Archive for February, 2009

Braking a drive system (2) – Finish

February 11, 2009
Braking methods

Brake motors
Motors fitted with mechanical brakes stop the load quickly and efficiently and provide holding torque at standstill. Their disadvantage is that the linings wear and require replacement from time to time. Brake lining wear can be reduced by combining mechanical braking with any of the electrical braking methods listed below.

Countercurrent braking
This involves switching a motor to the opposite rotational direction. After eceleration to standstill, the motor starts in the opposite direction unless the current is disconnected at the right moment. This method creates a very high braking torque, resulting in a large amount of heat being developed in the motor. Temperature monitors should always be used to protect the windings.

DC injection braking
DC braking can be performed with or without a frequency converter. With a frequency converter, a stop command makes the frequency converter switch to supplying the motor with direct current, developing a braking torque. The same effect can also be achieved using suitable DC excitation equipment. This method gives a considerably longer braking time than countercurrent braking, but its heat losses are much lower, so more frequent braking is possible. Still, its use is confined to applications in which braking accounts for a relatively small proportion of the running time.

Flux braking
Flux braking is a method based on increasing motor losses in a controlled way. This method is available in frequency converters based on DTC (Direct Torque Control). When braking is needed, the flux in the motor is increased, which in turn increases the motor’s capability to brake. When braking is not needed, DTC brings the motor flux down to its nominal value. Unlike DC braking, the motor speed remains controlled during braking.

Brake chopper and braking resistor
The braking chopper is an electrical switch that connects the DC bus voltage to a resistor, where the braking energy is converted to heat. During deceleration, the motor changes to generator operation and supplies energy back through the inverter. As brake energy cannot be fed back to the supply via the normal diode bridge, the brake chopper will turn on at a certain level and feed energy out via the brake resistor. Here, the energy is converted to heat and wasted, unless a separate heat recovery system is installed; additional ventilation for the room may be required. The chopper works even during loss of AC supply. This method is used when the braking cycle is needed occasionally, when the amount of braking energy with respect to motoring energy is small, or when braking operation is needed during mains power loss. Other solutions may be considered when the braking is continuous or regularly repeated.

Controlled mains bridge – anti-parallel thyristor solution
The diode rectifier bridges can be replaced by two thyristor controlled rectifiers, allowing the power flow to be reversed, effectively feeding mechanical energy back to the supply network, saving energy. However, the DC bus voltage is always lower than the AC supply voltage in order to maintain a commutation margin, which may cause a drop in torque. Additionally, the cos phi varies with loading, total harmonic distortion is higher than in IGBT regenerative units (see below), and the braking capability is not available during mains power loss.

Controlled mains bridge – IGBT solution
The IGBT, or insulated gate bipolar transistor, is a type of semiconductor power switch that can replace the anti-parallel thyristor. It has a low amount of supply current harmonics in both motoring and regeneration, as well as high dynamics during fast power flow changes on the load side. It also offers the possibility to boost the DC voltage higher than the respective incoming AC supply. This can be used to compensate for a weak network or increase the motor’s maximum torque capacity in the field weakening area. The IGBT solution is useful when the braking is continuous or repeating regularly, when the braking power is very high, when space savings can be achieved compared to the braking resistor solution, when network harmonics limits are critical, or when energy savings are targeted.
Common DC bus
When a process consists of several drives where one motor may need braking capability while others are operating in motoring mode, the common DC bus solution is a very effective way to reuse the mechanical energy. A common DC bus solution uses the DC bus as the channel to move braking energy from one motor to benefit the other motors.

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Braking a drive system (1)

February 11, 2009
When is braking needed?

In many applications, being able to stop safely and precisely is as important as being able to start and accelerate quickly. Obvious examples include cranes, elevators and ski lifts, but quick and precise stopping is also necessary in machine tools, feed equipment and many other processes.

Design of the braking system

Evaluating the braking requirement goes back to the mechanical fundamentals of the process. Typically, there will be a requirement to brake the mechanical system within a specific time. There may also be sub-cycles in the process where a moving load forces the motor to operate as a generator. Any devices used for braking must be dimensioned for the braking power required. This depends on braking torque and speed; the higher the speed, the higher the power. In electrical braking systems, this power is transferred as a certain voltage and current; the higher
the voltage, the lower the current needed for the same power.

Motors & Energy Saving (6) – Finish

February 1, 2009
8. Electric Motors Checklist

Motors & Energy Saving (5)

February 1, 2009

7. Diagnostic Tools

• Thermography – An infrared thermometer or camera allows for an accurate, non-contact assessment of temperature. Applications for motors include bearing and electrical contact assessments on motor systems and motor control centers.

• Ultrasonic analyzer – Electric motor systems emit very distinct sound patterns around bearings. In most cases, these sounds are not audible to the unaided ear or are drown-out by other equipment noises. Using an ultrasonic detector, the analyst is able to isolate the frequency of sound being emitted by the bearing. Changes in these ultrasonic wave emissions are indicative of changes in equipment condition-some of these changes can be a precursor to component degradation and failure.
• Vibration analyzer – The rotational motion within electric motors generates distinct patterns and levels of vibration. Using a vibration analyzer and signature analysis software, the analyst can discern the vibration amplitude of the point on the motor being monitored. This amplitude is then compared with trended readings. Changes in these readings are indicative of changes in equipment condition.
• Other motor analysis – Motor faults or conditions including winding short-circuits, open coils, improper torque settings, as well as many mechanical problems can be diagnosed using a variety of motor analysis techniques. These techniques are usually very specialized to specific motor types and expected faults.

Motors & Energy Saving (4)

February 1, 2009

6. Maintenance of Motors

Preventative and predictive maintenance programs for motors are effective practices in manufacturing plants. These maintenance procedures involve a sequence of steps plant personnel use to prolong motor life or foresee a motor failure. The technicians use a series of diagnostics such as motor temperature and motor vibration as key pieces of information in learning about the motors. One way a technician can use these diagnostics is to compare the vibration signature found in the motor with the failure mode to determine the cause of the failure. Often failures occur well before the expected design life span of the motor and studies have shown that mechanical failures are the prime cause of premature electrical failures. Preventative maintenance takes steps to improve motor performance and to extend its life. Common preventative tasks include routine lubrication, allowing adequate ventilation, and ensuring the motor is not undergoing any type of unbalanced voltage situation.

The goal of predictive maintenance programs is to reduce maintenance costs by detecting problems early, which allows for better maintenance planning and less unexpected failures. Predictive maintenance programs for motors observe the temperatures, vibrations, and other data to determine a time for an overhaul or replacement of the motor. Consult each motor’s instructions for maintenance guidelines. Motors are not all the same. Be careful not to think that what is good for one is good for all. For example, some motors require a periodic greasing of the bearings and some do not.

Motors & Energy Saving (3)

February 1, 2009
4. Safety Issues

Electric motors are a major driving force in many industries. Their compact size and versatile application potentials make them a necessity. Motors are chosen many times because of the low vibration characteristics in driving equipment because of the potential extended life of the driven equipment. The higher rpm and small size of a motor will also make it a perfect fit for many applications.

Motors can be purchased for varying application areas such as for operating in a potentially gaseous or explosive area. When purchasing a motor, be sure to check the classification of the area, you may have a motor that does not meet the classification it is presently in! For example, a relatively new line of motors is being manufactured with special external coatings that resist the elements. These were developed because of the chemical plant setting in which highly corrosive atmospheres were deteriorating steel housings. They are, for the most part, the same motors but have an epoxy or equivalent coating.

5. Cost and Energy Efficiency
An electric motor performs efficiently only when it is maintained and used properly. Electric motor efficiencies vary with motor load; the efficiency of a constant speed motor decreases as motor load decreases. Below are some general guidelines for efficient operations of electric motors.
• Turn off unneeded motors – Locate motors that operate needlessly, even for a portion of the time they are on and turn them off. For example, there may be multiple HVAC circulation pumps operating when demand falls, cooling tower fans operating when target temperatures are met, ceiling fans on in unoccupied spaces, exhaust fans operating after ventilation needs are met, and escalators operating after closing.
• Reduce motor system usage – The efficiency of mechanical systems affects the run-time of motors. For example, reducing solar load on a building will reduce the amount of time the air handler motors would need to operate.
• Sizing motors is important – Do not assume an existing motor is properly sized for its load, especially when replacing motors. Many motors operate most efficiently at 75% to 85% of full load rating. Under-sizing or over-sizing reduces efficiency. For large motors, facility managers may want to seek professional help in determining the proper sizes and actual loadings
of existing motors. There are several ways to estimate actual motor loading: the kilowatt technique, the amperage ratio technique, and the less reliable slip technique. All three are supported in the Motor Master Plus software.
• Replacement of motors versus rewinding – Instead of rewinding small motors, consider replacement with an energy-efficient version. For larger motors, if motor rewinding offers the lowest life-cycle cost, select a rewind facility with high quality standards to ensure that motor efficiency is not adversely affected. For sizes of 10 hp or less, new motors are generally cheaper than rewinding. Most standard efficiency motors under 100 hp will be cost-effective to scrap when they fail, provided they have sufficient runtime and are replaced with energyefficient models.

Motors & Energy Saving (2)

February 1, 2009
3. Key Components

3.1 DC Motor
• Field pole – The purpose of this component is to create a steady magnetic field in the motor. or the case of a small DC motor, a permanent magnet, field magnet, composes the field structure. However, for larger or more complex motors, one or more electromagnets, which receive electricity from an outside power source, is/are the field structure.

• Armature – When current goes through the armature, it becomes an electromagnet. The armature, cylindrical in shape, is linked to a drive shaft in order to drive the load. For the case of a small DC motor, the armature rotates in the magnetic field established by the poles, until the north and south poles of the magnets change location with respect to the armature. Once this happens, the current is reversed to switch the south and north poles of the armature.
• Commutator – This component is found mainly in DC motors. Its purpose is to overturn the direction of the electric current in the armature. The commutator also aids in the transmission of current between the armature and the power source.

3.2 AC Motor
• Rotor
– Induction motor – Two types of rotors are used in induction motors: squirrelcage rotor and wound rotor. A squirrel-cage rotor consists of thick conducting bars embedded in parallel slots. These bars are short-circuited at both ends by means of short-circuiting rings. A wound rotor has three-phase, double-layer, distributed winding. It is wound for as many poles as the stator. The three phases are wyed internally and the other ends are connected to slip-rings mounted on a shaft with brushes resting on them.
– Synchronous motor – The main difference between the synchronous motor and the induction motor is that the rotor of the synchronous motor travels at the same speed as the rotating magnetic field. This is possible because the magnetic field of the rotor is no longer induced. The rotor either has permanent magnets or DC-excited currents, which are forced to lock into a certain position when confronted with another magnetic field.
• Stator
– Induction motor – The stator is made up of a number of stampings with slots to carry threephase windings. It is wound for a definite number of poles. The windings are geometrically spaced 120 degrees apart.
– Synchronous motor – The stator produces a rotating magnetic field that is proportional to the frequency supplied.

Motors & Energy Saving (1)

February 1, 2009
1. Introduction

Motor systems consume about 70% of all the electric energy used in the manufacturing sector of the United States. To date, most public and private programs to improve motor system energy efficiency have focused on the motor component. This is primarily due to the complexity associated with motor-driven equipment and the system as a whole. The electric motor itself, however, is only the core component of a much broader system of electrical and mechanical equipment that provides a service (e.g., refrigeration, compression, or fluid movement).

Numerous studies have shown that opportunities for efficiency improvement and performance optimization are actually much greater in the other components of the system-the controller, the mechanical system coupling, the driven equipment, and the interaction with the process operation. Despite these significant system-level opportunities, most efficiency improvement activities or programs have focused on the motor component or other individual components (Nadel et al. 2001).

2. Types of Motors

2.1 DC Motors
Direct-current (DC) motors are often used in variable speed applications. The DC motor can be designed to run at any speed within the limits imposed by centrifugal forces and commutation considerations. Many machine tools also use DC motors because of the ease with which speed can be adjusted. All DC motors, other than the relatively small brushless types, use a commutator assembly on the rotor. This requires periodic maintenance and is partly responsible for the added cost of a DC motor when compared to an alternate-current (AC) squirrel-cage induction motor of the same power. The speed adjustment flexibility often justifies the extra cost.

2.2 AC Motors

As in the DC motor case, an AC motor has a current passed through the coil, generating a torque on the coil. The design of an AC motor is considerably more involved than the design of a DC motor. The magnetic field is produced by an electromagnet powered by the same AC voltage as the motor coil. The coils that produce the magnetic field are traditionally called the “field coils” while the coils and the solid core that rotates is called the “armature.”
• Induction motor – The induction motor is a three-phase AC motor and is the most widely used machine. Its characteristic features are:
– Simple and rugged construction.
– Low cost and minimum maintenance.
– High reliability and sufficiently high efficiency.
– Needs no extra starting motor and need not be synchronized.
An induction motor operates on the principle of induction. The rotor receives power due to induction from stator rather than direct conduction of electrical power. When a three-phase voltage is applied to the stator winding, a rotating magnetic field of constant magnitude is produced. This rotating field is produced by the contributions of space-displaced phase windings carrying appropriate time displaced currents. The rotating field induces an electromotive force (emf).
• Synchronous motor – The most obvious characteristic of a synchronous motor is its strict synchronism with the power line frequency. The reason the industrial user is likely to prefer a synchronous motor is its higher efficiency and the opportunity for the user to adjust the motor’s power factor. A specially designed motor controller performs these operations in the proper sequence and at the proper times during the starting process.