Mill power

by Emily Wilson
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Most flour mill managers spend the majority of their time dealing with labor or customer relations, maintaining production schedules and targets, or on other issues related to operations, production, delivery, sales, marketing, laws and regulations. They may not fully appreciate the hidden costs involved in the inefficient use of electricity.

Flour milling is a highly competitive business. The profit margin is so small that even a fraction of a penny per hundredweight of flour counts. In this kind of environment, it is important to pay attention to the cost-effective and cost-efficient use of electricity. Improved methods, practices and actions related to electrical energy use can be the difference between profit and loss.

Electricity bills are based on "energy" and "power." The two terms are often used interchangeably, but must be distinguished in order to calculate electricity costs. Energy is the total amount of work done in a given period of time while power is the time rate of doing the work.

To illustrate the difference, imagine that two men are asked to lift a 50-pound bag of flour from the floor to the top of a 3-foot-high table. The first man lifted the bag to the table in 1 second while it took the other man 3 seconds to do the same task. Both did exactly the same amount of work; in other words, both spent the same "energy." But the first man did the work at a rate of 150 foot-pound per second (power) while the second man did the work at a rate of 50 foot-pound per second.

Likewise, electricity bills are based on energy and power. Electrical energy is expressed in kiloWatt hours (kWh) while electrical power is expressed in kiloWatts (kW).

ELIMINATING INEFFICIENCIES. There are four main reasons for electrical energy inefficiencies in a flour mill. These include the inherent nature of the energy-consuming devices, poor design and selection of equipment and systems, poor energy usage and management indifference.

Poor power factor, which is generally caused by the first three reasons, is one of the main components of electrical energy inefficiency. Power companies monitor such inefficiencies and penalize the user with higher electric costs.

If a flour mill could convert all of the energy that is delivered to its facility into useful work, such as running all motors with full load, the electrical efficiency would be close to 100%. But it is virtually impossible to have a 100% efficient system. The goal, then, is to optimize the level of energy utilization efficiency or reduce the energy inefficiencies to the lowest possible level.

Power companies generally monitor the fraction or percentage of energy converted by each industrial customer into usable work. This fraction is known as power factor (PF), which is defined as the fraction of true power (active or usable) compared to the apparent power (the power that is delivered to the plant by the utility). The PF can be expressed either as a fraction or as a percentage.

The maximum value of PF is 1 or 100%, and the minimum value is 0 or 0%. Power factors of 1 and 0 mean that all or none of the energy delivered by the utility has been converted into work, such as movement of a motor shaft or creation of heat.

To explain the concept of PF, assume that a horse is used to move a rail car forward on a track. In the first scenario, the horse runs down the middle of the track. In the second scenario, the horse runs alongside the track. In the third scenario, the horse runs perpendicular to the track.

Obviously, the first scenario will be more effective and efficient to move the rail car along the track than the second scenario, and the third scenario will not result in any usable work at all. The first scenario is analogous to a PF of 1; the second has a PF of between 1 and 0; and the third has a PF of 0.

In the second scenario, to move the car along the track at the same speed as the first scenario will require a larger and stronger horse and harness. In the third case, no matter how big the horse is and how hard it works, the usable work will be zero.

Likewise, in a flour mill, if the PF is less than 1 then higher generation, transmission and distribution capabilities will be needed. Power companies pass these additional costs associated with larger capacities along to the inefficient users.

The characteristics of the electrical loads (equipment) mainly dictate the value of the PF in a flour mill. Accordingly, electrical loads are divided into four classes: resistive, inductive, capacitive and any combinations of resistance, inductance and capacitance.

A load consisting only of resistance has a PF of 1. Electric heaters and incandescent lamps are examples of resistive loads. Coils made of large diameter copper wire with a large number of turns and various capacitors are examples of inductive and capacitive loads, respectively. Both of these types have a PF of 0.

This means that no useful energy, such as heat, light or movement of a motor shaft, can be obtained from inductive and capacitive loads. All the electrical energies supplied to an inductive load are used to magnetize the coil. Similarly, for a capacitive load, all energies are spent to charge and discharge the capacitor.

It would be ideal if the load characteristics of a flour mill could be totally resistive, but resistive load is very minor in most mills. Motor load, which is a combination of resistive load (resistance of the copper conductors in a motor) and inductive load (coil formed within the motor), constitutes the main load characteristics of a flour mill. Therefore, the PF values of a motor lies between 0 and 1. Smaller and older motors have lower PFs than larger new ones.

The PF values vary with the size of the motor and the load on the motor. Power factor is generally higher for higher horsepower motors and maximum at full load, and gradually decreases as the load decreases. Under full load and rated conditions, the PF of a new motor greater than 30 hp is generally more than 0.8, while the PF for a fractional horsepower motor may be 0.2 or lower.

When analyzing the power factor, it is important to distinguish between the PF of an individual motor and the PF of a combined group of motors (all the motors in a flour mill). As more and more motor loads are added, the combined PF of the entire plant progressively goes down.

Utility companies are concerned about the energy usage of the entire flour mill, not the performance of an individual motor. Operating each motor at its highest PF, therefore, will generally result in the highest PF for the entire plant. However, this may not guarantee that the PF will meet the minimum value required by the utility company.

Since the value of PF depends on the load on the motors, it is obvious that the PF of the entire plant also depends on the load on the whole process. Process loads vary, and it is impossible to run a process under constant load. To be able to operate a flour mill at the desired PF, it is essential that various process-loading scenarios are thoroughly explored, defined, covered and adhered to.

LOAD EFFICIENCY. Load factor is defined as the ratio of the actual amount of load a motor carries with respect to its rated full load carrying capacity. Load factor can be expressed as a decimal or a percentage. For example, a 10 hp motor has been designed for a shaft load carrying capacity of 10 hp. If the motor is used to do work that needs only 4 hp, the load factor of the motor would be 4/10 or 0.4 or 40%.

Most electric motors are designed to run at load factors of 0.5 to 1.0. Often, motors are mismatched or oversized for the load they are intended to serve or have been rewound multiple times, which may cause poor load factors.

Motor efficiency must also be considered when discussing load factor because efficiency is affected by the load factor. Motor efficiency is defined as the quotient of the power output at the motor shaft and the electric power input to the motor, termed as "true power" or active power.

Motor efficiency is usually expressed in percentage. Maximum efficiency of a motor is usually near 0.75 or 75% load factor. A 10 hp motor has an acceptable load range of 5 to 10 hp with a maximum efficiency at 7.5 hp.

Motor efficiency tends to decrease dramatically below about 50% load. However, the range of good efficiency varies with individual motors and tends to extend over a broader range of larger motors.

A motor is considered under-loaded when it is operated in the range where efficiency drops significantly with decreasing load. Remember, power factor also drops off with decreasing loads, usually sooner but less steeply than the efficiency. Therefore, a low load factor results in high cost in three ways:

• By using too big a motor for too small a job, which means drawing unnecessary current from the lines;

• By lowering the PF; and

• By lowering efficiency.

The principles used in managing load factors are to select the right size motor for each equipment and to load the equipment fully to its rated capacity. Do not grossly oversize or undersize the motor and do not grossly under-load or run equipment empty for an extended period of time. Overloading motors can cause the motor to overheat and lose efficiency.

Many motors are designed with a service factor that allows occasional temporary overloading. For example, a 10 hp motor with a 1.15 service factor can handle 15% overload or 11.5 hp load for short periods of time without causing significant damage to the motor. Running motors continuously above the rated load, even if the service factor is higher than 1, reduces efficiency and motor life.

When a motor goes bad, it needs to be replaced. Sometimes the exact horsepower motor may not be available in spare stock. To prevent process shut down, larger-size motors are frequently used as a replacement. But if these larger motors are left in permanently, it will result in reduced load factors. The mill's maintenance and operation crew often do not understand the importance of replacing the large motors with an appropriate horsepower motor.

The most important step in managing motor load is the determining the load of a motor. There are four ways to estimate motor loads: measuring input power, calculating input power, measuring line currents and measuring motor slip.

ENERGY IMPROVEMENTS. The electric motor is the workhorse of all modern industries, including flour mills. A typical mill can have hundreds, even thousands, of motors in the plant.

Electric motors use approximately 57% of the total electrical energy consumed in the United States, and 47% of total motor energy consumption comes from the industrial sector.

It is estimated that motor energy consumption in the U.S. in 1988 was 1,574 million megawatt hour. There were an estimated 1.1 billion motors of 1/6 hp or larger in the U.S.; more than 85% of all motor energy usage in the U.S. is attributed to motors of 1 hp or larger, and 75% of those are motors of 7.5 hp or larger.

A small improvement in the energy efficiency of electrical motors will save billions of dollars and millions of barrels of oil annually. In 1992, the U.S. legislature passed the Energy Policy Act, which mandates the sale of only energy-efficient motors, under certain conditions.

Beginning in October 1997, the Act required most motors manufactured for sale in the U.S. to meet minimum efficiency standards. The Energy Policy Act applies to general purpose, T-frame, single-speed, foot-mounted, continuous-rated, polyphase, squirrel cage, induction motors of NEMA designs A and B.

An energy efficient motor produces the same shaft horsepower but uses less input power or active power (kW) than a standard efficiency motor. An energy efficient motor must meet certain efficiency values, and have higher quality and thinner stator lamination, more copper in windings, optimized air gap between the stator and rotor, reduced fan losses, closer machining tolerance and greater length.

Installing energy efficient motors in a flour mill can result in savings in kiloWatts and kiloWatt hours. Energy efficient motors are available in sizes of 1 to 500 hp, 3-phase and 2, 4, 6 and 8 poles. Energy efficient motors maintain fairly stable efficiencies under partial loads and can be rewound while standard motors cannot.

Other ways to save energy and cut electricity costs in a mill is to install reduced voltage starters, variable speed drivers and energy efficient lighting and to avoid non-essential activities in peak hours. Many utilities encourage their industrial customers to lower peak demand by offering drastically reduced rates for a "peak shaving" rate schedule. This obligates the mill to "shave" or reduce demand during peak loads.

Good operation and maintenance practices, such as appropriate greasing of bearings and other friction-creating moving parts, maintaining recommended tensions in drive belts, using cog belts on drives and regular cleaning of filters, can also save electricity costs.

Ekramul Haque is a professor in the Department of Grain Science and Industry at Kansas State University, Manhattan, Kansas, U.S. This article was presented at the Association of Operative Millers' 2000 conference.

Estimating the cost savings of an energy efficient motor compared to a standard efficiency motor

(estimates for kW and kWh)

kW saved = hp x 0.746 x L [100/Es – 100/Ee]

hp = motor nameplate rating

L = load factor

Es = efficiency of standard motor

Ee = efficiency of energy efficient motor

Total annual cost savings = (kW saved x 12 x monthly demand charge + kWh saved x energy charge)

For example, a 75 hp, totally enclosed, fan-cooled, energy efficient motor having 94.9% efficiency and operating 8,000 hours annually at an average load factor of 0.75 will save U.S.$484 per year compared with a 91.6% efficiency standard motor if the demand and energy charges are U.S.$5.35 per kW and $0.03 per kWh, respectively. Simple payback period for an energy efficient motor in this example would be about two years.

Electric motor efficiency levels required by Energy Policy Act of 1992

Nominal full-load efficiency

Open motors

Enclosed motors

Number of poles

6

4

2

6

4

2

1

80.0

82.5

--

80.0

82.5

75.5

1.5

84.0

84.0

82.5

85.5

84.0

82.5

M

2

85.5

84.0

84.0

86.5

84.0

84.0

o

3

86.5

86.5

84.0

87.5

87.5

85.5

t

5

87.5

87.5

85.5

87.5

87.5

87.5

o

7.5

88.5

88.5

87.5

89.5

89.5

88.5

r

10

90.2

89.5

88.5

89.5

89.5

89.5

15

90.2

91.0

89.5

90.2

91.0

90.2

H

20

91.0

91.0

90.2

90.2

91.0

90.2

o

25

91.7

91.7

91.0

91.7

92.4

91.0

r

30

92.4

92.4

91.0

91.7

92.4

91.0

s

40

93.0

93.0

91.7

93.0

93.0

91.7

e

50

93.0

93.0

92.4

93.0

93.0

92.4

p

60

93.6

93.6

93.0

93.6

93.6

93.0

o

75

93.6

94.1

93.0

93.6

94.1

93.0

w

100

94.1

94.1

93.0

94.1

94.5

93.6

e

125

94.1

94.5

93.6

94.1

94.5

94.0

r

150

94.5

95.0

93.6

95.0

95.0

94.5

200

94.5

95.0

94.5

95.0

95.0

95.0

 

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