Aug. 06, 2024
However, todays 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?
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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. Todays motor windings can be rated from to V, with a very common value of 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 V magnetic wire may have a reference to NEMA MG-1 , 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 RPM). If it has a 10:1 CT rating, it can produce that same torque down to 180 RPM. If it is a :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 ( RPM) and 4-pole ( RPM motors). However, 6, 8, 10, and 12-pole (, 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) / )].
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 its 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.
Here are questions we might ask to help you identify the electric motor youre looking for.
Shopping for electric motors can be tricky. There are just too many details and specifications which you must be certain of when looking for one.
Of course, the easiest scenario is that you have the motor you want to replace, and you just provide the vendor with the motors identification plate which is usually affixed to the side.
Shown is the ID plate for a TechTop TA2BTAI aluminium motor
Most motors have such a plate that has important specifications, including:
Some information, such as mounting options (foot vs flange, or both), are typically added post-manufacturing, as they could vary depending on the customer order. Therefore, you cant rely on the identification plate for that information.
This article walks you through the questions to point you in the right direction when choosing an electric motor, when you should consider:
Studies generally indicate an average lifespan of over 12 years for some motor types under normal operating conditions. Nevertheless, it depends heavily on several factors, such as:
Read more about them in detail in these supplementary articles about:
The most apparent difference, as the names suggest, is the type of current that powers the motor.
Alternating current (AC) motors:
In comparison, direct current (DC) motors:
We sell a wide range of AC induction motors from TechTop Australia. Heres their latest catalogue.
Generally, single-phase motors:
In comparison, three-phase motors:
In addition, there are dual voltage motors that:
Are designed to use either of two stated voltages (usually 240 V and 480 V) but are wired differently to maintain the motor output
Are commonly used in country areas where 240V is not available, so they have the option to take one of the three phases from the mains supply and run with a neutral to get 480V
Cast aluminium electric motors:
In comparison, cast iron electric motors:
Consider these two remarkably similar motors in terms of specifications, and how one (the cast aluminum motor) is significantly lighter than the other (the cast iron motor).
The speed of a motor is derived from its frequency rating and the number of poles it has. This is rated by the number of revolutions per minute (RPM).
Generally, the more poles (or pole pairs), the slower the speed of the motor, but the greater its torque.
For example, a 2-pole motor will be twice as fast as a 4-pole one, which will be twice as fast as an 8-pole one, and so on.
For more information, please visit Asynchronous Motor Suppliers.
To calculate rated RPM:
[120 x Frequency] # of poles
So, using the Australian standard of 50 Hz, the speed of a 2-pole motor is:
[120 x 50 Hz] 2 = 3,000 RPM
That of a 4-pole motor:
[120 x 50 Hz] 4 = 1,500 RPM
That of an 8-pole motor:
[120 x 50 Hz] 8 = 750 RPM
Also, its important to note the difference between synchronous and operating speeds:
*Using an automotive analogy, synchronous speed is like brake horsepower (engine power measured at the flywheel), while operating speed is like wheel horsepower (engine power measured at the wheels, factoring in inertia, power transmission inefficiencies and vehicle weight).
Important: If you have a three-phase motor and you want to operate the machine at different speeds, youll need a variable frequency drive (sometimes called a variable speed drive). As suggested by the name, it controls the frequency of the electricity powering the motor, which, together with the number of poles, dictates the speed of the motor.
Otherwise, you will have to use a gearbox to alter the speed.
The rule of thumb is that you should go no more than +/- 20% of the rated speed. So, for instance, you should not run a motor rated at 3,000 rpm any faster than approximately 3,600 rpm or any slower than approximately 2,400 rpm, or youll damage it.
Speed rating is a critical factor when choosing a motor, as is the power rating.
This number tells you the output capacity of the motor, or the amount of work that can be completed in each amount of time. In terms of small electric motors, this is (often) rated in kilowatts and horsepower.
*Heres a kilowatt-horsepower calculator for your convenience.
Of course, this is not to be confused with torque, which is the actual rotary force produced by the motor the one that turns the shaft. Torque is quantified in Newton metres (Nm).
To emphasise, aside from speed rating, power rating is the other critical factor when deciding which electric motor to choose.
In terms of compatibility, proper fitting and alignment of the shaft with the component to be moved (eg. fan, pulley, etc) is critical to the function, safe operation and life span of the electric motor.
In general:
This tells you how the electric motor can be installed.
Here are the most common examples:
On the TechTop motors we sell, the foot mount can be repositioned to the side of the motor, so it is installed with the terminal box on the side instead of the top, as shown in the image.
According to the TechTop Installation and Maintenance Manual, the motors should ideally be mounted to a rigid base to prevent excessive vibration. Shims may be used if the foundation is uneven and should be placed next to and between the motor mounting bolts.
*Mounts and size chart (page 28 of the TechTop catalogue)
Important: Installation must only be done by qualified personnel. When improperly installed or used, rotating equipment can cause serious or fatal injury. As always, please refer to the installation manual.
Keep in mind that different manufacturers may use different nomenclature when labeling their motors.
Below is an example from TechTop motors:
*Shown is the part number logic for the TechTop TM Series of aluminium single-phase motors
Specific considerations when buying TechTop motors:
Important: Choose the correct option for your application as motors used in the wrong application will fail quickly.
Make sure:
This refers to the ingress protection (IP) ratings of the motor, which is particularly important in some use cases, such as in marine and mining applications. It is possible to have the motor upgraded to higher IP ratings as needed. The TechTop motors we sell are mostly IP55: protection against dust limited ingress (no harmful deposit) and low-pressure jets of water from all directions.
Read more about electric motor IP ratings.
Do not:
Refer to our motor selection guide for more details.
(Refer to our content library's sub-index of articles about belt-driven systems and electric motors for more information.)
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