The brake is protected against short-circuiting of the brake resistor, and the brake transistor is monitored to ensure that short-circuiting of the transistor is detected.

A relay/digital output can be used for protecting the brake resistor against overloading in connection with a fault in the frequency converter.

In addition, the brake makes it possible to read out the momentary power and the mean power for the latest 120 seconds. The brake can also monitor the power

energizing and make sure it does not exceed a limit selected in par. B-12 Brake Power Limit (kW). In par. B-13 Braking Thermal Overload, select the function to

carry out when the power transmitted to the brake resistor exceeds the limit set in par. B-12 Brake Power Limit (kW).

NB!

Monitoring the brake power is not a safety function; a thermal switch is required for that purpose. The brake resistor circuit is not earth leakage protected.

Over voltage control (OVC) (exclusive brake resistor) can be selected as an alternative brake function in par. B-17 Over-voltage Control. This function is active for

all units. The function ensures that a trip can be avoided if the DC link voltage increases. This is done by increasing the output frequency to limit the voltage from

the DC link. It is a very useful function, e.g. if the decel time is too short since tripping of the frequency converter is avoided. In this situation the decel time is

extended.

3.9.1 Mechanical Brake Control

For hoisting applications, it is necessary to be able to control an electro-magnetic brake. For controlling the brake, a relay output (relay1 or relay2) or a programmed

digital output (terminal 27 or 29) is required. Normally, this output must be closed for as long as the frequency converter is unable to 'hold' the motor, e.g. because

of too big load. In par. E-24 Function Relay (Array parameter), par. E-20 Terminal 27 Digital Output, or par. E-21 Terminal 29 Digital Output, select mechanical brake

control [32] for applications with an electro-magnetic brake.

When mechanical brake control [32] is selected, the mechanical brake relay stays closed during start until the output current is above the level selected in

par. B-20 Release Brake Current. During stop, the mechanical brake will close when the speed is below the level selected in par. B-21 Activate Brake Speed

[RPM]. If the frequency converter is brought into an alarm condition, i.e. over-voltage situation, the mechanical brake immediately cuts in. This is also the case

during safe stop.

In hoisting/lowering applications, it must be possible to control an electro-mehanical brake.

Step-by-step Description

•

To control the mechanical brake any relay output or digital output (terminal 27 or 29) can be used. If necessary use a suitable contactor.

•

Ensure that the output is switched off as long as the frequency converter is unable to drive the motor, for example due to the load being too heavy or

due to the fact that the motor has not been mounted yet.

•

Select Mechanical brake control [32] in par. E-2# before connecting the mechanical brake.

•

The brake is released when the motor current exceeds the preset value in par. B-20 Release Brake Current.

•

The brake is engaged when the output frequency is less than the frequency set in par. B-21 Activate Brake Speed [RPM] or par. B-22 Activate Brake Speed

[Hz] and only if the frequency converter carries out a stop command.

AF-650 GP Design Guide

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