GB2432734A

GB2432734A - Apparatus for the braking of inverter driven induction motors

Info

Publication number: GB2432734A
Application number: GB0609041A
Authority: GB
Inventors: Robert Carter
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 2005-05-20
Filing date: 2006-05-09
Publication date: 2007-05-30
Anticipated expiration: 2026-05-09
Also published as: GB0510302D0; GB2432734B; GB0609041D0

Abstract

A driver( 1) includes a controller to add a reverse AC vector to a driving vector of an inverter drive motor (9) in order to brake the motor (9). In the preferred form the controller includes a set of pulse stretchers (5-9) which are responsive to an applied trapezoidal waveform to adjust the pulse widths of input signals to an inverter (2) thereby adding an extra contra-rotating voltage vector to the main output voltage vector.

Description

pparatus for the braking of inverter driven induction motors This

invention relates to the braking of induction motors, when driven by a voltage source inverter.

Induction motors are driven by application of varying voltages in a number of phases. In order to brake such motors three techniques have been proposed or implemented.

In a first technique, called DC injection braking, the AC driving voltage applied to the motor is stopped and a DC voltage is applied to the motor instead to produce a stationary field which opposes the motion of the rotor.

This technique suffers from the drawback of a lack of control due, in particular, to determining when the motor has stopped. The braking can also only be used to bring the motor to a complete stop. This means that if it is desired to slow the motor to a particular speed it has to be stopped using the DC injection and then re-started and brought up to the desired speed.

A second technique is called regenerative braking. It requires the reduction of the frequency of the normal driving vector. This produces braking torque and puts that energy onto the DC link capacitors.

In a third technique, called compound braking, DC is added to the main rotating voltage vector, which is being reduced in frequency to slow the motor. Such a technique is described in JP 60-187275. This confers some of the advantages of both the two earlier techniques including greater control of the braking and eliminates the need for an external resistor. It is difficult to control and leads to an oscillating torque.

Furthermore, some of the switching elements are heated more than others.

The unbalanced heating may lead to premature failure of one or more of the components.

According to the invention there is provided a driver for an inverter driven induction motor comprising: means to provide a rotating drive voltage vector to in use drive the motor; and means to provide an opposing braking AC voltage rotating in the opposite direction to that of the drive voltage vector which is added to the drive voltage vector.

Preferably, the rotation speed of the opposing voltage is less than 2Hz.

This is preferred since this has been found to reduce the possibility of the transistors in the inverter being overheated. The reverse rotation frequency is chosen with reference to the thermal time constants of the inverter cooling system to prevent large temperature variations in the switching transistors.

Preferably, control means are provided to monitor the inverter DC link voltage and to add or remove the opposing AC voltage in an attempt to prevent this voltage reaching dangerously high levels Preferably, the opposing voltage is provided by application of a trapezoidal wave shape to the inverter. This shape is more convenient to produce than a sinewave and has better harmonic content than a square wave.

In the preferred embodiment, pulse stretchers are provided coupled between a pulse width modulated waveform generator and the inverter.

The pulse stretchers are coupled to a source of the trapezoidal waveform.

A specific embodiment of the invention will now be described by way of example only with reference to the drawing in which: Figure 1 shows an induction motor controller in accordance with the invention including a waveform generator, and a motor controlled by the controller; Figure 2 is an explanatory diagram showing waveforms generated by the waveform generator of figure 1 to be applied to a pulse stretcher; Figure 3 is an explanatory diagram showing the effect of a pulse stretcher used in the specific embodiment with figure 3a showing an input waveform and figures 3b, 3c showing the resultant stretched waveform; and Figure 4 is a space phasor diagram of a compound braking voltage vector.

As is shown in figure 1, an induction motor controller 1 comprises an inverter 2, a Pulse Width Modulated Waveform Generator 3, a waveform generator 4 and pulse stretchers 5, 6 and 7. A controller 8 is provided to control the various components. This is a microprocsor and it will be appreciated by a person skilled in the art that a number of the depicted blocks may be provided by one or more suitably programmed microprocessors.

The inverter 2 is coupled to positive and negative DC link lines and includes a number of switching elements to produce from the DC voltage the required three phase AC voltages to drive an induction motor 10.

This part of the apparatus will be familiar to a person skilled in the art.

The inverter 2 is responsive to control inputs on three control input lines 11, 12 and 13 to vary the switching of the switch elements in order to generate the desired three phases to be applied to the motor 10.

The control inputs are provided by the combined actions of the PWM generator 3, the waveform generator 4 and the pulse stretchers 5 to 7.

The PWM generator 3 is of a conventional type and provides three signals which are coupled to the pulse stretchers 5 to 7. The pulse stretchers 5 to 7 are responsive to the signals received from the waveform generator 4. This in turn is controlled by the controller 8 via control line 9. Part of the processor 8a is allocated to monitor the DC link voltages When it is desired to brake the motor 10, the frequency of the PWM generator 3 and hence the motor drive vokage vector is reduced. This generates a negative (braking) torque on the motor shaft. The mechanical load energy is recovered in this operation and results in the DC link voltage of the inverter rising. The controller 8 detects via its voltage monitoring function 8a that a voltage threshold is crossed on the DC link and it instructs via line 3a the waveform generator 4 to output the three phase control signals to the pulse stretchers. The waveform of the control signals for the three phases is shown in figure 2 and each is a repeating trapezoidal shaped voltage pulse. The figure shows the three phases 20 to 22.

The effect of the control signals is to stretch the pulses of the signals introduced into the inverter 2. It causes a delay on the rising and the falling of the pulses and provides an AC braking vector on the main driving voltage vector produced by the inverter and input to the motor 10. A PWM waveform 30 is shown in figure 3a without the effect of the pulse stretchers, in figure 3b the rising edge is delayed to give a lower voltage and, in figure 3c, the falling edge is delayed by the pulse stretchers to give a higher output voltage. The logic signal depicted in figure 3a is produced by the PWM generator 3 and applied to the pulse stretchers 5, 6, 7 along lines 3a. The resultant stretched logic signals of figures 3B, 3C are applied to the inverter 2 along lines 11, 12 and 13.

A phasor diagram is shown in figure 4. It shows the main voltage vector progressing in direction of arrow 41 with the braking vector 42 rotating in the reverse direction 43 added to it. The preferred reverse rotational speed is about 2Hz.

The controller 8 continues to monitor the DC link voltage and when the DC link voltage reduces below a preset threshold it removes the AC braking vector.

Claims

Claims 1. A driver for an inverter driven induction motor comprising:

means to provide a rotating drive voltage vector to, in use, drive the motor; and means to provide an opposing braking AC voltage vector rotating in the opposite direction to that of the drive voltage vector which is added to the drive voltage vector.

2. A driver as claimed in claim 1 wherein the rotation speed of the opposing voltage vector is less than 2Hz.

3. A driver as claimed in claim 1 or claim 2 comprising control means to determine when a DC link voltage crosses a threshold and to apply or to remove the opposing AC voltage in response to the threshold being crossed.

4. A driver as claimed in any preceding claim comprising a pulse width modulation generator to provide pulse width modulated control signals to an inverter to produce the phase signals to drive the motor and means to delay or advance the rising or falling edges of the pulse width modulated control signals.

5. A driver as claimed in claim 4 wherein the means to delay or advance comprises pulse stretchers controlled by a control waveform.

6. A driver as claimed in claim 5 wherein the control waveform is a trapezoidal waveform.

7. A driver as claimed in any preceding claim in combination with an induction motor.