Question 1

How do AC electric motors work?

Accepted Answer

AC induction motors, sometimes called squirrel cage motors, work by using alternating current electricity to create an electromagnet. The magnet in the stator has poles that switch polarity with the current, causing the rotor to be rotated at a set speed.

Question 2

What information do I need before buying an electric motor?

Accepted Answer

Replacing a motor with a drop-in replacement is typically the easiest. You either need the part number to get an exact match or some basic technical info to get a comparable product. Most important are:

Voltage

Speed

Frame Size

Enclosure

Also important, although often standardized, are things like service factor, design, insulation, and other details. In general, being able to supply a picture of the motor nameplate is the best as it provides all this information and more.

Question 3

How do I choose the right motor for my application?

Accepted Answer

Choose a motor based on the equipment that needs to be run and the environment it will be installed. Pumps, fans, compressors, and other equipment will have specifications for the power, speed, mounting method, and other sizing details. Your environment will help you decide the enclosure, cooling, and mounting that works for your situation.

Read more in our Motor Buying Guide.

Question 4

What motor enclosure do I need?

Accepted Answer

Your motor enclosure will be dependent on the environment you install it the motor in. The most common industrial motors are either open drip proof (ODP) or totally enclosed fan cooled (TEFC). TEFC offers more protection and is cooled by a fan attached to the shaft, while ODP has a more open design that’s better for clean environments.

There are many other types of enclosures. You can learn more about motor enclosures here.

Question 5

Why are there two sets of numbers on my motor nameplate?

Accepted Answer

Many motors have two sets of numbers for one of two reasons. First is based on the frequency of your electrical system, typically one set of ratings for 50Hz and one for 60Hz. In these cases, you often see the entire set of technical specifications duplicated with a separating line.

The other reason is that many motors can accept multiple voltages and have different ratings depending on what is input. One common setup for this would for a motor that accepts 208-230/460 in voltage. In that case you would often see ratings with a hyphen or slash between them, such as 5.2-5.6/3.2 amps, with each of those ratings matching the corresponding voltage.

Learn more on our How to Read a Motor Nameplate article.

Question 6

What happens if a motor is oversized or undersized?

Accepted Answer

Undersized motors are likely to have issues. They will often overheat, trip protection devices, or fail prematurely because they can’t handle the required load.

Oversized motors may run but they often operate inefficiently, especially at partial load. This can lead to poor power factor (and associated penalties) and higher energy costs.

Question 7

Should I size a motor based on horsepower, amps, or load

Accepted Answer

Because you’re looking at mechanical output, you would be focused horsepower or torque requirements for the load. Amps are still important for electrical compatibility, but using that alone isn’t a complete enough picture.

Question 8

What service factor do I need?

Accepted Answer

Your service factor will depend on the application your motor is running. More demanding applications typically need more service factor. Common service factors include 1.15 or 1.25, correlating to 115% or 125%.

As an important note, if your motor is running on a VFD, you should consider your service factor to be 1.0 no matter what the original nameplate says.

Question 9

Are all motors with the same specifications interchangeable?

Accepted Answer

If every specification were the same between two motors, they would be interchangeable. The trouble that often comes up is that the basic specifications are a match, but someone misses one of the less common technical considerations. Most people know to check horsepower, frame size, voltage, speed, and enclosure. Many don’t think about checking torque design, mounting methods, or other considerations. Also evaluate for any modifications that were made that don’t appear on a nameplate.

Question 10

The NEMA frame size on a replacement motor is close but not exact – will it work?

Accepted Answer

T-frame NEMA specification motors will have most of their dimensions complying to a certain standard. The typical format is three numbers followed by the letter T (i.e., 284T, 326T, etc.). If there are additional letters after the T, they have a standard meaning and usually require more investigation. S typically means short shaft, C means a C-face mounting, Z means a custom shaft. If there is a letter before the numbers, the information is less standardized.

If you have room to work with and can change how or where the motor is mounted, you still might be able to make other configurations and frame sizes work. Learn more here.

Question 11

What is inrush current and why can it be a problem?

Accepted Answer

To magnetize a stator and build inertia, the motor requires an initial surge of electrical current, called inrush current. When started across the line, that initial inrush is often in the range of 600-700% of the motor’s nameplate rated amps. This inrush causes mechanical stress on the motor and other equipment. It also means that surge has to be supplied by the utility and pass through all the equipment feeding into the motor, which causes electrical concerns.

Question 12

How should I start and run my motor?

Accepted Answer

The three main methods to start a motor are starters, soft starters, and variable frequency drives. Each has benefits and costs. To determine which is right for your situation, you’ll evaluate your need for eliminating inrush current and ongoing speed control. Learn more in our article Motor Control Methods Explained.

Question 13

What’s the difference between U and T frame motors?

Accepted Answer

When a motor fits a frame standard, certain dimensions are the same regardless of brand or other factors. These are often distance between mounting holes, height and width of the shaft, and other important measurements. U frame motors were introduced in the 1950s, and T frames were introduced in the 1960s. The T frame standard has taken over and is the North American standard.

Question 14

What’s the difference between NEMA (or T frame) and IEC (or metric) motor frames?

Accepted Answer

NEMA frame motors, also called “T frame” motors, comply to the National Electrical Manufacturers Association (NEMA) standards for critical dimensions. This is the standard used in much of North America. IEC motors, also called metric motors, comply to the International Electrotechnical Commission (IEC) standards and is the standard in much of Europe.

Question 15

Why are slower motors usually bigger?

Accepted Answer

The nameplate speed of the motor is determined by the amount of poles. More poles means each alternating current rotates less distance. This means that slower motors have more material, are larger, and tend to have more torque than a similarly rated motor at a faster speed.

Learn more about how motors work and their speeds here.

Question 16

What are dual-speed motors?

Accepted Answer

Dual-speed motors utilize separate windings or other methods to have two configurations with different numbers of poles. Depending on how the motor is connected and configured, it can run at either speed, although it’s not possible to switch the speed during operation with these motors.

Question 17

Why is motor cooling such a big deal?

Accepted Answer

Many parts of a motor’s rating are based around managing the heat that is produced during running. Excess heat is one of the leading causes of insulation breakdown and motor failure. Insulation ratings, enclosure types, and even bearings are specified to withstand certain heat levels. Heat damages these components of a motor, so the more cool you can keep a motor the longer it will operate.

Question 18

Are premium efficiency motors worth the money?

Accepted Answer

Although usage ranges, many customers will see that they spend about 6 times the purchase price of a motor on electricity to run the same motor every single year. Over its lifetime, the energy cost grows to outpace the initial motor cost, especially as electrical rates increase.

Because of regulations and standards, it’s also very difficult to find motors that don’t comply with the NEMA premium efficiency standard. The standard new motors are held to is also likely to get even more strict in the future as stress on the grid increases.

Question 19

What is a classified or explosion proof motor?

Accepted Answer

Some motors are designed to have flame paths, non-sparking components, and other design features that make them safer to operate in areas with combustible materials or gases. These are typically certified by Underwriters Laboratories (UL) and any modifications or repairs would require them to be re-listed and certified. The ratings for which environments need which motors is a system broken into class, division, and groups.

Learn more in our Electric Motor Buying Guide.

Question 20

Can VFDs run any motor?

Accepted Answer

Most standard AC induction motors can be controlled by VFDs. Some motors, such as permanent magnet motors, can only be controlled by advanced VFDs. The bigger concern with adding a VFD to a motor system is protecting the motor from the power quality issues a VFD generates. Having an inverter duty motor and other filters can help mitigate these issues.

Question 21

What is an inverter duty motor?

Accepted Answer

Variable frequency drives, also called “inverters,” can cause damage to motors through reflective wave phenomena (dV/dt) and electric discharge machining (EDM, or shaft currents). Inverter duty have different components and construction to mitigate some of these issues. These may include better insulation, upgraded bearings, or other measures. This is all according to the NEMA MG1 Part 31 standard.

Learn more in our Electric Motor Buying Guide.

Question 22

What happens if I run a standard (non-inverter duty) motor on a VFD?

Accepted Answer

Many standard motors now come inverter duty as a default because of the growing prevalence of VFDs. In the case of a motor that is not inverter duty, a VFD increases the likelihood of bearing damage (such as pitting, fluting, or other EDM) due to shaft currents and winding failures due to voltage spikes and excess heat.

Question 23

When do I need shaft grounding or bearing protection?

Accepted Answer

Bearing protection is always recommended when a VFD will run the motor or might be added in the future. The level of the bearing protection often depends on the criticality and cost of the motor. More basic motors may be fine with a shaft grounding ring on the drive end, while larger and more expensive motors may want to evaluate a mixture of insulated ceramic bearings with shaft grounding rings.

Question 24

When do I need dV/dt filtering?

Accepted Answer

VFD output filters are good anytime a motor is especially critical or expensive, including the cost to access and replace the motor. If you’re still not sure, a general rule of thumb is that you should look at an output reactor or dV/dt filter when the cable length from VFD to motor is above 100 feet. You should look at a more advanced filter, like a sine wave filter, when the distance is over 1000 feet or the cables or motor are submerged.

Question 25

What are the most common motor failures?

Accepted Answer

From data of motors passing through a motor shop we work with, approximately half of the motor failures have bearings as the primary failure symptom, with half of those being related to lubrication issues – too much or too little grease, mixing incompatible greases, or contamination in the grease. The rest of the failures spread through many reasons, including winding failures, environmental issues, and many other reasons.

Learn more on their article How to Reduce Motor Downtime and Keep Your Motor Happy.

Question 26

How can I extend the life of my motor?

Accepted Answer

Proper maintenance on a motor is the best way to extend its life. Look at manufacturer recommendations for your specific motor and create a maintenance schedule that follows their guidelines. Both too much and too little grease can be a problem, as can mixing greases, so getting it right is crucial. Also evaluate the motors and perform some form of periodic vibration and health monitoring.

Question 27

Should I repair or replace my motor?

Accepted Answer

For industrial motors, a general rule of thumb is that somewhere in the 75 to 100 horsepower range is when it starts becoming economical to repair rather than replace a motor. Sometimes we’ll see that other factors influence this, with custom or difficult to replace motors making sense to repair if they’re smaller, while motors with extra repair costs (like UL re-listing for classified motors) may lead you to avoid repairing slightly larger motors.

Motor FAQ - Buying, Replacement, Protection, and Maintenance

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