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Kollmorgen Support Network

UL Cross Section | 07 Nov 2013 | |

UL Cross Section

UL problem: Wire cross-section of motor cable is smaller than the infeed

Example:
S724 with 30 A power supply input fuse for wiring with 6 mm² and 4 mm² motor cable for motor with 18 A rated current.

Power supply connection

The S724 is wired to the three-phase power supply input / energy supply input with a wire cross-section of AWG 10 (approx. 6 mm²), each for the power supply phases and PG. Acc. to UL, one prespecified fuse of 30 A must be used in the three power supply phases (type and size acc. to the manual, chapter on UL Markings).

This arrangement with the specified fuse is accepted and approved as such by UL.

Protective function of servo amplifier

The servo controller S724, which is configured with the correct motor data, acts current-limiting and safely protects the motor, and therefore also the motor cable, against overcurrent. The current measurement and current limiting were checked during the UL acceptance.

Requirement

Wiring acc.to manual, BTB contact switches off the line protection in case of a fault.

Motor phase cycle short-circuit and motor phase ground fault are detected by the controller
=> the output stage is disabled and the BTB network is reset
=> BTB contact switches off the power supply contactor
=> Resistance to short-circuiting, switching is sufficiently fast

Motor connection

Based on this protective function of the S724, the wire cross-section of the motor cable can be dimensioned as follows using the rated current of the motor:

Acc. to UL508C, Chapter 40, Table 40.3: "Ampacities of insulated conductors" (p. 76), the wire cross-section is taken from this table for 125% of the rated motor current.
(The same table is also located in UL508A in Table 28.1 !)

In the example, the rated motor current Irated=18 A.
In this case, 125% of the rated current amount to 22.5 A, and for copper as the wire material, a wire cross-section of AWG 11 (approx. 4 mm²) is sufficient. (Table values are incremented in AWG and have to be interpolated!)

This procedure for dimensioning the wire cross-section of a motor cable was confirmed by Underwriter Laboratories / Frankfurt!

UL acceptance test

If a UL acceptance test is required, the protective functions of the servo amplifier should be tested:

  • Testing the current limitation
  • Testing switch-off in case of short circuits

Testing the connector and line temperature (less than 75 °C) at 125% of the rated motor current and the wire cross-section selected according to Table 40.3. (Notice: at a max. ambient temperature of 40 °C!)

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