Comparison of Motor Speed Control Methods

Why motor speed control?

It is often desirable to control the motor speed, usually for reasons process control for such variables as flow or pressure. Such applications as fans and pumps often have varying output requirements, and control of the motor speed is more efficient than mechanically limiting the process output with such devices as throttling valves or dampers.


The reason for this is due to the fact that for centrifugally-based processes (such as fans and centrifugally-based pumps), the following relationships exist:

Torque = RPM²
Power = RPM³

So, for these types of processes the torque required to turn them is proportional to the square of the speed.

But, the power required to turn them is proportional to the cube of the speed, and this is what makes motor speed control economically attractive. To further this argument, consider the energy wasted when mechanical means such as the throttling valves or dampers are used to control a process which is being driven from a motor running at full speed.

 

It is clear that motor speed control can be used to save energy by reducing wasted energy used to mechanically control the process.

Adjustable-speed drives (ASDs)

By far the most commonly-used AC motor control method is the use adjustable-speed drives. In most commercial and industrial environments these have supplanted virtually every other motor speed control method.
An adjustable-speed drive works on the principle of varying the frequency to vary the speed of the motor. Recall that from equations above the synchronous speed of a motor is a function of both the system frequency and the number of poles of the motor. By varying the frequency, the motor speed may be varied so long as the motor is equipped to dissipate the heat at reduced speeds.

Unlike soft-starting, specialized definite-purpose inverter-rated motor designs are preferred since reduced-speed operation can cause thermal issues and overspeed operation can result in safety issues.

Further, pulse-width modulated (PWM) drive outputs can cause repetitive voltage overshoots referred to as ringing, which can reduce the life expectancy of a general-purpose motor. The motor manufacturer should be consulted before applying a general-purpose motor in an adjustable-speed drive application.


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Connections For Testing Insulation Resistance Of Electrical Equipment

Testing Insulation Resistance //

The following schemes show how to connect a Megger insulation tester to various types of electrical equipment. The schemes also show in principle how equipment must be disconnected from other circuits before the instrument is connected.

These illustrations are typical and will serve as guides for testing insulation resistance of practically all types of apparatus and conductors. Before proceeding with tests, read the part on Preparation of apparatus for test at the bottom of this article.

REMEMBER! The Megger insulation resistance tester measures whatever resistance is connected between its terminals. This may include series or parallel leakage paths through insulation or over its surface.

Electrical equipment to test //

1. AC Motors and Starting Equipment
2. DC Generators and Motors
3. Wiring Installation
4. Appliances, Meters, Instruments and Other Electrical Apparatus
5. Control, Signaling and Communication Cables
6. Power Cables
7. Power Transformers
8. AC Generators

Important //

• Preparation of apparatus to test
• Safety precautions
• Video tutorials (46 min.)

1. AC Motors and Starting Equipment

Connections for testing the insulation resistance of a motor, starting equipment and connecting lines, in parallel. Note that the starter switch is in the “on” position for the test. It is always preferable to disconnect the component parts and test them separately in order to determine where weaknesses exist.

 

 

 

Figure 1 – Testing the insulation resistance of a motor

 

 

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The Next Generation of Advanced Robotics

The Next Generation of Advanced Robotics


If you work with robots, you’ve noticed them getting faster, smaller for a given load capacity
and more advanced in their overall capabilities. At the same time, today’s robots have increasingly
stringent uptime requirements.

But it’s not just the robots that keep evolving. Components like cables and connectors must keep
pace with today’s advanced robotics technology. Without a reliable source for power and signal, even
the sturdiest robots will fail.

In this paper, we’ll show you new trends in advanced robotics, as well as some tips for properly
specifying and installing cables into your robot.


TOP TRENDS IN ROBOTICS TECHNOLOGY.

Several trends are shaping today’s robotics technology and helping expand its use beyond well established automotive applications. In general, robots are becoming smaller, lighter, more nimble,
easier to operate and less expensive. As robots become more lightweight and flexible, their mobility is also increasing. For example, six-axis robots are now commonly installed on platforms to move from one task to another. Cables must meet these new mobility demands, with companies such as
Lapp offering plug-and-play solutions that connect robots to a so-called “seventh axis,” which serves to move the robot into various positions. In this case, cable carriers with pre-installed cables are supplied by Lapp as complete units for ease of installation.

Torsion-rated cables are specifically designed to withstand increased torsional stresses and feature a different construction than continuously flexing cables. Continuous flex cable conductors are wrapped tightly to allow relative movement of individual conductors, whereas robotic cables feature
significantly longer lay lengths to compensate for more stressful torsional forces. In torsion applications, the following parameters must be considered: maximum speed and acceleration;
minimum bending radii of cables and wires; combined weight of all moving components (including the carrier track and related cables); and desired life expectancy of the complete system.

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How to Monitor Wind Speed?

Wind Speed – Mystery Or Not?

When considering wind power, most people ask what the average annual wind speed is and how to get that number. The usual response is that you must monitor the wind speed at your site for at least 12 months, preferably longer, to determine whether a wind generator will work for you.

Sounds too long? Well, yes and no…

For a home system, this isn’t necessary. The costs involved in collecting wind data may not be justified when compared to the total cost of a small wind machine.

There is no economic formula to determine this, but it doesn’t make much sense to spend $1,200 on instrumentation if your wind machine costs only $3,000. You can get close to the actual number by making an educated guess using the empirical methods.

Options For Monitoring The Wind Speed

If you decide to monitor wind speeds, you have several options.


1 The first is to buy a weather anemometer and record observations on a regular basis. This is the least expensive way to collect wind data, but it has disadvantages. For the data to be valid, you must be methodical in collecting it.

Recording one instantaneous wind speed per day won’t do.

If you can’t record multiple wind speeds throughout a day, the quality of your data is questionable.

 

 

 

 

2 A second option is to automate data collection by installing a data collection board in a personal computer. This method works, but the computer must stay on all the time, and there are additional costs. Since these are not plug-and-play components, some computer hardware and software knowledge is necessary.









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Maintenance of Electrical Equipment In Buildings

Purpose of Maintenance

Apart from safety, maintenance is needed to keep plant in an acceptable condition. Maintenance of this kind must be reviewed on an economic and energy efficiency basis.
While it is appreciated that breakdown of plant may result in costly interruption of normal building operation, it must also be borne in mind that stopping plant for maintenance can also cause a loss in production.
Equipment on continuous and arduous duty, e.g. switchboards, motor control centres (MCCs), air-handling units, chiller plant etc., require more attention than that which is lightly loaded and rarely used.

Initial Steps For Economic and Energy Efficiency

Apart from the above considerations there will be the question of whether to repair or replace faulty equipment. This requires analysis of the past and future maintenance costs and the benefits of new equipment.

There has been much operational research carried out into such things as the probability of breakdown, replacement and repair limits, and overhaul policies. This obviously requires considerable effort and expertise and may need the services of a specialist consultant.

However, some simple initial steps can be taken as far as the economic and energy efficiency is concerned for maintenance of electrical equipment in buildings.

1. Standardisation of Equipment

The use as far as possible of standard items such as switchgear will help both in buying, stockholding and replacement of components on the most economic and convenient basis.

Motor control center at the BCI plant (photo credit: hotmixmag.com)

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9 Recommended Practices for Grounding

Basis for safety and power quality

Grounding and bonding are the basis upon which safety and power quality are built. The grounding system provides a low-impedance path for fault current and limits the voltage rise on the normally non-current-carrying metallic components of the electrical distribution system.
 
During fault conditions, low impedance results in high fault current flow, causing overcurrent protective devices to operate, clearing the fault quickly and safely. The grounding system also allows transients such as lightning to be safely diverted to earth.
Bonding is the intentional joining of normally non-current-carrying metallic components to form an electrically conductive path. This helps ensure that these metallic components are at the same potential, limiting potentially dangerous voltage differences.

 
Careful consideration should be given to installing a grounding system that exceeds the minimum NEC requirements for improved safety and power quality.

 

Recommended practices for grounding

1. Equipment Grounding Conductors
2. Isolated Grounding System
3. Branch–Circuit Grounding
4. Ground Resistance
5. Ground Rods
6. Ground Ring
7. Grounding Electrode System
8. Lightning Protection System
9. Surge Protection Devices (SPD) (formerly called TVSS)

 

1. Equipment Grounding Conductors

The IEEE Emerald Book recommends the use of equipment-grounding conductors in all circuits, not relying on a raceway system alone for equipment grounding. Use equipment grounding conductors sized equal to the phase conductors to decrease circuit impedance and improve the clearing time of overcurrent protective devices.

Equipment grounding conductor

Bond all metal enclosures, raceways, boxes, and equipment grounding conductors into one electrically continuous system. Consider the installation of an equipment grounding conductor of the wire type as a supplement to a conduit-only equipment grounding conductor for especially sensitive equipment.
The minimum size the equipment grounding conductor for safety is provided in NEC 250.122, but a full-size grounding conductor is recommended for power quality considerations.

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8 Energy-Efficiency Improvement Opportunities In Electric Motors

Energy Saving Opportunities

When considering energy-efficiency improvements to a facility’s motor systems, a systems approach incorporating pumps, compressors, and fans must be used in order to attain optimal savings and performance.
In the following, considerations with respect to energy use and energy saving opportunities for a motor system are presented and in some cases illustrated by case studies. Pumping, fan and compressed air systems are discussed in addition to the electric motors.
Potential energy-efficiency improvements:

1. Motor management plan
2. Maintenance program
3. Using of energy-efficient motors
4. Rewinding of motors
5. Proper motor sizing
6. Using Adjustable speed drives (ASDs)
7. Power factor correction
8. Minimizing voltage unbalances

1. Motor management plan

A motor management plan is an essential part of a plant’s energy management strategy. Having a motor management plan in place can help companies realize long-term motor system energy savings and will ensure that motor failures are handled in a quick and cost effective manner.

The Motor Decisions MatterSM Campaign suggests the following key elements for a sound motor management plan (CEE, 2007):

1. Creation of a motor survey and tracking program.
2. Development of guidelines for proactive repair/replace decisions.
3. Preparation for motor failure by creating a spares inventory.
4. Development of a purchasing specification.
5. Development of a repair specification.
6. Development and implementation of a predictive and preventive maintenance program.\

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Cable Glands For Full EMI Protection

Winning the war against electromagnetic interference (EMI) on today’s factory floors requires attention to detail. One of those details involves the ability of cable glands to contribute to a reliable grounding system.

Left unprotected, cable glands transmit electrical noise that can wreak havoc on motor-driven industrial processes. Many types of cable glands feature shielding to keep EMI at bay, yet the effectiveness of that shielding  can vary widely from product to product.

The ease of installation can vary as well. Some shielding options have difficult termination methods or grounding connections that can drive up labor cost and time.  

With SKINTOP^® MS-M Brush, our engineers have addressed both the shielding and installation issues. As its name suggests, this cable gland features a brush-type grounding connection that:

•  Enhances EMI control. Unlike other connection methods, such as the grounding clamps commonly used on drive systems, the brush provides continuous 360° contact around the cable’s screen braid. This continuous contact protects against EMI by lowering the resistance of current ground path and providing a low impedance connection between the cable shield and the housing.

•  Eases Installation. Making an EMI-free connection between cable and the SKINTOP cable gland is simple. Installers simply insert the cable, push the screen braid into the brush and tighten the cable gland assembly. Other grounding methods require far more effort to make the termination. Earth sleeves, for example, can serve as an effective grounding method, but their termination process requires precision measurements and exacting preparation of the stripped area.

In addition to its innovative brush-type connection, SKINTOP^® MS-M Brush offers IP 68 protection and resists temperatures up to 100ºC. Until recently, it was available only in metric sizes, ranging from M–25X1.5 to M–110X2.0. This month, we’ve introduced NPT sizes from ¾” to 2”.

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New Industrial Ethernet Cables For Tray Applications

Industrial Ethernet offers plenty of flexibility when it comes to designing complex automation networks.  Until recently, you couldn't say the same about data cables used in these networks.

CAT 5e Ethernet cables have traditionally been intended for stationary applications. And in the early days of industrial Ethernet, that limitation didn't matter much. Modern automation systems, however, increasingly require a new breed of Industrial Ethernet cables that can withstand more flexible conditions.

Our new ETHERLINE® Tray cable has been designed with flexibility in mind.  With its PLTC rating, the ETHERLINE® Tray can be used in cable trays, interconnecting the Industrial Platform with the Commercial Infrastructure.  The new ETHERLINE cable features four twisted pairs of stranded copper, and can handle gigabit speeds. 

Reliable Data Transmission.  Reliable data transmission is a must-have if you want to control production equipment over an Ethernet network. Unfortunately the electrical noise that pervades most manufacturing environments makes error-free data transmission a challenge. The new ETHERLINE cables combat electrical noise with a rugged, flex-friendly foil and braid EMI shield.

The construction of the ETHERLINE TRAY CAT.5e consists of a polyolefin-based insulation and a PVC jacket. The cable has a 600V rating making it ideal for tray applications alongside power cables. When used in flexible applications, the cable can withstand operating temperature from -25 to 80ºC.  They have a characteristic impedance of 100 Ω, ±15 Ω, at 100 MHz. 

Important Approvals. The new ETHERLINE cables have a wide range of approvals, which lets them serve in many different settings. These approvals include:

• CMR per UL 444
• CMG per UL 444
• PLTC per UL 13
• AWM 2570 80°C 600V
• c(UL) CMG
• RoHS II

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