Analysis of Starting, Speed Regulation and Braking Relationships of Asynchronous Motors

For motor products, its basic working state can be divided into starting state, speed regulation state and braking state, adjustment state generally refers to the normal operating state of the motor needs to change the speed of the situation, the more common variable poles and frequency control; starting state of the braking state is also the process of motor speed change.

We have also talked about in previous articles, motor starting process, smaller starting current and larger starting torque is very ideal, how to pinpoint these two parameters, is always a motor manufacturer and user are more concerned about the problem.

1.Analyzing the Starting Process of a Motor

For squirrel cage asynchronous motors, when the grid capacity allows direct starting can obtain a large starting torque, but the starting current is relatively large, is 5 to 7 times the rated current. When the grid capacity is small, the motor can not be directly started, generally used to reduce the starting voltage starting to reduce the starting current, but when the reduced voltage starting, starting torque will be reduced to varying degrees. Medium but for the winding motor, in the rotor circuit into the resistance, can meet the small starting current and starting torque of the dual purpose. In view of this reason in some of the starting difficulties in the equipment and occasions, often use the winding motor. The following are some examples of wire-wound motors

For squirrel cage motors, the use of skin effect to select the use of deep groove type and double squirrel cage rotor, increase the rotor resistance during the motor starting process, can better reduce the starting current, but the starting torque will not be reduced too much. In this case, in the normal operation of the motor, the rotor current frequency is very low, the skin effect basically disappears, the rotor current will be relatively uniformly distributed in the rotor guide bars, the rotor resistance returns to normal, and the efficiency parameter index of the motor can be guaranteed.

The starting of asynchronous motor has a great impact on the factors, and by the high harmonics of the magnetic field generated by the additional torque, how to weaken the additional torque generated by the high harmonics, we need to improve the design of the winding (such as short torque winding), the rotor slot slopes to control the stator, rotor slots with the number of choices. We have already described this aspect in detail in the previous article, so we will not repeat it here.

2..Speed Control of Asynchronous Motors  

We have previously introduced the motor variable pole speed control, frequency conversion speed control and rotational rate speed control. The more commonly used asynchronous motor speed control methods are squirrel-cage motor variable speed control and wire-wound rotor motor circuit series resistance. With the maturity of frequency conversion technology, frequency conversion speed control has reached a relatively wide range of applications, especially under the guidance of the national energy-saving policy, the development of frequency conversion motor progress is relatively fast.

For the same multi-speed motor, there are two-speed, three-speed and even four-speed variable speed effect, the focus is on the stator winding rotating magnetic field changes to achieve the purpose of speed regulation. The stator winding of multi-speed motor has single winding structure and double multi-winding structure, the winding design and production process of this kind of motor is relatively complex, using different winding tap and wiring modes to realize the purpose of highlighting a harmonic and offsetting another harmonic, so as to realize the pole change. Variable-pole speed motors often encounter problems, including poor starting performance at a certain speed or electromagnetic noise, these problems need to be designed and tested during the process of full justification, to avoid problems in the use of the process.

braking of asynchronous motors

Asynchronous motor braking is also a problem to be solved in production practice, must master how to realize the reverse braking, generator braking and energy braking methods. These methods are through the motor itself state changes to realize the braking method. With the development of motor control technology and braking requirements demand, auxiliary electromagnetic braking technology has been widely recognized and applied, such as loss of power brake is widely used in motor products, that is, on the basis of ordinary motors, increase the loss of power braking device. This is to increase the power loss braking device on the basis of ordinary motors