Archive for the ‘Energy Saving’ Category

Electrical Systems: Power Factor Correction (2)

November 29, 2009

Power Factor Charges
Many utilities charge for low power. To measure power factor, the most common type of utility meter measures the total kVAr-hours and kVA-hours over the billing period and calculates the average power factor as:

PF = Cos [ ArcSin (kVArh / kVAh) ]

The most common methods of charging for low power factor are:

1. Adding a demand penalty when the power factor dips below a set amount (usually 90%)

2. Basing the demand charge on the supplied power Ps (kVA), rather than the actual power used Pa (kW).

3. Basing part of the overall charge on the reactive power kVAr, which increases as power factor decreases.

Electrical Systems: Power Factor Correction (1)

November 29, 2009
Resistive devices, like electric resistance heaters and incandescent lights transform all the power supplied to the device into heat or useful energy. Inductive devices, like motors, use some of the power supplied to the device to energize the inductive windings and create a magnetic field. This power, called reactive power, is alternately stored and given up by the windings, but is not used to do actual work. When this happens, the line supplying power to the device now carries the actual power used by the device and the reactive power created by the device.

Actual power used by the device is measured in kW, reactive power created by induction devices is measured in kVAr, and the apparent power in the supply lines is measured in kVA. The mathematical relationships between these types of power are described by the “power triangle” shown below. For example,

The ratio of the actual power consumed by equipment (Pa) to the power supplied to equipment (Ps) is called the power factor.

PF = Pa / Ps = kW / kVA = cos Phi

Devices which generate/require large amounts of reactive power in relation to actual power consumed have low power factors. Such devices include:

• Motors
• HID and fluorescent lights with low PF ballasts
• Devices which convert AC power to DC power such as:
• DC drives
• Welding machines
• VFDs
• Induction furnaces

Fully loaded motors generally have a power factor of about 80%. However, if the motor is under loaded, the fraction of reactive power (for the coil) to actual power (for mechanical work) increases and the power factor decreases.

Two potential problems are associated with low power factor. First many utilities have explicit or implicit charges for low power factor. Second, low power factor increases the current, and hence losses, in transformers and the electrical distribution system. These losses cost money and generate excess heat in the electrical distribution system, which may shorten equipment lifetime or cause production shut downs. These potential problems are discussed in the sections that follow.

Variable Frequency Drives (4) – FINISH

May 2, 2009
4. Basic Operation of a PWM Inverter (VFD)

In this section we will discuss how the five basic drive system components work together. After this discussion we shall include a detailed, component level, discussion of operation.

The rectifier circuit of a pulse width modulated drive normally consists of a three phase diode bridge rectifier and capacitor filter. The rectifier converts the three phase AC voltage into DC voltage with a slight ripple. This ripple is removed by using a capacitor filter. (Note: The average
DC voltage is higher than the RMS value of incoming voltage by: AC (RMS) x 1.35 = VDC)
The control section of the AFD accepts external inputs which are used to determine the inverter output. The inputs are used in conjunction with the installed software package and a microprocessor. The control board sends signals to the driver circuit which is used to fire the inverter.
The driver circuit sends low-level signals to the base of the transistors to tell them when to turn on. The output signal is a series of pulses, in both the positive and negative direction, that vary in duration. However, the amplitude of the pulses are the same. The sign wave is created as the average voltage of each pulse, the duration of each set of pulses dictates the frequency.
By adjusting the frequency and voltage of the power entering the motor, the speed and torque may be controlled. The actual speed of the motor, as previously indicated, is determined as:

Ns = ((120 x f) / P) x (1 – S)

where: N = Motor speed; f = Frequency (Hz); P = Number of Poles; and S = Slip.

Variable Frequency Drives (3)

May 2, 2009

3. Basic Drive System

The AFD consists of several basic components:

  • Line Voltage – In this case 3-phase AC voltage.
  • Input Section – Consists of a rectifier and filter. Transforms the AC voltage into DC voltage.
  • Control Section – The control board accepts real world inputs and performs the required operations. The tasks are performed by a microprocessor.
  • Output Section – This section includes the base drive circuits and the inverter. The base drive signals are low level signals that tell the inverter to turn on.
  • Motor – Already described.

Variable Frequency Drives (2)

May 2, 2009
2. Constant Torque Loads

Direct Current electric motors, eddy-current clutches, transmissions, etc. used to be the best way of controlling process speed. With present AC drive technology, greater speed control and fewer losses can be realized. Additionally, there are fewer moving parts that would have to be maintained.

Vector drives can deliver full rated torque from full speed to zero RPM. Torque can be controlled, with precision, allowing even large motors to position loads much like servo motors. This allows for greater flexibility of control over the other methods of speed control.

Variable Frequency Drives (1)

May 2, 2009

1. Variable Torque Loads

Variable loads offer a tremendous opportunity for energy savings with AFD’s. The areas of greatest opportunity are fans and pumps with variable loads.

Fan and pump applications are the best opportunities for direct energy savings with AFD’s. Few applications require 100% of pump and fan flow continuously. For the most part, these systems are designed for worst case loads. Therefore, by using AFD’s, fluid affinity laws can be used to reduce the energy requirements of the system (Fig. 1).

Fig.1 Pump and Fan Affinity Laws
Eq. 1: N1 / N2 = Flow1 / Flow2
Eq. 2: (N1 / N2)^2 = Head1 / Head2
Eq. 3: (N1 / N2)^2 = T1 / T2
Eq. 4: (N1 / N2)^3 = HP1 / HP2
By using the affinity laws, you can determine the approximate energy savings:
Ex. 1: 250hp Fan Operating 160 hrs / Week
hp1 / hp2 : (N1 / N2)^3
100% spd = 40 hrs = 100% ld = 250hp
75% spd = 80 hrs = 42% ld = 105hp
50% spd = 40 hrs = 13% ld = 31hp
kWh / wk = (hp) x (0.746) x (hrs / eff)
250 x 0.746 x (160 / 0.95) = 31,411kWh/wk
Assuming no loss of efficiency at reduced speeds:
(250 x 0.746 x (40/0.95)) + (105 x 0.746 x (80/0.95)) + (31 x 0.746 x (40/0.95)) = 15,422 kWh
By using an AFD the approximate kWh savings per year would look like:
(31,411 – 15,422) x 50 = 800,000 kWh/yr

Motors & Energy Saving (6) – Finish

February 1, 2009
8. Electric Motors Checklist

Motors & Energy Saving (5)

February 1, 2009

7. Diagnostic Tools

• Thermography – An infrared thermometer or camera allows for an accurate, non-contact assessment of temperature. Applications for motors include bearing and electrical contact assessments on motor systems and motor control centers.

• Ultrasonic analyzer – Electric motor systems emit very distinct sound patterns around bearings. In most cases, these sounds are not audible to the unaided ear or are drown-out by other equipment noises. Using an ultrasonic detector, the analyst is able to isolate the frequency of sound being emitted by the bearing. Changes in these ultrasonic wave emissions are indicative of changes in equipment condition-some of these changes can be a precursor to component degradation and failure.
• Vibration analyzer – The rotational motion within electric motors generates distinct patterns and levels of vibration. Using a vibration analyzer and signature analysis software, the analyst can discern the vibration amplitude of the point on the motor being monitored. This amplitude is then compared with trended readings. Changes in these readings are indicative of changes in equipment condition.
• Other motor analysis – Motor faults or conditions including winding short-circuits, open coils, improper torque settings, as well as many mechanical problems can be diagnosed using a variety of motor analysis techniques. These techniques are usually very specialized to specific motor types and expected faults.

Motors & Energy Saving (4)

February 1, 2009

6. Maintenance of Motors

Preventative and predictive maintenance programs for motors are effective practices in manufacturing plants. These maintenance procedures involve a sequence of steps plant personnel use to prolong motor life or foresee a motor failure. The technicians use a series of diagnostics such as motor temperature and motor vibration as key pieces of information in learning about the motors. One way a technician can use these diagnostics is to compare the vibration signature found in the motor with the failure mode to determine the cause of the failure. Often failures occur well before the expected design life span of the motor and studies have shown that mechanical failures are the prime cause of premature electrical failures. Preventative maintenance takes steps to improve motor performance and to extend its life. Common preventative tasks include routine lubrication, allowing adequate ventilation, and ensuring the motor is not undergoing any type of unbalanced voltage situation.

The goal of predictive maintenance programs is to reduce maintenance costs by detecting problems early, which allows for better maintenance planning and less unexpected failures. Predictive maintenance programs for motors observe the temperatures, vibrations, and other data to determine a time for an overhaul or replacement of the motor. Consult each motor’s instructions for maintenance guidelines. Motors are not all the same. Be careful not to think that what is good for one is good for all. For example, some motors require a periodic greasing of the bearings and some do not.

Motors & Energy Saving (3)

February 1, 2009
4. Safety Issues

Electric motors are a major driving force in many industries. Their compact size and versatile application potentials make them a necessity. Motors are chosen many times because of the low vibration characteristics in driving equipment because of the potential extended life of the driven equipment. The higher rpm and small size of a motor will also make it a perfect fit for many applications.

Motors can be purchased for varying application areas such as for operating in a potentially gaseous or explosive area. When purchasing a motor, be sure to check the classification of the area, you may have a motor that does not meet the classification it is presently in! For example, a relatively new line of motors is being manufactured with special external coatings that resist the elements. These were developed because of the chemical plant setting in which highly corrosive atmospheres were deteriorating steel housings. They are, for the most part, the same motors but have an epoxy or equivalent coating.

5. Cost and Energy Efficiency
An electric motor performs efficiently only when it is maintained and used properly. Electric motor efficiencies vary with motor load; the efficiency of a constant speed motor decreases as motor load decreases. Below are some general guidelines for efficient operations of electric motors.
• Turn off unneeded motors – Locate motors that operate needlessly, even for a portion of the time they are on and turn them off. For example, there may be multiple HVAC circulation pumps operating when demand falls, cooling tower fans operating when target temperatures are met, ceiling fans on in unoccupied spaces, exhaust fans operating after ventilation needs are met, and escalators operating after closing.
• Reduce motor system usage – The efficiency of mechanical systems affects the run-time of motors. For example, reducing solar load on a building will reduce the amount of time the air handler motors would need to operate.
• Sizing motors is important – Do not assume an existing motor is properly sized for its load, especially when replacing motors. Many motors operate most efficiently at 75% to 85% of full load rating. Under-sizing or over-sizing reduces efficiency. For large motors, facility managers may want to seek professional help in determining the proper sizes and actual loadings
of existing motors. There are several ways to estimate actual motor loading: the kilowatt technique, the amperage ratio technique, and the less reliable slip technique. All three are supported in the Motor Master Plus software.
• Replacement of motors versus rewinding – Instead of rewinding small motors, consider replacement with an energy-efficient version. For larger motors, if motor rewinding offers the lowest life-cycle cost, select a rewind facility with high quality standards to ensure that motor efficiency is not adversely affected. For sizes of 10 hp or less, new motors are generally cheaper than rewinding. Most standard efficiency motors under 100 hp will be cost-effective to scrap when they fail, provided they have sufficient runtime and are replaced with energyefficient models.