Autotransformer Application Question

[zai_jnr]

Dear All,

This is my first time using auto transformer in motor system design.
From my reading, the sequence of starting auto transformer is (using 3 magnetic contactors in system design) :

1) Initially, The Star Switch is closed
2) Then, The Start Switch is closed to energize the autotransformer
3) The motor is connected at a selected reduced voltage tap on the autotransformer and start to turn and accelerate.
4) After a predetermined period the Star Switch will open.
5) After a mili-sec delay, the Run Switch will close, connecting full line voltage to the motor.
6) The Start Switch will then open and the motor will be at operational speed.


My question is,
1) how to to determine value of step no 4?
2) what the ideal delay timer for step no 5?

Autotransfomer that I'm working is for start up run of motor compressor . All the sequence control by PLC.

Thanks a lot for ur reply.

 

[BobB]

Auto transformer starter? Have forgotten all that stuff - use a VSD. Used to rewind these things in the 1960s - and secondary resistance starters. Google should bring up some hits - have not seen one for 50 years!

 

[ktmtragic]

Depending on the load. The normal range is between 05-5 seconds. The best option is to put a tong tester on the motor and determine when the motor amps are stable, this is the point at which to switch to full voltage.Start a little low and work from there. Too long in star is no good

 

[shooter]

An autotransformer is a really slow starter, so on average take 10-15 seconds.
make the time adjustable, and change it in the field, when the compressor is running on 80% of rpm you can switch over, any later is no problem, it just heats up the transformer.
You also need a restarter blocking time, to prevent restarts within 15 minutes or so.

 

[zai_jnr]

Actually, I'm already set delay timer 5 sec to switch from star sw to run sw.

The problem is, transformer heat up then got smoke, fortunately, OCR function to cutoff the power to auto transformer.

Is it normal, auto transformer will heat up during star and start sw close so long?

 

[osmanmom]

post your wiring diagram

 

[jraef]

Your description is pf an open transition starting scheme, which is very hard on the starter, the transformer and the motor. Do a search for the term "Korndorfer Starter", which is a variant of the Autotransformer starting scheme that allows for a closed transition and less problems. Most websites that describe it will give you the control scheme.

As to the timing, the official time value is the point at which your motor ceases to accelerate, usually at 80% speed as mentioned, minus 1/2 second. But that time is TOTALLY dependent upon the inertia in the system, so there is no one answer that applies to all situations. Adding to the complexity is the fact that all Autotransformers are built to handle a specific amount of heat, and generally the cheaper the transformer, the less heat it can handle**. That means you must weigh the cost of your transformer against the acceleration time of YOUR load when starting with reduced current. I do not however, know of any Autotransformer made to take more than about 15 seconds at the 80% taps, which equates to about only 25 seconds at the 50% taps. That doesn't mean EVERY transformer will handle that, and it sounds as though yours is one of those at the opposite end of that spectrum if it started smoking at 5 seconds!

So just out of curiosity, why Autotransformer and not a Reduced Voltage Solid State soft starter? They are now LESS expensive and infinitely easier to implement.

** You should always buy the autotransformer with an embedded thermal switch in the core that you wire into the controls to drop out the contactor coils BEFORE things start to smoke. And also, once transformer insulation gets to the point of smoking, it's failure is imminent. Be prepared for the worst now, and again, I suggest replacing it with an RVSS soft starter.

 

[zai_jnr]

Your description is pf an open transition starting scheme, which is very hard on the starter, the transformer and the motor. Do a search for the term "Korndorfer Starter", which is a variant of the Autotransformer starting scheme that allows for a closed transition and less problems. Most websites that describe it will give you the control scheme.

As to the timing, the official time value is the point at which your motor ceases to accelerate, usually at 80% speed as mentioned, minus 1/2 second. But that time is TOTALLY dependent upon the inertia in the system, so there is no one answer that applies to all situations. Adding to the complexity is the fact that all Auto transformers are built to handle a specific amount of heat, and generally the cheaper the transformer, the less heat it can handle**. That means you must weigh the cost of your transformer against the acceleration time of YOUR load when starting with reduced current. I do not however, know of any Auto transformer made to take more than about 15 seconds at the 80% taps, which equates to about only 25 seconds at the 50% taps. That doesn't mean EVERY transformer will handle that, and it sounds as though yours is one of those at the opposite end of that spectrum if it started smoking at 5 seconds!

So just out of curiosity, why Autotransformer and not a Reduced Voltage Solid State soft starter? They are now LESS expensive and infinitely easier to implement.

** You should always buy the autotransformer with an embedded thermal switch in the core that you wire into the controls to drop out the contactor coils BEFORE things start to smoke. And also, once transformer insulation gets to the point of smoking, it's failure is imminent. Be prepared for the worst now, and again, I suggest replacing it with an RVSS soft starter.

Dear jraef,

Thanks a lot for your reply. Really appreciated.
Actually the system is design by other person/programmer. Then this guy already resign from his company A. His boss call me to complete the job.That why for system design, I need to study back one by one what actually this previous programmer planned before.

Why using autotransformer? Base on info that I get from company A , supplier (company B) for HVAC system was requested to use autotransformer for motor startup.Because there only have 3 terminal connection to motor. Initial plan, company A want to use start delta connection. That why they use this auto transformer as a motor start up.

I'm not sure what the issue for heated up/smoke auto transformer before. But I'm found out this HVAC system got some wiring issue with blower fan motor when rotation is not follow the standard. The seq of system should be :

1) Blower Fan ON
2) After 3 sec, Condensor fan turn ON
3) After 5 min, Compressor POwer ON (using autotransformer)

After modify wiring of motor blower fan, the issue solved. Still question mark, why auto transformer heat up/smoking during first trial.

 

[shooter]

The auto transformer heats up when there is current flowing thru it.
When trying this too often it will get too hot.
The failure can beoccur when the relais are not in correct position, the rotor is locked, the transformer is too small, not connected correct etc.
check also the liquid valve operation.

 

[MikeVT]

I had some experience a few years back, replacing an auto transformer start with a Solid State soft start. One pretty big GOTCHA we ran into was there was considerably less starting torque with the solid state soft start, than with the auto transformer start.

The application was a tough one. A 350hp motor driving a roller mill making feed pellets. It took a LOT of repeated explanations and fuse replacements before it sunk in to the operators, that if the roller mill ever stopped with product in it - you HAVE TO clear it out to restart the roller mill. They used to just restart the motor with the auto transformer. They probably shouldn't have. That may be why it burnt up. But the electronic soft start would not even get close to starting that mill, with product in it.

 

[jraef]

... After modify wiring of motor blower fan, the issue solved. Still question mark, why auto transformer heat up/smoking during first trial.

I'm guessing, but it may have been some sort of problem with the interlocking of the blower motor with the compressor starter, intended to not allow the compressor to start until after the blower was started and running. If the electrical interlock was in the wrong (logic) place, it may have interfered with the sequence in the starter. If you end up with an "incomplete sequence" in that type of starter, the transformer is left in the circuit too long and overheats quickly. After you modified the blower, it may have indirectly fixed the other problem too.

I had some experience a few years back, replacing an auto transformer start with a Solid State soft start. One pretty big GOTCHA we ran into was there was considerably less starting torque with the solid state soft start, than with the auto transformer start.

The application was a tough one. A 350hp motor driving a roller mill making feed pellets. It took a LOT of repeated explanations and fuse replacements before it sunk in to the operators, that if the roller mill ever stopped with product in it - you HAVE TO clear it out to restart the roller mill. They used to just restart the motor with the auto transformer. They probably shouldn't have. That may be why it burnt up. But the electronic soft start would not even get close to starting that mill, with product in it.

Technically, the LINE current is lower using an RVAT starter, because of the transformer effect. So for example if you use the 65% taps on an RVAT, the motor current will be reduced by 65%, but the LINE current into the transformer will be at 42%. With an RVSS, the line current AND the motor current will be both at 65%. So for the SAME amount of motor torque, the line current will be 23% lower. If you then adjust the RVSS so that it only draws 42% current from the line, it will give you 42% current, instead of 65%, so 23% less PER UNIT of line current.

But what happens is that if you were mainly after the current reduction because you had a limited supply available and used an ammeter on the line side of the starter to determine the torque, that looked to you as if the RVSS was giving less torque. If you measured the MOTOR current, you would have seen it was less and if you turned up the setting on the RVSS to where it delivered 65% current to the motor, it would have started the same.

This is the only possible advantage that RVAT starting still has, and only if the required accelerating torque fits into one of the tap settings on the transformer. But what I've see a lot in pellet mills and jaw crushers that have similar problems with loaded re-starting, the operators often have figured out how to go AROUND the transformer start and punch in Across-the-Line to avoid having to dig it out. In that case, I would just give them a soft starter with dual ramps, and set the second ramp to being 500% current limit.