Ten Tips for Proper Motor Selection

You know how to match your motor to your supply voltage. What you may not know are the many other criteria for selecting a motor that will perform well over a long life.Selecting the wrong motor can mean downtime at the worst possible time. Then there's the added expense of repair and so on. Here are ten tips to help you do the job right. Tip 1: Get the big picture. Match the motor to its environment:

You know how to match your motor to your supply voltage. What you may not know are the many other criteria for selecting a motor that will perform well over a long life.

Selecting the wrong motor can mean downtime at the worst possible time. Then there's the added expense of repair and so on. Here are ten tips to help you do the job right.

Tip 1: Get the big picture. Match the motor to its environment: heat, humidity, corrosion, accessibility, hazardous location classification, and particulate concentration. Consider physical mounting, the diameter and length of the output shaft, and the amount of tolerable noise. You'll also need to know the types of torque you'll be dealing with.

Most people know to specify a Totally Enclosed Fan Cooled motor (TEFC) or other type of non-vented motor (such as a hermetic motor) for wet applications. You may want to consider such a motor for dusty environments, as well. The Photo shows a motor that runs a pump. Notice the dust on the piping and vents. At the very least, this motor should have a filtration system to prevent the dirt from getting into its vents. One plant uses white, half-in.-thick polyfiber rectangles that adhered with loop and faster over motor vents, to solve this problem.

Tip 2: Go for a drive. Can your motor run at the reduced speeds a variable speed drive requires? If the frame size isn't large enough to dissipate the heat from reduced cooling fan speed, the motor will fail prematurely. Ask the manufacturer to help match your motor and drive. You can often do this with a trip to their Web site (see www.ecmweb.com for industry links). One of this article's sidebars has some drive selection tips, another gives key criteria for selection. Select the proper motor and drive combination, and you will have excellent reliability, economy, and performance.

Tip 3: Answer the call of duty. The features that make a motor efficient reduce its ability to handle the heat of frequent starts. Selecting the right motor design and size for the duty cycle allows you to balance operating economics against maintenance economics. The duty cycle may require you to go with a higher temperature rating. Art. 100 of the National Electrical Code (NEC) defines duty (see "Motor duty definitions" sidebar) cycles.

Tip 4: Move your body. If a load has a high inertia (hard to move), you place a high demand on your motor to accelerate it. Fans, blowers, pumps, and mixers are examples of high inertia loads. How do you match motor and load-inertia? Your motor vendor has rules of thumb to help you select the motor size.

Here's a simple approach that works well for an existing installation. Observe and record the acceleration time for the motor to reach full speed during an across the line start. Observe the inrush current; it should plummet just seconds after the motor starts to turn. If the motor maintains a high current (compared to its nameplate rating), it's still accelerating. This means you need a bigger motor for that load. If you have to restart the motor to "get the load going," your motor is undersized.

Maybe you have a problem with the load itself. For example, your motor may be the correct size, but your gearbox has bad oil. You'll have to determine if the load you are driving is, indeed, the desired load. If not, you must incorporate some sort of monitoring, administrative controls, or overload protection to ensure your replacement motor will drive only the loads you want it to drive.

Tip 5: Know your ABCs (D and E, too). The original article shows a graph of motor curves, and has an accompanying table that helps you understand the graph. Until your motor's curve drops down (this happens as you approach full speed) you are developing extra torque; and requiring extra current.

With the Design B motor, the highest torque demand occurs at about 80% full speed. The Design B, when run under a continuous heavy load, incurs high heating until the motor stops accelerating. If the load requires substantial driving torque, as high-inertia loads do, the net torque available for acceleration is less. This increases the time to reach full speed.

If your application requires frequent starting and/or reversing, the Design D will outlast a comparable Design B. The Design D produces higher average torque from zero to full speed. Thus, the Design D will move the load more rapidly than will the Design B. The drawback to the Design D is its higher price and lower efficiency.

Tip 6: Insulate or be polar. You can specify a motor with better insulation. If you already use the maximum Class H system, you need some other means of improvement. You could choose a slower speed motor. A lower base speed means more poles. By replacing a 4-pole, 1750 rpm motor with the more expensive 8-pole, 875 rpm motor, you cut the effect of inertia by a factor of 4 to 1. Lower reflected inertia means less acceleration and less heat. A drawback here is you may need a different gearbox or belt/pulley arrangement. Make sure you look at the whole system before deciding on a new rpm value.

Tip 7: Watch where you're running. A motor can encounter a wide variety of loads during normal operation. If you size it for the "average load," it may not be able to handle all of its loads. Most motors have a service factor of 1.15 (some go as high as 1.40). The motor can handle, for short periods, 115% of its rated horsepower without overheating. Consider sizing the motor for its normal loading plus a demand factor for the occasional demand surge. Your normal load should be 75% to 95% of the motor's capacity.

When you've determined the load and torque/horsepower requirements, factor in slip. Every induction motor needs some slip to work. If you can't find slip on the nameplate, you can calculate it (see "Calculating Slip" sidebar).

Tip 8: Close the case. You may need a damp-duty motor. All motor components, not just insulation, deteriorate faster when exposed to moisture. See "Rules of thumb" sidebar.

Motors exposed to wide fluctuations in temperature or humidity can easily accumulate moisture. The standard cure is to use heaters installed within the motor. Heaters maintain the motor at a temperature usually about 10 DegrF above that of the surrounding air. The typical control scheme interlocks the heaters with the motors. The heaters are on when the motor is off, and off when the motor runs.

Tip 9: Calculate circuit protection. Some premium efficiency motors have inrush currents as high as 1800% of full-load current. Thus, complying with the NEC (which allows a maximum of 1300% starting current under special conditions) and preventing nuisance tripping becomes a problem. Another problem is that a high setting in the motor protection circuit may allow the upstream breaker to trip first.

Fault currents progress in steps. They usually begin at twice-locked rotor value, rise to three times that value, and then rapidly progress to motor failure. These failures start at low fault current levels and quickly cascade in a chain reaction fashion. A device called the Motor Circuit Protector (MCP) clears low-level faults (resulting from motor failures) quickly. Other devices are also fast, but MCPs account for parameters unique to motor circuits.

Tip 10: Watch the windings. You can choose from several motor winding schemes. Don't do so lightly. Some winding schemes work well for reducing across the line starting current, but may have adverse effects on your distribution system or on other motor drives in that system. Know your system wiring method (4-wire grounded wye, for example) and what other types of loads you have before deciding on any winding scheme you are not already using.

TAGS: Design
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