Introduction to Variable Frequency Drives

Question 1

What is the main action of a variable frequency drive?

Answer 1

To change the frequency and voltage to a motor, which will change it’s synchronous speed, allowing smooth speed control through a vaariety of loads.

Question 2

What are some other names of VFDs?

Answer 2

• Adjustable frequency drive;
• Adjustable speed drive;
• Inverter;
• AC drive.

Question 3

What are the three main reasons for installing VFDs?

Answer 3

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

Question 4

Which amazing features can VFDs provide?

Answer 4

• Speed control;
• Torque output control;
• Ramped soft starting/deceleration;

Question 5

What is a variable torque load and what are some examples?

Answer 5

One which requires more or less torque with a change in speed;

Fans, blowers and centrifugal pumps (HVAC applications).

Question 6

What are the power and torque requirements of a variable torque load at half speed?

Answer 6

1/8 Power and 1/4 Torque.

Question 7

How is energy savings achieved with VFDs?

Answer 7

When a variable torque load does not have to run at full capacity, the voltage and therefore current to a motor can be reduced while still providing adequate output for the task.

Question 8

What are the power and torque requirements of a constant torque load at half speed?

Answer 8

1/2 Power and 100 % Torque (Constant torque).

Question 9

What is Open-loop, as opposed to Closed-loop?

Answer 9

Open loop will only run according to pre-programmed parameters;

Closed loop will also respond to feedback to control output to a motor.

Question 10

By which means could a VFD recieve feedback?

Answer 10

• Directly through a programmable logic controller (PLC);
• A proportional integral derivative controller (PID);
• Other instrumentation equipment which regulates a process.

Question 11

What are a few processes which typically use closed loop feedback?

Answer 11

• Machine tools;
• Textile manufacturing;
• Paper or plastic film manufacturing;
• Other processes which require ultra-precise speed and position control.

Question 12

How can a VFD provide increased equipment life?

Answer 12

It can provide ramped soft starts which reduce the inrush current to the motor and inrush torque experienced by a delicate load.

Question 13

Before VFDs what was the common way to provide soft stop and starts?

Answer 13

DC motors.

Question 14

What is of primary concern when installing a VFD?

Answer 14

Harmonic currents;

In particular, Triplen harmonics (3rd, 9th, 15th order);

This is because they are-0 phase meaning they add together in all three phases because it works out that their peaks line up therefore they combine for large unexpected currents, particularly in wye systems.

Question 15

What are the main problems associated with VFD harmonic currents?

Answer 15

• Excessive neutral current;
• Additional hysteresis loss in transformer cores;
• Harm to the connected motor.

Question 16

What can help reduce harmonic problems?

Answer 16

• Line reactors (inductors) installed on the line side of the VFD;
• Harmonic filters;
• Built in harmonic reduction within the VFD.

Question 17

Which other devices are very likely to produce harmonic currents?

Answer 17

Rapidly switching solid state devices;

• Fluorescent ballasts;
• Computer power supplies;
• Other electronic equipment.

Question 18

How do we minimize RFI in VFD cabling?

Answer 18

By providing a shield with a drain wire (low impedance) path to the VFD;

An RFI filter on the input wires is often reccomended.

Question 19

Why must cables be impedance matched with the motor they serve?

Answer 19

Because otherwise higher than normal voltage spikes may occur;

High cable capacitance can cause this mismatch, this is typically a factor of cable length.

Question 20

What is the typical VFD bonding path?

Answer 20

3 smaller conductors; or

1 conductor as large as the circuit conductors.

(This is to provide more surface area for the high frequency currents, due to skin effect high frequency currents tend to only flow on the outside of a conductor).

Question 21

What are some important installation concerns when installing VFD cable near other sensitive devices?

Answer 21

• Be at least 300 mm away control/signal cables and other RFI sensitive devices;
• Do not run parallel to control/signal cables;
• If you are to cross control cables, do so at 90°.

Question 22

To avoid cable capacitance which is too great, what must be done?

Answer 22

Keep VFD cabling no longer than 15 m.

Question 23

What are three concerns when connecting standard motors to VFDs?

Which type of motor should be connected to a VFD?

Answer 23

A motor may be designed to cool itself properly only within rated speed;

High voltage spikes from VFDs could damage standard winding insulation;

High carrier frequencies can induce voltage into the shaft of a motor which can arc across the bearings creating pits/damage.

Inverter duty motors.

Question 24

What bonus features could make up an inverter duty motor?

Answer 24

Thinner rotor core laminations;

Better winding insulation;

Shaft bonding rings, to prevent buildup of voltage;

Insulated or non conductive bearings on the non drive end;

Separately fed blower motor for cooling (Totally enclosed blower cooled [TEBC]).

Question 25

What are the three basic sections of a VFD?

Answer 25

Rectifier (converts AC to DC);

DC link (Filters the DC);

Inverter (Converts DC to AC at desired frequency).

Question 26

How does the DC link smooth the DC voltage?

Answer 26

By using capacitors and inductors.

Question 27

Are the outputs of a VFD true sinusoids?

Answer 27

No, they are Pulse width modulated DC, which simulates 3 Φ AC.

Question 28

What is the amount of pulses per cycle dependant upon?

Answer 28

The carrier frequency.

Question 29

What is meant by time period?

Answer 29

It is the time in which one cycle at the carrier frequency is carried out in.

T = 1/f.

Question 30

How is average voltage per time period modified?

Answer 30

By changing the length of time which the DC signal is applied within the time period;

Longer time is higher voltage for the specific time period.

Question 31

Which components of a VFD produce the pulse width modulation?

Answer 31

Transistors capable of rapid switching.

Question 32

Why is voltage to frequency ratio important?

Answer 32

If the voltage is not reduced along with frequency the current would be excessive at low frequency, because X_L will lower with frequency.

Question 33

If frequency is too low what may happen to a constant torque load?

Answer 33

It could stall;

To prevent this, V/Hz is adjusted to provide higher V, I and T at lower frequencies (Torque Boost).