Industrial Brakes Guide - Brake switching Information
Spring applied brakes with DC coils can be operated from low voltage supplies, for example 12, 24, 36 and 48V DC. Switchgear should be sized for the relevant current. However many applications utilise an AC supply with a rectifier that converts to DC voltage. When switching the brake on the AC side of the rectifier, brake engagement times are extended by a factor of 3-4 over catalogue values.
The simplest form of connection to a motor, in parallel with the rectifier and brake coil, further extends the engagement time. This is because the motor which is already switched off but still turning continues to excite the brake. With falling loads such as hoists, lifts and cranes, it is ESSENTIAL to switch the brake coil on the DC side of the supply. A spark suppressor is required to prevent inductive voltages from damaging the brake coil or rectifier. Reducing the brake torque also prolongs brake engagement times.
The disengagement time is not influenced by AC or DC switching. It can only be shortened by over-excitation of the coil, for example by using a force voltage rectifier.
Three types of Rectifier are available
- Full wave rectifier which gives a ratio of 1.1 between AC input and DC output, i.e. 230V supply gives 209V DC. Rectifiers are chosen to match available brake stator voltages which are up to a maximum of 250V DC. Optionally the rectifiers can include spark suppression and terminals for DC switching.
- Half wave rectifiers which give a ratio of 2.2 between supply and output, for example when using a 400V supply and a 180Vbrake coil. Spark suppression and DC switching are optional.
- Forced voltage rectifiers which use a combination of half and full wave rectification to over-voltage the brake coil for a short period and so speed brake release. Forced voltage rectifiers can alternatively be used to achieve a lower holding voltage which reduces energy consumption and speeds brake engagement.
Modular options are readily available to customise the brake for specific duties and mountings, although not every option is available for every size of brake.
- Manual release, also known as hand release, allows the brake to be disengaged without applying current. Combining a manual release with a monitoring microswitch can prevent starting whilst the release is in use.
- Brake seal is a rubber protective ring that fits on the outside of the brake.
- Mounting flange and friction plate are optional mounting surfaces. Brakes require a ferrous friction surface that is flat with turned smoothness and is square to the shaft.
- Long life variant has mechanical strengthening for high frequency applications and ratings in excess of 10 million operations.
- Microswitches fitted to the brake stator can monitor operation or wear.
- Terminal box connection instead of the flying leads which are the norm.
- Cold Climate Version to suit operation in ambient temperatures from -40°C to +40°C.
- Double brakes that are piggy-back mounted for increased safety, often used in stage machinery.
Further advice on sizing and installation
Spring applied brakes are established products and a high volume of knowledge on selection and installation is in existence Particularly for performance-critical and safety sensitive applications, advice should be sought from specialists.