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What's the difference between a 1HP and 2HP electric motor?

Slightly different dimensions: Diagram

They look almost identical:
0.75kW, 1HP
1.50 kW, 2HP

Here are the specs:
Specifications
Specifications

Same top speed (RPM). Same Voltage. Difference in current required. What is physically different between the two? What causes one to draw more current than the other? Is it simply more winds on the coil? Thicker gauge wire? Larger stator/rotor? How is it they have the same max RPM? Shouldn't more power mean higher speed in the absence of other variables (e.g. weight)? What is the extra current spent on if not speed? Torque? Or is the extra current only drawn under load; in response to a physical resistance (i.e. drag) on the driveshaft?

These might seem like seperate, unrelated questions but really im just trying to determine: what's the same, what's different; and to understand: why.

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  • \$\begingroup\$ Those motors have a rotation speed synchronous to the mains frequency less nominal slip. \$\endgroup\$ Sep 13, 2020 at 20:58
  • \$\begingroup\$ Which model numbers are they? \$\endgroup\$ Sep 13, 2020 at 21:02
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    \$\begingroup\$ @voices: Despite the similarity in the photos, the 2 hp motor is bigger than the 1 hp motor. Check page 25 and compare the size of the 80 frame to the size of the 90 frame. \$\endgroup\$
    – JRE
    Sep 13, 2020 at 21:18
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    \$\begingroup\$ Electrical current drawn by the motor, is roughly proportional to the mechanical torque required by the load. The 2Hp motor is capable of driving a load with more torque, at the cost of requiring more input power. The design of the rotor and stator (not visible in the photos) is what provides the capability for more torque, at the cost of more current. \$\endgroup\$
    – MarkU
    Sep 13, 2020 at 21:23
  • \$\begingroup\$ the prior answers seem to be correct. the main difference is a different characteristic of the coils. we can assume slightly increased wire diameter on the stronger motor to cope with higher current. More current then results in higher torque. \$\endgroup\$
    – schnedan
    Sep 13, 2020 at 22:51

3 Answers 3

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Same top speed (RPM).

Those induction motors have speed dependent on the incoming mains frequency, so they will be nominally the same, regardless of rated power

Same Voltage.

They are intended to run from the same voltage.

Difference in current required. What is physically different between the two? What causes one to draw more current than the other? Is it simply more winds on the coil? Thicker gauge wire?

Probably a different number of turns on the stator, which might require a different guage wire.

Larger stator/rotor?

Any given frame size motor will likely use the same diameter rotor. You have a frame 80 and frame 90 motor there, so they will be slightly different.

How is it they have the same max RPM?

Their max rpm, under no load, will approach 1500 rpm, this is the synchronous speed with a 50 Hz mains frequency.

Shouldn't more power mean higher speed in the absence of other variables (e.g. weight)? What is the extra current spent on if not speed? Torque? Or is the extra current only drawn under load; in response to a physical resistance (i.e. drag) on the driveshaft?

Speed is nominally constant, due to the AC frequency being constant, so the other variable, torque, changes. 'Slip' is defined as the % reduction in speed below synchronous. The current drawn by the motor is more or less proportional to the slip. As the motor load increases, the speed will drop, the slip increases, the current and so torque increases, until it reaches the rated speed of 1440 rpm at the rated power, either 1 or 2 hp depending on the motor.

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HP = Torque x 5250 / RPM, so given that it will be the same RPM at the same frequency, it's really just the torque that is different here.

The physical differences that lead to the motor being capable of developing more torque, and thereby more HP, come from the windings in the stator. Current is what generates torque and resistance and impedance are what control current. So to reduce the resistance, they use larger conductors and to reduce the impedance they use fewer turns in the coils. In some sizes that can be accommodated within the existing mechanical structure, but at some point it requires changing the depth and shape of the slots in the stator frame that the coils fit into. So once you get to a point of less resistance / inductance, more current flows in the motor and it produces more torque.

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What's up with RPM

How is it they have the same max RPM? Shouldn't more power mean higher speed in the absence of other variables (e.g. weight)? What is the extra current spent on if not speed? Torque? Or is the extra current only drawn under load; in response to a physical resistance (i.e. drag) on the driveshaft?

They're the same RPM because they are induction motors. (also would be equal if they were synchronous motors).

What you're expecting is series-wound or shunt motor behavior. These are DC things, which add a great deal of maintenance or complexity: Brushes.*

Remember -- this is AC power. It came from a spinning generator. The 3 phases are already rotating - peaking in sequence: A-B-C-A-B-C. A motor is simplicity itself: 3 windings in the stator, wire them up to each phase, and now you have a rotating magnetic field in the stator. It is rotating at 50 Hz, the agreed AC line frequency.

(we'll come back around to the single-phase wrinkle).

So with the stator rotating at 50 Hz (or 25 Hz if you have wound it with double the poles), the rotor is simplicity itself. A permanent magnet would give a synchronous motor, but even cheaper, a trick with induction lets you use an entirely passive aluminum "squirrel cage" rotor, giving nearly synchronous speed.

If you can live with 2880, 1440 or 720 RPM, it's cheaper* all around.

With single-phase, it's like bicycle pedals - If you start a bike with the pedal at absolute top, it won't go down - sideforce is needed to start it, but once in motion it is not needed. That's what the capacitor and extra winding do. Again once the motor is spinning it is not needed.

Anyway, you are correct that the extra current is not drawn unless a load is placed on the motor. At that point, the motor will bog -- or not -- depending on if it can handle the load.

The motor has a torque max around its stated frequency (e.g. 1440 RPM). Below that, torque gets worse and worse. If you've ever pushed the limits of a table saw, you are well familiar with how easy it is to bog or stall the motor by asking too much of it. The load (machine) needs to be designed to be suited to this torque curve. This is why trams and electric trains don't use induction motors (at least, not without a VFD).

When you need a motor that "downshifts" and makes more and more torque, you need a DC series-wound motor, which is what trains use. Or a VFD tuned to have the same effect.

Size

A motor of twice the power needs twice the mass of copper windings.

It's pretty much that simple. They are making twice the magnetic flux, or so they say**. Whether it's longer or thicker wire makes little difference. (longer wire most likely means they are paralleling).

Without the shaft (compute L - E), the motors are 156 dia x 255 length, vs 174 dia x 295 length. So one motor is 6,205,680 cylindrical mm and the other is 8,931,420 cylindrical mm. That's considerably less than double the package size, but given the things that don't need to expand such as case thickness or rotor diameter, that may be about right.





* Unless you really, really, really want to do a Variable Frequency Drive, using silicon to convert AC to DC then back to AC at any frequency you want.

** And these motors are cheap Chinesium. Notice the obvious misspelling in the lower label, and the "weird font" in the upper. That's a Chinese language font, in a style fit for Chinese characters, which includes the 52 Roman letters done in a matching style. Also the prominent CE mark, which they fake because there are no consequences for doing so, unless your boots are inside the EU.

*** In North America, 60 Hz, 3450 RPM, 1725 RPM or 885 RPM.

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