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Buck-Boost Transformer

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drivetr.

Member
Yesterday, a 230 volt, DC, 5 HP motor stopped working.The breaker was not tripped or the fuses open. It was found to have two brushes worn-down to its rivets and another had a broken brush holder. The source of power comes from a 208 volt, single phase,50 amp breaker. From the breaker, it goes to a step-up transformer raising the voltage to 230 volts. From the transformer to goes to 40 amp fuse block. The voltage reads 118 volts to ground and the other reads 68 volts to ground at the fuse block because the secondary of the step-up transformer is ungrounded. After repairing the motor, it was discovered the controller was damaged as well. The controller has a 30 amp fuse protecting the motor, and this fuse checked good as well. Does having a ungrounded secondary have a negative effect? How can the situation be improved so in the event the brush goes to ground, the controller is protected from being damaged?
 
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bphgravity

Senior Member
Location
Florida
Re: Buck-Boost Transformer

At what point does this system rectify to DC, in the controller? Section VIII of the NEC covers grounding requirements for DC systems. Article 250.20 requires the AC portion of a rectified system to be grounded, but allows the DC portion to not be grounded per 250.162(A)(Ex2). :confused:
 
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shocker3218

Senior Member
Re: Buck-Boost Transformer

It's been about 10 years since I worked on DC drives, but I seem to recall the negative side of the dc bus always being tied to ground. When a short occurred, the rectifier fuse would blow and you would loose dc bus voltage. Wish I could remember more. Most of the drives I was working on were in the range of 50 to 500 horse dc.
 
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