Sparks Electrical News August 2018

MCCs AND MOTOR PROTECTION

11

HOW TO SELECT A MOTOR

By Bryan Sisler, AutomationDirect. Adapted from an article in Control Engineering

T here are many things to consider when se- lecting a motor, including application, op- erational, mechanical, and environmental is- sues. Generally speaking, the choice is either an ac motor, a dc motor, or a servo/stepper motor. Know- ing which one to use depends on the application and if there are any special needs required. A constant or variable torque and horsepower will be required for the motor depending on the type of load the motor is driving. The size of the load, the required speed, and acceleration/deceleration – particularly if it is fast and/or frequent – will define the torque and horsepower that is required. Requirements for controlling motor speed and position also need to be considered. • Variable horsepower and constant torque • Variable torque and constant horsepower • Variable horsepower and variable torque • Positional control or torque control. Variable horsepower and constant torque appli- cations include conveyors, cranes, and gear-type pumps. In these applications, the torque is constant because the load doesn’t change. The required horsepower may vary depending on the application, O ffice and administration buildings are generally equipped with PCs, servers, networks and telecommunication systems. Should these systems experience failure, work processes and operations can be brought to a standstill. Lightning and surge protection specialist DEHN Africa, the local arm of global company DEHN + SÖHNE, offers comprehensive surge protection solutions. Protection of power supply systems The table shows examples of the types of surge protective devices that can be used inside an administrative building to protect it from surges. Combined arresters such as DEHNvenCI (1) and DEHNguard® (4) surge arresters can be used to protect power supply systems. DEHNrail (5), SFL Protector (9) or DEHNsafe (11) protect terminal devices from surges and reduce induced voltages and switching overvoltages to safe values. Protection of information and telecommunication systems To ensure safe operation, both data and voice transmission require adequate protection elements. Networks are typically designed in the form of universal cabling systems. Even if fibre optic cables between building and floor distributors are standard today, copper cables are typically installed between the floor distributor and the terminal device. Therefore, the HUBs, bridges or switches must be protected by NET Protector LSA 4TP (8). DEHNpatch (6) cables are used to protect terminal devices. The DEHN equipotential bonding enclosure (2), which can be fitted with LSA disconnection blocks and lightning current carrying DEHNrapid® LSA plug-in SPD blocks, can be provided for information technology lines extending beyond the building. To protect the telecommunication system, NET Protector (10) can be installed in the floor distributor to protect the outgoing lines to the system telephones. A Motor load types There are four types of motor loads:

inverter duty ac motor with an encoder is often used for tight torque control in steel or paper lines as well as similar applications. Different motor types While there are two main motor classifications – ac and dc – there are over three dozen types of motors used in industrial applications. While there are many motor types, there is a great deal of overlap in industrial applications and the market has pushed to simplify motor selection. This has narrowed practical choices for motors in most applications. The six most common motor types, which fit the vast majority of applications, are brushless and brush dc motors, ac squirrel cage and wound rotor motors, and servo and stepper motors. These motor types fit most applications with the other types used only in specialty applications. Three main application types The three main applications for motors are constant speed, variable speed, and position (or torque) con- trol. Different industrial automation situations require different applications and their own sets of ques- tions. For example, a gearbox may be required if the

which makes constant speed ac and dc motors a good choice. An example of a variable torque and constant horsepower application is a machine rewinding paper. The material speed remains constant, which means the horsepower doesn’t change. The load does change, however, as the roll diameter increases. In small systems, this is a good application for dc motors or a servo motor. Regenerative power also is a concern and should be considered when sizing the motor or choosing the energy control method. AC motors with encoders, closed-loop control, and full quadrant drives may be beneficial for larger systems. Fans, centrifugal pumps, and agitators require variable horsepower and torque. As the motor speed increases, the load output also increases along with the required horsepower and torque. These types of loads are where much of the motor efficiency discussion begins with inverter duty ac motors using variable speed drives (VSDs). Applications such as linear actuators, which need to move to multiple positions accurately, require tight positional or torque control and often require feedback to verify correct motor position. Servo or stepper motors are the best option for these applications, but a dc motor with feedback or an

top speed is less than the motor’s base speed. This may also allow a smaller motor running at a more efficient speed. While there is a great deal of information online on how to size a motor, users must account for many factors because there are many details to consider. Calculating load inertia, torque, and speed requires the user to know about parameters such as total mass and size (radius) of the load as well as friction, gearbox losses, and the machine cycle. Changes in load, speed of acceleration or deceleration, and the application’s duty cycle also must be considered or the motor may overheat. After the motor type is selected and sized, users also need to consider environmental factors and motor enclosure types such as open frame and stainless housing for wash down applications. Application, motor knowledge Manufacturers offer a wide selection of motors for industrial applications. Stepper, servo, ac, and dc mo- tors can meet most industrial automation require- ments, but the ideal motor depends on the applica- tion. Whether it’s a constant speed, variable speed, or position control application – users should work closely with the motor and drive supplier to select the right motor for the application.

SURGE PROTECTION SOLUTIONS FOR OFFICES

THE ROLE OF MOTOR PROTECTION E lectric motors are exposed to many kinds of disturbances and stress. Part of the disturbance is due to imposed external conditions such as overvoltage and un- dervoltage, over frequency and under frequency, harmonics, unbalanced system voltages and supply interruptions, for example, auto reclosing that occurs in the supply- ing network. Other possible causes of external disturbances are dirt in the motor, cooling system and bearing failures, or increase of ambient temperature and humidity. Stress factors due to abnormal use of the motor drive itself are frequent successive start-ups, stall and overload situations, including mechanical stress. The above stress and disturbances deteriorate the winding insulation of the motor mechanically and by increased thermal ageing rate, which may eventually lead to an insulation failure. The purpose of the motor protection is to limit the effects of the disturbances and stress factors to a safe level, for example, by limiting overvoltages or by preventing too many start up attempts. If, however, a motor failure takes place, the purpose of the protection is to disconnect the motor from the supplying network in due time. Thermal behaviour and thermal protection Motor overload condition is mainly a result of abnormal use of the motor, harmonics or unbalanced supply voltages. They all increase the motor losses and cause additional heating. As the temperature exceeds the rated limits specified for the insulation class in question, the winding insulation deterioration accelerates. This will shorten the expected lifetime of the motor and may lead at some point to an electrical fault in the winding. Thus, the thermal overload protection, in addition to the short circuit protection of the motor, can be considered as being the most important protection function. Usually, authorities require that motors are equipped with thermal overload protection. Thermal model The thermal behaviour of the stator and the rotor during start-ups and during constant overload situations differ significantly from each other. Because of this, the dynamics of the motor heating and cooling are typically modelled separately for the stator and for the rotor. Implementing the thermal overload protection in this way, it can be set to follow the thermal state of the motor optimally, and good and accurate protection against both short and long-time overload conditions can be accomplished, which allows full use of the available capacity.

data protection module (12), for example, can be used for the system telephones. Protection of building automation systems Failure of building automation systems can have serious consequences. If the air-conditioning system fails as a result of surges, a data centre may have to be disconnected or a server may have to be shut down. As shown in the table, bus systems such as KNX / EIB or LON can be protected by BUStector (13), DEHNconnect (14) or BLITZDUCTOR® XT (15). Availability is increased if surge protective devices are installed according to the particular system and concept.

Enquiries: +27 (0)11 704 1487

Surge protective device

Part No.

• DEHNven CI DVCI 255 FM

961 205

1

• DEHN equipotential bonding enclosure • LSA disconnection block • DEHNrapid LSA

2

906 101

907 996 907 401

• Equipotential bonding bar • DEHNguard modular TNS

563 200

3

4

952 405

• DEHNrail modular

5

953 205

• DEHNpatch

6

929 100

• DEHNlink ISDN 1

7

929 024

• Net Protector LSA 4 TP for 8 ports • 19”enclosure for 3X NET Pro

8

929 036 929 034

• SFL Protector 19”

909 251

9

• NET Protector TC 2 LSA • 19”enclosure for 3X NET Pro

10

929 072 929 034

• DEHNsafe

924 370

11

• Data protection module DSM TC 1 SK

12

924 271

• BUSector

13

925 001

• DEHNconnect SD2 MD 48 • BLITZDUCTOR XT ML4 BD 48 • BLITZDUCTOR XT base part

14

917 942

15

920 345 920 300

• DEHNrail modular

953 405

16

Source: www.electrical-engineering-portal.com

SPARKS ELECTRICAL NEWS

AUGUST 2018