
- 220 A
- 265 A
- 330 A
- 500 A
2. Spectrum of voltage and current harmonics is presented in figures below.
Which figure shows the current and which figure depicts the voltage?
- Signal 1 is voltage, signal 2 is current
- Signal 1 is current, signal 2 is voltage
3. What section of the torque-speed envelope below depicts the constant power range?
- section 1
- section 2
- section 3
4. What is the unit of total harmonic distortion (THD)?
- %
- A/s
- A/mm2
5. Motor runs with 64% torque at 80% of its nominal speed.
What is the current drawn from the supplying network in % of nominal input current?
- approx. 50%
- approx. 67%
- approx. 80%
6. What is the function of the DC bus inside the variable frequency drive?
- Smooth the DC voltage or DC current
- Intermediate energy storage
- Both functions above
7. AFE type of VFD is supplied from a 6.6 kV grid and feeds 6.6 kV asynchronous motor. What is the expected relationship between grid and motor current?
- Grid current is higher than the motor current
- Motor current is higher than the grid current
- Both currents are equal
8. What is the range of the time constant of a capacitor-based DC link (i.e. cap bank of voltage source inverter)?
- several milliseconds (depending on model and topology)
- 2…5 seconds (depending on model and topology)
- 15…30 minutes (depending on discharging resistor)
9. What is the typical efficiency of a medium voltage VFD at full load?
- 80…85%
- 92…95%
- 97…99%
10. How many windings has an input transformer supplying a VFD with 24-pulse diode rectifier?
- 4 (four)
- 5 (five)
- twenty four (24)
11. What is the purpose of the shield (screen) of power cables in inverter fed motor applications?
- To minimize electromagnetic interference (EMI)
- To provide a path for common mode currents
- Both above answers
12. What is the typical range of modulation index of an inverter?
- 0…1.15
- 0…63 Hz
- 0…5 V/s
13. Nominal DC link voltage (mean value) of a 3-level inverter is 5’000 V. What is the approximate DC link voltage when the motor runs at 50% speed?
- 2’500 V
- 4’200 V
- 5’000 V
14. What technologies can be used to adjust the speed of driven load?
- VFD, steam turbine and clutch
- VFD, gas turbine and fluid coupling
- VFD, gas turbine and bypass valve
15. What stands the acronym FWP for in context of variable speed motors?
- First World Problem
- Framework Programme
- Field Weakening Point
16. Can you use VFD for motors in hazardous area?
- Yes, if the motor is equipped with corresponding warning labels.
- Yes, if the motor is installed in an area with restricted access.
- Yes, if the motor is properly designed and certified for specific inverter.
- No, motors in hazardous area must not be supplied from VFDs.
17. What is the motivation of user to install a variable frequency drive?
- Significant energy savings
- Improved process control
- Inherent motor soft starting
- One or more above items (depending on application)
18. What is the starting current of VFD fed motor?
- 200…350% of nominal current depending on total inertia and load torque
- 150…250% of nominal current depending on motor construction and process dynamics
- It depends on selected speed ramp and system inertia but normally far below nominal current
19. What types of motors are mostly used for variable speed applications?
- asynchronous, BLDC and SynRM motors
- asynchronous, synchronous and PM motors
- asynchronous, PM and BLDC motors
20. What is another (historical) name of a power switch or a power stack?
- transducer
- repeater
- gear
- valve
21. What is the most common cooling medium of liquid-cooled VFDs?
- inhibited mineral oi
- deionized water
- liquid nitrogen
- vegetable oil
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