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I'm making this question here because I'm a little bit confused with what I've been seeing from some old and new 1/2 hp motors.

I've bought a 1/2 hp induction motor (4-pole, single phase, capacitor start and no run capacitor) and both its standstill and pull up torque seem quite tame. Having a ~ 2 inch diameter pulley on its axis, I can easily stop it whenever I want with one hand. I'm sure that up to 3 times that torque could still be held with one hand only.

This matched well with what I saw in two bench drills, both of which also use 1/2 hp, 4-pole induction motors, the only difference being they use both a start and a run capacitor. In both cases I could stop them by holding the chuck with a single hand, and even in the highest reduction (with reasonable effort but still), which was about 1:3.5 in both cases.

I always thought that to be kinda weak but ok.

Then, last week I saw a 60 year old drill with another motor that should theoretically be similar (1/2 hp, 4-pole). But it was a 3-phase motor, which, as long as I'm concerned, should have a lower standstill torque than a single-phase motor with start capacitor, given their torque vs slip curves.

But I had no luck holding it. Even using adherent gloves and using both hands I couldn't stop it from starting in any way and I couldn't stop it while running either.

It was also in the max torque (~ 1:3.5) relation, so maybe I could stop it in a 1:1 relation, but didn't feel like it. While others could be even stopped with one hand, this one didn't even seem to slow down while using two hands.

Is there something I'm not considering here? Once all of them have the same power rating, nominal speed and reduction, I'd expect them to be at least similar in torque, but the older one seems to be just so much stronger (and the motor itself is also much bigger and heavier).

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    \$\begingroup\$ I believe you are comparing "apples to oranges" here with your single-phase capacitor-start induction motor vs. a 3-phase induction motor. 3-phase motors need no starting circuit as they are self-starting and you get a much greater starting torque than you do with any sort of single-phase induction motor. \$\endgroup\$
    – jwh20
    Oct 15, 2020 at 19:54
  • \$\begingroup\$ The motor geometry will play a huge role in apparent torque. There are many motor topologies. Induction motors notoriously "slip" and have lower torque. A synchronous, servo, or BrushLess DC (can look like three-phase) can have much more torque. Without comparing apples to apples, real-world comparisons are just a shot in the dark. \$\endgroup\$
    – rdtsc
    Oct 15, 2020 at 21:13
  • \$\begingroup\$ @jwh20 That's definitively not comparing apples to oranges. They could even be completely different types of motors, as long as I know their standstill torque I could directly compare the torque required to prevent them from starting. Regarding stopping them from max speed then comes both the top speed and the moment of inertia of the rotor, but these are all induction motors of the same top speed and same mechanical output. Of course their torque vs speed curve can be considerably different, but not by such a great factor. \$\endgroup\$
    – user2934303
    Oct 15, 2020 at 21:14
  • \$\begingroup\$ @rdtsc Yes, but I'm not comparing totally different motors here, like induction motors against synchronous or brushed motors. They're all induction motors of the same nominal power output and nominal speed. Plus, note I'm not talking about a single point in the torque vs speed curve, like just their standstill or just their pull up torque. Instead the torque from standstill to pull up (nearly all the curve) was much larger in the 3-phase motor. That's what I'm finding strange, plus the fact that, looking at their charachteristic curve, the standstill torque is not that great in a 3-phase motor \$\endgroup\$
    – user2934303
    Oct 15, 2020 at 21:27

1 Answer 1

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How reliable are motor power ratings through time...?

The published data and the nameplate markings have been very reliable for motors manufactured to international standards. Such motors meet or exceed the stated ratings. Older motors may have been more conservatively designed and tend to exceed the stated ratings. When the NEMA motor standards were established, the standard dimensions for a given rating were larger. At some point, insulation was improved, increasing the safe operating temperature and making it possible to meet the same performance standards with smaller dimensions. As designs and manufacturing methods improved, it is likely that designers became less conservative and motors less likely to exceed the stated ratings.

With regard to single phase motors, Puchstein, Lloyd, Conrad, Alternating Current Machines, 3rd Edition 1954, in introducing the section on single-phase induction motors, says "The single-phase induction motor displays less satisfactory operating characteristics than the polyphase machine..." In general, single-phase machines are designed for home appliances and three phase machines are designed for industrial use.

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