Motor starts/hr.

Status
Not open for further replies.
K

kthbrwn

Member
Have a motor (not original)on a climate control compressor set-up, that is cycling way more than it probably should. Right now going to assume it is excessive air leaks throughout building, i.e. system was sized correctly.
This problem came to attention when the circuit breaker for electric motor was found tripped. Is it the thermal portion of the c.b. "tripping" because it to does not have time to cool off from the inrush currents?
I ask because I don't think the windings have shorted or ground out yet, seeing how the c.b. reset and the motor runs.
 
Re: Motor starts/hr.

If a compressor starts with high head pressure, the CB might trip, or the thermal breaker on the compressor might trip.

If the unit is cycling too often, that could mean that there is a problem with the condensing unit, say a fan or restricted air flow, or perhaps too little or too much refrigerant.
 
Re: Motor starts/hr.

Another thought/question on the occasional tripping of the 20 amp c.b.
The motor is a 3/4 hp, 115v. Table 430.148 has full load current of 13.8.
The breaker is a SqD QO I believe, at least it's in a NQO series panel. These are considered a inverse time breaker, correct? Therefore if the breaker is increased to a 25 amp (table 430.52 allows up to 250 percent), perhaps this would take care of the trip.
By the way, this has happened to another unit with identical motor and a 20 amp breaker.
What consequences should one weigh when considering upsizing breakers as I suggest.
The units cycle about 8x an hour and are in the room with the boilers, i.e. it gets warm. Are these conditions contributing to my problem?
 
Re: Motor starts/hr.

Generally I would consider a regular 20A QO bkr.
as bordering on too small for a 3/4 HP motor like you describe.
Do not hesitate to use a 25A or 30A.
There would not be a real disadvantage to the larger branch ckt. OCP.
I assume the motor running overload protection is properly provided.
 
Re: Motor starts/hr.

What you must determine first is to whether the breaker is tripping thermally of magnetically. A breaker which trips thermally is most likely warm to the touch but even more so it can not be reset and closes immediately after it trips as it must be given time to cool down.
A breaker that trips instantaneously can be reset and closed immediately.
Here's is the way I look at it. The TM breaker must be considered from its magnetic calibration aspect not the thermal when considering 430.52. When looking at the article are you applying the breaker because of its thermal (overload) protection capability or its magnetic (intantaneous) for short circuit protection?
The thermal element of a breaker is really not intended for motor protection. If the TM breaker is tripping because of thermal its trip curve is, with almost all certainly, inside of that of the overload relay or internal thermal protection which should be integral to your motor. If the motor was in danger of being overheated either on OLR of the internal thermal protection integral to your motor should provide protection if the motor was started to frequently.
Because neither an OLR nor the internal thermal protection in the motor can detect a motor fault that's where the magnetic element of the breaker comes into play which provides shirt circuit protection. If your installation allows you to go up towards that 250% what you are looking for is an advantage of the higher magnetic calibration which is usually 10x the rating of the breaker (+-20%.
I assume that you have already considered if the manufacturer has specifies a HACR rated TM breaker size similar to that which is often specified with common home central A/C units and it is not a requirement.
Also, you're correct in stating that a breaker can derate in warmer ambients which makes it prone to nuisance tripping but that's what it's suppose to do because the wire there is also warmer.
 
Re: Motor starts/hr.

You may have another issue soon. For every 10 degrees above the motor temperature rating that the motor operates in it is "half-lifed". In other words it will only continue to operate for a term that is 50% of the designed life of the motor. Heat increases resistance and ampacity increases. Starting current is not the problem, short cycling is. :)
 
Re: Motor starts/hr.

by templdl:
If the motor was in danger of being overheated either on OLR of the internal thermal protection integral to your motor should provide protection if the motor was started to frequently.
Click to expand...
I believe this would be correct, and since my motor's internal overload did not trip, I tend to think a 25 amp breaker is better suited for this 3/4 hp's start-up.
Aceman, can you tell me at what point the act of "short cycling" begins with a motor/belt driven compressor. I was told 6x/hr., read no more that 12x/hr..... I'm not sure.
 
Status
Not open for further replies.
Top