auto transformer timing

[jes1983]

I have set a auto transformer to start a 1500 hp motor. The tap is at 50% the transition to 100% is at 8 seconds. The load is on the motor is set to engage at 9.2seconds ( gas turbine.) My questions are, what is correct time between transitions? Should the transition be when the motor current at its minimum or when the current starts dropping off? Thanks

 

[jraef]

The real determining factor is motor torque. You want to transition when the torque is no longer increasing. Transitioning too soon runs the risk of defeating the purpose, because you may end up with a transition spike that is just as high as if you had started Across-the-Line (DOL). But torque is difficult to measure, so the generally accepted method is to look at motor speed; 80% speed minimum, 90% speed optimal. You can make an educated guess based on current if you have no other way to figure what's going on, but that's not the best method because current can be affected my may other factors. If you are forced to use current only, look for when the current stops increasing, BEFORE it drops to running load amps.

Good reading on the subject from another member's website:

http://www.lmphotonics.com/m_start.htm#AutoTransformer

Or jettison the entire contraption and put in a Solid State Soft Starter.

"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[jes1983]

jraef thanks for the input. I have invested to much time and money on this contraption to disolve it. I ran a test and measured the current, maxed at 350 amps @4 seconds, and dropped off to 300 amsp minimum, it never dropped to running amp. When it transitions to 100% the current jumped to 400 amps. Should i decrease my transition time to avoid the spike at transition?thanks

 

[rhatcher]

For a smooth transition with a minimum current spike you want to switch to across-the-line voltage (run) when the starting current is at a minimum. Specifically, the current will start high because of inrush and as the motor accelerates and the slip decreases (speed increases) the current will drop. At some point the current will stabilize at a minimum value. This is when you want to make the transition to the run contactor.

The minimum current on the starting tap may still be higher than the full load current of the motor if the motor is still slipping and has not accelerated to near synchronous speed.

This can happen if the available motor torque is less than required to accelerate the load to full speed. This is often the case unless there is a clutch in the system or the load is variable torque such as a fan or pump.

However, the basic rule is the same; once the current stops dropping and the motor is no longer accelerating it is time to switch to the run contactor.

Keep in mind that the motor acceleration is not based on soley only the transition timer. The voltage tap is the major factor.

The motor torque is reduced proportional to the square of the voltage. The more commonly used 65% tap provides about 42% of the motor torque for acceleration. A 50% tap provides about 25% of the motor torque for acceleration.

When picking a voltage tap, a lower starting torque will require a longer acceleration time and will be a greater risk of overheating the motor due to the extended time at high starting current. The benefit is that the peak inrush current is less and the resulting voltage drop (sag) is less. If you can accelerate to near synchronous speed in a reasonably short time at a low voltage tap then that is a desirable outcome.

A higher tap will accelerate the motor faster with less risk of overheating, but the inrush current and voltage drop will be greater.

The balance is to find the highest tap that your power system can supply without unacceptable voltage drop and to set the transition time at the moment that the minimum current occurs.

Finally...I will caution that operating induction motors for extended time at high slip (extended start time) or for repeated starting cycles without proper cool-off time can quickly cause major damage to the rotor. This is especially true for large motors with high inertia driving large loads with high inertia. This sounds like the case you are describing. You should check the motor nameplate for the motor starting duty (or contact the manufacturer) to be sure that you do not exceed the manufacturer's recommended ratings.