Hitachi SJ300 Series Inverter, What is an, Torque and, Constant Volts, Hertz Operation, The Purpose of Motor Speed Control for Industry

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SJ300 Inverter

Introduction to Variable-Frequency Drives

The Purpose of Motor Speed Control for IndustryWhat is anInverter?

Hitachi inverters provide accurate speed control for 3-phase AC induction motors. You connect AC power to the inverter, and connect the inverter to the motor. Many applications can benefit from the use of variable-speed drives 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

The term inverter and variable-frequency drive are related and somewhat interchangeable. An electronic drive for an AC motor controls 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.

Started Getting

Power				Variable-frequency Drive
Input
		Converter		Internal DC Bus			Inverter
	L1/R
									Motor
						+
	L2/S
			Rectifier		+			U/T1
	L3/T							V/T2

W/T3

–

Torque andConstant Volts/Hertz Operation

The simplified drawing of the inverter shows three double-throw switches. In Hitachi inverters, the switches are actually IGBTs (isolated 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.

In the past, AC variable speed drives used an	Output
open loop (scalar) technique to control speed.
	voltage
The constant-volts-per-hertz operation	V
maintains a constant ratio between the applied
	100%
voltage and the applied frequency. With these
conditions, AC induction motors inherently		Constant torque
delivered constant torque across the operating
speed range. For some applications, this scalar
technique was adequate.					f
Today, with the advent of sophisticated micro-	0
processors and digital signal processors			Output frequency	100%

(DSPs), it is possible to control the speed and torque of AC induction motors with unprece-

dented accuracy. The SJ300 utilizes these devices to perform complex mathematical calculations required to achieve superior performance. The technique is referred to as sensorless vector control. It allows the drive to continuously monitor its output voltage and current, and their relationship to each other. From this it mathematically calculates two vector currents. One