My friend G asked Ms. Can: "Why does the power of some motors change when you change the brand? Does the price of a motor also change when you change the brand?" "Hehe, those are two separate things and shouldn't be confused," Ms. Can replied, explaining: First, it's impossible for the price of a motor to change when you change the brand; second, the power of some motors does indeed change when you change the brand because the operating mode changes.
The question raised by Xiao G actually touches upon the issue of motor ratings, which is closely related to the applicable load conditions of the motor. The ratings of electromagnetic devices such as motors and transformers are usually determined from both mechanical and thermal considerations, and their lifespan is determined by the highest temperature they can withstand without being excessively shortened.
Motor lifespan is closely related to its operating temperature.
The aging of insulating materials is affected by both time and temperature, making it a chemical phenomenon involving slow oxidation and sudden curing, leading to damage to mechanical durability and dielectric strength. In many cases, considering the aging rate, the lifespan of an insulating material can be expressed as an exponential function:
Lifetime = AeB/T………(1)
In equation (1), A and B are constants; T is the absolute operating temperature. Based on past experience, it is known that the failure time of organic insulating materials will be halved when the temperature rises by 8℃-14℃. Therefore, the approximate lifespan-temperature relationship can be obtained from this.
Insulation life test
Generally, working conditions should be simulated as much as possible. These conditions usually include: (1) thermal shock caused by heating to the test temperature. (2) continuous heating at this temperature. (3) thermal shock caused by cooling to room temperature or lower. (4) vibration and mechanical stress that may be encountered in actual work. (5) exposure to a humid environment. (6) dielectric test to determine the insulation state.
Motor quota
The motor rating is defined by the motor manufacturer as the total value of the electrical and mechanical outputs, along with their duration and sequence. Typical examples include:
(1) Continuous work quota.
(2) Short-time work quota.
(3) Periodic work quota.
(4) Non-discrete constant load duty cycle.
Factors affecting motor rating
(1) The heating and heat dissipation conditions of the motor.
(2) Motor structure.
(3) Usage environment.
(4) Cooling method.
(5) Work system.
(6) Insulation class.
Quota issues in specific applications
The most commonly used motor rating is the continuous rating, which defines the output power that the motor can continuously handle without exceeding specified limits (DC generators are expressed in kW, AC generators in kVA at a specific power factor, and motors in horsepower or kW). For intermittent loads, periodic loads, or loads with varying load rates, short-time ratings can be given, defining the load that the motor can handle within a specified time. The standard time limits for short-time ratings are 5, 15, 30, and 60 minutes.
Motor ratings specify speed, voltage, and frequency, as well as permissible voltage and frequency variations. However, in many motor applications, the load cycles over a wide range. A typical crane or elevator's work cycle is a good example. From a thermal perspective, the average heat generation of the motor must be determined by a detailed study of the losses at each stage of a work cycle. This can be done using the same method as determining the effective value of the periodically changing current, and the motor rating can be selected based on this result.
In equation (2), the constant introduced by the reduced ventilation due to the pause is approximately equal to 4 for an open-type motor. The duration of a complete working cycle must be shorter than the time required for the motor to reach its stable temperature.
