Hitachi SJ200 Series manual 31

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Introduction to Variable-Frequency Drives

Getting Started

Introduction to Variable-Frequency Drives

The Purpose of Motor Speed Control for Industry

Hitachi inverters provide speed control for 3-phase AC induction motors. You connect AC power to the inverter, and connect the inverter to the motor. Many applications benefit from a motor with variable speed, in several ways:

•Energy savings - HVAC

•Need to coordinate speed with an adjacent process—textiles and printing presses

•Need to control acceleration and deceleration (torque)

•Sensitive loads - elevators, food processing, pharmaceuticals

What is an Inverter?

The term inverter and variable-frequency drive are related and somewhat interchange- able. An electronic motor drive for an AC motor can control the motor’s speed by varying the frequency of the power sent to the motor.

An inverter, in general, is a device that converts DC power to AC power. The figure below shows how the variable-frequency drive employs an internal inverter. The drive first converts incoming AC power to DC through a rectifier bridge, creating an internal DC bus voltage. Then the inverter circuit converts the DC back to AC again to power the motor. The special inverter can vary its output frequency and voltage according to the desired motor speed.

Power			Variable-frequency Drive
Input
	Converter		Internal				Inverter		Motor
L1			DC Bus
					+			U/T1
L2
		Rectifier		+
								V/T2
L3
						–		W/T3

The simplified drawing of the inverter shows three double-throw switches. In Hitachi inverters, the switches are actually IGBTs (insulated gate bipolar transistors). Using a commutation algorithm, the microprocessor in the drive switches the IGBTs on and off at a very high speed to create the desired output waveforms. The inductance of the motor windings helps smooth out the pulses.