Horse Power types:
Not all motors are the same even if both motors claim 1.5 HP. They are all built differently. Even the copper windings can be made of high or low grade copper. The enclosures of the motor can be made of low or high carbon steel. Some AC motors create high EMF and some much lower, even if they are the same horsepower. As a rule, the smaller the motor, usually in the cheap machines, which have a higher HP rating, usually have a higher EMF rating to go with it. A bigger motor, if it is AC, creates less EMF because it is not being worked as hard.
Horsepower related to vibration machines in my opinion is intentionally misrepresented by many of the brands out there. According to the dictionary, one (1) horsepower is 746 watts of power. So, if you have a 2 HP motor from two different companies, one model can be more or less powerful than the other, that is why I wouldn't buy based on horsepower alone. (PHP, peak horsepower vs CHP, continuous horsepower). Just for simplicity's sake, peak horsepower is what the power the motor can do without anyone on it, meaning it is not a measurement for the heaviest person. When you are looking at HP ratings, you want to get the continuous duty ratings, not just the peak.
Even More Confusing HP issues:
To make picking out a motor or a vibration machine more confusing, some of the companies are gearing up their "smaller" motors, often the size of a coke can, to have it produce extra wattage. Well, in the short run, a smaller motor that is running at higher RPMs can produce the same amount of power that a larger motor running at low RPMs, but in the long run the smaller motor will run hotter, have more problems and break down quite a bit faster.
It's all a little more complicated than that (to determine HP you need to know the voltage, amps and efficiency of the motor), but the bottom line is to make sure the HP rating you see on the machine is a continuous duty motor rating. A peak rating doesn't do me any good at all -- how powerful a machine is when you're not on it is meaningless.
So say it with me "I want a big motor, running at low RPMs and I want to know its continuous duty rating." It may all sound confusing but it is the difference between a machine with a motor that will burn out in 6 to 12 months and one that will last over 5-10 years or longer.
When looking at horsepower, make sure you know if it is bare motor speed HP or constant speed with an actual person's weight on it. If you need an amplitude of 5 mm and the plate only goes up to 3 mm if your weight is over 250, then you know you need more wattage.
Horsepower is Meaningless if you don't know the wattage and amperage.
Horsepower can mean peak hp, average hp, running hp, output hp, input hp, or even startup hp with bare motor speed. Wattage is what tells you how powerful the motor is.
Is a higher wattage, higher horsepower machine better?
First, most people don't know if a 1.5 horsepower, 500 watt motor is better than a 2 horsepower, 300 watt machine. Before knowing which is better, you need to know the amperage (amps) of the machine. Most companies FAIL to tell you the amperage on their website. You can have a 1.5 horsepower, 300 watt machine and 2 amps or you can have 1 amps and this can make the difference between a machine that has the same specs and one is 1999 and the other 800 dollars. A huge difference. A stronger motor must have a strong frame. If the frame is plastic, having a very strong motor can damage the housing that holds the motor so you won't usually get a high amperage motor in a machine less than a 1000 dollars. I did tests where I took an amp meter to see if the motor was pulling more amps than the motor rating allowed. If the machine was rated at 300 pounds, I held weights to see if the motor pulled more amps than the 15 amp outlet is rated for. If you have a 15 amp outlet and the surge when you turn it on goes over 15 amps, not good.
Many vibration machines, especially when heavier people are on them, draw more amperage from the outlet. Poorly designed machines create more friction and much of the current is converted into heat. The better motors companies give sensitivity to creating less heat and motor efficiency. The better motors deliver more power using less wattage. They are bigger motors, usually 2 horsepower, that doesn't have to strain so hard and pull more amperage than needed. A motor can also have the same specs but use aluminum wires while some brands use copper wires.