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Over Horsepower Motors

Happy you got to take in a Nooner, Techman. Now that you are again focussed on work :) ......

I would install only the appropriate motor for the job unless you are in a pinch and will replace it with the appropriate one as soon as you can. When a motor is unloaded it can burn out, drawing too much amperage for the duration, not to mention wasted electrical energy. Any reason you feel so compelled? Just curious.


  • Techman
    Techman Member Posts: 2,144
    Over HorsePower Motors

    Afternoon! Just stopped by the office for a Quickie, so here goes. I think I know that putting a 7 1/2 HP evap motor in a unit that had a 5HP evap motor(OEM size), woun't run very long , something happens to the motor.So going from ,say,an evap 1/2 upped to a 3/4 ? Cond motor from a 1/4 to a 1/3?Thanks ! Sucked up enough BTU's from office AC for now, so back out into it !Enjoy your day!
  • Techman
    Techman Member Posts: 2,144

    Hi Brad!The 7 1/2 HP on the 5HP unit was my last Bosses idea and it didn't work very long.The 3/4 in place of the proper 1/2 was a new customers 5 year old problem ,thatwas solved with the right sized motor , and the 1/3 cond motor in place of the proper 1/4 is an agreement I had with the motor shop mechanic .The wondering is all mine !Just trying to be a sharper mechanic,thanks!!
  • Christian Egli_2
    Christian Egli_2 Member Posts: 812
    No fan of the not non-overloading squirrel cage fan, not!

    I think there is something else going on with the fan in your evaporator. We are talking about a squirrel cage type fan, right? No? It seems the first motor burned out, then another larger one? Perhaps there is something very wrong with the RPM, the CFM air flow, the rotation, the pulleys, the capacitors (if?? one phase), or the voltages. Is there a lack of air ducts and thus lack of back pressure?

    Many possible problems.

    The way to tell at a glance if an electric motor is being overloaded is to see how hot it is. Hand burning hot is too hot, and if you have proper thermal protection on the motor, it should be popping out of service. All bigger motors need protection beyond just fuses. Also, since motors installed inside an air stream are very well cooled, naturally, you can guess which way some unscrupulous manufacturers may go as far as sizing their motors.

    *** Overloading an electric motor

    An electric motor can give out a whole lot many times it power rating - but just for an instant. This is how mechanical cash registers were electrified. Like automobile starters, the tiny electric motor is grossly overloaded, but you don't run it long, it doesn't get too hot to melt.

    Pedestal or bench grinders where a motor is stuck between two grinding wheels is an application where the motor needs to be way overloaded, you can't grind anything otherwise without the RPM slippage. Try it out, turn the grinder on and see how hand-burning hot everything gets within 10 minutes already.

    That said, 10% permanent overload is usually acceptable.

    *** Underloading an electric motor

    Underloading a motor is fine. If it were a crime, motors turning all by themselves with nothing attached should... ??? disappear in a flash of smoke??? They don't, they're very happy left alone! However, using a 300 hp motor to run a coffee grinder is an embarrassing waste of energy, mildly, to say the least.

    With underloading, you get a loss of efficiency. Roughly speaking, at a 60% load you're running with a 10% waste of electricity; a 50% load will drop you to 20% waste. If we believe (which I don't) 5 hp is your actual power need, and you cater to it with a 7.5 hp electric motor, you're then working your motor at a 66% load which still gives you less than 10% inefficiency. It's acceptable.

    *** Undervolting an electric motor

    You probably know how motors are labeled 208-220 V, leaving you some leeway, or so it seems. Yet the motor is exactly the same except for the label. It's just that we're counting on the +/- 10% tolerance from the utilities to solve our voltage problem. Motors get into serious trouble when undervolted. Thus this dual voltage motor is simply meant to run on a nominal 220V. When supplied with very standard 240V it will be a little overfed and everything is perfect. 250V is good too. On the low end dropping to 200V is the limit of acceptable for the motor operation. You see how you can plug this motor into a 208V outlet, and it will survive. It will be happy at 208+10%, 228V, but it will be starved at 208-10%, 187V. Under such conditions, the motor will overheat and self destruct. Is this perhaps what's going on?

    *** The non overloading fan and its opposite the ratty squirrel cage fan

    This is an HVAC application where low cost and low noise are highly desired, in spite of slight inefficiency issues. The type or fan you have is most likely a squirrel cage with forward blades that form cups throwing the air away. These fans are neat but dangerous because they are not the non-overloading type. How do you overload a fan? by allowing it to blow higher and higher CFM rates.

    Oppositely, non-overloading fans have power curves that peak and then demand less and less power as you increase CFM (and drop the head at the same time). Design the motor for the peak demand and you're safe. These fans have paddle like blades that slap the air. The backwards blade.

    And so, the squirrel cage fans are different. One, they are not as efficient, but mainly, as you increase CFM, power demand keeps increasing, up and up and up, until nothing stops the motor from melting itself in a sweat. With these fans, you have to throttle the flow.

    Is your air handler not connected to any ducting? Or is it perhaps lacking flow restrictors or filters? The ducting would normally restrict the CFM rate and thus the power demand, without it, you're overloading your motor. Squirrel cage fans cannot be run without the shroud and its flow restriction.


    There, that's about all I'm thinking of right now. Typical fans on outdoor AC units are the axial type. These have a whole other set of properties but they are of the non-overloading type and thus safer to deal with in a power struggle with the electric motor.

    This talk may all seem familiar to water circulator problems. Circulators are generally speaking of the non-overloading type. They'll either churn butter or suck themselves into a cavitation disorder if you don't throttle the flow or get a proper piping match.

    But with steam heat, there is no fans and no pumps to make your life miserable... Of course you have to be a fan yourself (and I had to come up with this whole lecture just to say that... :)

    Techman, I hope I helped your arguments at work. Best regards, Christian.
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