High starting currents from A/C compressor.

Status
Not open for further replies.
Hard start kit plus a soft starter on same compressor motor somewhat conflict with one another I would think.

First thing is making sure you are not starting with residual pressure in the system from the last run. Delay timers made for this are pretty inexpensive, maybe only $10 -15 at HVAC supply house. If you still need hard start kit when there is no residual pressure, you probably have a compressor that isn't going to last much longer. Low volts, undersized circuits, small transformer - all add some the characteristics of a soft starter, but if low volts is on the service/feeder and not just the branch circuit other items will see the effects as well and will be easily noticed with lighting particularly LED lighting.
 
There are two initial current profiles on an AC induction motor; the “starting current” which is associated with the LRA, and what iriginally as called “inrush current”, which is the same as inrush current in a transformer. It is the magnetizing current inrush that takes place in the first half cycle of applied voltage where there is no mutual induction yet, only the resistance of the motor winding wire to slow down the rise of current to the level of the Available Fault Current in the system at that point. This magnetizing current can be as high as 20x the FLA, but is of such short duration that it is rarely discussed. Back when we all used analog meters nobody saw that current outside of a lab, because the meters were too slow to react. But now with high sampling rates on digital meters, it can get picked up on “peak” settings. I suspect this is what you have seen when you read over 200A on startup. In other words that’s an artifact of the type of meter you are using, it’s not unusual in and of itself. Magnetizing current is usually not going to cause flicker because of the extremely short duration. I would not keep pursuing this as a cause if it were me.

it’s highly more Ike’s that there is a loose or high resistance /corroded connection somewhere in the lighting circuit that is making the LEDs flicker and the slight voltage drop from the real starting current of that compressor is being amplified across that high resistance connection. But because LEDs draw so little current when running, it’s not showing up on a thermal scan.

*The term “inrush current” has become synonymous with starting current but was initially only referring to magnetizing current. So please don’t clog up the thread with attempts to correct me by posting all of the definitions that have adopted that meaning, I get it.
 
Jraef said:
*The term “inrush current” has become synonymous with starting current but was initially only referring to magnetizing current. So please don’t clog up the thread with attempts to correct me by posting all of the definitions that have adopted that meaning, I get it.
Click to expand...
This isn't a plumbing forum, lets not talk about clogs :)
 
210928-1644 EDT

Jared:

Can you provide a scope single shot display of an induction motor that displays any inrush current. I have only seen starting current.

At locked rotor a motor looks like an RL series circuit. At application of power to an LR series circuit the initial current is zero, if there was no stored energy in the inductor at t = 0. Initial first cycle current remains relatively constant until the motor starts to reach a reasonable speed.

See my discussion on "1-phase induction motor starting current" posted in the General Electrical Forum on 27 Feb 2016. I have some actual waveforms shown there, and absolutely no inrush current. These do clearly show starting current. This thread also shows that various persons have completely different definitions of the same words, and that others totally lack an understanding of electrical circuit theory.

.
 
Was the HVAC tech made aware of this issue at the time of inspection, or were they just there to preform a basic preventative maintenance? If they weren’t made aware then they should be called back so they can troubleshoot the issue. They most likely wouldn’t pick up on something like that at a maintenance unless the concern of dimming lights was specifically mentioned. It’s more involved to troubleshoot something like that and most people don’t want the time spent investigating that sort of issue if it doesn’t noticeably exist.

I’ve seen more than once where thermal imaging misses poor connections. At this point I would be getting my hands on tools to verify the torque on the lugs and make sure nothing is loose.
 
pjones said:
Was the HVAC tech made aware of this issue at the time of inspection, or were they just there to preform a basic preventative maintenance? If they weren’t made aware then they should be called back so they can troubleshoot the issue. They most likely wouldn’t pick up on something like that at a maintenance unless the concern of dimming lights was specifically mentioned. It’s more involved to troubleshoot something like that and most people don’t want the time spent investigating that sort of issue if it doesn’t noticeably exist.

I’ve seen more than once where thermal imaging misses poor connections. At this point I would be getting my hands on tools to verify the torque on the lugs and make sure nothing is loose.
Click to expand...
They still more likely to do a couple basic tests and say it is beyond their ability and to talk to an electrician. If it starts, running amps and volts are within acceptable range they won't do much more. VFD driven or ECM motors they maybe get more involved in, but those won't have a high surge of starting current from the supply line either.
 
kwired said:
They still more likely to do a couple basic tests and say it is beyond their ability and to talk to an electrician. If it starts, running amps and volts are within acceptable range they won't do much more. VFD driven or ECM motors they maybe get more involved in, but those won't have a high surge of starting current from the supply line either.
Click to expand...

Like in any field, it depends on what tech you have and their experience. I know many that would knock this out of the park easy…. Some others wouldn’t know where to start. Find the right tech and your problem is solved.
 
Status
Not open for further replies.
Top