Auto-transformer starting 1

[BL1]

We are about to install a 10ft fan with a 2500hp motor.We
are going to use an auto-transformer (5000kva) to supply the motor.This will be used to ventilate a large mine with multiple coal seams.We will eventually add another fan of the same.My question: are there any draw backs to using auto-transformer starting on this set up?Thanks in advance for any replies.

 

[jraef]

There are advantages and drawbacks to every form of starting. You just need to decide which ones outweigh the others in YOUR application.

The drawbacks to RVAT starting, in no particular order:
1) Size and weight. It is the biggest (physically) method available.
2) 2-step process. No matter how good you think you get the timing set, there is ALWAYS a second surge of current when the starter transitions from Start to Run. Contrary to popular belief, the magnitude of that second surge is almost the same as if you started Across-the-Line (DOL), just of a much shorter duration. So essentially it is just like when you were confronted by a playground bully as a kid and offered a choice: trade one big long slug in the gut for one small hit on the arm followed by a big quick punch in the jaw.
3) Parts that wear out. This is only relevant when compared to solid state soft starting.
4) Limited duty cycle. The transformer can only take the thermal stress of starting maybe once or twice per hour, sometimes even longer. That is something to discuss with your supplier. If you only start once per day, this is irrelevant of course.
5) Limited adjustability. You will typically get 3 sets of taps on the transformer, 50%, 65% and 80%. If one of those does not allow enough starting torque, you must jump up to the next, which may be more than you want.

Advantages of RVAT:
1) Most efficient use of line current vs motor current. In any other form of RV starter, line current = motor current. In the RVAT, the transformer action allows you to have a line current reduction ratio proportional to the transformer voltage ratio. This can be advantageous in limited supply situations such as generators.
2) In MV applications like yours, slightly lower initial cost than some other forms. Given the larger footprint this advantage may end up negated at installation, but again, only you can determine that. At 2500HP you will likely be around 3-5% less expensive than solid state. In Low Voltage (for the benefit of future readers), RVAT is actually more expensive now than solid state.

JRaef.com
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