|
Introduction
Although motors may appear to be the least complicated component in the specification of mining equipment, this paper begins to demonstrate that mining applications present an immense matrix of application criteria to properly specify, design and build motors.
To obtain the correct motor for the specific job, communication, expectations and knowledge must be exchanged between the mine operator, driven equipment manufacturer and motor supplier.
Operating conditions
Basic motor specifications begin with determining the motor nameplate horsepower and rpm. These are determined by the driven equipment supplier and are based upon a steady state equipment operation.
Next is the determination of the available power voltage. The mine operator or engineering consulting firm must determine the most effective power source, taking horsepower and ampere values of the entire system into consideration.
The frequency (Hz) rating is determined by the power system available at the site. Because the mining market is global with many frequency and voltage combinations, the frequency value cannot be assumed. It is important to the motor manufacturer in the proper design of a motor, which is different for Chile (50 Hz) than Argentina (60 Hz).
Ambient temperature is often overlooked as a design criterion. Ambient temperatures below –30° C (–22° F) can require special bearing lubricant and material requirements. Conversely, ambient temperatures above 40° C (104° F) cause the allowable motor temperature rise to be lower, which effectively derates the motor design.
The altitude at the site can also affect the motor selection when installation elevations exceed 1,000 m (3,281 ft). The lower density of air at higher altitudes results in a decreased cooling media for the motor. The derate factor is 1% of the specified temperature rise for each 100 m (328 ft) of altitude in excess of 1,000m (3,281 ft).
Driven equipment torque requirements
To properly select ac induction motors for any application, the speed vs. torque requirements of the driven equipment must be understood. It is an easy mistake to believe that a 298-kW (400-hp), 1,200-rpm motor, with a full load torque rating of 245 kg-m (1,773 ft-lb.), which would function well in a centrifugal pump application, would also work aptly in a crusher application. However, the load torque requirements of a centrifugal pump during starting are typically less than 30% of full load torque, while a full gyratory crusher would have load torque requirements at start-up of more than 100% of the motor's full load rated torque.
The distinction must be understood between the running condition of the driven equipment, which dictate the hp and rpm of the motor, and the starting load condition of the driven equipment, which dictates the motor starting characteristics. The National Electrical Manufactures Association (NEMA) classifies the torque characteristics of motors as follows:
– Locked-rotor torque (LRT): the percentage of rated full load torque the motor generates at initial rotation of motor shaft.
– Pull-up torque (PUT): the lowest percentage of rated full load torque the motor generates during starting.
– Breakdown torque (BDT): the highest percentage of rated full load torque the motor generates prior to reaching full load speed.
Motors that do not have sufficient starting torque for the driven equipment will stall during starting. A stall condition requires the mine operator to lower the starting load before attempting to restart the equipment. In the case of crushers or mills, this means the removal of aggregate from the machine. Excessive stall conditions also damage the motor due to excessive current flow in the stator and rotor.
Design specifications
Motor enclosure.
The motor enclosure defines the degree of protection for the motor windings. The selection of the motor enclosure is typically left to the discretion of parties other than the motor manufacturer. However, the motor manufacturer can choose to provide an enclosure that exceeds the requirements of that specified by the purchaser.
Open enclosures.
Open type enclosures present a lower cost option to the mining industry, although as the NEMA definition implies, the degree of protection for the motor windings is diminished. "An open machine is one having ventilating openings that permit passage of external cooling air over and around the (stator) windings of the machine."
Electrical specifications
The electrical design criteria of a motor are often assumed by the motor vendor at the time of quotation, unless a specification is submitted by the customer or consulting engineer.
Service factor.
The service factor (SF) of the motor is the level of overload the motor is capable of maintaining above the nameplate power rating. A service factor of 1.0 or 1.15 is most common, although 1.05, 1.1 and 1.25 are occasionally used.
A service factor of 1.0 indicates the motor is specified and designed to not operate above the nameplate horsepower. Service factors above 1.0 indicate the motor is suitable for continuous operation at the nameplate hp multiplied by the SF.
Temperature rise.
The temperature rise of a motor is the specified maximum level of stator temperature increase over a specified ambient temperature.
An 80° C (144° F) rise by resistance at 1.0 SF at 40° C (104° F) ambient at a maximum elevation of 1,000 m (3,281 ft) has become the basic motor industry standard.
Starting method.
This is a subject that is frequently overlooked until a motor will not start at the job site. When a motor has been sold on the assumption of full nameplate voltage (across the line) starting, and the customer intends to use an auto transformer or some other type of reduced voltage starter, the potential exists for starting problems.
Motor torque performance is based upon 100% nameplate voltage. Motor torque output varies as the square of the voltage change.
The motor vendor can evaluate, and design the motor for reduced voltage starting, with the submittal of starting voltage, load inertia value and load speed vs. torque curve.
The use of an adjustable speed drive (ASD) is becoming a more frequent application occurrence. Fan and pump motors are being installed with ASDs for the purpose of energy savings via speed variation instead of the use of valves and dampers. ASDs are also being applied with motors for crushers and conveyors for more precise process control.
Efficiency.
Specifying a minimum motor efficiency is occasionally seen in the mining market. Similar to temperature rise, discussion of efficiency represents an opportunity for a paper itself. When specifying a motor, if efficiency is to be a criterion, it must be defined in terms of method of acquisition and in the interpretation of the value (guaranteed minimum, nominal, calculated or typical).
Mechanical specifications
The mechanical design criteria includes some items that must be specified by the driven equipment manufacturer, some that can be assumed and some that must be dictated by the motor manufacturer.
Accessory equipment
The specification of the motor accessory equipment is primarily the choice of the motor purchaser. These items represent additional cost and will typically not be included by the motor vendor unless required by the operating conditions.
Conclusions and recommendations
For optimum motor performance and customer satisfaction, the application of AC induction motors to mining process equipment must be understood by the mine personnel, the consulting engineer(s), the driven equipment suppliers and the motor manufacturer. This understanding requires a transfer of critical information between all parties.
К началу страницы
|