Selection of AC induction motors for cement plant applications

August 12th, 2014, Published in Articles: Vector

The selection of AC induction motors for cement mill applications, with reference to fan, kiln and vertical mill applications.

Cement plant applications present an immense matrix of application criteria to specify, design and build motors properly.

Operating conditions

Basic motor specifications begin with determining the motor nameplate horsepower (hp) and revolutions per minute (rpm). These are determined by the process equipment supplier and are based on a steady state equipment operation.

Next, the available power voltage is determined. The plant operator, process equipment supplier or engineering consultant must determine the most effective power source, taking load hp and amp values of the entire system into consideration.

The hertz (Hz) rating is determined by the power system available at the site. The Hz value cannot be assumed because the cement market is global and has many Hz and voltage combinations.

Ambient temperature is often overlooked as a design criterion. Ambient temperatures below -30ºC can require special bearing lubricants and materials. Conversely, ambient temperatures above 40ºC may result in a decrease in the allowable motor temperature rise, which effectively de-rates the motor output.

The altitude at the site can also affect the motor selection when installation elevations exceed 1000 m. The lower density of air at higher altitudes results in a decreased cooling medium for the motor. The derate factor is 11% of the specified temperature rise for each 100 m of altitude over 1000 m.

Driven equipment torque requirements

The speed versus torque requirements of the driven equipment must be understood to select AC induction motors for any application properly. It is an easy mistake to believe that a 400 hp,
1200 rpm motor, which would function well in a low-inertia fan application, would also work aptly in a kiln application. However, the load torque requirements of a fan pump during initial starting are typically less than 30% of full load torque, while a full kiln could have load torque requirements of over 100% of full load torque.

The distinction must be understood between the running condition of the driven equipment, which dictates 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 minimum torque which the motor will develop at rest with rated voltage. It is expressed as a percentage of rated full load torque generated by the motor 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 which 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.

The motor manufacturer can, however, choose to provide an enclosure which exceeds the requirements specified by the purchaser.

Totally enclosed fan cooled (TEFC) enclosures are the most common enclosures used in the cement industry. A totally-enclosed machine has no free exchange of air between the inside and the outside of the case, but it is not sufficiently enclosed to be termed “air-tight”.

The two major types of TEFC motor are totally enclosed fin cooled and totally enclosed air to air cooled or TEAAC (see Fig. 1). The fin cooled variant is defined by the cooling fins that cover the main structure of the enclosure (see Fig. 2). This frame is typically constructed of cast iron, although welded steel fin and aluminum cast construction are occasionally offered.

Fig. 1: Totally enclosed air-to-air cooled.

Fig. 1: Totally enclosed air-to-air cooled.

TEAAC motors are equipped with an air-to-air heat exchanger on the top of the motor stator. In a TEAAC enclosure, the hot air from the stator is forced around the tubes that channel the cooling air. Available tube materials on TEAAC motors include aluminum, copper and stainless steel.

Fig. 2: Totally enclosed fin-cooled.

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 with ventilating openings which permit passage of external cooling air over and around the windings of the machine.

The primary open-type enclosures in the cement industry are the Weather Protected Type II (WPII), shown in Fig. 3. The WPII enclosure includes a minimum of three 90º turns of the inlet and exhaust air to limit the ingress of airborne contaminants.

Fig. 3: A WPII enclosure.

The WPII type motor can also be supplied with filters on the air intake (galvanised steel or stainless steel are most common). The advantages include a greater hp / stator weight ratio and lower cost. By allowing the ambient air to pass directly through the motor rotor and stator, the open enclosures cool the motor better, allowing for more hp output than with a TEFC or TEAAC enclosure.

The primary disadvantage of open enclosures is that airborne dust from the cement environment can build up inside of enclosures and cause the units to overheat. The airborne contaminants can also “sand blast” the stator winding insulation if filters are not in place.

A totally-enclosed water air-cooled machine (TEWAC) is cooled by circulating air which, in turn, is cooled by circulating water. It is provided with a water-cooled heat exchanger for cooling the internal air and a fan or fans, integral with the rotor shaft or separate, for circulating the internal air.

The TEWAC enclosure provides the greater hp/stator weight of an open-type motor with the stator protected by the motor enclosure. This enclosure will provide the highest hp ratings of all enclosed motors. These ratings are either unachievable or cost prohibitive on TEFC motors.

The obvious drawback of the TEWAC enclosure is its water requirements. The supply water must be pumped, cooled and kept clean.

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.

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 or 1,15 is most common. A service factor of 1indicates that the motor is specified and designed to not operate above the nameplate hp. Service factors above 1indicate the motor is suitable for continuous operation at the nameplate hp, multiplied by the SF.

The temperature rise of a motor is the specified maximum level of motor stator temperature increase over a specified ambient temperature. Temperature rise encompasses a diverse matrix of combinations as shown in Table 1.

Table 1: Temperature rise encompasses a diverse matrix of combinations.
HP Voltage Method of determination Temperature rise, °C
Class of insulation system
A B F H
HP ≤ 1500 All Resistance 60 80 105 125
HP ≤ 1500 All Detector 70 90 115 140
HP > 1500 V ≤ 7000 Detector 65 85 110 135
HP > 1500 V > 7000 Detector 60 80 105 125

 

An 80ºC rise by resistance at 1 SF over a 40ºC ambient has become the basic motor industry standard. This represents a class B temperature rise. However, the customer has the option to specify alternatives.

Class F is the industry standard temperature endurance rating for AC induction motor insulation.

The starting method is frequently overlooked until a motor will not start at the job site. When a motor has been sold on the assumption of direct-on-line (DOL) starting, and the customer intends to use an auto transformer or some other type of reduced voltage starter, the potential starting problems arise.

Motor torque performance is based on 100% nameplate voltage. Motor torque output varies as the square of the voltage change. Therefore, with an auto transformer starter with a 65% tap setting, the 65% voltage results in only 42,2% of the nameplate starting torques (assuming no line drop).

The use of adjustable speed drives (ASDs) is becoming a more frequent. ASD application requires specific information about the particular ASD, the load characteristics and the speed range requirements for the motor vendor to design the motor appropriately. Table 2 details alternate starting methods with resultant motor torque outputs.

Table 2: Alternative starting methods.
Starting method Voltage applied Percent of full voltage
starting current
(on line side)
Percent of full voltage
starting torque
Direct-on-line  100 100 100
90 90 81
Autotransformer   80 66 64
65 45 42
50 27 25
Series reactor   80 80 64
65 65 42
50 50 25
Solid state soft start Adjustable (Volts applied/
volts rated)
(Volts applied/
volts rated)2
Variable frequency drive Adjustable Adjustable Adjustable
Star–delta 100 33,3 33,3

The speed torque curves shown in Fig. 4 demonstrate a situation in which the motor could start successfully at 100, 90 and 80% voltage, but it would stall at approximately 70% speed if only 65% voltage were applied.

Fig. 4: These speed torque curves demonstrate a situation where the motor could start successfully at 100, 90 and 80% voltage, but would stall at approximately 70% speed, if only 65% voltage were applied.

Evaluation of the power distribution system of a processing area can result in inrush amp limitations placed upon the motors. Inrush amps are the amp draw of the motor during starting.

Locked rotor amps (LRAs) is the common designation. The units for LRA are typically percentages of the full load amp value.

Motors designed for high starting torques have higher LRA values than standard torque motors. This is due to the higher flux density and/or higher resistance required in the motor’s rotor cage.

Limiting the LRA level can result in larger motor sizes. Given the load data, the motor vendor can evaluate and design the motor for reduced voltage starting, and lower inrush, a process most effectively performed during the quotation stage.

Mechanical specifications

The mechanical design criteria include 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.

Direction connection of the motor shaft to the driven equipment/gear box will be assumed unless a belt drive arrangement is specified. Horizontal mounting on level ground will be assumed by the motor supplier unless specified otherwise. However, in the case of kiln drive motors, it is common for the motors to be oriented on a 3º incline, in which case a process equipment supplier specification is helpful.

A particular challenge in motor design can be height restrictions on vertical mill applications. It is common for the motor to be mounted beneath a table or structure surface so that a standard-height motor may not fit.

Some equipment, including vertical mills, may include the requirement for an inching drive provision on the motor. This necessitates a second shaft extension on the non-drive end (NDE) of the motor to which an inching drive assembly is connected (see Figs. 5 and 6) on two vertical mills.

Fig. 5: Inching drive on NDE of mill drive motor.

Fig. 6: Inching drive.

Although both sleeve and anti-friction bearings are available on most motors larger than 440 frames, the connection of the load and the speed of the motor can dictate the choice. The advantage of sleeve bearings is that, theoretically, they will provide an infinite life. However, they do have limitations. Sleeve bearings cannot be applied to belted applications. They can also require supplemental oil supply in ambient temperatures higher that 40ºC and on the larger frame sizes. Sleeve bearings also have minimum speed requirements, which can be an issue with inching drive situations (see Figs. 7 and 8.

Fig. 7: Basic sleeve bearing design.

Fig. 8: Sleeve bearings.

Anti-friction bearings provide the greatest flexibility in application, but they do have a finite life. Anti-friction bearing life is specified in terms of L10. A minimum L10 life of 100 000 hours is typical for direct connection applications. For these applications, deep groove ball bearings are used on both ends of the motor as standard. Roller bearings can also be applied on larger motors.

Belted duty dictates the use of a roller bearing on the drive-end of the motor to provide higher levels of side loading capacity and longer bearing life. An L10 life of 17 500 hours minimum is common for belted applications.

The specification of the motor accessory equipment is primarily the choice of the motor purchaser. These items represent cost adders, and will not be included by the motor vendor unless required by the operating conditions.

The available accessories for protecting the stator windings include space heaters, abrasion resistant treatment on the end-turns, anti-fungus treatment, surge protection (lightning arrestors and surge capacitors), metering current transformer (CT) and differential CTs.

Overheating decreases motor life. The available accessories for monitoring the stator for temperature include resistance temperature detectors (RTDs), thermocouples (TCs) and thermostats. The specific type of RTD or TC is required for final motor design.

Protection of the motor bearings includes the diverse options of special shaft seals, vibration protective devices and temperature monitoring devices (RTDs or TCs).

Conclusion

For optimum motor performance and customer satisfaction, the application of AC induction motors in cement process equipment must be understood by the cement plant personnel, the process equipment suppliers and the motor manufacturer. This requires a basic understanding of motors by the cement plant operator and process equipment supplier to “spec out” the motors, and specific application knowledge by the motor manufacturer to properly design and manufacture the motors. Transfer of information between all parties is essential due to the vast amount of variables and design factors that exist.

Contact Keshin Govender, Siemens, Tel 011 652-2412, keshin.govender@siemens.com

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