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 motor’s identification plate which is usually affixed to the side.
Shown is the ID plate for a TechTop TA2B0223TAI 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 can’t 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.
Is it an AC or DC motor?
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. Here’s their latest catalogue.
Is it a single-phase or three-phase AC motor?
Generally, single-phase motors:
In comparison, three-phase motors:
In addition, there are dual voltage motors that:
Do you prefer cast aluminium or cast iron?
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).
What’s the speed rating?
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.
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, it’s 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, you’ll 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 you’ll damage it.
Speed rating is a critical factor when choosing a motor, as is the power rating.
What’s 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.
*Here’s 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.
What is the shaft diameter and length?
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.
What’s the mounting position?
This tells you how the electric motor can be installed.
Here are the most common examples:
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.
Mind the manufacturer nomenclature
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.
Other important considerations
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.
Refer to our motor selection guide for more details.