The Current Magnitude (04.001) is compared to an upper and lower threshold by a comparator with hysteresis to give torque present and drive output open circuit detection functions respectively.

BC Lower Current Threshold (12.043) and BC Upper Current Threshold (12.042) are given as a percentage of Motor Rated Current (05.007). BC Upper Current Threshold (12.042) should be set to

the current level that indicates that there is magnetising current and sufficient torque producing current in the motor to deliver the required amount of torque when the brake is released. The output of

the comparator remains active after this level has been reached unless the current subsequently falls below BC Lower Current Threshold (12.043) which should be set to the required level to detect

the condition where the motor has been disconnected from the drive. If BC Lower Current Threshold (12.043) ≥ BC Upper Current Threshold (12.042) then the upper threshold applies with a

hysteresis band of 0. If BC Lower Current Threshold (12.043) = BC Upper Current Threshold (12.042) = 0 then the output of the comparator is always one.

Frequency detection

The frequency comparator is used on starting, to detect when the motor frequency has reached a level where the motor can produce the required amount of torque to ensure that the motor rotates in

the demanded direction when the brake is released. BC Brake Release Frequency (12.044) should be set to a level slightly above the motor slip frequency that is likely to occur under the highest

expected load that is applied to the motor when the brake is released.

The brake apply frequency threshold is used to ensure that the brake is applied before the motor frequency reaches zero and to prevent the motor rotating (in the reverse direction due to an

overhauling load for example) during the brake apply time. If the frequency falls below BC Brake Apply Frequency (12.045), but the motor is not required to stop (i.e. reversing direction without

stopping) then Reference On (01.011) will be one, and so the brake is not applied. This prevents the brake from activating and de-activating as the motor passes through zero speed. If the frequency

falls below BC Brake Apply Frequency (12.045) and Reference On (01.011) = 0 then the brake will be applied.

Brake Release

When both Current detection and Frequency detection conditions are met, the ramp is held and the brake released after the BC Brake Release Delay (12.046). When the brake is released, the ramp

is held for a further period ( BC Post-brake Release Delay (12.047) ) after which the ramp is released.