GB2441570A - Electromechanical brake operating arrangement for an inverter-driven motor - Google Patents

Abstract

A motor (1) is controlled by an inverter (2) and an electromechanical brake (3). The electromechanical brake (3) is powered from the supply (line in) to the inverter via a circuit comprising transistors (4, 5) and diodes (6, 7, 8). The transistors may be controlled by braking signals (not shown) from the inverter. The circuit is clamped to the DC bus of the inverter (DC+, DC-). The circuit thus employs two transistors to disconnect both braking solenoid terminals from the supply line and also uses the DC link voltage already present in the inverter as a clamp voltage, resulting in a fast but controlled switch-off speed of the EM brake.

Description

1 2441570 Electric Motor Braking System This invention relates to

electric motors and their breaking mechanisms.

Electromechanical brakes consist of a static disc held against a disc attached to the motor shaft by spring pressure and released by the action of an electrical solenoid. Such devices are often used as a motor holding brake where it is required that the motor shall be locked against rotation when power is not applied to the motor.

When a motor is used direct cm line (i.e. when the motor terminals are directly connected to a constant AC voltage) it is convenient to connect the solenoid terminals to the same point as the motor terniinals as shown in figure I. In this way the brake is released as power is applied to the motor and re-engages as power is disconnected.

When a motor is used with an inverter it is rio longer possible to connect EM brake solenoid terminals to the same point as the motor terminals since the voltage source used to drive the motor is no longer constant amplitude but varies with desired motor speed. It is therefore necessary to connect the solenoid terminals separately to the line voltage and to provide a method of interrupting this separate connection at the same I'he state of the art solution to the above problem is to use either a relay or a thyristor as a switch to disconnect the separate connection to the brake solenoid terminals, see figure 2. A thyristor suffers from the problem that it can only switch off while no current is passing through it. leading to delays at switch off, while a relay suffers from the problem of early contact failure in application where the brake solenoid is switched on and off repeatedly.

The invcntiofl comprises a system to control an electric motor comprising an inverter, having at least two voltage input lines and which outputs modulated voltage to the motor, and an electrically controlled brake, said brake terminals being connected to Iwo terminals of said line input, via two transistors, the bases of which arc controlled by a braking signal. and one of each other terminal connected of each transistor connected to a brake terminal and an input line.

Preferably the voltage output from the transistor and the brake terminals is clamped. This is preferably done in an enhancement advantageously using the DC bus of the inverter.

Although transistors are referred to, this term should be interpreted hereinafter as any transistor device including MOSFETs and the like. Any reference to base, emitter and collector should he interpreted therefore as equivalent terminals in such devices (e.g. gates of a MOSFET).

The proposed solution essentially uses two transistors to disconnect both solenoid terminals from the line voltage and also to use the DC link voltage already present in the inverter as a clamp voltage giving a fast hut controlled switch off speed to the EM brake.

The invent ion will now he described with reference to figure 3 which shows motor I controlled by an invertcr 2 and brake 3. The brake is controlled by two transistors 4, 5. The braking signal is applied to the base of the transistor. by any suitable means including remote activation. This will then let current IS through from their line inputs of the inventor to the brake. A pair of diodes 6, 7 is inserted between the transistors and line input as shown. Diode 7 shown in broken outline is optional, without this, if' the top of the brake coil is accidentally shorted to earth then there is a direct path from the bottom input conductor through the freewheel diode to earth which results in device failure under these conditions.

Furthermore a further diode 8 connects the motor brake terminals as shown. This "free-wheel" diode ensures than a negative voltage is not output to the coil (of motor). These three diodes altogether essentially form a half-wave rectification input.

inverters that are typically used for motor control have a DC bus which is manipulated buy the inverter to output modulated for the motor terminals. Additionally in the example shown, the DC+ and DC-from this bus is made use of and connected to the brake terminals as shown output via two further diodes. This embodiment results in a further neat enhancement to form a "floating gate drive" such that the DC output is used as a clamp voltage for the transistors. Clamping is preferably required because, during braking. the motor flux in the motor coil causes fly-hack (back-EMF) which needs to be dissipated. It should be understandable to the skilled person that clamping of the transistor may be performed by other means, and the resulting back-EMF dissipated otherwise. By using the enhancement the back-EMF does not need to he dissipated hut is essentially re-fed back to the overall system; the energy is fed back. Further more this arrangement keeps the voltage symmetrical about the line voltage.

The advantages of the proposed system are that when the inverter disconnects the solenoid from the line voltage, the motor shaft is locked quickly and repeatedly because the current in the solenoid coil is collapsed quickly and in a controlled manner across the inverter DC link. Also no extra fusing is required to protect the connections to the brake solenoid since the transistor circuit can provide an over-current trip. The circuit combines the operating lifetime of a thyristor with a repeatable and fast locking of the motor shaft.

Figure 4 shows a further embodiment of the invention. The diode 6\ is swapped with transistor?? and an extra diode 9 is added across the transistor. It is now possible to usc a power supply referenced to DC-to power the transistor gale drivers by bootstrap'. This is a technique where a circuit referenced to a power switching signal is powered from a supply referenced to the highest or lowest voltage which the switching signal attains in operation. In this case a power supply at DC-is used, DC-being at the same potential as the most negative input phase.

Claims (6)

1. Claims 1. A system to control an electric motor comprising an inverter,

   having at least two voltage line inputs and which outputs modulated voltage to the motor, and an electrically controlled brake.

   said brake terminals being connected tO two terminals of said line input, via two transistors, the bases of which arc controlled by a braking signal, and one of cach other terminal connected of each transistor connected to a brake terminal and an input line.
2. 2. A system as claimed in claim 1 wherein the voltage output from (lie transistor and the brake terminals is clamped.
3. 3. A systeni as claimed in claim 2 wherein said clamping of voltage uses the DC bus of the inverter.
4. 4. A system as claimed in claim 3 wherein one of the brake terminals is clamped to the DC negative of the DC bus and the other clamped to the DC positive of said bus.
5. 5. A system as claimed in claim 3 including a diode located each between the DC positive/DC negative and the brake terminals.
6. 6. A system as claimed in claims I to 5 including a diodes located between either or both transistors and their respective line inputs.