Introduction To VFD’s

Question 1

What can a VFD do

Answer 1

1. Control speed
2. Control torque output
3. Provide a ramped soft start and ramped deceleration

Question 2

What are 3 reasons for installing a VFD

Answer 2

1. Energy savings
2. Accurate process control
3. Extended equipment life

Question 3

What are the main concerns when installing a FVS

Answer 3

1. The horsepower
2. The voltage
3. FLA
4. Service factor

Question 4

Describe harmonic currents

Answer 4

1. Produced by non linear loads (where impedance changes as voltage changes)
2. The size of harmonic current decreases as it’s harmonic number increases
3. Even numbered harmonics cancel out in a three phase system
4. Low odd numbered harmonics can create problems due to the magnitude of these harmonics being generally larger

Question 5

What do harmonic currents cause

Answer 5

1. Excessive neutral current on a three phase system even though the phases seem to be balanced
2. Additional hysteresis losses in transformer iron core resulting in transformer overheating
3. Harm to the motor that is connected to the VFD

Question 6

How can harmonic problems be reduced

Answer 6

1. Inductors called line reactors installed on the line side of the VFD
2. Harmonic filters
3. Built in harmonic reduction in the VFD

Question 7

How should VFD cables be installed

Answer 7

1. 300mm away from control and signal cables and other RF sensitive devices
2. Not run parallel to control or signal cables
3. If you have to cross control cables, it must be at an angle of 90 degrees

Question 8

How long can VFD cables be

Answer 8

1. Kept as short as possible (some recommendations are for no greater than 15m)

Question 9

What are some features of an inverter duty motor

Answer 9

1. Thinner steel laminations in the rotor core that reduces iron losses
2. Better winding insulation
3. Shaft bonding rings to prevent build up of induced voltage between the shaft and bearings
4. Insulated or ceramic coated bearings on the non drive end to prevent circulating currents
5. A separately fed blower motor to provide cooling

Question 10

What are the 3 basic sections of a VFD

Answer 10

1. Rectifier
2. Dc link
3. Inverter

Question 11

Describe the function of a rectifier

Answer 11

Changes the incoming AC power supply to DC

Question 12

Describe the function of a DC link

Answer 12

Filters the DC voltage using capacitors and inductors

Question 13

Describe the function of an inverter

Answer 13

Converts DC to AC at desired frequency