Soft start on air compressor

[hillbilly1]

I have a customer that is having issues with about 50% of the HPS hid’s going out every time the compressor starts. 30 hp line voltage starter, 208 volt three phase. It’s an obvious voltage drop issue, looking at what options I have. The compressor people say it’s working perfect, but I hear it struggling to start, and I don’t hear the unloader firing after it shuts off. Inrush is 333 amps. POCO is a little high (216 volts) , but drops to 200 on start up at the service, but running is at 208. Looks like they have one or two parallel 4/0 aluminum 150’ to the service, which is 800 amp. The compressor feeder is also a little on the small side, as it is #1 cu @ 130’. Anyway, can softstarts be used on air compressors? I’ve used them on fire pumps and large refrigeration compressors, but not on piston type air compressors.

 

[Barbqranch]

I would first make sure that it unloads so the head pressure is 0 before trying to start. We have a 15 hp compressor, and it starts without struggling.

 

[drcampbell]

If the unloader's functioning correctly, a soft-starter is not necessary.
If the unloader's not functioning correctly, a soft-starter probably won't work.

 

[ptonsparky]

Unloader first.

333 amps would not be unusual for that size of motor but length of time it stays there would be a problem. #1 is adequate.
2 - 4/0 conductors for an 800 amp service?

What is the voltage at the fixtures when they drop out? 200 should not be a problem.

 

[ActionDave]

I agree with checking the unloader.

How many starts per day? Put in a bypass valve on the air line, get rid of the pressure switch, and let the compressor run all day.

 

[Cow]

I agree with the others, the unloader needs to be addressed first.

Have the lights going out always been a problem since the compressor was installed, or did it just start more recently?

 

[hillbilly1]

Just got in from the job, I too believe it’s the unloader, but if their compressor people come back and say it’s not, then I’m looking for the next step. While I was doing the four hour drive home....... I got to thinking, it was weird that it was only dropping part of the lights, not all. They are using the breaker to turn them off and on, so it’s not a magnetically held contactor dropping out. They just moved into this building, and all of the bay lights are working.........that’s unusual, because most landlords and previous tenants tend to not fix anything before they leave. Hmmm, I wonder if the contractor that repaired them, used the 240 volt tap on the replacement ballasts, instead of the 208? Since the poco is a little high (216) the fixtures would fire. That would have a net 40 volt drop on those fixtures when the compressor starts. I’m having my local tech go back out with a lift to check that. I will get back with that answer later.

 

[hillbilly1]

I agree with the others, the unloader needs to be addressed first.

Have the lights going out always been a problem since the compressor was installed, or did it just start more recently?

Compressor was existing, new tenant in building, so I don’t know if it was a problem with the previous tenant.

 

[hillbilly1]

I agree with checking the unloader.

How many starts per day? Put in a bypass valve on the air line, get rid of the pressure switch, and let the compressor run all day.

Yeah, at 70 amps, they would love that power bill! Looks like it is used only a couple of times a day.

 

[hillbilly1]

Unloader first.

333 amps would not be unusual for that size of motor but length of time it stays there would be a problem. #1 is adequate.
2 - 4/0 conductors for an 800 amp service?

What is the voltage at the fixtures when they drop out? 200 should not be a problem.

Utilities get away with a lot smaller wire than we can! LOL! Building load without the compressor was 50 amps per phase, poco was probably using load history to size their wire.

 

[hillbilly1]

If the unloader's functioning correctly, a soft-starter is not necessary.
If the unloader's not functioning correctly, a soft-starter probably won't work.

It would be for reducing voltage drop on the service, not so much for easier motor starting. Doubt getting utility to upgrade their lines. Direct burial cables not in conduit. The store manager said the fiber contractor cut one of them and just put duct tape on the end of it! Poco found it that way, and properly repaired it.

 

[ptonsparky]

It would be for reducing voltage drop on the service, not so much for easier motor starting. Doubt getting utility to upgrade their lines. Direct burial cables not in conduit. The store manager said the fiber contractor cut one of them and just put duct tape on the end of it! Poco found it that way, and properly repaired it.

Time to check that underground again. We repair a lot of repairs along with the ignored damage that was done when the first repairs were made.

Back to the original problem. You know the first thing you need to do by now.

 

[drcampbell]

It may be necessary to replace/augment the unloader with a slow-closing solenoid valve to use a soft starter, unless the soft starter can provide full torque at reduced speed. (which pretty much means a full VFD)

Most unloaders close the switch contacts and the bleed valve simultaneously, so the compressor will be almost fully loaded as soon as the unloader tube is filled -- one revolution or less. Meanwhile, the motor will not be up to speed yet; still soft starting.

 

[hillbilly1]

It may be necessary to replace/augment the unloader with a slow-closing solenoid valve to use a soft starter, unless the soft starter can provide full torque at reduced speed. (which pretty much means a full VFD)

Most unloaders close the switch contacts and the bleed valve simultaneously, so the compressor will be almost fully loaded as soon as the unloader tube is filled -- one revolution or less. Meanwhile, the motor will not be up to speed yet; still soft starting.

That’s what I’m afraid of when using a soft start in this application. Didn’t know if it had been done before, and figured if it had, somebody on here would have done it, and know the pitfalls.

 

[StarCat]

Typical unloaders on " reciprocating " air compressors are mechanical and integrated into the operating pressure control. There is a tab which moves with the switch mechanism that trips the unloader valve when the switch opens. You should hear at least a 5 second bleed off depeneding on the size rig you have after the cycle. These valves are know to be a problem if the system is outside in freezing weather because there is always moisture in the air stream that runs through them. Heat cable can work its its done carefully, but these units are mostly meant to be in mechanical rooms. I recently retrofitted 2 24 CFM units to electric unloaders and solved some issues. The unloader valve itself can fail, but usually the failure is open and not closed. On some switches they can be replaced, and I have used a Square-D unloader vakve to repair an off brand pressure switch in a pinch. If the unit is not unloaded at start it can create some unusual destruction of the starting contactor that is atypical. In the recent past I found an inrush calculator on line that is somewhat useful if you have all the necessary motor nameplate data. I found the normal inrush on these 7.5 HP units to be less than what was projected which is not a bad thing. The electric unloaders are pretty slick and work very well for setting up a dual crosswired alternating array. You employ a Normally open Solenoid valve which closes with the start of the cycle and install an orifice on the outlet so you get the correct bleed off time. There is an outfit called Tameson that has Solenoids rated for that duty and pressure. They are not high end but ok. You should be able to witness the unloader on this type of system bleed off for a few seconds after the cycle ends. This is critical.

 

[kwired]

If unloader isn't working you possibly may not be able to start it at all.

2 - 4/0 is plenty of conductor for this motor- but means little until you figure what other load is at the time. Size of transformer could be a contributing factor as well. Just because you have 800 amps service disconnecting means don't mean the demand isn't or has been in the past low enough they only have a 75 kVA transformer.

As said even with soft starter this high torque type of load may still need significant amount of current just to get it going and is possible you could need VFD just to get more torque at those lower starting voltages

 

[hillbilly1]

My tech checked the lights, and they are tapped for 208. Customer refuses to have unloader checked, ( I think they do their own maintenance since it is a rental repair facility) so I’m just going to quote them a soft start and upgrade the wire and conduit size and be done with it. All I can do at this point.

 

[ptonsparky]

I have never had anyone come in and check the unloader for me. It was done by me or my help as part of the trouble shoot.

 

[hillbilly1]

W

I have never had anyone come in and check the unloader for me. It was done by me or my help as part of the trouble shoot.

We don’t go any further than electrical, it’s always the mechanical contractor to troubleshoot and repair anything on their end. Of course all of our customers are big corporations. You know, the bigger the company, the smaller the brain! LOL!

 

[kwired]

W

We don’t go any further than electrical, it’s always the mechanical contractor to troubleshoot and repair anything on their end. Of course all of our customers are big corporations. You know, the bigger the company, the smaller the brain! LOL!

But when the complaint is an electrical problem and you determine the real cause is a mechanical issue - you need to be right or you look bad in the end. Many times had motor overloads trip and had to find the mechanical issue and explain why it trips the overload, even if it is as simple as you can't run/feed the item that fast and/or with that much material in/on it.