Vfds

Question 1

What does a Rectifier do?

Answer 1

AC to DC

Question 2

What does an Inverter do?

Answer 2

DC to AC

Question 3

What does a VFD do?

Answer 3

Can change the sync speed to enable an induction motor to run at any speed within a certain range

Question 4

Other names for VFD

Answer 4

AFD- adjustable frequency
ASD-adjustable speed
Inverters
AC drives

Question 5

What does a torque boost do?

Answer 5

Prevents a constant torque lid from stalling out

Question 6

Can a VFD control torque output?

Answer 6

Yes, ramped soft start bud ramped deceleration
Can also provide variable or constant torque

Question 7

Why are VFDs used?

Answer 7

Energy savings
Accurate process control
Extended equipment life

Question 8

Variable torque loads require _of power and _of torque when ran at half speed?
Examples?

Answer 8

1/8 of power
1/4 torque
Ex. Fans, blowers , centrifugal pumps (HVAC)

Question 9

Constant torque load require _ amount of power when at half speed?
Example of constant torque loads?

Answer 9

1/2 power if the speed is 1/2
Ex. (Piston pumps), compressors,mixers or conveyors

Question 10

VFDs run with ‘open loop’
Preset values but no_?
Examples

Answer 10

Doesn’t receive feedback from the motor
Ex. Lights, sprinkler

Question 11

VFDs run with ‘closed loop’
Preset values but _?
Examples?

Answer 11

They respond to feedback input from sensing devices
Ex. PLC (programmable logic controller)
PID ( proportional integral derivative controller)
Needs precise control

Question 12

Examples of closed loop VFDs

Answer 12

Machine tools, textile manufacturing, paper or plastic film manufacturing

Question 13

Impedance matching of equipment for VFD referral to?

Answer 13

Motor and cable

Question 14

Go can you control excessive heat building up in VFD?

Answer 14

Separately fed blower motor (totally enclosed fan cooled motor)

Question 15

Can a rectifier connect single and three phase ac input?

Answer 15

Yes

Question 16

What is surge voltage amount that VFD could be placed further than recommended?

Answer 16

1000v

Question 17

VFD to motor cables should be as short as possible
What is max length?

Answer 17

No greater than 15m

Question 18

PWM- pulse width modulation
What does it do?

Answer 18

Reverses the polarity of DC voltage output by VFD which simulates an output of three AC voltages 120degrees apart

Question 19

How a VFD changes frequency?

Answer 19

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

Question 20

Are the output Voltages of VFD true sinusoidal three phase AC voltages?

Answer 20

No- voltages r pulsed DC voltages . PMW method

Question 21

What are examples of linear loads?

Answer 21

Resistor - current proportional to voltage
Impedance remains the same

Question 22

Non-linear loads? What gets produced?

Answer 22

Impedance changes as voltage changes.
Harmonic currents are produced

Question 23

What is RFI?

Answer 23

Radio frequency Interference
Can cause problems with monitoring, metering and sensing equipment

Question 24

How to reduce RFI?

Answer 24

•VFD cables require shielding or continuous metal armour in construction
•RFI filter on input power cables to VFD

Question 25

How to reduce harmonics?

Answer 25

Inductors - line reactors installed on line side
• harmonic filters
•built in harmonic reduction in VFD

Question 26

What can harmonics due negatively?

Answer 26

•excessive neutral current
•hysteresis loss and overheating
•harm motor

Question 27

What is the formula for the length of a time period?

Answer 27

Reciprocal of the carrier frequency
T= 1/f
Ex T=1/5khz = 200 useconds