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Introduction

Torque

Flux, Voltage

Torque

Capability

0

0.5

1.0

1.2

1.5

Speed ( X 50/60)

Figure 1-3 Torque Reduction above Base Speed

Therefore in order to control the speed of a standard AC motor, the applied frequency and voltage must be controlled.

Although it is difficult to control voltage and frequencies at these high powers, the use of a standard induction motor allows a cost-effective speed control system to be built.

1.2The Variable Frequency Inverter

An electronic converter that converts Direct Current (DC) to Alternating Current (AC) is known as an inverter. Electronic speed controllers for AC motors usually convert the AC supply to DC using a rectifier, and then convert it back to a variable frequency, variable voltage AC supply using an inverter bridge. The connection between the rectifier and inverter is called the DC link. The block diagram of a speed controller (often called an inverter) is shown in Figure 1-4.

+

Supply

Motor

-

Rectifier

DC Link

Inverter

Figure 1-4 Inverter Block Diagram

MICROMASTER Applications Handbook

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