OT: Wye Start / Delta Run Troubleshooting

Greetings,
I have a 300hp motor controller that has burnt out resistors. First a little history. Last year one of the 3 resistors burnt out. We replaced it. Six months later all 3 resistors have burnt up. There is a 4 week lead time on the replacement resistors. We need the motor to run way before then. If anyone is familiar with this type of motor control, I would appreciate it if they could help me find a way to prevent any further resistor problems and or get the motor running without them.

Motor = 300hp 3 phase 460 volt 6 wire
Starter = wye start / delta run
Resistor value = 1.2 ohm (wattage = ???)
Application = Ammonia refrigeration compressor drive

I have attached a PDF with the schematic of the motor starter. It is an AllenBradley brand of starter. After looking at the schematic I get the feeling that the resistors are only in use for a short time during the transition from start to run. This design is supposed to prevent arcing of the contactors during this transition period. I could be wrong in my thoughts on this one and that is why I'm requesting help.
Thanks for reading and any suggestions
BD

It looks like a closed transition star-delta starter. As you say, the resistors are only in circuit during the changeover from star to delta. The idea is not to interrupt the current in the motor winding during the transition, which is what happens in a normal S-D starter.

The timer value you have shown is only 2 seconds in star. This doesn't seem very long for quite a big motor. It needs to accelerate to a reasonable speed, otherwise the transition to delta is basically a DOL start. I wonder if the current is too high when the transition occurs, which is what is burning the resistors.

I can't see why it wouldn't work without the resistors. I would look at timing of the transition. You need to be sure the arc in the star contactor is extinguished before closing the delta contactor. I always use a dedicated S-D timer, though some people here use a PLC (there's a thread in this forum somewhere). If your upstream protection is a circuit breaker, you could in trouble, as the transient can easily trip the mag protection.

BD,

I think that AutomaticLeigh has hit the nail on the head...I check one of mine (smaller 100 hp) its at 5sec, I don't think you can do any harm running it to long in star, but switching to fast will be a large draw.

Do know what the FLA is on the name plate?, and have you clamped it on start up?

I'm sure Dick or one of the drive boy's could tell you if you are in the ball park on the current draw vs time

The star time definately seems too low, but then it depends on the application.

Is it an unloaded or loaded start ? (Has the compressor Unloaders ? Or is there an unloader between the suction and discharge?)

Have you checked the motor current on startup in WYE and changeover to DELTA with a clamp-on ?

I did a 150Hp 415V 6 Wire 3 Phase Star Delta Grasso 6 Cylinder reciprocating refrigeration compressor about 8 years ago and this had a 20 seconds star (WYE) time before changeover to Delta. The compressor unloaded 66% on startup through internal unloaders.

Increase your start/run timer to around 10 seconds, use a ammeter to check you motor current after it gets to synchronous speed the current should drop off to a lower value, use that time frame from the start to current drop point time to set your start/run timer.

As for replacement resistors, if you know the ohms and wattage, calling a large industrial drives distributor like Galco in Highland Park MI and asking for that value should get the job done. These distributors would be stocking such items a snubber brake resistors for AC drives.

As for how to set the transition timer, that's a bit out of my field but, it seems to me that optimum would be to see the same inrush current at transition as at start. The problem would be measuring the inrush. If you have a way to do that, it seems to me that setting the timer too short will give you a higher transition inrush than starting and setting it too long will give you a smaller inrush than starting.

Am I on track here or have I just shown my ignorance again?

I agree with DickDV, in the past I have used a clamp on and a fluke 192 to watch the current when starting. When the current begins to level of in wye it's time to switch.

2 seconds seems awful low for any type of load with a motor this big

You`re getting alot of good info here and i`m not familiar with a Ammonia refrigeration compressor and how much it takes to start one, but i`m pretty sure 2 seconds isn`t enough start time. We`ve left 300 hp bandmill and chipper motors from 30 seconds to 2 minutes before switching into delta. Now something else to think about? It looks like from the photo of the resistors that this didn`t happen in a split second and that`s all we`re talking about when M1 timer brings in 2S and the aux. on 2S drops out 1S and the aux. on 1S should bring in 2M. Here lies what could be an intermitant problem if the NC aux. on 1S didn`t close the resistors would stay in the circuit and let all thier smoke out! This looks like what is happening. Now if you will add some logic to your plc that proves what`s suppose to happen has happened if it doesn`t happen in a timely fashion drop everything out. Now with a little rigging you can make an open transition starter out of the one you have. It doesn`t need to be closed transition to start a motor, but looks better on paper.Most well built starter have a timer checking on the timer doing the switching like timer M1 is supposed to switch in 15 seconds now let timer XX2 be set for 17 seconds and if timer M1 doesn`t work or it does work, but what ever it was suppose to bring in didn`t let timer XX2 drop everything out. If you don`t put some logic like this in to prtect everything and you wire it for a regular Y delta starter you stand the chance of bring your motor up, because if the starter doesn`t go through its sequence right it will leave the motor running in a star and may not have enough torque to do the work. :site:

Hi,

Thanks for the feed back guys.

I set the timer at 2 sec because it seems to be finished accelerating after 1 second. I've never used a clamp on ammeter to verify the amp draw during start up. I always just play it by sound and sight. The only amp clamp we have that goes that high is a digital unit. The digital unit doesn’t display the inrush very well. Can anyone recommend a proper amp clamp to use that will show the curve in real time? I have checked the run amps before. The name plate says 250 FLA. It will draw around 240 amps fully loaded. The reciprocating type compressor has 100% unloading ability. The rotating compressor assembly turns fairly easily so there isn't any large load at start up. The unloading sol. valves won't energize for 10 seconds after start. Of course this is in a PLC timer and is adjustable. The unloading valves are hydraulic in nature and depend on oil pressure from the crankshaft driven oil pump. That is the only control the PLC has besides closing the start circuit and monitoring the thermal overload relays. The transition timer is a little palm sized device attached to the contactor in the same manner as an auxiliary contact. I will adjust the timer to 5 seconds and then see what the amp clamp shows. I think I'll take Thomas's suggestion to heart and replace all the auxiliary contacts since they are 17 years old. I think it would be a good idea to add some wiring back to the PLC to monitor the contactors also. If a closed transition starter isn’t necessary, I may go ahead and isolate the resistor leads and run it that way until the replacements arrive. I’m not sure how to do the math to figure out the resistor wattage. Anyone know how to figure the wattage? I tried to figure it and came up with over a hundred thousand watts! That can't be right can it? Again thanks for all the help!

BD

As for the resistor wattage you can call your supply house and give them the physical dimension of the resistor and they will be able to give you the wattage. From the picture I would say its more along the line of 100-150 watt.

bikerdude said:

I set the timer at 2 sec because it seems to be finished accelerating after 1 second. I've never used a clamp on ammeter to verify the amp draw during start up. I always just play it by sound and sight. The only amp clamp we have that goes that high is a digital unit. The digital unit doesn’t display the inrush very well. Can anyone recommend a proper amp clamp to use that will show the curve in real time? I have checked the run amps before. The name plate says 250 FLA. It will draw around 240 amps fully loaded. The reciprocating type compressor has 100% unloading ability. The rotating compressor assembly turns fairly easily so there isn't any large load at start up. The unloading sol. valves won't energize for 10 seconds after start. Of course this is in a PLC timer and is adjustable. The unloading valves are hydraulic in nature and depend on oil pressure from the crankshaft driven oil pump. That is the only control the PLC has besides closing the start circuit and monitoring the thermal overload relays. The transition timer is a little palm sized device attached to the contactor in the same manner as an auxiliary contact. I will adjust the timer to 5 seconds and then see what the amp clamp shows.

Click to expand...

Use the clamp you have. run in star and watch the current untill it levels out, the change over to delta shouldnt really occur until the compressor is up to 75-80 % speed minimum. As its an unloaded start id set the Wye Delta to a second before the unloaders kick in.

Even on an unloaded start a 300Hp will need more than 2 seconds to get away.

As for a clamp on that gives a curve in real time, i dont know of any, but if you have a scope then id go along those lines.

I have a wye/delta motor (40HP I think) that requires 9 to 15 seconds before the switch. The OEM suggested that we monitor current just as other here have said. He also suggested having two separate overload relays, since each mode has it's own current rating in our case.

This motor is used to crank up to speed, a 300 pound flywheel so the current takes about 5 seconds to settle down, much longer than typical loads. We set the time safely above our longest measurement (about twice).

How fast the motor comes up to 85% speed really depends on the combination motor+load.
I have had large blowers with y-D starters where the startup time was more than one minute !

Apart from that, as far as I understand the contactor 2S should only be activated for a very short time.
When the timer has finished 2S (resistors) will activate, next 1S (star) will be interrupted, next 2M (delta) will activate and finally 2S (resistors) will be interrupted.
I would check how long time 2S is on, it should be a fraction of second only. If it is on for too long then I suspect a defective auxilliary contact somewhere.

Hi,

I went in on Sunday and did some testing of the motor. First some corrections from my previous post. Motor FLA is 395. There seems to be no timer on the unloader coils. I replaced what aux. contactors we had on the shelf. I didn't have any late break type. I will get those ordered today. The aux. contact on 2M is supposed to be a N.C. late break type. It appears to be a regular type! Someone must have replaced it over the years with the wrong type or the drawing is wrong. I did some testing of the components with the multimeter at first, after that I left the 3 phase locked out and substituted an alternate source (extension cord) of power to the control circuit. Set the timer for 7 seconds and hit the "Hand" selector switch. Contactors 1M & 1S came on. 7 seconds later 2S came on briefly while 1S & 2M flip flopped. Seemed right so I removed all the testing tools, isolated the resistor leads with some shrink tubing, closed the cabinet door and reconnected the power. Time for the real test. The motor ramped in less than 2 seconds, energized the unloader valves, picked up a few tons of refrigerant and then a few seconds later switched to delta. It seems to work just as before the resistors burned up. The amp clamp showed that the current dropped off at around 1.7 seconds from what I could tell. At this point I think I'll just go ahead and run it the way it is after replacing the remaining aux. contacts and programming a few timers in the PLC. One timer to delay the unloaders until a few seconds after 2M has closed. Another timer to prove that 1S is not closed longer that 1 second. Maybe another to check that 2M is closed within 10 seconds from start command. I'll throw the resistors into the mix when they arrive next month. The only problem I can see about having the wrong type aux. contact is on 2M (normal instead of L.B.) is that the resistors (2S) will drop from the circuit a little earlier than planned. Is that a bad thing? I guess it kind of defeats the purpose of having the resistors in the first place. It is turned into a semi-closed transition.
Thanks for the help and don't hesitate to chime in if you see something I have screwed up.
Later BD