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Thread: electrical over current protection

  1. #1
    Bronze Member mikilianis's Avatar
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    electrical over current protection

    I have a problem that I must please explain to an irate engineer ( must be a mechanical ) why he must pay for 8 new 6 pole 2.2kw extractor fans. What caused them to burnout, the Gent.wants to know if the correctly rated fuses were used. Now let me explain the circuits in question , the fans, 16 in all ,are part of a heat pump producing hot water for a hospital, they are exposed to the elements and are about 7years old. I do not know the I.P. rating but I suspect the I.P. protection is diminishing as for protection there are two fans in parallel and supplied through a C H G G 10 ( info on circuit diagram ) in addition there is also circuit protection via thermistor.Now my argument is that the fuses ( slow blow or delay blow I do not know ) are not there to protect the motors and should one blow as in this case, the motor will continue to run untill the thermistor detects an increased temp in the windings and cuts the circuit out,not very efficient as the motor has a constant draught passing over the housing so it won`t overheat instantly ( being an extractor fan and extracting cold air ) so what we have is not a very effective protective system, What is required is a 3 Phase D Curve m.c.b. and or a motor protection circuit breaker like a telemecanique G V M 2 or whatever make, there are plenty on the market, Now what I would like to know, What is coarse current and fine current protection.Does anyone have any more info on fuses types and action.
    Thanks Mikilianis

  2. #2
    Diamond Member AndyD's Avatar
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    Circuit breakers and fuses don't make good protection for motors because you have to size them to accommodate starting currents which can be at least twice the running current depending on the motor. A direct on line starter is commonly used protection. The thermal overload in a DOL starter is specifically designed to protect motors against most common causes of failure such as phase failure, locked rotor etc.

    Thermistor protection is very effective if it's embedded in the windings and correctly connected, their response times are surprisingly fast.

    If the failures are mechanical ie bearings, check for reverse running which may (depending on blade-set characteristics) cause marginal current over draw which in turn causes higher motor temperatures which may or may not trip the thermistor circuit. Also variable frequency drives can cause induced rotor voltages which lead to premature deterioration of bearings.

    All said and done, seven years external use doesn't sound too bad for a motor.

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    Platinum Member desA's Avatar
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    On my heat-pumps operating in SE Asia, I install phase-protection voltage hi/low devices. If the incoming voltage exceeds set hi/low settings, the system stops for a pre-set time. (The main reason is to protect the compressor, which is probably around similar rating to your fans. Of course, compressor start-ups are tougher than fans, but the principle remains the same.)

    After this time-out, the system tries to re-start. If the voltage is within target range, the system will operate, if not, the system will time out again.

    This allows the motors to cool down after each re-start. The trick is to use a decent time-out period, to protect the system from overheating. These devices are not very expensive - perhaps a few hundred rands each.

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    Diamond Member AndyD's Avatar
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    With refrigeration compressors nearly all manufacturers stipulate a maximum number of starts per hour due to the heat produced during the start cycle. There's lots of other factors in refrigeration such as sub-cooling from saturated vapour return through the suction side etc which affects motor temperatures.
    This wouldn't be so critical with fan units which I'm assuming are TEAO motors. Mikilianis, you say 'info on circuit diagram' can you show us the diagram?

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    Platinum Member desA's Avatar
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    Quote Originally Posted by AndyD View Post
    With refrigeration compressors nearly all manufacturers stipulate a maximum number of starts per hour due to the heat produced during the start cycle. There's lots of other factors in refrigeration such as sub-cooling from saturated vapour return through the suction side etc which affects motor temperatures.
    The main issue would actually be the compressor suction temperature. Too hot & the motor windings run into trouble. Sub-cooling, in itself, is not an issue to worry about. Compresor discharge temps at start=up tend to lag due to thermal inertia & so are not usually a factor, unless the restarts are too frequent.

    The maximum number of restarts is typically around 6-10 re-starts per hour. This is easily managed with a timer.

    Big fans, with large blades, & lots of inertia, will be sure to have some kind of current limit. As they age, & bearings tighten up, the startup load will probably need taking care of. Timers allow some level of re-start staging - this can be useful.
    Last edited by desA; 18-Jan-10 at 02:03 PM.
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    Diamond Member AndyD's Avatar
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    Quote Originally Posted by desA View Post
    The main issue would actually be the compressor suction temperature. Too hot & the motor windings run into trouble. Sub-cooling, in itself, is not an issue to worry about. Compresor discharge temps at start=up tend to lag due to thermal inertia & so are not usually a factor, unless the restarts are too frequent.

    The maximum number of restarts is typically around 6-10 re-starts per hour. This is easily managed with a timer.

    Big fans, with large blades, & lots of inertia, will be sure to have some kind of current limit. As they age, & bearings tighten up, the startup load will probably need taking care of. Timers allow some level of re-start staging - this can be useful.
    I'm not sure about the merits comparing compressors and fans starting characteristics, they are a completely different animal for many reasons.

    Almost all compressors from 1/4 horse (300watts) upward have assisted start PSC, CSCR, RSIR etc. Even gas pressure equalization aside, a compressor has a considerable mechanical load from the crank, con rods, pistons, valves etc from zero RPM onwards.

    Fans would be DOL start all the way up to 5kW 4-pole depending on blade-set characteristics. Only then would a Star Delta starter or a soft starter by employed and sometimes even then the deciding factor for this is often the ability of the blade to withstand the mechanical stresses of direct starting. The inertia of the blade-set isn't really a major factor from a motor point of view and the load of the blade-set from the air movement only becomes a factor at higher revolving speeds by which time the motor has achieved better torque characteristics. The only point of friction in a direct drive fan is the two motor bearings (maybe a rubber dust seal). These bearings are always very low drag (not tight), any friction produces lots of heat at 1440 rpm for a 4-pole motor which breaks down the lubricant and catastrophic failure rapidly ensues.

    For any readers that are interested in a fantastic article about very basic motor theory that's not too condescending try this one.

  7. #7
    Platinum Member desA's Avatar
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    The OP mentioned a heat-pump application, in a hospital.

    With this kind of application, the fan operation is often controlled in order to maintain a certain air movement over the evaporator. The control could be simple on/off, or speed control - via a number of mechanisms. It would appear, from the OP, that a bank of fans is used.

    This method of operation would tend to be a lot more onerous in terms of motor life, than a simple continuous run operation. If voltage-control is used, for instance, & the supply voltage were to be low, this could lead to premature motor burnout.

    I'd say, that if the OP were to provide more information on his particular application, the merits, or de-merits, of fan control strategy could be further developed.

    Wherever the quality of power supply voltage is in question, the use of over/under voltage protection, together with time delay, is recommended in the SE Asian region. Based on what I've been hearing about SA's power of late, similar issues would be in play.
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    Diamond Member AndyD's Avatar
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    Quote Originally Posted by desA View Post
    This method of operation would tend to be a lot more onerous in terms of motor life, than a simple continuous run operation. If voltage-control is used, for instance, & the supply voltage were to be low, this could lead to premature motor burnout.
    Many refrigeration fans are external rotor motors. The whole blade set of the fan is actually manufactured as part of the external rotor. Because the rotor, where a large percentage of the heat is developed, is directly in the airflow and because there is a massive amount of air passing over the motor you can use voltage to control speed. Manufacturers such as Systemair supply only voltage transformer type speed controllers for this type of fan but they do state a minimum rotation speed of around 600 rpm (from memory) on a 4-pole motor.

    Under any other circumstances though, I totally agree that voltage speed control is not a good idea.

    I also would be interested to know more details.

    Were 8 out of 16 motors faulty due to a single event or was this an accumulation of motors going down over time to the point where maybe an HP switch kicked out bring this to light?

    Were all the failures similar in nature and were the failures electrical or a combination of mechanical and electrical. Did anything stand out if any of them were workshop post-mortemed?

    Why is the customer of the opinion that a motor of seven years in age is to be considered premature or preventable failure.

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  10. #9
    Platinum Member desA's Avatar
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    Excellent comments AndyD.

    Perhaps it's time for the OP to update with further details.
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