Mike Dolt Enterprises
Mike Dolt Enterprises
User: Bassmangsj, Location: Wisconsin
Asked on: Friday, March 18, 2011 at 9:00 am EDT
Answered on: Tuesday, May 14, 2024 at 2:59 am EDT
Question: I design electrical systems to be used with welding robots. The problem I am having is with the ocpd supplied with the robot. The robot itself requires 208-3 phase. On the robot panel is a 30 amp 3-pole breaker that is supplied by robot manufacturer. If the robot is to be installed in a facility that does not have 208 vac I install a transformer with the robot.The robot manufacturer says to install the transformer downstream of the 30amp breaker. Then 208 vac will be sent directly to the robot controller using a "plug" that fits the high power unit of the robot, so I can not go through another fuse or cb.I have 3 transformers I can use based on the size of the robot. All are 480/240 delta to 208 wye, 18kva, 10kva, or 4 kva.Am I wrong to assume the 30amp breaker is too large for the primary of this transformer when I can not protect the secondary because of the "plug" required for the high power unit of the robot?I figure the primary of the 4kva at 480v should at 6amps (125% of 4.8A), the 10kva at 15amps, the 18kva at 30amps.Thanks in advance for any advice.
Yes, it is reasonable to assume that the 30 amp breaker is too large for the primary of the transformer.Reasoning
The secondary of the transformer will provide 208 VAC, which is the voltage required by the robot. The 30 amp primary breaker is used to protect the transformer from overcurrents. However, you cannot install another fuse or circuit breaker downstream of the transformer due to the "plug" required for the robot's high-power unit. Therefore, the 30 amp primary breaker is not providing any real protection for the transformer or the robot's wiring.
Recommended Approach
Select a transformer with a primary current rating that is appropriate for the load. Based on your calculations, you should choose the
4 kVA transformer, as its primary current rating of 6 amps is within the acceptable range for a 480V primary voltage. Install the transformer downstream of the 30 amp breaker as specified by the robot manufacturer. Ensure that the primary wiring and protection devices are rated for the transformer's primary current rating.
Additional Considerations: The secondary of the transformer (208 VAC) will not have overcurrent protection. This is acceptable in this case because the robot's internal wiring and components are designed to handle the maximum current draw. It is important to use a transformer that is specifically designed for welding applications, as these transformers typically have special features to handle the transient currents and voltage spikes associated with welding. Flag for review