Considerations When Choosing an AC Induction Motor

There are many aspects to consider when choosing an ac induction motor. Basic design compliance by motor manufacturer with NEMA standards including system supply voltage, horsepower rating, and then considering the application itself . The following brief preliminary information helps when specifying the correct motor.

However, today’s AC induction motors are not often given the design consideration the application deserves. The thought is "one size (or type or manufacturer) fits all" and the selection of a motor only depends on its voltage, power, and base speed. However, there are many other aspects to consider such as supply voltage, horsepower rating, and the application itself. What is the motor's purpose? Does it meet the NEMA efficiency standards? Does it meet the environmental conditions? Does it meet the site service conditions?

Induction motor fundamentals 

There are starting point for specifying a motor and supply voltage, the horsepower rating, and the base speed for the particular application are the driving questions. However, other important consideration is the enclosure type, the environmental or IP ratings, duty conditions and total motor efficiency. Will the motor have mounting feet, a flange on the drive end, or can it have both? What are the environment's characteristics? Is rain or falling water a possibility? Is dirt or dust exposure a potential issue? In these instances, total enclosed fan cooled (TEFC) or total enclosed non-vented (TENV) motors are a good option. If the environment is clean, dust free and dry, an open drip proof (ODP) motor may be sufficient.

Inverter duty considerations

Due to incentives from local power companies combined with the advantages and benefits of changing the operating speed of motors, variable frequency drives (VFDs) are becoming a common methods of controlling motors and load speed. In these cases, consideration needs to be given to proper cooling as the VFD slows the motors speed (what is the motors turn-down ratio)? There are several features a motor suited for use with a VFD should have; however, two stand out at the top.

The first consideration is the voltage rating of the magnetic wire used in the motor windings. Today’s motor windings can be rated from 1000 to 1700 V, with a very common value of 1200 V. Typically, the greater the distance between the drive and the motor the greater the magnitude of the voltage spikes which can reach the motors windings from the VFD. A motor that has a 1700 V magnetic wire may have a reference to NEMA MG-1 2003, Part 31, Section 4, which states that a motor shall resist corona inception voltage (CIV) damage at voltages up to 1,600 V. Does your motor manufacturer meet these conditions?

The second consideration may be the constant torque (CT) turn-down ratio of the motor, often listed a "xx:1 CT". This expresses how slow the motor can run and still deliver the same torque as it would at rated speed. Below this value the torque production capabilities of the motor decrease. For example, consider a 10 HP motor with a base speed of 1,800 RPM. It can produce 29 lb-ft of torque at rated speed (approximately 1800 RPM). If it has a 10:1 CT rating, it can produce that same torque down to 180 RPM. If it is a 1000:1 CT, then it could produce 29 lb-ft of torque as low as 1.8 RPM.

Confirm with your manufacture how the motor stays cool if it has the typical shaft mount fan and the motor is running at a fraction of the rated speed. Does the motor fan move sufficient air at low speeds to maintain the thermal margin within the motor frame? If the motor is running at low speed and producing high torque for an extended time, how much heat is produced and will the motors insulation life be reduced?  Should you consider a different cooling method of cooling?  Do you need to choose a fixed blower-cooled motor design? The blower has its own motor, separately controlled-not by the attached VFD. The airflow across the motor is constant and sufficient to keep the motor cool-even at low speed or zero speed.

Choosing between horsepower and torque

The base speed of the motor is another important aspect when choosing an ac induction motor. It is common to see 2-pole (3600 RPM) and 4-pole (1800 RPM motors). However, 6, 8, 10, and 12-pole (1200, 900, 720 and 600 RPM) motors can be found as well. The base speed of a motor is related to the number of poles the motor contains by this equation: [RPM = (120 x design frequency) / # poles]. As a side note; although related, typically as the number of poles increase so does the size as well as overall costs.

Users might also want to swap speed for torque, depending on the application. In general, as the speed of the motor increases, the torque decreases, which is also true of gearboxes and belt and chain drives. This relationship is explained by this equation: [HP = (torque x base speed) / 5252)].

The horsepower and base speed of the motor, along with any associated gearing, can be selected to meet the original goal, which is reducing the cost of equipment, size, and lead time for replacement components.

What are IP Ratings in Electric Motor

Ingress Protection (IP) ratings are now typically used to define the motors condition and classify how well it’s protected against both solids (dust) and liquids. Most motor manufacturers can offer in a TEFC enclosed enclosures an IP54, or IP55 rating as standard.  A typical IEEE-841 motor are increased to a IP56-rated as a premium cast iron offering. As a brief explanation IP55 is rated as “Dust Protected” (5) and “Protection Against Water Jets From Any Angle” (5).  The IP55 rating meets or exceeds most general industrial applications.

Motor manufacturers can accomplish this level of protection by various means, usually it adding features to the motor (shaft seals / enclosure sealant / terminal box gaskets) or precision machining and rabbiting of fitted surfaces. Cast iron motors surfaces heat and cool at different rates and thus certification of the enclosure is required. When a motor manufacturer certifies a IP rating, they take this into consideration in order to protect the motors core and coils to guarantee internal protection. Motors IP ratings can very between IP20 and IP65 with optional features requirements added. I hope you find this helpful. Let me know of your comments.