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Three phase motors

motor internally protected? Most 3phase motors are not. If the motor is not internally protected it should not be controled with a "contactor". It should be controlled with a magnetic starter which contains the overload protection. Two pole three phase mag starters have not been manufactured for quite a while. They had a major fault in that they do not prevent against single phasing. Maybe someone has changed the motor and not changed the heaters in the mag starter to match the current draw of the new motor. Sometimes if the ambient temperature at starter location is different than the motor it will effect heater performance. I'm just trying to think of things that will cause problems when amp draw is less than nameplate.


  • Barrington White
    Barrington White Member Posts: 37
    Three phase motors

    What could cause three phase motor to overheat. A three phase motor on a belt driven blower. 208/230 v, 6.3 amps. But motor is only pull 5.8 amps. The winding are giving off 2.2 ohms. The wires are good I'm getting 2.4 ohms off the wires. It connected to a 2 pole contactor.
  • Techman
    Techman Member Posts: 2,144

    Happy 4th to All!The amp's are ok,if it is pretty much the same on all 3 legs and the volts are very close L1-L2, L2-L3, L1-L3 .And the resistance is darn near identical 1-2,1-3 ,2-3. A lot of belt drive motors are "open" motors which means they have an air opening[s] for ventilation ,and an internal fan blade to cool off the windings. Blocked vent holes or dirty fan blade can lead to overheating.Or some "drive" pullys have their own rotation sensitive "fan blades".Or a few other possibilites!
  • Empire_2
    Empire_2 Member Posts: 2,343
    3# Phaze motors

    As long as you are under or at the name plate amp draw, you should be OK. The problem probably occurs when you are not there and can be due to many factors. Condition of the contactor, possible phase imbalance, brown out or spike in voltage etc... The do make a control called a Linebacker which will monitor your power source to the load and kick it out if there is a problem. It also tell you what the problem was that occurred. Little on the pricey side, but well worth it.

    Mike T.
  • Empire_2
    Empire_2 Member Posts: 2,343
    Hello Bob

    I have seen, way to often manufacturers only install 3 pole contactors w/ no overload protection as you described. That's to cut cost. When you get into upper end equipment they are installed,.which I prefer. After about 5 HP or so you will always see them which is great.....

    Mike T.
  • Christian Egli_2
    Christian Egli_2 Member Posts: 812
    Qualify overheating

    Olden motors were built as big as washing machines and they were cool. Nowadays, marketing brochures tout the so-called modern efficiency of super motors that can be built real tiny and have huge nameplate horsepower. There is always progress of course, but such miniaturization comes at your own risk. The more modern the motor is, the more it gets really hot.

    If you believe the horsepower rating has been faked a bit too much on the label, then install a new motor with a bigger physical size and now what would seem a way oversized label.

    Check with the manufacturer on what they think is too hot, then measure your motor. Too hot to touch with the hand has sadly become normal, particularly in hot summer weather. You know it's too hot when it starts smelling like it's fried, and then it's too late too.

    Thermal protection that works is a must.

    If you are on 208V, then visible overheating is that much more to be expected. If you have a transformer with taps, you could try bumping up your nominal voltage.

    For the rest, kiss and hug your power meter for the three phase you have and be extremely glad you are not fighting with a single phase motor and all the baggage that comes with that. I love three phase power and too bad for Edison and his elephants. Tesla knew it, Morgan made it happen.
  • Barbarossa
    Barbarossa Member Posts: 89

    Don't forget Carl August Rudolph Steinmetz when waxing fondly over Telsa.
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