Diagnostics for external brake resistors

Diagnostics for external brake resistors 1:

Reference values for the external brake resistor:

P-0-0207; P-0-0208; P-0-0209; P-0-0210; P-0-0218; P-0-0219; P-0-0220; P-0-0221

This chapter provides basic information on dimensioning and configuration of external brake resistors. The parameters described in this chapter can be recorded with the TwinCAT Scope View for diagnostic purposes.

If an internal brake resistor is used, the configuration takes place via the IDNs:

If an external brake resistor is used, the configuration takes place via the IDNs:

Diagnostics for external brake resistors 2:

"Actual continuous power" is an arithmetically averaged value of the actual power. Even after several typical machine cycles the value of P-0-0210 (P-0-0209 internally) must always remain below the value of the continuous power of the brake resistor.

A cycle time of 100 s is assumed for calculating the brake resistor power. Monitoring should therefore take place over this time as a minimum. The permissible value for the continuous power can be found in P-0-0207 (internal brake resistor) or P-0-0208 (external brake resistor):

Diagnostics for external brake resistors 3:

"Actual peak energy" indicates the greatest measured braking energy, much like a drag indicator does. The situation with the maximum required peak braking power, e.g. emergency stop, is supposed to be determined and tested. In the process, the value of P-0-0218 (internal) or P-0-0219 (external) should stay considerably below 100%.

For more exact observations it is possible to record the parameters with the software oscilloscope. If for example the velocity or the position is recorded, a better allocation of the current braking power to the machine cycle is possible. To this end the parameters must be inserted into the process image beforehand:

Diagnostics for external brake resistors 4:

Further information on external brake resistors can be found at:

Information on mechanical and electrical installation, dimensions and technical data can be found in the system manual for the AX5000 servo drive under: „Accessories – External brake resistor – AX2090-BW5x-xxxx“.

For further analysis peak values are recorded in the parameters P-0-0220 (internal brake resistor) or P-0-0221 (external) with respect to the duty cycle.

Duty cycle (DC) = switch-on time / cycle time (machine cycle) x 100%

Beckhoff specifies an overload factor for the brake resistors, depending on the duty cycle (see diagram below):

Diagnostics for external brake resistors 5:

Multiplied by the power rating, the permissible short-term power results from the overload factor dependent on the duty cycle:

Short-term power = Power rating x Overload factor (ÜF)

For the AX5000 energy values were calculated for the supported brake resistors from the power rating, duty cycle and overload factor and saved as parameters. They can be found for the internal brake resistor in P-0-0207 and after selecting an external brake resistor P-0-0208.

Sample: AX2090BW50-1600 with 47 ohm and 1600W power rating

Overload factor for 1% duty cycle: 30

1% duty cycle for 100s cycle time corresponds to 1s

1s x 1600W x 30 = 48000J

Correspondingly the other saved values result:

Duty cycle [%]

40

20

10

1

0.1

Overload factor

2.2

3.9

7

30

144.375

Peak E. [J]

140800

124800

112000

48000

23100

When the brake resistor becomes active, the software program calculates the current energy values, related to the duty cycle and presents the maximum values in P-0-0220 or P-0-0221. Both parameters are reset when the device is restarted (=0). The values entered in operation are preserved until the next restart or until they are overwritten by the user. They can be compared with the permissible values in P-0-0207 (internal brake resistor) or P-0-0208 (external). As a result it may be possible to influence the braking power in critical regions by modifying the cycle.

The greatest percentage value is entered in P-0-0218 or P-0-0219 (in %) (see above).