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For example, the very fast voltage rise times associated with insulated gate bipolar transistor (IGBT) technology contributes to precise motor speed control. But it can also lead to voltage spikes that damage cables of poor quality, or ones that are improperly insulated.

Other possible concerns with use of variable frequency drives (VFDs, variable speed drives, VSDs) are the potential for acoustical motor noise and motor heating, when currents, induced by pulse width modulated (PWM) switching, flow in improperly grounded motor shafts. The result can be damaged bearings.

 Another benefit of variable frequency drives (VFDs) is the soft-start capability in which motors are ramped up to speed instead of being abruptly thrown on line. This useful feature of VFD (VSD) reduces mechanical stresses on the entire motor system and leads to lower maintenance costs, as well as a longer motor life.

And another benefit of this solid state electrical device is improved process control. In fact better process control with VFD is the original application. Since industrial process throughput in most operations depends on a range of variables, a motor that is only able to operate at a constant speed, even when one or more process variables change might contribute to creation of scrap, not to mention wasted energy.

With a variable frequency drive (VFD, motor speed can be changed almost instantaneously to adapt to changing process conditions, thus improving process control. (to be continued)

Below is the image of World class vector control variable speed drive with same quality as ABB, Siemens from Shenzhen POWTRAN Technology Co., Ltd.:
  

The output is a series of narrow voltage pulses having constant amplitude, but sinusoidally-varying widths. Internally, variable frequency drives (VFDs) consist of three main sections, including an AC-to-DC converter based on a rectifier bridge of diodes, a DC bus that filters and smoothes out the rectifier output, and a DC-to-AC inverter to change the DC back to AC.

The inverter, which is most typically based on insulated gate bipolar transistor (IGBT) technology, creates the variable voltage and frequency output that will control the motor’s speed. A microprocessor in the VFD, with programming resident in firmware, governs the overall operation of the device.

Fans are also used in power plants and the chemical industry. In these 2 cases, the fans need to be adjusted according to the main process. In power plants, the main process changes due to varying demands for power at different times of the year, day or week. Likewise, the need for variable speed drives (VSD) differs according to the process.

Continued from the Direct Torque Control (DTC) of Frequency Converters (ac drives, frequency changers) (part 1)  , below we still introduce this advanced control technique of frequency inverters (ac drives, variable speed drives, VSDs).

The heart of direct torque control (DTC) is its adaptive motor model. This model is based on the mathematical expressions of basic motor theory. The adaptive motor model of DTC requires information about the various motor parameters, like stator resistance, mutual inductance, saturation coefficiency, etc.

The algorithm captures all these details at the start from the motor without rotating the motor. But rotating the motor for a few seconds helps in the tuning of the model. The better the tuning, the higher the accuracy of speed and torque control.

 As the torque producing component in vector control is controlled only after transformation is done, and is not the main input reference, the vector control is known as "indirect torque control".

Vector control variable frequency drives from Shenzhen POWTRAN Technology Co., Ltd.: 

The most challenging and ultimately, the limiting feature of the field orientation, is the method whereby the flux angle is measured or estimated. Depending on the method of measurement, the vector control is divided into two subcategories: direct vector control and indirect vector control. 

In the direct vector control, the flux measurement is done by using the flux sensing coils or the Hall devices. This adds to additional hardware cost and in addition, measurement is not highly accurate. Therefore, the direct vector control is not a very good control technique for frequency changers (variable speed drives, VSDs).

Sinusoidal Pulse Width Modulation (PWM)

In this method of Sinusoidal Pulse Width Modulation (PWM), the sinusoidal weighted values are stored in the PICmicro microcontroller and are made available at the output port at user defined intervals.

The advantage of Sinusoidal Pulse Width Modulation (PWM) is that very little calculation is required. Only one look-up table of the sine wave is required, as all the motor phases are 120 electrical degrees displaced.

The disadvantage of Sinusoidal Pulse Width Modulation (PWM) is that the magnitude of the fundamental voltage is less than 90%. And the harmonics at Pulse Width Modulation (PWM) switching frequency have significant magnitude.

Six-Step Pulse Width Modulation (PWM)

The inverter of the frequency converter (ac drive) has six distinct switching states. When it is switched in a specific order, the three phase AC induction motor can be rotated.

In this type of control (scalar control, v/f Control), the motor is fed with variable frequency signals generated by the Pulse Width Modulation (PWM) control from an inverter, using the feature rich PICmicro microcontroller. Here, the V/f ratio is maintained constant in order to get constant torque over the entire operating range. Since only magnitudes of the input variables - frequency and voltage -  are controlled, this is known as "scalar control". Generally, the drives with such a control are without any feedback devices (open-loop control). Hence, a control of this type offers low cost and is an easy-to-implement solution.

In such controls, very little knowledge of the motor is required for frequency control. So, scalar control (v/f Control) is widely used. A disadvantage of scalar control (v/f control) is that the torque developed is load dependent, as it is not controlled directly. Also, the transient response of such a control is not fast due to the predefined switching pattern of the inverter.

2. Find the full power required to run this ac motor by calculating the full load wattage.

A basic electrical formula for wattage is voltage times amperage (w = v X a). Multiply the 480 volts times 20 amperes and the operation wattage is equal to 9600 w. All frequency inverters (variable speed drives, VSDs, variable frequency drives, VFDs) are rated in kilowatt (kW). One kilo is equal to 1,000. The ac motor will use 9.6 kW of electrical power.

3. Find the maximum power that variable frequency drive (VFD) can provide according to the specifications.

In the example above, a 10 kW (10,000 watt) frequency inverter will be needed to power the electric ac motor. The important point here is that it is always better to use a slightly larger drive unit than one that is too small to provide full power. Find the full load amperage of frequency inverters (variable speed drives, VSDs, variable frequency drives, VFDs) when it is providing full power to the motor at 480 volts. Divide the 10 kW by 480 volts to find that 20.83 amperes will be required.

These AC drives control the speed of AC motors in a very accurate fashion. In most all applications the adjustable frequency drives provide overcurrent protection for the motors. The main feed of electrical power to the frequency drive must still have some form of overcurrent protection to safely power the drive unit. Sizing the overcurrent protection, regardless of the specifications provided with the drive unit, may still require calculation.

How to size overcurrent protection for frequency inverter? Let's follow these 4 steps:

1. Obtain the electrical specifications from the electrical alternating current (AC) motor's nameplate tag. The nameplate data tag is generally placed on the motor, near the topside of the exterior frame. The operational voltage, the full load amperage, the horsepower and the power factor rating are recorded on the motor nameplate.