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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.

A typical name plate on an AC induction motor is shown below. Generally the name tag includes: type, horsepower (H.P.), amps, volts, hertz, rpm...
Typical name plate on AC motor:

Below I explain these parameters of the typical name tag on ac induction motor.

Volts: Rated terminal supply voltage.
Amps: Rated full-load supply current.
H.P.: Rated motor output.

We have introduced 5 design types of the National Electrical Manufacturers Association (NEMA) in our previous articles. Below we explain the motor standard: the International Electrotechnical Commission (IEC).

The International Electrotechnical Commission (IEC) Torque-Speed Design Ratings practically mirror those of NEMA. The IEC Design N motors are similar to the National Electrical Manufacturers Association (NEMA) Design B motors, the most common motors for industrial applications. The International Electrotechnical Commission (IEC) Design H motors are nearly identical to NEMA Design C motors.

The IEC Duty Cycle Ratings are different from those of the National Electrical Manufacturers Association (NEMA)'s. Where NEMA usually specifies continuous, intermittent or special duty (typically expressed in minutes), the International Electrotechnical Commission (IEC) uses nine different duty cycle designations (IEC 34-1).

The standards, shown in the table below, apart from specifying motor operating parameters and duty cycles, also specify temperature rise (insulation class), frame size (physical dimension of the motor), enclosure type, service factor and so on.
 

Savings from variable speed drives (VSDs, variable frequency drives, VFDs, frequency converters, ac drives) come from reduced load of the fan, pump, or driven device. With fans and pumps, energy consumed is proportional to the cube root of shaft speed. If shaft speed is reduced by 10%, flow is reduced by 10%, while power consumption is reduced by 27%. And if shaft speed is reduced by 20%, power is reduced by 49%.

Compared to throttling as a means of flow control, speed reduction provides dramatic energy savings. Throttling to reduce flow in a fan or pump backs the device up on its operating curve, increasing pressure and often increasing power consumption.

Applications of energy efficiency variable speed drive (vsd, frequency converter):

The most common applications of ariable speed drive (vsd, frequency converter, adjustable frequency drive) are for pumps and fans to balance flows, and meet changing system needs. For example, Variable speed drives (frequency inverters) can be very cost-effective in retrofit or new construction of heating, ventilating, and air conditionin (HVAC) systems. Many heating, ventilating, and air conditionin (HVAC) systems were designed with constant flow pumps and fans that are throttled to meet changing operating conditions.

Variable speed drive (vsd, frequency converter, adjustable frequency drive) is also useful for loads, such as elevators, water and wastewater pumps, boiler fans, cooling towers, cranes and conveyors.

2. Inverter stage:

Electronic switches - power transistors or thyristors - switch the rectified DC on and off, and produce a current or voltage waveform at the desired new frequency. The amount of distortion depends on the design of the inverter and filter.

3. Control system:

An electronic circuit receives feedback information from the driven motor and adjusts the output voltage or frequency to the selected values. Usually the output voltage is regulated to produce a constant ratio of voltage to frequency (V/Hz). Controllers may incorporate many complex control functions.

Converting direct current (DC) to variable frequency alternating current (AC) is accomplished using an inverter.

Since an induction motor rotates near synchronous speed, the most effective and energy-efficient method to change the motor speed is to change the frequency of the applied voltage. Variable frequency drives (frequency changers) convert the fixed-frequency supply voltage to a continuously variable frequency, thereby allowing adjustable motor speed.

A VSD (frequency converter, variable speed drive, variable frequency drive, VFD) converts 60 Hz power, for example, to a new frequency in two stages: the rectifier stage and the inverter stage. The conversion process of frequency changers incorporates three functions:

1. Rectifier stage:

A full-wave, solid-state rectifier converts three-phase 60 Hz power from a standard 208, 460, 575 or higher utility supply to either fixed or adjustable DC voltage. The system may include transformers if higher supply voltages are used. (to be continued)

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.

Variable frequency drives (frequency inverters, VFDs, variable speed drives) allow more precise control of processes, such as water distribution, aeration and chemical feed. Pressure in water distribution systems can be maintained to closer tolerances. Waste-water treatment plants can consistently maintain desired dissolved oxygen concentrations over a wide range of flow and biological loading conditions by using automated controls to link dissolved oxygen sensors to variable frequency drives (frequency inverters, VFDs, variable speed drives) on the aeration blowers.

Energy savings from variable speed drives can be significant. Affinity laws for centrifugal pumps suggest that even a small reduction in motor speed will highly leverage the energy savings.

Variable speed drives (frequency inverters, VSDs, VFDs, variable frequency drives) can reduce a pump's energy consumption by as much as 50%. A variable frequency drive controlling a pump motor that usually runs less than full speed can substantially reduce energy consumption over a motor, which is running at constant speed for the same period.