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  1. #1
    Join Date
    Jul 2012
    Posts
    18

    amp draw on universal air handler motor

    Please excuse this long question, and it being a strange thing to ask about. I wanted to be complete about the whole situation so the events and the system are described as best as possible.

    I read the thread on universal replacement motors
    http://hvac-talk.com/vbb/showpost.ph...90&postcount=1
    and noticed no mention of amperage drawn by the motor.

    My a/c guy put a universal replacement motor in my Janitrol (Goodman) GMP-075-3 furnace after the 20 year old G.E. 1/3HP 1075RPM motor's bearings finally started to make noise. He didn't have the OEM GE in stock on the truck, and I didn't want to wait lest the motor actually stop running over the weekend. My a/c contractor is great, worth every cent, especially when the a/c decides to act up on a Friday. The new universal replacement barely fits BTW. It is quite long for the application.

    The original blower motor drew about 4.5A, but the new one draws 8A. The paperwork says 8 Amps but says nothing about amps vs. HP. This is the type of 1/3 1/4, 1/2 3/4 1HP motor where the capacitor size determines the horsepower. He used the size called for in the new motor's papers to do 1/3HP as spec by the furnace & the original motor. I suspect it is capable of running more than 1/3HP because there are only two capacitor choices, and the wheel size would make a difference in HP produced.

    It's all interesting, but I'm not thrilled about the extra 3A being drawn (for what -to make the cool humming sound as it spools up?) and we'd thought this was an anomaly but another one draws the same Amps in another furnace. He does not have an explanation for this and there is nothing technical in the literature but a hookup chart.

    Using only an amp clamp and no way to check power factor it's not possible to tell if the true Wattage is close to 1KW or is just a case of 1KVA with the true Wattage being more in line with 245 or whatever it takes to make 1/3HP.

    OK so finally the question please:
    Have others seen this higher amp draw on 1HP universal replacement motors operating with a low-HP capacitor selection?
    Is this considered normal?

    Thank you,

  2. #2
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,776
    Does it seem like your getting a ot more air flow?
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    How many times must one fix something before it is fixed?

  3. #3
    Join Date
    Aug 2004
    Location
    North Richland Hills, Texas
    Posts
    14,914
    IMO, a 1 hp "rescue" type motor is a very poor choice of motors when replacing a 1/3hp motor. I'd only do it as a temporary fix if I was out of the appropriate motor.

    That type of motor virtually always gives less than desirable performance when used at the lower end of what it is rated for.
    They make to much noise, use more electricity, generate more heat, and limit the speed taps available for tuning the airflow for the cooling and heating modes.
    The parts cost is also roughly double the cost of a 1/3 hp GE service replacement motor.
    If more government is the answer, then it's a really stupid question.

  4. #4
    Join Date
    Jul 2004
    Location
    Massachusetts
    Posts
    6,829
    I'd suggest that if you're not happy, you should be speaking with your AC company about the repair and your dissatisfaction. If they're a good company and you say they are, then they'll most likely be more than happy to get the correct motor. Many companies have moved from specific motors to some type of universal motor due to limited space on trucks to stock a bunch of motors. In most instances, having the correct substitute motor isn't a problem but in your case, I'd agree that the motor selected is probably not the best choice. We use ECM brushless motors as our standard. Most of the time they work out fine, until we run into a system with very high static pressures. We don't carry the 1.0-HP on our trucks as we just don't replace that many 3/4 or 1.0-hp motors so the smaller, 1/6 to 1/2 hp Evergreen covers our needs. The only time we need to get OEM is for some of the more unique blowers with the one-off mounts that never match up to any of the universals. Sometimes, you just can't win and the homeowner has to wait for the OEM part.
    If YOU want change, YOU have to first change.

    If you are waiting for the 'other guy' to change first, just remember, you're the 'other guy's' other guy. To continue to expect real change when you keep acting the same way as always, is folly. Won't happen. Real change will only happen when a majority of the people change the way they vote!

  5. #5
    Join Date
    Jul 2012
    Posts
    18
    Thanks for these replies and good points.

    1. Since the job was done I have a very noticeable increase in airflow. I like that. I don't know if the increase is due to the the removal of a thick layer of encrusted lint and old cat hair that he pulled off the blower wheel's blades, or to an increased speed of the wheel, or both

    Neither of us has a tach. What might be an acceptable overspeed on a 1050RPM blower wheel? I ask because we will possibly measure the RPM. (if not with a tach then with electronic and optical equipment I have for other reasons)

    2. rescue motor=poor choice for low HP. I agree. It was an emergency -so we had to make do with what was on the truck.

    3. I've already been speaking with them about it and there is no conflict. The post was not meant to express dissatisfaction but to learn about these motors. He told me that if I wasn't happy with it, he'd put the GE in but was out of them at the time. Without exposing prices, the rescue motor was an overly expensive motor for a 1/3HP application in my opinion. I'll likely go to the original if we can predict it will make sufficient airflow.

    4. Static: I don't know what pressures are considered high in this case. With the current arrangement, pressure in the plenum is +0.25" H2O, and in the return plenum it is -0.35". I regret I had not written down the previous values.

    The system is a 3 tons, cooling a poorly insulated house built in 1948. Is that a reasonable set of pressures? It does make very good airflow, and the system works better since the new motor was installed.

    But I refer again to #1. We don't know the wheel speed and so can't judge 100% why the flow is better.

    Later it maybe be possible to further improve the return pressure but the existing return setup is too weird to describe to the point where it needs pictures and then y'all would laugh. At least I am not responsible for that design but that's another story.

    Thank you!

  6. #6
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,776
    The supply pressure(static) needs to be read between the A/c coil and the furnace. good chance its twice what you read above/after the coil.

    How much faster is ok, depends on the SF of the motor.

    I think he has the motor set up wrong. And its running at 3/4HP rating, or close to it.
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  7. #7
    Join Date
    Oct 2010
    Location
    Baton Rouge, LA
    Posts
    1,113
    If its suppose to draw 8amps and is drawing 8 amps then its working properly. The unit and electricity surrounding the fan doesn't change the factory design but it can negatively affect it. If you are getting the OEM design then you are getting what you should.
    If you're too "open" minded, your brains will fall out.
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  8. #8
    Join Date
    Jun 2001
    Location
    Moore, Oklahoma, United States
    Posts
    4,159
    The extra energy used by the motor may be offset by the efficiency gained by the higher airflow. Especially true if airflow was too low to begin with, as typically is the case on many installs. How much temperature drop are you getting across the coil?

  9. #9
    Join Date
    Jul 2006
    Location
    Virginia
    Posts
    4,615
    The multifit motors and rescue motors suck ,but thats what we carry also i wonder if he replaced a 825 rpm with a 1075 rpm and also a blwer motor will draw alot more current with the door off that it will with the door on

  10. #10
    Join Date
    Jul 2012
    Posts
    18
    OK. I was reading the pressure after the coil. I should be able to move the line to before the coil/after the furnace. It goes filter-blower-furnace-coil-ducts. So I'll move it.

    When we saw the amp draw, we both checked the wiring. It's connected per the diagram, which is nothing more than color codes. It won't hurt to look again but the paperwork says 8A, does not mention any other values regardless of HP setting.

    PSC motors seem to draw almost the same juice through the main winding regardless of the capacitor in use, within reason. The cap controls the torque via current in the split winding. If it is truly connected properly, then I have to face the fact that it's a 1HP winding and by nature it's drawing what it's supposed to, in this case 8A. Sort of what I was thinking before, some of it is possibly just reactive power but I'm not a motor expert.

    Temperature drop across the coil was about 20 degrees or just a hair more IIRC last time checked, right after the new motor was put in. It is too early to be sure if it's more efficient, but it seems so. Sure the extra 4A of draw, assuming worst case a 1.0 PF, would be 480W, 7.7KWh per day, 231KWh a month, that's $23.10 cost to gain the efficiency. usually the bill is about $400 in this 100+ weather, so time will tell if it seems to be lower.

    It is airflow-challenged but we've been fixing that as T&$ allows. The next improvement there will be time consuming and has to wait until cooler weather when the a/c guy isn't so busy.

    I'll check the temperature differential again and be more precise, and also re-check the way the motor is connected. And relocate the gauge probe.

  11. #11
    Join Date
    Jan 2012
    Location
    indy
    Posts
    471
    more air more amps, could be on wrong speed but probthe half of cat he pulld of the wheel gave you more air more amps. 8 amps is on the higher side not off the charts, try the next lowest speed maybe?

  12. #12
    Join Date
    Jun 2001
    Location
    Moore, Oklahoma, United States
    Posts
    4,159
    High HP motors don't save much energy when used at lower speeds. My furnace has a 1/3HP blower and uses 500W on highest speed, 380W on lowest speed. I measured with a Kill-a-Watt meter, furnace total power. The motor uses all but 5W of the total power while in cooling mode. (5W is for board/transformer/thermostat/contactor)

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