Motor Starting Current

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big john

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Portland, ME
This was in a question-and-answer newsletter I received from this site:

Q7. The lights in my house blink every time my new 5-ton Carrier AC unit starts (180 Amps on start). All conductor terminal connections are secured, so I know that's not the problem. The builder replaced the 6 AWG aluminum to the compressor with 1 AWG, carrier changed the compressor to ensure it was not sticking, the utility replaced the transformer from 25 kVA to 50 kVA, and they changed the service from 2/0 to 4/0 cable. Why do the lights still blink in the house?

A7. The lights blink because the start up inrush current (180A) of new more efficient air-conditioners causes a short-pulse voltage drop on the service conductors. What can be done? Nothing.
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I'm amazed by that. The capacity of the service was doubled and it did nothing to reduce the problem. How would you calculate a resolution to that problem? In large buildings, all the office lights don't dim every time an air-handler fires up. What is done differently there than had been done in this case? Is 180 amps the peak instantaneous start-current or does that actually approach infinity?

-John
 
Re: Motor Starting Current

A 5 ton unit is big for most houses under 2500 square feet. A 5 ton unit should have higher starting current than indicated.

What is the FLA of this unit? The circuit breaker and disconnect switch should be examined. Measure voltage drop across each one.

This may be one of those "learn to live with it deals"
 
Re: Motor Starting Current

Bennie,

Unfortinately, I don't have any more information than what was available in that quote. On a side note: What does the measurement "tons" refer to concerning airhandling equipment?

-John
 
Re: Motor Starting Current

One ton of refrigeration is 288,000 Btu of cooling per 24 hours.

One ton of refrigeration is equivalent to 12,000 Btu/hr cooling capacity (i.e.- 288,000 Btu's divided by 24 hours equals 12,000 Btu/hr.).

For five tons, just multiply times five.

I hope this helps.

History of tonnage: One Ton of refrigeration is the amount of heat required to melt one ton of ice in a 24 hour period.

[ September 29, 2003, 02:22 AM: Message edited by: awwt ]
 
Re: Motor Starting Current

John,
The difference between the house and the office building is the size of the transformer that is feeding the system. The light dimming problem is a voltage drop issue and a good portion of the voltage drop that occurs when a motor load is started is the voltage drop within the transformer itself. While the house was being served by a 50kva transformer, it would not be unusual to have an office building served by a transformer 20 to 50 times larger.
In extreme cases, a week primary feed to the transformer can also contribute to the voltage drop, but that is not very likely with this small of a load. I did work in an industrial plant that had a 4500 hp motor and they were able to change power companies as a result of deregulation. The original company fed the plant substation with a 138 kV feeder and there were no problems. The new power supplier fed the substation with a 33 kV feeder. When they started up the 4500 hp motor the voltage would drop low enough that many of the other motor starters would drop out because the voltage sagged below the "hold-in" point on the 3 wire start/stop circuits.
Don
 
Re: Motor Starting Current

The 4500 hp was on a rolling ball mill at a cement plant. It is used to grind the fired limestone into cement powder.
Don
 
Re: Motor Starting Current

don; i have simuair experance with large electic motor it have 2500 hp large frame a.c. motor it was set up to run on 4160 volts every time the company have to start it. they have to info the POCO about 10-15 min lead time before they throw the switch on it due the line capitcy .


merci , marc
 
Re: Motor Starting Current

It has been a long time since I had to deal with this situation so I may be out in left field. In an industrial or commercial application, isn't it common to install capacitive banks to help offset inductive load? I believe most residential situations don't take this into consideration since it is usually quite minor. Is it possible that with the increased use of larger inductive loads as from A/C compressors, etc., we should be looking more closely?

Bob
 
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