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enter image description hereI'm looking for a 1/6 HP motor for my lathe and found a motor with specs I like except for low 1/60HP. So I found many online horsepower calculators and entered the motor plate info:115V, 1.1A, 60Hz, 1568 rpm and results were .137 to .166697 horsepower. That is a factor of 10x higher than plate marking. I tried this with other similar, low horsepower, motor specs and got similarly higher horsepower results. Why is there such a large difference between plate marking and calculators results? Or am I not understanding something?

I also used amps, PF, voltage and efficiency calculators entering plate markings. I then entered the lowest values of .8 PF and 65% efficiency values and got approx 1/10 hp results. That is quite a bit larger than 1/60 hp. Just added photo. Thanks

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  • \$\begingroup\$ The plate markings are correct. These low power motors are not very efficient. Those calculators are for larger motors (like 1/2 horse and above). \$\endgroup\$
    – user57037
    Commented May 9, 2021 at 17:27
  • \$\begingroup\$ Can you please provide a link to at least one of the horsepower calculators? \$\endgroup\$
    – user57037
    Commented May 11, 2021 at 7:51

2 Answers 2

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The horsepower of an induction motor can not be calculated from the current without knowing the motor efficiency and power factor. For single phase power, Hp = (V X I X eff. X pf) / 746. An online calculator that simply converts current and voltage to horsepower without taking that into consideration will not give a good answer. A tiny induction motor is likely to have a very low efficiency. The power factor is less than one, but perhaps not so low because of the extremely low efficiency.

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  • \$\begingroup\$ I also used amps, PF, voltage and efficiency calculators entering plate markings. I then entered the lowest values of .8 PF and 65% efficiency values and got approx 1/10 hp results. That is quite a bit larger than 1/60 hp. \$\endgroup\$
    – henrylr
    Commented May 9, 2021 at 15:38
  • \$\begingroup\$ Please edit your question to include that comment -- it's a relevant part of the question. \$\endgroup\$
    – TimWescott
    Commented May 9, 2021 at 15:40
  • \$\begingroup\$ I entered info in question. \$\endgroup\$
    – henrylr
    Commented May 9, 2021 at 16:19
  • \$\begingroup\$ By "entered the lowest values" do you mean selected the lowest alternatives offered by the online calculator? They may not be low enough. Can you post a link to the 1/60 Hp motor? \$\endgroup\$
    – user80875
    Commented May 9, 2021 at 16:46
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    \$\begingroup\$ Is there an external capacitor connected to the motor? If there is not a capacitor, it is probably a shaded pole motor, I believe that is the least efficient type of induction motor. The 1568 RPM rating indicates an unusually high slip motor, another indication of unusually low efficiency, \$\endgroup\$
    – user80875
    Commented May 9, 2021 at 17:30
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When you find a plate (or label) on an induction motor that specifies the power output, that is the most reliable source of information. The power listed on the plate will be the mechanical output power of the motor under the specified conditions.

So for this motor, if it is supplied with 115 VAC at 60 Hz, and the load is such that it is consuming 1.1 A and rotating at 1568 RPM, it will produce 1/60 HP.

These small motors are very inefficient. A clue to that is the low RPM (1568). That tells you that the motor is operating with high slip, because synchronous speed is 1800 RPM. In general with induction motors, high slip implies low efficiency.

The online calculators you are using are probably designed for larger induction motors that are quite a bit more efficient.

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  • \$\begingroup\$ 1800 rpm is for 4-pole motor? 2-pole may be 3600. \$\endgroup\$
    – user263983
    Commented May 9, 2021 at 17:45
  • \$\begingroup\$ @user263983 yes, at 60 Hz, 1800 RPM is synchronous speed for a 4-pole motor. I am assuming this is a 4-pole motor. If it is a 2-pole motor, then synchronous speed is 3600 RPM, just as you say. \$\endgroup\$
    – user57037
    Commented May 9, 2021 at 18:13
  • \$\begingroup\$ I've spent time on many sites with single phase electric motor formulas and haven't seen any specifically for small motors. I thought motor formulas like these - Power factor PF = P(W)(V(V) × I(A)), Torque T = V x I x pf / (2 x pi x N(rpm) / 60), Voltage V(V) = 746 x P(HP) / I(A) volts and others were applicable regardless of motor size or power. \$\endgroup\$
    – henrylr
    Commented May 9, 2021 at 19:37
  • \$\begingroup\$ @henrylr there are some formulas which are valid. For example P = V * I * PF * eff where P is power in watts, V is voltage in volts, PF is power factor (between zero and 1) and eff is efficiency (between zero and 1). If you know all those things you can calculate the output power of the motor. Generally you will not know all those things unless you get them from the motor plate or datasheet. \$\endgroup\$
    – user57037
    Commented May 11, 2021 at 7:42
  • \$\begingroup\$ The formulas you list appear to all leave out efficiency. I didn't check them carefully but any time you use power factor, you also need to use efficiency. \$\endgroup\$
    – user57037
    Commented May 11, 2021 at 7:43

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