On the mechanical properties of three-phase asynchronous motor, starting, braking and speed control

Abstract: The induction motor structure, reliable operation, low price, easy maintenance and a series of advantages, at present, asynchronous electric drive motor has been widely used in various industrial electrical automation field. For three-phase asynchronous motor mechanical starting characteristics, the main brief motor starting, braking, speed and other technical issues.

Keywords:: three-phase asynchronous motor, electric drive, mechanical properties, start, brake, speed control
Induction motor has a simple structure, reliable operation, low price, easy maintenance and a series of advantages, therefore, induction motor is widely used in electric drive system. Especially with the development of power electronics and AC drive technology has become more mature, making the asynchronous motor in a much higher speed performance. At present, the induction motor electric drive has been widely used in various industrial electrical automation field. to the mechanical properties of three-phase asynchronous motor starting, motor starting the main brief , brake, speed and other technical issues.

1 Mechanical properties of three-phase asynchronous motor text
The mechanical properties of three-phase asynchronous motor is the motor speed n and the relationship between the electromagnetic torque Tem. As the slip speed n and S has a certain relationship, so the mechanical properties are also used Tem = f (s is expressed . Three-phase asynchronous motor electromagnetic torque expression has three forms, namely, physical expression, the parameter expressions and practical expression. physical expression reflects the induction motor torque generated by the physical nature of the electromagnetic torque is described main flux and rotor active current generated by the interaction. parameter expression reflects the electromagnetic torque and power parameters and motor parameters of the relationship between the use of this type can easily analyze parameters on the impact of the electromagnetic torque and the kinds of human characteristics. practical expression is simple, easy to remember, is often used in the form of engineering calculations.

Maximum torque and starting torque overload capacity to reflect and start the motor, two important indicators of performance, maximum torque and starting torque is larger, the stronger the motor overload, start the better the performance.

The mechanical properties of three-phase asynchronous motor is a nonlinear curve, under normal circumstances, the maximum torque (or the critical slip is cut-off point, the linear segment for the stable operation of the district, and non-linear section of the unstable operation area. inherent mechanical properties of the linear segments are hard characteristics, the rated operating point slightly lower than the speed of the synchronous speed. artificial mechanical characteristic curve analysis of the shape parameter expression can be obtained, analysis of the key is to seize the maximum torque, the critical transfer slip and the amount of starting torque with these three parameters were studied.

2 three-phase induction motor starting
Small-capacity three-phase asynchronous motors can be started directly, larger cage motor can be a buck to start. Antihypertensive start into the stator in series resistance or reactance voltage starting, YD buck up and step-down autotransformer start. Stator series resistor or the motor voltage starting, the starting current decreases with voltage between the first side, and starting torque decreases with the square of the voltage relationship, which applies to light-load start. YD voltage starting is only applicable to normal operation when the motor delta connection, the starting current and starting torque are reduced to direct start-up 1 / 3, which also applies to light-load start. autotransformer voltage starting, the starting current and starting torque are reduced to Direct startup l/k2 (k for the auto-transformer turns ratio, suitable for larger load with startup.

Can be wound rotor induction motor rotor resistance or frequency sensitive rheostat in series start, the starting torque, starting current, suitable for medium and large asynchronous motor overload started. Reposted elsewhere in the Research Papers Download http:// www.hi138.com is a set of soft starter motor soft start, soft stop, light-load energy and various protection functions in one of the new motor control device, called the foreign Soft Starter. Its main components are connected in series in the power and the contrast between the three accused of the motor and its electronic control circuit in parallel thyristors. using cascaded in between the power and charged with motor soft starter, in different ways to control its internal thyristor conduction angle so that the motor input voltage from zero to the default function gradually increased until the start over, to give full voltage motor, is the soft start. In soft start process, increasing the motor starting torque, rotational speed is gradually increased. Soft Starter actually is a voltage regulator for the motor is started, the output voltage does not change just to change the frequency.

3-phase asynchronous motor brake
There are three three-phase asynchronous motor braking: braking, reverse braking (power of two opposite pick and pull reverse and regenerative braking down. These three mechanical braking curve, the energy conversion relationship and use, features etc, similar to DC motor braking.

4 three-phase asynchronous motor with speed control
Three-phase induction motor speed control method has changed very speed, the slip frequency control and variable speed control. Which includes variable speed wound rotor slip induction motor resistor in series with the rotor speed control, cascade control and step-down speed.

Pole change speed by changing the stator winding pole motor wiring to change the number of motor speed in order to achieve change. Variable speed control to a level very speed, variable speed when the stator pole connected in three ways: Y-YY , cis-Y-playing a string Y, D-YY. One way of Y-YY connection speed is constant torque mode, the other two are constant power speed way. Pole-changing speed, you should also swap the stator two-phase wiring, so as to ensure the motor speed after the shift change.

Modern AC variable speed frequency control is the main direction of technology, it can realize stepless speed regulation for constant torque and constant power load.

Wound rotor motor resistor in series with the rotor speed control method is simple and easy to implement, but the timing is a class, not smooth, soft and low speed characteristics, speed stability is poor, while the rotor copper loss of a large, low efficiency of the motor. String stage governor resistor in series with the rotor speed to overcome the shortcomings, but the equipment is much more complex.

Step-down speed asynchronous motor is mainly used for the occasion Fans Load, or high slip of the motor, the speed should be used while the negative feedback loop control system.

The voltage and frequency constant frequency alternating current is transformed into variable AC voltage or frequency device called "drive." In order to produce variable voltage and frequency, the device must first transform the AC power supply direct current ( DC, a process called rectification. then direct current (DC conversion to alternating current (AC, a process called inverters, AC to DC conversion devices for the inverter is called (inverter. for adjustable frequency inverter is a voltage adjustable We called the frequency inverter. inverter output waveform is simulated sine wave, the main motivation used in the three-phase asynchronous speed, also known as frequency converter. reposted elsewhere in the Research Papers Download http://www. hi138.com