Basic Concepts

Question 1

What is an adjustable speed drive?

Answer 1

A packaged device that converts input power into another dynamic power output. The power output is fed directly to a motor in such a manner that it controls the motors operations precisely. The drive considers such issues as motor load, motor speed, smooth stopping and starting, and, in some cases, the control of the motor shaft position. The drive itself typically includes features that protect the motor and the drive itself from damage as well as diagnostics.

Question 2

What is an inverter?

Answer 2

A bridge that converts DC power into AC power or AC power into a controlled form of AC power.

Question 3

What does the bridge refer to?

Answer 3

The power conversion components of the drive.

Question 4

What is a converter?

Answer 4

A bridge that changes AC power to DC power.

Question 5

What is needed for a drive to control motor speed precisely?

Answer 5

It must receive feedback from the motor indicating its actual speed. This feedback takes the form of a signal from a device mounted on the motor shaft. The device producing the signal is called a tachometer. An analog tachometer on a motor outputs AC or DC voltage depending on speed.

Encoders produce digital feedback about motor speed and other motor shaft characteristics such as position and direction of rotation. An incremental encoder provides feedback as a variable frequency pulse train or in digital word. A sinusoidal encoder produces a frequency-dependant sine wave based on speed. Both incremental encoders and sinusoidal encoders are low-voltage devices with differential quadrature outputs.

Question 6

What is a marker channel (Z)?

Answer 6

An output pulse once every full shaft turn.

Question 7

What is a tachometer?

Answer 7

A device on a motor shaft that generates a motor speed signal, AC or DC, whose output voltage depends on speed.

Question 8

What is an Encoder?

Answer 8

A digital device that can be used to feedback speed (velocity) or shaft position as a variable frequency pulse train or in digital word.

Question 9

What is meant by differential?

Answer 9

A signal or pulse train referenced to an inverted signal of itself (A,/A) or a common from another device. Using the signal as the difference, the 2 wires improve noise immunity.

Question 10

What is quadrature?

Answer 10

(A, /A, B, /B) A second pulse train is output 90o out of phase of the first. The control uses this to determine the motor’s rotation direction.

Question 11

What is a quadrant?

Answer 11

Describes the electrical operating capability of a drive and motor.

In a one quadrant system, the drive can only produce forward current and forward voltage, and the motor can only output positive shaft power and run in one direction.

In a four quadrant drive, the control can operate in both
positive and nega-
tive voltages and
currents, and the
motor can output
both positive and
negative (regenerative) shaft power and run in forward and reverse.

Question 12

What is regeneration?

Answer 12

Regeneration refers to the ability of a motor control to convert motor mechanical power back into electrical power.

Question 13

What is a vector?

Answer 13

A vector is a value that expresses both magnitude and direction. A three- phase AC input can be represented by vectors, which rotate at the line
frequency, 50 or 60 times a second.

Question 14

What is vector control?

Answer 14

Vector control for AC drives refers to its ability to resolve the motor’s output into vector components of flux current and torque current. Flux vector control can regulate both the torque current and the flux components. Torque vector control has a fixed flux and can regulate only the torque current.

Question 15

What is inertia?

Answer 15

Inertia is the tendency of a body in motion to stay in motion and resist a rotational change in speed. The effective inertia equals the motor inertia + the gearbox inertia + (the material inertia + the mechanical inertia ÷ the gear ratio2).

Inertia is a critical factor in all processes. A motor, its load, the gearbox, and the process equipment all contribute to inertia. This inertia must be overcome when accelerating or decel- erating a motor by applying or subtracting addi- tional increments of power. This process is called compensation.

Adjustable speed drives can compensate for inertia automatically. Compensations can be made for friction losses, motor windage losses, material bending losses, air resistance losses, changes in diameter, etc. Since actual losses may be nonlinear, compensation can be in the form of a table or curve based on speed and/or load.

Question 16

What is compensation?

Answer 16

Compensation · The extra power automatically provided during the acceleration or deceleration of a motor to maintain speed or tension.

Question 17

What happens if extra power is not added or subtracted to overcome inertia?

Answer 17

Undesirable process transients in speed and tension develop, causing strip breakage or web sagging.

Question 18

What is torque?

Answer 18

The rotational force generated at the
motor shaft. Torque is
measured at a specified distance
(radius) from the motor shaft. For
instance, a one-foot lever attached to
a shaft with a one-pound weight on it
generates one foot-pound of torque at the motor shaft.

Question 19

What is tension?

Answer 19

Tension is the amount of force on a process material (strip, web, sheet, etc.). It is a function of the motor’s output torque and roll diameter. For most materi- al transport devices, roll diameter is a constant. Roll diameter is a variable for a winder. Tension can be expressed in units of lbs., lbs./ft., kg., kg./m., etc.

The torque consumed by accelerating or decel- erating the motor and gearbox is subtracted or added to the resultant material tension. If this torque is not compensated for, material stretching, tearing, or breaking can result.

Question 20

What is stall tension?

Answer 20

Stall tension is a percentage of running tension that is requested from a drive when a process is stopped. Stall tension keeps wound material from unwinding and in readiness for a restart.

Question 21

What is taper tension?

Answer 21

Taper tension is a feature that reduces tension as the material diameter builds, or as motor speed increases.

Question 22

What is a joule?

Answer 22

A joule is an expression of energy in watt seconds. Watts are measured in voltage × amperes.