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DC injection brakes comply with woodworking regs

An ABB Automation Tech (LV Products) product story

Edited by the Engineeringtalk editorial team May 9, 2002

Entrelec MTE, a member of the ABB Group, has updated its Powerbrake range of DC injection brakes, designed to enable woodworking machines to stop safety and quickly.

Entrelec MTE, a member of the ABB Group, has updated its Powerbrake range of DC injection brakes, designed to enable woodworking machines to stop safety and quickly.

New features include thyristor integrity checking, increased braking time and automatic tripping should the time potentiometer fail.

Under the new PUWER (Provision of Use of Work Equipment Regulation 98) legislation all new woodworking equipment must already be equipped with an automatic brake that stops the tool in a sufficiently short time.

From December 2003 or 2005 (depending on the exact type of equipment) this standard will be extended to include second-hand and existing machinery already in use.

The Powerbrake has been developed to provide a way of meeting the PUWER 98 requirement which is much more reliable than a mechanical brake and around one tenth of the cost of installing a braked motor.

It is a solid state injection brake which provides frictionless, fast, smooth braking of single and three phase AC induction motors by injecting a controlled DC current into the motor windings; this introduces a stationary magnetic field which exerts a braking torque on the rotor.

In response to customer demand Entrelec MTE has introduced a number of important added safety features on the updated Powerbrake range for 2002.

The thyristor is the heart of the DC injection braking system.

Its integrity is now checked cycle by cycle during braking.

Should the thyristor or its supply fail then the unit will trip immediately.

The Powerbrake features user adjustable torque and time settings.

The maximum braking time has been increased from 20 to 30s, and should the time potentiometer become disconnected or the internal link is absent then the unit will now trip instantaneously.

The weight of the new units has also been reduced by around 40%, mainly by elimination of a mains transformer.

Entrelec MTE can supply the new Powerbrake units in three main versions suitable for use on woodworking machinery with motors up to 75kW: a DC injection brake module; an enclosed production brake; or an enclosed emergency/critical brake.

The DC injection brake module is an open style brake module for installation into a control system designed by the user.

The larger sizes comprise a separate power module and a control module.

The production brake is an enclosed unit used to increase productivity on a machine which would normally take a long period of time to come to rest.

The basic model without a starter is easily retrofitted to an existing installation to provide braking every time the motor is stopped.

A DOL (direct online) starter model incorporating start and stop pushbuttons is ideal for the machine builder, while a star delta model is available for use with larger motors.

A mushroom head stop button is available as an option, while a pair of terminals make it simple to add a remote stop button.

The enclosed emergency/critical brake is an enclosed unit designed for retrofitting to a machine to enable it to stop quickly in an emergency.

The brake is activated by a safety device, such as an emergency stop pushbutton or interlock switch and this disengages the AC supply to the motor before injecting the DC current.

Because these units inject in emergency situations only, and not on every stop of the motor, they are not suitable for making machinery PUWER98 compliant.