Four pointers for choosing an electric motor
1. DC motors
The benefit of a DC motor is that it can produce a greater torque and its speed can be readily changed by varying the voltage.It works well with weights that need speed adjustments often, like hoists in mines and rolling mills in steel factories.However, the advancement of frequency converter technology has made it possible for AC motors to modify the frequency in order to change the rotational speed.Although the cost of an inverter motor is not significantly more than that of a regular motor, the cost of the inverter takes up the majority of the equipment, therefore brushed DC motors also have the benefit of being less expensive.
The complicated structure of DC brushless motors is a drawback, and any equipment with a complex structure is inevitably going to have a higher failure rate.In contrast to an AC motor, a DC motor adds a slip ring, brush, and commutator in addition to the winding complexity (excitation winding, commutation pole winding, compensation winding, and armature winding).In addition to requiring a high degree of craftsmanship from the producer, post-maintenance costs are also somewhat substantial.As a result, DC gear motors are gradually becoming less common in industrial applications, albeit they are still helpful during the awkward situation's transition.Choosing an AC motor with an inverter program is advised if the user has more money since, although it is not specified, using an inverter has numerous advantages.
2. Asynchronous motor
The advantages of an asynchronous motor are its straightforward design, steady performance, ease of maintenance, and affordability. Additionally, the manufacturing procedure is the most straightforward.According to what I've heard from the old technician's workshop, the building of a brushless DC motor requires man-hours and can produce nearly as much power as two synchronous motors or four asynchronous motors.As a result, the most popular motors in industry are asynchronous ones.The rotor distinguishes wire-wound motors from squirrel-cage motors, two types of asynchronous motors.
A squirrel cage motor's rotor is composed of copper, aluminum, or metal bars.Aluminum is frequently utilized in less demanding applications and is reasonably priced. China is a major producer of aluminum.However, the great majority of my contacts are copper rotors since copper has superior mechanical qualities and electrical conductivity over aluminum.When it comes to resolving broken row issues, squirrel cage motors are significantly more reliable than winding type rotor motors.
The drawback is that it is challenging to manage loads with high beginning torque needs because a metal rotor cutting magnetic induction lines in a revolving stator field produces a little torque and a significant starting current.While extending the motor core's length can increase torque, the effort required is little.While extending the motor core's length can increase torque, the effort required is little.In order to generate additional torque, wire-wound motors energize the rotor winding through slip rings during startup. This creates a rotor magnetic field that moves in relation to the revolving stator magnetic field.Additionally, to lower the starting current during the starting phase, the water resistance is linked in series.
A complex electronic control mechanism regulates the water resistance to alter the resistance value during the startup procedure.It can handle loads like hoists and rolling mills.The overall cost of the equipment has gone up as the wire-wound asynchronous motor's slip ring, water resistance, etc., have increased in comparison to the squirrel cage motor.It has a smaller speed range and comparatively little torque.Compared to DC motors, it has a smaller speed range, a smaller torque, and a lower equivalent value.However, the grid is greatly impacted when an asynchronous motor absorbs reactive power from the grid since the stator winding is energized to create a rotating magnetic field, while the winding associated with the inductive components does not function.
High-power inductive equipment linked to the grid cause the grid voltage to drop and the brightness of electric lights to instantly decrease, according to intuitive experience.The usage of asynchronous motors will therefore be restricted by the power supply bureau, which is something that many factories need to take into account.Some major electricity consumers, including steel mills, aluminum facilities, etc.,To lessen the limitations on the use of asynchronous motors, they decide to build their own power plants in order to create their own autonomous power grid.While synchronous motors can supply reactive power to the grid through excitation devices, asynchronous motors must have reactive power compensation devices in order to meet high power loads. The more power a synchronous motor has, the more evident its advantages become, and a synchronous motor stage is formed.
3. Synchronous motor
Synchronous motors have the advantage of compensating for reactive power in addition to having an over-excitation condition.
1) Synchronous motor speed is carefully controlled by n=60f/p, which is strictly adhered to.
2) High operational stability: The asynchronous motor torque (proportional to voltage squared) can drop considerably when the grid voltage drops abruptly, but the excitation mechanism will typically force excitation to maintain stable motor operation.
3) a higher capacity to handle overload than the similar asynchronous motor.
4) Excellent operating efficiency, particularly for synchronous motors with low speeds.
Synchronous motors require frequency beginning or asynchronous starting since they cannot be started directly.Asynchronous start refers to a synchronous motor having a starting winding on the rotor that resembles the cage winding of an asynchronous motor. In order to create a closed circuit, an additional resistance that is roughly ten times the resistance value of the excitation winding is connected in series with it in the excitation circuit.In this way, the synchronous motor's stator is directly linked to the power grid, starts as an asynchronous motor, and when the speed reaches 95% sub-synchronous speed, the extra resistance is eliminated; frequency conversion begins with little Not much to say.Thus, the requirement for additional equipment devices for starting is one of the drawbacks of synchronous motors.
Excitation current powers synchronous motors; in the absence of excitation, an efficient motor operates asynchronously.With the same rotational speed and polarity as the stator, the excitation is a DC system that is added to the rotor.An excitation issue will cause the stepper motor to be out of step and unadjustable, triggering the "excitation fault" protection and causing the motor to trip.The synchronous motor's second drawback is that it requires a larger excitation device, which was previously supplied directly by a DC machine but is now mostly supplied by a silicon-controlled rectifier.According to the adage, the failure rate increases with the complexity of the structure and the number of devices.
Synchronous motors are mostly used for hoisting, milling, fans, compressors, rolling mills, pumps, and other loads, based on their performance characteristics.In conclusion, the general rule when selecting a motor is to prioritize those with a straightforward design, low cost, dependable operation, and easy maintenance, provided that the motor's performance satisfies the needs of production equipment.In this sense, squirrel cage asynchronous motors are superior to wire-wound asynchronous motors, AC motors are superior to DC motors, and AC asynchronous motors are superior to AC synchronous motors.Ordinary squirrel cage asynchronous motors, which are frequently found in pumps, fans, and other production equipment, are preferred for continuous operation with smooth loads and no specific starting or braking requirements.
Bridge cranes, mine hoists, air compressors, irreversible rolling mills, and other equipment that requires a greater starting and braking torque should start and brake more frequently.ought to employ an asynchronous wire-wound motor.Synchronous motors should be utilized in applications like medium and large capacity water pumps, air compressors, hoists, mills, etc. when speed adjustment is not necessary but speed must remain constant or power factor must be increased.The production machinery requires constant, steady, and smooth speed management if the speed range is more than 1:3.Other synchronous motors with frequency regulation, squirrel cage asynchronous motors, or excitation DC motors, such those found in big precision machine tools, gantry planers, steel rolling mills, hoists, etc., are suitable.requires the use of series-excited or compound-excited DC motors, a high starting torque, and mechanical features of soft production gear, such as big cranes, motor vehicles, and trams.
The electric motors' rated power
The output power, also known as shaft power or capacity, is the defining characteristic of larger motors and is referred to as the rated power of an electric motor.When people inquire about the size of induction motors, they typically mean the rated power rather than the motor's actual size.It is the most crucial metric for measuring the motor's capacity to draw the load, and it is also a necessary parameter to be supplied at the time of motor selection.Assuming that the motor can generate the necessary mechanical load, the basis of proper stepper motor capacity selection should be the most cost-effective and sensible choice of motor power.A gear motor will run overload and prematurely damage itself if the power is too small; conversely, if the power is too large, the equipment investment will rise and waste money, the motor will frequently run under load, and the efficiency and power factor of the AC motor will be low.
The dc gear motor's primary power is determined by three parameters.
(1) The motor's heat and temperature rise, which is the primary determinant of the motor's power.
2) Permitted short-term overload capability.
(3) It is important to take into account the asynchronous squirrel-cage motor's starting capacity.
Initially, the particular manufacturing equipment determines and chooses the load power based on its load need, temperature rise, and heat generation.The motor then chooses the rated power in advance based on the working system, overload requirement, and load power.Heat production, overload capacity, and starting capacity should be examined once the motor's rated power has been pre-selected.
The motor needs to be re-selected and calibrated until all of them are qualified if one of them is not.Consequently, one of the prerequisites is the working system; in the absence of one, the most traditional S1 working system is used; motors that require overload must also have an overload multiplier and the associated running time; The torque curve for beginning resistance and the load's rotating inertia must be provided in order to assess the starting capacity of an asynchronous squirrel cage motor that drives a fan and other significant rotating inertia loads.The rated power option above is based on the assumption that the ambient temperature is 40°C.
The motor's rated power needs to be adjusted if the surrounding temperature changes while it operates.The table below shows how the motor's power can be roughly raised or lowered based on theoretical calculations and practical application when the surrounding temperature varies.As a result, it is also essential to supply the ambient temperature in regions with harsh climates. For instance, in India, the ambient temperature must be set to 50°C.The power of the servo motor will also be affected by high altitude; the greater the altitude, the higher the motor's temperature rise, and the lower the output power. Additionally, the impact of the corona phenomenon must be taken into account by the motor employed at high altitudes.
The voltage rating
The line voltage in the rated operating mode is referred to as the rated motor voltage.The motor's rated voltage is chosen based on the motor's capacity and the enterprise's electric power system supply voltage.The voltage level of the power source at the location of use mostly determines the voltage level of the AC motor.The rated voltage is 380V since the low voltage network is typically 380V 3 kinds.
Low-voltage motor power grows to a certain degree, the current is constrained by the wire's capacity, or it is too expensive. The voltage must be raised in order to attain a high power output. The typical high-voltage grid supply voltage is 6000V or 10,000V, while some countries also have voltage levels of 3300V, 6600V, and 11000V. great-voltage motors' great power and robust shock resistance are their advantages; nevertheless, their enormous moment of inertia and challenging starting and braking are their drawbacks. The power source voltage and the DC motor's rated voltage must coincide. 110V, 220V, and 440V are typical. When the AC power supply is 380V, the DC motor's rated voltage should be 440V when using a three-phase bridge-type silicon controlled rectifier circuit power supply, and 220V when using a three-phase half-wave silicon controlled rectifier power supply.
The speed rating
The motor's rated speed is the speed at which it operates in its rated mode.Each motor has a rated rotational speed, as does the working machinery it pulls.It is important to keep in mind that the motor speed shouldn't be set too low because the lower the motor's rated speed, the more stages it has, the larger its volume, and the higher its cost.At the same time, the motor's speed shouldn't be excessively high.since doing so will make the transmission mechanism excessively intricate and challenging to maintain.Furthermore, the motor torque is inversely proportional to the speed while the power is fixed.
As a result, those with low requirements for starting and braking can assess a number of rated speeds in terms of initial investment, floor space, and maintenance costs before deciding on the rated speed. They also frequently start, brake, and reverse.However, aside from taking into account the initial expenditure, the length of the transition process has no bearing on productivity. This is mostly because the motor's rated speed and speed ratio are chosen to minimize the transition process's loss. For instance, the hoist motor requires a lot of forward and backward rotation, has a high torque, a low speed, a large motor volume, and is costly.The critical speed of the motor must also be taken into account when the motor speed is high. When a motor rotor is operating, vibration will occur, and the amplitude of the vibration will increase with speed until it reaches a maximum speed (also referred to as resonance). After this speed, the amplitude will gradually decrease and become stable within a specific range; the amplitude of the maximum speed is known as the rotor critical speed.
This speed is equivalent to the intrinsic frequency of the rotor.
Nearly twice the intrinsic frequency of the amplitude will increase once again when the speed increases; this is known as the second-order critical speed. Third-, fourth-, and other critical speeds follow.Vibrant vibration and a large increase in shaft bending will occur if the rotor is operating at the critical speed. Prolonged operation will also cause the shaft to bend and deform severely, possibly breaking.Since the motor's first-order critical speed is typically higher than 1500 rpm, the impact of critical speed is typically overlooked by conventional low-speed motors.
On the other hand, the effect must be taken into account for 2-pole high-speed motors with rated speeds close to 3000 rpm.The type of load to be driven, the motor's rated power, rated voltage, and rated speed can all be used to approximate the motor's specifications.However, if the load requirements are to be optimally satisfied, these fundamental characteristics are insufficient.Depending on the particular circumstance, additional characteristics must be supplied, such as frequency, operating system, overload requirements, insulation level, protection level, rotational inertia, load resistance torque curve, installation technique, elevation, ambient temperature, outdoor requirements, etc.
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