AN2388 datasheet(7/54 Pages) STMICROELECTRONICS

7 / 54 page

AN2388

Background

7/54

circuited, but are terminated to a set of slip rings. These are helpful in adding external

resistors and contactors.

In fact, it is possible to demonstrate that the slip frequency producing the maximum torque

(pull-out torque) is directly proportional to the rotor resistance. In wound-rotor motors, the

real rotor resistance can be increased by connecting external resistors through the slip

rings. This possibility allows for higher slip and hence, the pull-out torque at lower speed. A

particularly high resistance can deliver a high pull-out torque starting from zero speed. As

the motor accelerates, the value of the resistance can be reduced so that the motor

characteristic can follow the load requirement at different speeds. Once the motor reaches

the nominal speed, external resistors are removed from the motor coming back to work as

the standard induction motor.

This motor type (when external resistors are connected with the rotor) is ideal for very high

inertia loads, where it is necessary to generate the pull-out torque at almost zero speed and

accelerate to full speed in the minimum time with minimum current consumption.

The typical speed-torque characteristic of an induction motor is shown in Figure 3

Figure 3.

Speed-torque characteristic

The X axis shows speed and slip. The Y axis shows the torque. During start-up the motor

needs seven times the rated current and this depends on the interaction between stator and

rotor flux, the losses in the stator and rotor windings and losses in the bearing due to friction.

This over-current produces the torque necessary to spin the motor from zero speed.

During start-up, the motor is able to delivers 1.5 times the rated torque. This torque is called

locked rotor torque (LRT). Once the speed increases, the current that flows in the motor

reduces slightly. When the motor runs at approximately 80% of the synchronous speed, the

load can increase up to 2.5 times the rated torque. Any further growth in term of load could

take the motor to a stall condition.

As seen in the speed-torque characteristics, torque is highly nonlinear as the speed varies.

In all applications where it is necessary to regulate the speed a control strategy must be

used which is able to vary the frequency. One of the best known strategies is the simple

open loop method called Variable Voltage Variable Frequency or simply V/f method. This

method doesn’t allow management of the quantities in terms of phase but modifies only the

magnitude of the stator flux.

Pull-out Torque

Flux decreases

Stable zone

Flux = rated

Rotor speed

Unstable points

T, rated

1.5

1.0

Slip Speed

Locked Rotor Torque (LRT)

AN2388 датащи(PDF) 7 Page - STMicroelectronics

AN2388 датащи(HTML) 7 Page - STMicroelectronics

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54

датащи скачать

ссылки URL