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how big a motor

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I keep looking at all the plans and built power hammers of various flavors and there is one thing I can't figure out. How big a motor do you use? is there a guide line for hammer weight vs HP? I'd love to build one (helve hammer I think) but I dont' wanna just take a random guess at the motor size.

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There are several sites that list Little Giant Power Hammer Sizes and the recommended motor size.

Google "Little Giant Specifications" and they should come up.

These should provide guidance on what a mechanical hammer power requirements are for hammers opertaing at the same speed as a Little Giant.

Trust this helps

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Can you (or maybe you already have) post pictures of your hammers. Helve hammers have always had a special interest to me.

Phillip in China

A lightly loaded motor is not always better. They produce a low power factor load which increaases the amp draw. That is a 2 hp motor developing .9 hp will draw more current than a 1 hp motor developing .9 hp. It probably doesn't matter that much with 1 or 2 motors but if you have a factory full of lightly loaded motors it will matter.

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You'll find IRNSRGN's Helve hammer design in the blueprints BP0063 Helve Hammer. There is also a pic of a difrent one in one of the "show me your XXX" threads in this forum.

Another question. Motor speed. I'm guessing you want a slower-ish motor. Could you get a faster motor and gear/pulley it down? would that let you get away with a bit smaller engine?

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  • 1 month later...

HP is just RPM x Torque x a constant, so torque is directly proportional to Horse Power at any given RPM.

Starting torque is an other matter, and some motor designs are better than others.

I have a 34 Lb "Rusty" type hammer. I just measured the RPM and Load current:

Present speed: 198 BPM
Motor Type: TEFC, frame 56-75 (a bit less efficient than some others)
Voltage: 115 VAC
Rating: 1 HP
Speed: 1725 RPM under load
Full Load Current: 13.4 A at 115VAC

Present hammer speed: 198 BPM
Measured current: 10A at idle, 13-14A at full speed of the hammer.

As I mentioned in an earlier posting today, I had too small a flywheel initially, and the motor ran hot and made the needle on the clip-on ammeter swing between 10 and 26 A!

Now that I have upgraded the flywheel, and the motor is running close to the specified full load current, I suspect it is delivering close to the rated 1 HP.

The guides are UHMW and I give the ram a shot of silicone lube from time to time, so friction or viscous damping due to heavy oil is minimal.

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Hmmm, ok. But something is niggling at the back of my mind about torque, total number of windings in the motor, and manufacturers marking HP on their products based on an 'unloaded' measurement.

I'm pretty sure that the 2hp motors we used to open the gates on the Erie Canal (where I was lockmaster) would have more torque than a 2hp vacuum motor in my shop vac.

Am I imagining this or can I really just slap any 2hp motor on my 50# LG?

Edited to add:

Doesn't a flywheel also help to add torque?

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For practical purposes there are two kinds of motors used in the shop, and you have identified both of them: the induction motor (no brushes or commutator these days), and the universal AC/DC motor that has brushes and a commutator.

The induction motors in your shop are likely either 2-pole or 4-pole motors. They only run on AC power; 60 Hz in North America. If the motor has no load on it and has perfect bearings, it would run at close the synchronous speed: 3600 rpm for 2-pole or 1800 rpm for 4-pole motors. They develop no torque at this speed. Induction motors require "slip" in order to develop torque, so as they slow down (within reason), they develop more torque, and torqe x rpm is proportional to the horsepower produced. Maximum slip would be in theorder of 5-10 percent. Beyond that, these motors tend to stall.

This is why these motors are rated at, say 2 HP at 3450 RPM for a 2-pole motor or at 1725 RPM for a 4-pole motor - the standard speed ratings under load for these types of motors.

If the motor has a "continuous duty" rating, it can run at this speed and produce the rated amount or Horsepower all day long. It will draw the peak load current printed on the nameplate.

If you load this motor down more, it will run a bit slower, resulting in more slip below synchronous speed, and will produce more torque. As a result, it will produce more horsepower, and suck more current. On a cheap motor, the heat generated by the curren flowing through the (barely adequate) windings will cause the motor to overheat and trip the thermal switch or burn up. On those heavy motors you mentioned, they probably have extra-heavy windings, heavy castings, and plenty of surface area to radiate the heat, so they can probably produce twice the horsepower listed on the nameplate and just over twice the torque without burning up. They will no longer be running at the standard rated speed of 1725 rpm though.

Induction motors have a torque curve that peaks around 1700 rpm (4-pole motor) and drops off very rapidly below that. You have probably noticed this on your bench grinder. You can load it down and the motor slows down a bit. Load it down a bit more and it suddenly stalls.

Universal AC/DC motors run at a much higher speed, say 5000 rpm, limited by the windings and internal friction. Torque increases as the speed drops, but these motors can be loaded down to a much lower speed before they stall. They can produce lots of horsepower, but the rpm may vary all over the place. As they are loaded down, the current through the windings increases. The heat generated is proportional to the square of the input current, so as you go over the rated current, lots of heat is generated. Universal motors are usually compact and light weight, and need a lot of air movement. If you load them down, the rpm drops off, and the cooling fan moves less air. They overheat rapidly and as a result, burn out easily. Since universal motors run at a relatively high rpm, they are often coupled directly to a gear box as in a skill saw to get the rpm down and the torque up to a usable level.

As a result, both induction and universal motors can produce the same 2 horsepower, but when overloaded a bit, the induction motor can hang in there for a long time. The universal motor can outperform the induction motor in a heavy torque application, but only for a few seconds.

You also mentioned flywheels:
A flywheel can average out the torque requrements of the load, presenting a lower peak torque requirement to the motor. A typical example is a compressor or a power hammer. If you don't have a flywheel on a compressor, you can use a huge motor and overcome the compression stroke, but it is more effective to smooth out the load with a flywheel and use a smaller motor.

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