Monday, September 20, 2010

Electrical

23.1 Introduction
The interface between the miner and electrician requires an exchange of knowledge and good communications. For this purpose, the miner needs to understand the basic principles, use, and control of electricity. This chapter is intended only as a primer to a field of work that is far more extensive than can be adequately addressed in one chapter.

The main function of electricity in a mine is to provide mechanical energy to perform useful work. Miners think of electricity in terms of energy (“it takes 300 HP to start that 100 HP motor”) while electricians think in terms of amperage and voltage (“The draw of current at full load of 100 Amps for that 100 HP motor will increase to 350 Amps when started”).

The cost of electrical power for an underground mine may be 10% or more of the mine operating cost; therefore, efficient use warrants scrutiny by all concerned. The miner’s role in designing a proposed power system is to accurately describe the location and size of electrical equipment to be employed and provide details of planned future relocations and possible expansion of operations. Without this information, an electrical specialist cannot design an efficient and effective system.

2. Rules of Thumb
Power Consumption
• The power consumption for a typical open pit mine, including the concentrator (mill) will be approximately 60 kWh per tonne of ore mined and processed. While that of a typical underground mine including the concentrator will be approximately 100 kWh per tonne. Source: Jack de la Vergne
• The scale up factor for the power requirement at an underground mine is 1.85 for a doubling of mine capacity. Source: Jack de la Vergne
• Good demand factors for power systems range from 0.7 to 0.8, depending on the number of operating sections in the mine. Source: Morley and Novak
• The power consumption for a concentrator (mill) can be roughly approximated by adding 15 kWh/tonne to the Bond work index of the ore (determined by laboratory testing). Source: Jack de la Vergne
• Power consumption (energy portion of utility billing) for a mine hoist is 75% of RMS power equivalent. Source: Unknown

Motors
• AC motors operate very well at 5% over-voltage, but are likely to give trouble at 5% undervoltage. Source: George Spencer
• At 10% under-voltage, the life of fractional horsepower motors will be reduced to three years and the life of 3-phase motors reduced to five years. Source: Klaus Kruning
• For an AC motor, torque varies with the square of the voltage – a 10% loss in voltage is a 21% loss in torque (this is an important consideration for the head of a pump and the rope pull of a mine hoist). Source: Jarvis Weir
• A typical AC induction motor for regular mine service is supplied with a 300% breakdown torque. It operates at nearly constant speed within its normal working range, develops rated horsepower at approximately 97% of no-load speed, and a maximum torque of approximately three times full-load torque at about 80% of no-load speed. Source: Domec Lteé.
• A typical AC induction hoist motor is supplied with a 250% breakdown torque. In application, this means that the peak horsepower of a hoist motor should not exceed 1.8 times the RMS power. Source: Larry Gill
• For a DC hoist motor, the peak power should not exceed 2.1 times the RMS power for good commutation. Source: Tom Harvey
• An AC cyclo-converter hoist motor can have a peak/RMS rating as high as 3. Source: E A Lewis
• To permit overhung motors, the air gap for large direct drive DC hoist motors is typically 6mm (0.25 inch). Comparable cyclo-converter drives can have similar or larger gaps. Source: E. A. Lewis
• In operation, a typical 575-V AC motor will draw one amp per horsepower. A similar 440-V motor will draw 1¼ Amps per horsepower. Source: Bill Forest
• The brushes on an AC machine should be first set at a pressure between two and three pounds per square inch (15-20 kPa). Source: General Electric
• The brushes on a DC machine should be maintained at a pressure between three and five pounds per square inch (20-35 kPa). Source: General Electric
• The peak inverse voltage from a DC mine hoist motor will be approximately twice the supply voltage so the thyristor bank is designed accordingly. Source: Jim Bernas
• The rate of brush wear on DC motors and generators can be kept to an acceptable level if the air has a water vapour density above 5 mg/l. The sensitivity to atmosphere humidity increases at least proportionately to the speed (of rotation of the armature). Source: Gerald Tiley

Belt Drives
• The lower side of the belt loop should be the driving side. Vertical belt drives should be avoided. Source: General Electric
• 2½ times the diameter of the larger pulley will normally provide a safe working distance between centers. Source: General Electric 

Transformers
• For a typical mine circuit with multiple components, the capacity required for a transformer, measured in kVA, is approximately equal to the load expressed in horsepower. In other words, a load of 500HP normally requires a transformer with 500-kVA capacity. Source: Bill Forest

3. Tricks of the Trade
• The collector ring surfaces of an electric motor can be kept in better condition by occasionally changing the polarity of the brushes, especially where operating conditions are severe. Source: General Electric
• When it is required to dry the windings of an electric motor, it should not be heated above 900C (thermocouple) or 750C (thermometer). The heating rate should be controlled such that full heat is obtained after two hours. Source: General Electric
• Do not open a switch on a circuit carrying a large amount of current. Trip the circuit breaker first, then open the main switch. Always close the circuit breaker first; then close the switch. Source: General Electric
• When checking rotation for connection feeds, the motor should be uncoupled. One second at 1,750 RPM is 30 turns – more than enough to completely destroy a piece of equipment not designed to run backwards. Some operating equipment is very difficult to uncouple. In this case, the leads are properly tagged before disconnecting so that correct rotation is ensured when later reconnected. Source: Bert Trenfield
• The capacity of a transformer will be reduced from its rated capacity at 60 cycles per second (Hz) if operated at 50 Hz due to saturation of the magnetic circuit. The capacity of a transformer will be reduced from its rated capacity at 50 Hz if operated at 60 Hz due to increased impedance. You lose both ways. The capacity of the 50-Hz transformer operating at 60 Hz may be restored with forced ventilation, but the 60-Hz transformer operating at 50 Hz cannot. Source: Jim Bernas
• When starting up a generator that has tripped out, it is unnecessary to wait until the machine has come to rest. Source: General Electric
• Motor bearing wear can be determined by measuring the air gap between rotor and stator. Source: General Electric
• Electricians, not mechanics, should grease and lubricate electrical motors. Over-lubrication is not usually a problem for mechanical equipment, but it is harmful to electrical motors. Source: Largo Albert
• Motor generator sets are designed to produce, not receive, electrical power. A mine hoist will generate power into the mains when holding back an overhauling load (regenerating). In this case, a constant load at least equal to the hoist motor rating (such as a ventilation fan) should be incorporated into the generator grid. Source: Jim Bernas
• At altitudes less than 13,000 feet (4,000m), the textbook reduction factors for the capacity of an electric motor may usually be ignored. Source: George Greer
• One full sized generator (and a small emergency standby generator) better serve a mine development project at a remote location than two generators operating in parallel. The latter invites problems with synchronization. Source: Bill Forest
• Second-hand generators are often described by their standby capacity, which may be 20-25% more than their rating for continuous service at the mine. Source: Jack de la Vergne
• The capacity of new generators is often described by their continuous rating at unity power factor (“kVA”), which may be 20-25% more than their rating for typical mine service at 0.8 power factor (“kW”). Source: Jack de la Vergne
• Aluminum conductors are generally not favored at copper mining facilities even if they are technically appropriate. Copper is strongly preferred as standard for cables and bus conductors. Source: Julian Fisher
• Any electrical fault will take the path of least resistance. Without adequate grounding, the path could be someone rather than something. Grounding saves lives. Source: Julian Fisher
• A capacitor (installed for the purpose of power factor correction) is subject to high inrush currents of high frequency when another capacitor is nearby, but this is greatly reduced by even small values of inductance between the units. It is advisable not to install capacitors (whether on individual motors or in banks) too close to a bus. This way, there will always be some impedance between any two capacitors that are separately switched. Source: Fred Hampshire

4. Nomenclature
Table 23-1 shows common electrical abbreviations.
Table 23-1 Electrical Nomenclature


5. Laws and Formulas
V = IR (Ohm’s law) ER = VIcosθ (Single-phase real power)
E = I2R (Joule’s Law) EX = VIsinθ (Single-phase reactive power)
Z = (X2 + R2) ½ ER = √3VIcosθ (Three-phase real power)
θ = tan-1 X/R

6. Power Factor
Power factor (PF) is used to describe a property of an AC distribution system or piece of equipment.
PF is the ratio of actual power used in kilowatts (kW) to the apparent power drawn in kilovoltamperes (kVA). This is commonly illustrated as shown below.


A PF =1.00 is ideal and exists on all resistive loads, such as electric heaters and ovens. Units of electrical equipment containing windings, such as induction motors, transformers, welders, solenoids, belt magnets, lighting ballast, etc. have a PF less than one. The low PF is because a reactive power (kVAR) is needed to provide the magnetizing force necessary for to operate the device. As a result, the voltage is pushed out of phase with the current, causing the current to lag the voltage. The lag is measured in degrees and illustrated by the angle θ in the previous diagram.

Miners in North America tend to underestimate the importance of PF because, unlike Europe, North American utility companies do not normally bill extra for reactive power unless the mine PF is less than a specified value (usually 0.9); however, this practice is gradually being eliminated. In any event, PF correction is an important aspect to consider for any mine distribution system because reactive power has three other adverse effects: (1) heat generation, (2) premature failure of electrical components, and (3) increased voltage drop in power lines and cables.

Two common means exist to improve the power factor in a mine’s electrical system.
• Provide synchronous motors with a leading PF instead of unity PF
• Install capacitors in switched banks or on individual motors

7. Electrical Demand
Electrical experts often use different nomenclature for the same items, but “demand” is a term that is universal. Demand is a fundamental concept in designing power systems and determining electrical consumption.

The definition of demand is best explained by an example. Consider an underground dewatering pump powered by a 100-HP (75 kW) motor (nameplate rating) that pumps water from a sump. The operating characteristic of the system requires only 80 HP (60 kW) from the pump motor. (North American mines tend to oversize electric motors.) The pump operation is intermittent, controlled by level sensors in the sump, such that the pump only operates 45 minutes out of each hour.
• The connected load is 100 HP.
• The “load factor” is 0.8 (80 HP/100 HP).
• The “demand” is the amount of load that can be determined by a demand meter averaged over a specified time interval measured in kilowatts. If the 100 HP pump were fitted with a typical 15-minute demand meter and the pump engaged, the meter would indicate a demand of 30 kW (40 HP) after 7.5 minutes, 45 kW (60 HP) after 11 minutes and 60 kW (80 HP) at 15 minutes and thereafter. (As a result of the delay associated with measuring demand, inrush currents due to motor starting do not affect the demand values since their duration is so short.)
• The “average demand” of the pump is 45 kW (60 HP) (45 minutes/60 minutes x 60 kW).
• The “maximum demand” is a term that does not apply to the pump motor itself, but rather the total electrical system that includes the pump motor. It is defined and explained in the subsequent section.

The capacity (size) of the cable that feeds the pump motor is described in Amps and considers the nameplate HP without consideration of the load factor. If the line voltage to the three-phase pump motor is 440 v, its power factor is 0.8 and the motor efficiency is 96%, then the required cable amperage rating is (100 HP x 0.746 x 1,000)/(440 x 0.8 x 0.96 x √3) = 127 Amps. When the pump is starting up, the amperage draw is greatly increased, but this is not normally taken into account since the duration of starting is not long enough to overheat the feed cable. If the motor circuit is correctly fitted with a capacitor, the power factor becomes unity and in this case the required feed cable capacity rating would be reduced from 127 Amps to 102 Amps.

In an electrical distribution system, the connected load consists of all motors, heaters, transformers, etc. that are installed in the system. If all the loads were operating at 100% nameplate rating, simultaneously for the meter sample time interval, the demand would equal connected load. As indicated above, not all of the connected loads are operating at their maximum or nameplate values.

Similarly, not all of the loads are operating at the same time. From a billing point of view, the “maximum demand” is the highest value of demand as measured by the demand meter over the billing period (typically one-month). The “demand factor” is the ratio of maximum demand to the total connected load.

The “maximum demand” is the amount of power (in kW) that must be made available by the utility (or generating system) to the mine. If power factor is less than unity, the utility must supply additional power to compensate for the inefficient use of power on the site. The amount of power that must be provided is equal to the real power used (kW) divided by the power factor (less than unity) and expressed as kVA. Various billing structures exist, but generally if the power factor is poor, the utility uses the kVA value in its power calculations instead of the kW resulting in a penalty being applied to the customer. When estimating demand (and consumption) for a proposed mining facility, it is customary to omit consideration of this penalty on the assumption that appropriate power factor correction will be incorporated into the design of the mine’s electrical circuitry.

When estimating demands for a new mine, it is generally assumed that the power required by individual loads will be the nameplate data times the load factor. The summation of all the loads will provide a value of demand that assumes that these loads are all running simultaneously. To obtain “maximum demand,” electrical engineers simply apply a “diversification factor” (based on their experience from similar mines) to the total to reflect the fact that all the loads do not operate simultaneously or always at the assumed load factor.

Maximum Demand (kW) = Σ[connected load x load factor] x diversification factor The maximum demand in conjunction with the power factor is used to estimate power transmission line capacities, transformer capacities, and on-site generator requirements.

8. Power Consumption and Cost Estimate
In estimating the amount of power that will be consumed by a motor or other energy consumer, the following formulae are used.
• Motor kW = Motor HP x 0.746
• Load factor (Lf) = Running Load/ Nameplate Rating
• Utilization Factor (U) = Per unit running time/ Per unit time
• Unit Energy Consumption (kWh)= kW x Lf x U x operating hours
Operating hours may be in intervals of a day, week, or month. Typically, utilities calculate consumption over a period of a month. This value should be close to the value obtained by multiplying average demand by interval hours. Table 23-2 is a spreadsheet example for a proposed 2,500-tpd underground mine applying these
explanations.
Table 23-2 Tabulation of Estimated Power Consumption and Cost for a 2,500 tpd Mine
(Operations are 24 Hours per Day, Seven Days per Week)



Utility Rates
Energy cost per kilowatt-hour: $0.046 on peak, $0.034 off peak
Demand cost per kilowatt: $8.86 (of maximum demand measured in monthly billing period)
Peak time energy cost per day = 14/24 x 194,479 kWh x $0.046 = $5,219
Off peak energy cost per day = 10/24 x 194,479 kWh x $0.034 = $2,755
Demand cost per day = 6,805 kW x $8.86/30 days = $2,009
Total power cost per day = $9,983
Cost per ton mined = $9,983/2,500 tpd = $3.99/ton

Typical Power Consumption for a Mill (Concentrator)
Table 23-3 shows typical power consumption for a mill.
Table 23-3 Kilowatt-hour per Tonne Processed (Typical Values)



Power Consumption for a Mine Hoist
The mine hoist is a major consumer of electrical power; therefore, particular attention is paid to estimating mine hoist power consumption. A common misconception is that the consumption is directly related to the RMS power routinely calculated to determine the power requirements of a hoist drive. Energy consumption is a function of the average energy expended and not the RMS of heating values.
Example
Determine the energy consumption in kWh/ton for the following skip hoist.
Facts: 
1. The hoist line speed is 3,000 fpm
2. The hoist cycle time is 160 seconds
3. The friction loss in the shaft and headgear is 6% of external work
4. The aerodynamic drag loss is 1% of external work
5. The hoist is direct driven (no gear reduction)
6. The drive is an AC motor equipped with a cyclo-converter
7. The copper losses in the motor, converter and transformers are 18%
Solution: 
1. External work = 2,000 x 3,000 x 0.746 x 160/(33,000 x 3,600) = 6.03 kWh/ton
2. Loss due to friction and drag = 6.03 X 0.07 = 0.42 kWh/ton
3. Losses in drive train (none because of direct drive) = 0.00 kWh/ton
4. Copper losses = 6.03 x 0.18 = = 1.09 kWh/ton
5. Reactive power losses (none) = 0.00 kWh/ton
6. Loss for test runs, shaft inspection, brake tests = 6.03 x 0.02 = 0.12 kWh/ton
Power consumption per ton hoisted = 7.66 kWh/ton

The utility billing normally includes a demand charge in addition to the basic energy tariff. A common misconception is that the demand of the mine’s power system is affected significantly by the peak power spike in the hoist cycle. This is not true because a demand meter works by averaging the demand over a much longer period of time (15, 20, or 30 minutes). Even the new generation demand meters (that average over a shorter time interval) are not affected.

9. Standard Electrical Motor Sizes
Table 23-4 shows standard electric motor sizes.
Table 23-4 Standard Electric Motor Sizes


10. Full Load Current for AC and DC Motors
Table 23-5 shows the full load current (amperes) for three-phase AC and DC motors with normal torque characteristics, running at full load.
Table 23-5 Full Load Current Data


1 For unity power factor (for 90% and 80%), multiply Amps by 1.1 and 1.25, respectively.

11. Transmission Line Data
Table 23-6 shows transmission line data.
Table 23-6 Transmission Line Data


12. Ratings of Motor Circuit Fuses and Breakers
Table 23-7 shows ratings of motor circuit fuses and breakers as a percentage of full load current based on the Canadian Electrical Code. Table 23-8 shows the same data based on the American National Electrical Code.
Table 23-7 Fuse and Breaker Ratings (CAN)


(1) Resistor or reactor starting.
(2) Autotransformer or star-delta starting.
1 Or at not more than 250% of the motor locked rotor current, when given, except that the
ratings need not be less than 15 A.
Table 23-8 Fuse and Breaker Ratings (USA)


1 For motor code letters A through E, some ratings may be slightly reduced (see code)
2 Full voltage, reactor, or resistance starting

13. Fuse Ratings Required for Motor Applications
Table 23-9 shows the required fuse ratings for motor applications.
Table 23-9 Required Fuse Ratings


14. Worldwide Power Grid Supply System
Frequencies
Table 23-10 shows the worldwide power grid supply system frequencies.
Table 23-10 Worldwide Power Grid Supply System Frequencies
(All frequencies shown in Hz)