Horsepower, RPM's Voltage Rating, Wattage, Cycles, Overload Protection, Current Rating.
All of the above Even though I hate the unit of horsepower, it is the convention that the industry uses nonetheless. It refers to the mechanical output power, in contrast with electrical input power. I would prefer it be listed in Watts, but it is rare you will see that. To move a given mechanical load at a given rotational rate, you need to have at least that amount of mechanical output power. It can be greater, if that is what is available. RPM are important to keep the same, because that is how fast it is rated to rotate. And the load should be moved at the speed at which it is rated to be moved. You will find that there is a relationship between the RPM and the electrical frequency, which is based on the geometry of the magnets and coils. Often times, it is an integer fraction or integer multiple of 3600 RPM in the USA, or 3000 RPM in much of the rest of the world. The voltage rating is CRUCIAL for you to get correct. If you get this wrong, it can mean overheating and failure. Make sure you also get the electrical topology correct (single phase-to-neutral, single phase-to-phase, 3-phase wye, 3-phase delta, etc). Wattage is also a power rating, except it refers to the electrical input power. You might think that if you get voltage and current correct, then you also get wattage correct. And mostly, this is true. Although motors aren't simple resistive loads, and do have a difference between their kVA and kW. The Wattage will account for the real power consumed, the kW. With current rating, this is important because you need to place it on a circuit with large enough wires to handle its continuous current without overheating. The correct overcurrent protection device (fuse or breaker) must be installed on the circuit to trip in case of an overload. For non-motor circuits, the rule is overcurrent device rating = 1.25*max rated current. Not the case for motors, since the current is a little more interesting. I'm not sure what it is exactly.
Does the motor need to start under a high-torque load? Not all motors are good at this. Like motors to run air compressors are different than those that run a fan. Is the motor going to be under constant load or intermittent load, like a saw vs. a blower.
[1] Size to fit in the machinery space. [2] 3 phase or single phase or DC motor. [3] Voltage to meet power source. [4] For AC motor, 50c/s or 60c/s [5] VA rate for AC motor or wattage for DC motor. [6] Torque at full load. [7] Maximum speed rate. [8] Cost. [9] How long it takes for delivery.
All those are important. Starting torque, ambient temperature, explosive atmosphere, water and dust, delta, Y or single phase and more are sometimes important. Neil
If you are replacing an AC Motor in an existing system, the primary factor would be, the reason why you want to replace an existing motor. Is the speed low? Is it noisy? Is it overheating? Is the efficiency low? Does it break down often? Or would you save more space by replacing a foot mounted motor by flange mounted motor? Or you just can't stand the look of it?! (asthetics) Once you identify the reason, go for a motor that addresses this problem keeping all other specficiations the same or slightly higher.
