# Why is this 240V / 20A motor labelled 3HP?

Here's the motor badge:

As far as I know, 115 V x 40 A = 230 V x 20 A = 4600 watts which is equivalent to ~6.2 HP, all presumably @ full load.

So what does the "3 HP" mean?

I know that in some cases the "marketing HP" is often loosely based on reality but usually that is in the sense of inflating the HP number to exaggerate capability. This obviously doesn't fit that pattern.

• input power isn't the same as output power. Plus, that ampere rating is probably a peak current, not a continuous average operating current. Commented Aug 2, 2020 at 17:27
• It may be different in your country and for a motor in particular, but the nameplate current rating is most often for fuse and wire dimensioning, with worst case and some margin in mind. Laptop adapters are a good case here, nameplate voltage times current = way more power than output despite running very cool (good efficiency). Commented Aug 2, 2020 at 20:55
• Note that it says 40/20 amp. This makes me strongly suspect it's really drawing 40 amps at startup and 20 continuous. Commented Aug 3, 2020 at 5:07
• @LorenPechtel No, the 40/20 AMP are related to the two alternative voltages 115/230 VOLTS. (40*115 = 20*230) Commented Aug 3, 2020 at 5:59
• @Marcus Müller: No. I am quite confident the motor conforms to NEMA standards. The current is the RMS current that would be observed under dynamometer test conditions for continuous operation with a steady load of 3 Hp with the voltage and frequency marked on the nameplate.
– user80875
Commented Aug 3, 2020 at 12:43

The current marked on a motor should be the continuous, full-load operating current. The power marked on a motor should be the full-load mechanical power delivered by the shaft. A 3 Hp motor should deliver about 2240 watts. 2240/4600 is 0.487. That tells you that the motor's full-load power factor multiplied by its full load efficiency is 0.487. For a rough estimate, you could say that full load efficiency = full load power factor = square root of 0.487 = 0.7. That is not very good, but single phase motors generally have a lower efficiency than a comparable 3-phase motor. The power factor can vary quite a bit.

Since this motor was likely made more than 55 years ago (postal zone rather than zip code) and is a design type (repulsion-start, induction run) that was abandoned long ago, you can pretty safely assume that it is not very good in terms of efficiency and power factor.

More About Interpreting Motor Nameplates

There are online resources that explain in detail how to interpret the nameplates (rating plates) of motors that have been manufactured to international standards. Note that rating plates or labels that have less information than the one pictured in the question likely do not conform with international standards. Interpret those with great caution.

NEMA is the National Electrical Manufacturers Association of the United States. It is a standards developing organization led by people associated with manufacturing companies. NEMA is accredited by ANSI, the American National Standards Institute. ANSI is a private, not-for-profit organization dedicated to supporting the U.S. voluntary standards and conformity assessment system. ANSI is the official U.S. representative to the International Organization for Standardization (ISO) and, via the U.S. National Committee, the International Electrotechnical Commission (IEC).

Motors and generators that have a rating plate like the one pictured in the question are generally assumed to conform with the applicable NEMA Motors and Generators standard, such as ANSI/NEMA MG 1-2016. Data marked on the rating plate should be interpreted with reference to that document. There is an ongoing effort to “harmonize” NEMA MG 1 with the similar IEC standards. The basic meanings of the data items have been pretty well harmonized. Items like “CODE F” and “DESIGN M” must be interpreted by referring to tables published in the standard. It may be possible to find important MG1 information and tables quoted or reprinted in documents that are freely available online.

• Does this mean that at full load, the motor is dissipating 4600-2240=2360W as heat? Commented Aug 3, 2020 at 17:49
• No. The 4600 volt-amperes include some power that represents energy that is transferred back and forth between the magnetic field in the motor and the source. Look for questions here or information elsewhere about power factor and reactive vs. real power. The 4600 volt-amperes is apparent power, a combination of real and reactive power.
– user80875
Commented Aug 3, 2020 at 18:46

'3 HP' marked on the motor name plate would be based on the measured full load torque at the rated speed (HP = full load torque kgm * full load speed RPM / 716).

The power factor of a single phase induction motor would generally be between 0.5 and 0.8. Assuming a power factor of 0.65 the motor input HP would be 230 x 20 x 0.65 / 735 = 4 and its efficiency 3/4 = 75%.

Brush lifting Repulsion Start / Induction Run motors are museum pieces. Very cool to watch them start, especially if you can record it from the brush end and play back in slow motion. But they are not so fun to own, because they were horribly inefficient.

• I can imagine that some brush lifting mechanisms are very cool to watch, but the Master mechanism is a really clever design that fits in a little copper housing that is the same diameter as the commutator and only about 30% of the commutator length. You can barely see the moving parts through a narrow slot between the end of the commutator and the housing. A coil spring band holds the little copper segments away from the commutator. When centrifugal force overcomes the spring, the segments pivot out a fraction of an inch and short the commutator segments to the copper housing.
– user80875
Commented Aug 4, 2020 at 16:14