paul smith belts-Electric motor

Chapter VII of the motor selection

7-1 7-2 motor heating and cooling of the motor insulation materials and allow the motor temperature rise of 7-3 work load motor power calculation 7-4 7-5 Summary of motor capacity selection

Summary

various motor during operation will have a loss, these losses on the one hand reduces the efficiency of the motor is running, the other losses into heat, the temperature rise. Motor temperature and the ambient cooling medium temperature difference, called the temperature, it is an important motor performance.

motor insulation is the worst part of the heat resistance. So the heat is directly related to the motor life and reliability. To limit the impact of heat on the motor, the motor temperature does not exceed a certain value, on the one hand to control the loss of motor parts, so that heat is reduced; the other hand to improve the motor's cooling system to improve the heat transfer and heat dissipation to the motor's internal heat conduction and distribution quickly out.

motor temperature rise is not only determined by the size of the load, but also with the duration of the load, that is the way with the motor work. According to the motor is running on different machines and different work load, has its corresponding load power calculated by the load on the motor power to complete the selection.

7-1 motor heating and cooling (top)

motor heat from the motor, ie a current flows through the stator winding copper loss generated in the core changes in magnetic flux produced when the iron losses asics australia online, bearing friction and mechanical loss generated by additional losses. The heat generated by the motor, the first transfer by conduction to the outer surface of the motor, and then by radiation and convection to distribute heat from the motor to the outer surface to the surrounding cooling medium.

First, heat conduction in motor

the formation of a variety of different sources of heat loss, heat loss into, it will flow through different materials, distributed by the motor to the outer surface of the outside. For the heat generated by the stator windings, first spread through the insulation core, and then spread from the core case. As current flows through the conductor to be met with resistance, as the conduction of heat in the process of the motor should be met with resistance, known as the thermal resistance. As resistance is inversely proportional to the same conductivity coefficient, thermal resistance is inversely proportional to thermal conductivity. Thermal conductivity of different materials vary widely, generally a large thermal conductivity of metal, insulating materials, low thermal conductivity, thermal conductivity of air minimum. Therefore, to improve the heat transfer process inside the motor, increasing the thermal conductivity has mainly taken the following two measures:

(1) the use of high strength, good thermal conductivity of insulating materials. Ensure the insulation strength as far as thinning the thickness of the insulation layer to reduce the thermal resistance of insulation.

(2) the use of impregnation methods to drive away the air out of the tank, that is to fill up the tank with paint all the gaps. Winding Varnish can not only improve thermal conductivity, but also to enhance the insulation performance. In addition, after winding dipping into a solid whole, the mechanical properties are greatly improved.

Second, the electrical heating process

various losses in the motor generates heat, so that the motor temperature. Motor temperature rises, they would by radiation and convection to the ambient air heat. When the heat generated by the heat and the heat are equal, the motor temperature will not rise to a stable value. The following study of the motor temperature changes over time in the process. Motor temperature rise in the process is extremely complex, there are in the motor windings, core, bearing several different sources of heat such, there must exist between the heat source heat exchanger, coupled with the heat of each part of the same conditions without , so to accurately calculate the temperature distribution in the motor is very difficult. In order to simplify the problem, we only consider homogeneous solid isothermal heating process. Namely, that the surface points of the cooling conditions exactly the same, the body temperature of the same points.

in the heating process, the heat generated by the object can be divided into two parts: one part of the object stored in the internal temperature of the object, and partly distributed to the surrounding air to go. In this way, the heat balance equation for the object

(9-1)

in style in that the heat generated per second object is generated at the time of total calories. Said heat coefficient, that is the object temperature is too taken per unit area per second shed heat; said cooling area; said temperature; that heat, that is, per unit mass of the object is increased by the heat required; that the quality of the object .

assumption, the temperature, according to the initial conditions, from (9-1) yields the final expression for the heating process

; (9-2)

where - is the object of the final temperature. Objects, the greater the heat generated per second, the higher temperature. On the contrary, the object of cooling coefficient and thermal area is larger, the lower the temperature. To increase the heat factor, the general motors are equipped with fans to increase air convection. To increase the cooling area, enclosed motor housing are equipped with cooling ribs.

- temperature curve is the time constant, in theory, only time, to reach a stable temperature. But in fact when, at this time that has reached thermal steady state.

type (9-2) represents the heating process curve shown in Figure 9-1. Began to heat, the heat generated by all the motor used to improve their temperature paul smith belts, so temperature rises rapidly. With the motor temperature, motor temperature difference between the surrounding medium increases, the heat outside the motor is gradually increasing, so that the temperature will gradually slow down, as shown in Figure 9-2 to form temperature curve.

Figure 9-1

body heat homogeneous isothermal curves 9-2 homogeneous isothermal body The cooling curve

Third, the motor cooling

If the temperature reaches its final value, the motor stops running, the motor will not generate heat, so. At this point, because the heat inside the motor gradually distributed to the surrounding air to the motor temperature began to decline. In the cooling process, to this condition into (9-1), the solution was then the expression for the cooling process

; (9-3)

cooling process shown in Figure 9-2, is an exponential curve.

four ventilated motor cooling

cooling of the motor determines the motor's temperature rise, temperature rise has a direct impact on the life of the motor and the rated capacity, we can see, the problem of motor cooling is important.

motor cooling medium is directly or indirectly to motor away the heat of the material: such as air, hydrogen, water and oil. According to the different cooling media, the general cooling of the motor can be divided into two categories: gas cooling and liquid cooling.

small motor is generally used for air ventilation and cooling. Cooling can be divided according to their natural cooling, from the ventilation cooling, forced air cooling and air cooling pipes and a few ways.

(1) natural cooling the surface of this motor alone radiation and natural air flow to obtain cooling, do not install any special cooling device, only a few hundred watts for a small motor.

(2) This self-ventilated motor cooling fan driven by its own supply of cooling air to cool the hot parts of the surface and interior.

(3) forced air cooling the cooling air is independent of the motor-driven fans and blowers supply. Its characteristics are adjusted according to load size and the blower fan speed to control the air supply of the motor, thereby reducing the ventilation at low load losses.

(4) ventilation and cooling pipes of the cooling air through the motor of this pipeline to introduce or exclude. When the indoor air quality, there are cotton, dust and other powders, the use of more favorable ventilation pipe, so that can draw clean air from outside to cool the motor.

7-2 to allow the motor insulation and temperature (top)

motor temperature is a measure of an important indicator of motor performance, it characterized the motor's internal losses, the physical quantities that characterize the motor heat. Motor internal thermal resistance is the worst part of the insulating material.

First, the level of insulation materials

electrical insulating materials commonly used in the heat level and temperature limits shown in Table 9-1.

; Table 9-1

Insulation

A

E

B

F

H

temperature limit

105

120

130

155

180

materials, for example

impregnated treated organic materials: such as paper, cotton, etc.

polyester film, the three cellulose acetate film

Mica , mica paper

mica, asbestos, glass fibers, synthetic resin, made sticky and economic

mica, asbestos, glass fiber, silicon-organic resin as adhesive and economic

when the insulation is shown in Table 9-1 when the limit temperature, motor life can be up to 15 to 20 years. If higher than the temperature indicated in Table 9-1 continuous operation, the motor life will decline rapidly. According to statistics, A-class insulation for every temperature rise of 8 to 10, the insulation life will be shortened by half. Hyundai Motor is the most widely used E-Class and B-class insulation.

Second, to allow the various parts of the motor temperature rise

motor temperature and the ambient cooling medium temperature difference as the motor temperature rise, with that, that is,

; (9-4)

heating and cooling of the motor that is temperature, rather than temperature.

When the motor insulation material used to determine, the motor maximum allowable temperature on the set, this time limit the temperature rise depends on the temperature of the cooling medium. General motors in the cooling medium is air, its temperature varies with the regions and seasons, in order to create a year-round throughout the country can apply to the motor, and a clear, unified inspection standards, national standards: the temperature of the cooling air as 40oC. In this environment, temperature, motor winding temperature limits: E-class insulation to 75oC, B class insulation to 80oC.

motor is running, the output power is larger, the greater the current and losses, the more the higher the temperature, but the maximum temperature must not exceed the maximum allowable insulation temperature. Therefore, the motor to allow long-term maximum output power (ie, the motor capacity or rated power) by the maximum allowable insulation temperature limit, or maximum allowable capacity of the insulation temperature is determined. Indicated on the motor nameplate rated power refers to the standard ambient temperature (defined as China 40oC) and work under way, the temperature does not exceed the maximum allowable insulation temperature of the maximum output power.

7-3 motor work (top)

electrical work, the temperature rise is not only determined by the size of the load and load duration of a relationship, the same motor, different lengths of time if the work is able to bear the load power is also different. In order to meet the needs of different loads, motor manufacture, according to the different duration of the load, the motor was divided into three kinds of work or three working system.

A continuous (long term) work system

its features are: motor continuous long working hours, their working hours> (3 ~ 4) T, up to several hours or even tens of hours, so the motor temperature can reach a stable value. Such work is the production of mechanical system with a pump, fan, paper machine, machine tool spindles.

Second, the short-time work system

its features are: motor working time is short, <(3 ~ 4) T, the temperature reached during working hours less than stable value; stop a long time,> (3 ~ 4) T, the motor temperature drops and the ambient temperature is sufficient, as the temperature enough to zero. Belong to the system of production machinery such work has assisted exercise machines, such as sluice gates from the closed machinery.

motor short-term work, often only by the overload capacity and ability to start the restrictions, short-time work system designed specifically for the motor, the overload capacity and starting torque are larger. China's production of motor short-term work system, its working hours 15min, 30min, 60min, 90min four kinds of fixed.

Third, the repeated short-time work system

repeated short-time work system, also known as intermittent periodic duty. Its characteristics are: work and stop periodically alternating, but the working time and stop time is short, <(3 ~ 4) T, <(3 ~ 4) T, and duty cycle requirements. Working temperature increases, but reach a constant value; stop when temperature decreased, but lower than zero. The temperature rise at the end of each cycle than the beginning of the temperature rise, so that after several cycles, there will be a period of growth and land temperature are equal, then reached a stable temperature fluctuations state, that is the highest temperature and lowest temperature fluctuations between the average temperature change. Belong to the system of production machinery for such work have cranes, elevators, rolling auxiliary machinery, automatic machine tools work in some institutions.

in duplication of work system, the rated load time and the whole cycle is called the duty cycle. Standard duty cycle rate of 15%, 25%, 40% and 60%. Each cycle is 10min.

cycle work, the electrical heating and cooling process is staggered, so it reaches a temperature lower than the continuous operation, as shown in Figure. Whether a fixed period of work or short-term fixed motor motor cycles do not work according to their work quota fixed for long-term or short-term continuous operation, otherwise it would overheat and damage the motor.

7-4 motor load power calculation (top)

motor load changes according to load size is divided into two categories.

1, constant load: in the operation, the size of the load is essentially constant, such as pumps, fans, large spindle and so on.

2, changes in load: in the operation, the larger size of the load changes, but most of the cyclical variation, such as the bench planer, mine hoist, etc.

First, the calculation of constant load power

a variety of production machinery in different load power formula can be found in the design manual. Here are some commonly used mechanical load power production formula for selection of these motors produce mechanical reference.

(1) linear motion of the mechanical

where - load force (ie, static resistance) (N);

- velocity (m / s);

- transmission efficiency.

(2) rotation of the production machinery

where - load torque (ie, static resistance torque) ();

- rotation speed (r / min);

- transmission efficiency.

(3) pump machinery

where - liquid flow, that is per second displacement; ()

- the proportion of the liquid ();

- total drainage height (m)

- pump efficiency. Low pressure centrifugal pump = 0.3 ~ 0.6; high-pressure centrifugal pump = 0.5 ~ 0.8; piston pump = 0.8 ~ 0.9.

- transmission efficiency.

(4) blower type of production machinery

where - liquid flow, that is pressed into the second gas inhalation or volume; ()

- gas pressure ();

- blower efficiency. Large blowers = 0.5 ~ 0.7; medium-sized centrifugal blower = 0.3 ~ 0.5; small impeller blower = 0.2 ~ 0.35.

- transmission efficiency.

Second, the calculation of changes in load power

motor load operation is characterized by changes in output power are constantly changing, so the loss of motor and motor it generates heat and temperature are constantly changing, but over time, the motor temperature rise that reached a kind of steady state fluctuations. For changes in load can only find the average power or average torque:

where, ..., - the time period of the load power, according to the constant load formula;

, ..., - each time the load torque;

, ..., - various segments of the time. And for a period of time, use that.

7-5 motor capacity selection (top)

choice due to its electrical capacity and load the type of work, here we only introduce continuous duty motor capacity selection.

1, the motor under constant load capacity options

constant load motor capacity selection is relatively simple, just follow the load power can be selected. That

because the general is based on constant load motor continuous design, such as load power does not exceed the motor rated power, the motor temperature rise will not exceed the allowable value.

If the actual ambient temperature and ambient temperature (40oC) difference is large, for safety and full use of the motor, the output power can be different with Pe. From the heat point of view, when the ambient temperature is below 40oC, the motor can increase power use; when the ambient temperature higher than 40oC, the motor power should reduce the use of motor response when the output power to be amended. Correction method has the following two:

1) calculation

2) estimation method

2, change the value of the motor under load capacity options

taking into account the over-current motor during normal working hours than the large, but variable loss and current proportional to the square, so the more serious motor heat. However, average power and average torque Mzd Pzd not reflected in increased motor heating over the course of the case, therefore, pre-press-type motor rated power

Pe = (1.1 ~ 1.6) Pzd

in the above equation, if the transition process in the whole project in a larger proportion of the work, then the coefficient (1.1 to 1.6) should be chosen too large values.

for short-term work system and repeated short-term work system is more complex motor capacity selection, long work for the system, its calibration is more complex herve leger 2009, not one by one in this introduction. But people in engineering practice, summed up some of the production of mechanical motor power selected practical methods. These methods are relatively simple, but there are some limitations. As: Statistical analysis:

this method is the same type of production machinery used for statistical analysis of motor power, motor power and the type of production to identify the main parameters of the relationship between the machine and then the actual situation, set the corresponding index. For example, China's machinery manufacturing industry has summarized the different types of machine tool main drive motor power P of the statistical analysis of the formula is as follows:

1, lathe

P = 36.5D1.54 (kW)

where D - maximum workpiece diameter m

2, vertical lathe

P = 20D0.88 (kW)

where D - maximum workpiece diameter m

3, radial drilling machine

P = 0.0646D1.19 (kW)

where D - the maximum drilling diameter mm

4, cylindrical grinder

P = 0.1KB (kW)

where B - Wheel width mm

K - consider the grinding wheel spindle bearings with different coefficients, when the rolling bearing, when K = 0.8 ~ 1.1, the use of a sliding bearing, K = 1.0 ~ 1.3 .

5, Horizontal Boring

P = 0.004D1.7 (kW)

where D - boring bar diameter mm

6, planer

where B - Table width mm

example: China's C660 lathe workpiece diameter 1250mm, with the above formula to calculate the main drag of the motor power is P = 36.5 × 1.251.54 = 52kW. The actual selection Pe = 60kW electric motor. Actual evidence, the choice of motor is suitable.

Summary (top)

In this chapter, study the electrical heating and cooling, electrical insulation materials and the allowable temperature rise, motor works, motor load and motor power calculation capacities.

study sections in this chapter, you should pay attention to the question is:

1, generated when the motor is running into all kinds of losses are heat, heat the motor gradually increased. Motor temperature is higher than the surrounding medium temperature, we began to distribute heat to the surrounding cooling medium, and finally, all the heat generated by the motor distributed to the surrounding medium, no longer increases the temperature of the motor to reach the thermal steady state, when the temperature as the temperature stability of the motor. Motor temperature and the stability of the difference between the ambient temperature as stable temperature.

2, electrical heating and cooling process of the temperature rise is an exponential function with time of growth and decline.

3, to improve the cooling conditions can reduce the temperature, motor temperature rise can reduce the need to understand the measures taken.

4, the motor in the worst part is heat-resistant insulating material, so the motor's rated power by the maximum allowable temperature insulating material (that is, the motor maximum allowable temperature) to decide. Motor rated power is the standard ambient temperature (40oC) and work under way, the temperature (or temperature rise) does not exceed the maximum allowable insulation temperature (or maximum allowable temperature rise) is the maximum output power.

5, according to the different duration of load, the motor was divided into three modes, continuous-time work system, short-term and repeated short-time work system work system.

6, this chapter according to the type of load, the load on the two power calculation: constant power load calculations and the calculation of average power load changes. Constant load depending on the production machine has a different formula, the value for the variable load only the first segment obtained by means of its constant power load, the calculated average power.

7, motivated the choice of capacity and its working methods and the type of load, constant load motor capacity selection is relatively simple, just follow the load power can be selected. Ie; value for variable load electric motor capacity selection, taking into account the over-current motor during normal working great, but variable loss and current proportional to the square, so the more serious motor heat. However, average power and average torque Mzd Pzd not reflected in increased motor heating over the course of the case, therefore, pre-press-type motor rated power

Pe = (1.1 ~ 1.6) Pzd

people in engineering practice, summed up some of the production of mechanical motor power selected practical methods. Such as statistical analysis.