The Professor: Variable-Frequency Drives

October 5, 2009
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Figure 1: Energy savings can be accomplished through the use of variable-frequency drives (VFD). (Photo courtesy of Ferris State University.)

The U.S. Department of Energy has indicated that 65 percent of electrical energy used in commercial and industrial systems comes from electrical motors powering centrifugal pumps and fans. Also, the United States government is the single largest user of energy in the country. Of the government’s total energy usage, 45 percent comes from the heating, cooling, and ventilating of buildings.

The HVACR industry is one industry where both centrifugal pumps and fans are employed on an ongoing basis everyday. Because of this, today’s HVACR systems are huge energy consumers. Creating higher-efficiency HVACR systems offers a tremendous opportunity for great electrical and monetary savings, thus conserving our nation’s valuable energy resources.

Many of these savings can be accomplished through the use of variable-frequency drives (VFD) (Figure 1). The world’s first mass-produced VFD was introduced in 1968. Computerization, the reduced cost of electronics, and the interfacing with direct digital control (DDC) systems, along with their energy savings, have made VFDs even more popular today. In fact, VFDs for evaporator and condenser fans and compressor motors are often coming as standard equipment in many newer, high-efficiency, rooftop air conditioning systems incorporating greener, more efficient, HFC-based refrigerants like R-410A.

When retrofitting systems with VFDs, payback periods are ranging from one to two years depending on the type, size, and application of the system. This is a very short payback period yielding fast cost savings, considering that most commercial systems have life cycles of 15 to 20 years. It is estimated that motors incorporating VFDs and linked to the building’s DDC system, are up to 65 to 75 percent more efficient than motors operating at a constant speed at line voltage.

REDUCED CAPACITIES AT PART-LOAD DEMAND

It is usually only on the hottest and coldest days of the year that systems must operate at 100 percent capacity with full fan, compressor, and/or centrifugal pump speeds. Most of the time, system motors can operate much more energy efficiently at reduced capacities and speeds. Systems with variable-speed fan motors have the ability to deliver variable air volume (VAV) flows. This allows the airflows to exactly match the system’s heating and cooling demands and offers the opportunity to save electrical energy and money.

HIGHER HUMAN COMFORT LEVELS

VFDs also contribute to the overall comfort level within the building by regulating air and water flows according to the instantaneous heating or cooling load. Closer system temperatures and humidity set points can be controlled with variable air and water flows within the HVAC system, resulting in higher human comfort levels. Noise levels are also reduced with the use of VFDs on system motors.

Figure 2: Circuit diagram of a three-phase VFD.

LOWER MAINTENANCE COSTS AND LONGER EQUIPMENT LIFE

The elimination of motor line starting shock can be eliminated by soft starting the motor through a VFD and gradually ramping it up to its required speed for the associated heating or cooling load at that time. Reduced maintenance costs and down times can be realized from soft starting the motors instead of starting the motor at full speed and drawing locked rotor amperage (LRA). VFDs also eliminate short cycling of motors, which will result in longer life of the motors and driven equipment.

Major compressor manufacturers are also manufacturing compressors with VFDs to control the speed of the compressor motor. This simply adds to a more accurate control of the capacity of the system. Systems incorporating VFD technology on a combination of motors including evaporator and condenser fans, compressors, and centrifugal pumps for chilled-water systems optimize cost savings and energy efficiency.

Other motors offering cost-saving opportunities within systems include:

• Cooling tower fans;

• Cooling tower water pumps;

• Make-up air fans;

• Exhaust fans;

• Air-handler fans;

• Booster fans;

• Centrifugal hot water pumps; and

• Centrifugal cool water pumps.

Figure 3: Sine waves and coding.

FUNDAMENTALS OF AC MOTOR SPEEDS

The following formula can be used to determine the no load (synchronous speed) of an alternating current (ac) motor in revolutions per minute (rpms).

rpm = (Hz) x (60 seconds/minute) (# of pole pairs)

Where:

Hz = is the frequency of the voltage in cycles/second

rpm = Revolutions/minute or motor speed

Notice that the rpms have units of rpm on one side of the equation, but (Hz) has units in cycles/seconds. So, the conversion from seconds to minutes comes from multiplying by 60 seconds/minute. What is left of the equation after this time conversion from minutes to seconds is that rpms are governed by nothing but the frequency (Hz) and the number of poles. As one can see from the equation, the more poles the motor has, the slower it will turn. Also, as the motor’s frequency decreases, the rpm or speed of the motor will decrease.

Conversely, as frequency increases, the motor’s speed will increase. Conventional motor speeds are controlled by the number of poles, and the frequency is constant at 60 Hz. Somehow, the number of poles has to be changed usually through a wiring scheme. It is much easier to change the frequency (Hz) of the voltage coming into the motor with electronics than it is to increase or decrease the number of poles in the motor. This is where VFDs come into play.

The equation can be rewritten into the simpler form that follows:

rpm = (Hz) x (60 seconds/minute) (# of poles/2)

By simply multiplying the numerator and denominator (top and bottom) of the equation by 2, the equation now becomes:

rpm = (Hz) x (120 seconds/minute) (# of pole pairs)

Rewriting the equation again without the units and it becomes:

rpm = (Hz) x (120) (# of poles)

This is the form of the equation used in most books. However, do not get the 120 confused with voltage. It is not 120 volts!

Figure 4: The longer the power device is on, the higher the output voltage will be. The less time the power device is on, the lower the output voltage.

OPERATION OF THE VFD

A VFD can consist of three separate electronic sections. The three electronic sections’ functions and their accompanying electronics components are:

• Rectification (diodes) or converter section;

• Filtering (capacitors and inductors) or dc bus section; and

• Switching (transistors) or inverter.

Rectification: In a circuit diagram of a three-phase VFD (Figure 2) all three lines of the three-phase power go through diodes in the form of a bridge rectifier. Diodes let current pass only in one direction. The bridge circuit of diodes actually rectifies (changes) the three-phase ac voltage to pulsating dc current (dc). The diodes actually reconstruct the negative half of the waveform into the positive half. So, the dc bus section sees a fixed dc voltage. The output of this section is actually 12 half-wave pulses, which are electronically 60 degrees apart.

Filtering: The filtering section of the VFD simply makes the pulsating dc smoother or filters out its imperfections. These actually can be more than two capacitors wired in parallel with one another, but in series with the bridge rectifier and an inductor. The capacitance of capacitors adds when they are in series. There is synchronous charging and discharging of the capacitors with the three phase input voltage. This makes a pure dc signal from the half-wave signal of the bridge rectifier. The capacitors in parallel filter the voltage wave and the inductor will filter the current wave. Both the inductor and capacitors work together to filter out any ac component of the dc waveform. The smoother the dc waveform, the cleaner the output waveform from the drive.

Switching: The switching or transistor section of the VFD produces an ac voltage at just the right frequency for motor speed control. This section is also called the inverter section. This section converts the dc back to ac. There are two transistors for each output phase. These transistors act as switches for current to flow.

Inverters consist of an array of transistors that can be switched on and off. When the control system sends a signal to the base connection of the transistor, the transistor turns on and allows current to flow through it. When the signal is dropped, the transistor turns off and no current will flow. The base or controlling part of the transistor is controlled by a pre-programmed microprocessor which fires the transistors at appropriate times in six steps. Each set of transistors is connected to a positive and negative side of the filtered dc line (Figure 2).

Today’s inverters almost all use these transistors to switch the dc bus on and off at specific intervals. This process is also known as pulse width modulation (PWM). Inverters produce the correct frequency of voltage and current to the motor for the desired speed. This is where the term variable-frequency drive (VFD) originates.

In other words, an inverter creates a variable ac voltage and frequency output. Inverters can actually control motor speed down to about 50 percent of their rated speed and up to about 120 percent to their 60-Hz rated speed.

MOTOR SPEED

The motor’s speed is controlled by supplying the motor’s stator (stationary) coils small pulses of voltages. At low speeds, the voltage pulses are short; at high speed, the pulses are longer. The PWM pulses are sine coded, meaning that they are narrower at the part of the cycle close to the ends. This makes the pulsating signal look like a sine wave to the motor (Figure 3).

This output wave is not an exact replica of the ac input sine waveform. Instead, there are voltage pulses. The VFD’s microprocessor or controller signals the power device to “turn on” the waveform’s positive half or negative half. The longer the power device remains on, the higher the output voltage will be. The less time the power device is on, the lower the output voltage (Figure 4).

The speed at which power devices switch on and off is referred to as the switch frequency or carrier frequency. As the switch frequency increases, the resolution and smoothness of the output waveform increases. However, as the switch frequency increases, the heat in the power device increases.

Publication date: 10/05/2009

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