The speed of an induction motor controlled by a VFD is primarily changed by altering the:

• Number of poles

• Supply voltage only

• Frequency of the applied voltage

• Rotor resistance

Answer explanation

The speed of an induction motor controlled by a VFD is primarily changed by altering the frequency of the applied voltage. Increasing frequency increases speed, while decreasing it reduces speed, making this the key factor.

What is the correct formula for synchronous speed?

• Ns = (60 × f) / p

• Ns = (f × p) / 60

• Ns = (p × 60) / f

• Ns = (120 × f) / p

Answer explanation

The correct formula for synchronous speed (Ns) is Ns = (120 × f) / p. This formula relates the frequency (f) in Hz and the number of poles (p) in a motor, providing the speed in RPM.

In a conventional induction motor, which of the following remains constant?

• Supply voltage

• Frequency

• Number of poles

• Speed

Answer explanation

In a conventional induction motor, the number of poles is fixed during design and does not change. Supply voltage, frequency, and speed can vary based on operating conditions, making 'number of poles' the correct constant.

The output of the VFD inverter section is:

• DC voltage

• Single-phase AC

• Variable frequency three-phase AC

• Constant frequency AC

Answer explanation

The VFD inverter converts DC voltage into variable frequency three-phase AC to control motor speed and torque, making this the correct output choice.

The input rectifier section of a VFD converts:

• AC to DC

• DC to AC

• AC to AC

• DC to DC

Answer explanation

The input rectifier section of a VFD (Variable Frequency Drive) converts AC (Alternating Current) to DC (Direct Current) to provide a stable power source for the inverter stage, making 'AC to DC' the correct choice.

The DC bus section of a VFD typically includes:

• IGBTs

• Capacitors and inductors

• Diodes only

• Resistors

Answer explanation

The DC bus section of a VFD typically includes capacitors and inductors, which help in smoothing the DC voltage and managing current flow. IGBTs and diodes are part of other sections, while resistors are not primary components here.

One common method to reduce harmonic distortion is to use:

• Step-up transformer

• Line reactor

• Phase converter

• Neutral grounding resistor

Answer explanation

A line reactor is used to reduce harmonic distortion by providing impedance that limits the flow of harmonics in electrical systems, thus improving power quality. Other options do not specifically target harmonic distortion reduction.

VFDs can cause interference known as:

• EMI/RFI

• DC offset

• Static induction

• Phase imbalance

Answer explanation

VFDs (Variable Frequency Drives) can generate electromagnetic interference (EMI) and radio frequency interference (RFI) due to their switching actions, making EMI/RFI the correct choice for the type of interference they cause.

To protect cables from high-frequency interference, use:

• Parallel conductors

• Shielded cables

• Thicker insulation only

• Plastic conduits

Answer explanation

Shielded cables are designed to protect against high-frequency interference by using a conductive layer that blocks external electromagnetic fields, making them the best choice for this purpose.

The inverter section of a VFD consists mainly of:

• SCRs

• IGBTs or MOSFETs

• Capacitors

• Transformers

Answer explanation

The inverter section of a VFD primarily uses IGBTs or MOSFETs for switching and controlling the output voltage and frequency, making them essential components in modern variable frequency drives.