Tips for selecting small electric motors

Selection of the most appropriate electric motor for small applications results from analysing its load requirements for each application, as well as other more specific considerations such as work, system efficiency, motion requirements for the application, or the wear it is subjected to during its daily operation. We bring you some tips for you to select the right small electric motors. Keep reading! 

As a purchase manager, it may be the first time you are tasked with selecting the most suitable motor or gear motor for the latest project your company is developing. You are aware that there is a wide spectrum of actuator systems, but which one can offer the best performance for your application?

Questions you should answer when selecting small electric motors

• Under which environmental conditions will the electric motor operate?

Manufacturers set limits where the motor’s operation is no longer optimal under high or low-temperature conditions. Also, ask yourself whether the manufacturer has performed corrosion tests, since some machines degrade when exposed to saline mist or other environmental conditions.

• What protection rating does the motor require?

The motor’s spec sheet should include an alphanumeric code, known as the IP rating, which specifies the degree of protection its materials have against the ingress of dust or fibres, accidental contact and ingress of liquids. The most common ratings in electric motors are IP21, IP22, and IP55, even though it will all depend on the type of application.

• How will the electric motor be placed within the setup?

It should be clear to you how the motor shaft’s position will relate to that of the output shaft of the speed reducers. Positions can be of three types: horizontal position, special application, and vertical position.

• What power rating does the application require?

In order to answer this question, you will need to know what the rotational speed and required torque will be. With this information, you will be able to select a motor that fits the power rating requirements of the application; in other words, the power a motor is capable of providing without its temperature damaging the motor’s insulation material due to overload.

• What duty cycles will the electric motor be subjected to?

Duty cycles are related to the power required by the application. There are four types:

1. Continuous operation: the load is constant during long periods of operation, and is enough to reach a stable temperature, as is the case for slush machines, for example.
2. Variable continuous operation: operation periods are long, but the load level is not always the same, as is the case for handheld power tools.
3. Intermittent operation: the motor’s operation periods are separated by rest periods where there is no connection to a power grid. The relation between operation time and the total duration of the working cycle will provide its duty cycle. This type of operation is seen in certain applications such as farm ventilation, window opening and incubators.
4. One-hour operation: the motor operates for one hour at a constant level that exceeds the continuous operation level, but the temperature does not reach stabilisation, and therefore, does not damage the insulation material.

• Which rotational speeds will the motor require?

Knowing the rotational speed is paramount, since by combining it with the torque –force at which a motor can hold its step, measured in Newtons·Centimetre (N·m or N·cm)– you will be able to obtain the power required by the machine to perform the small actuation. Depending on this rotational speed, electric motors may be asynchronous or synchronous.

• How many start-ups will be required per operation?

The number of start-ups limits the life cycle of small electric motors, since at start-up, motors require a larger current than normal in order to go from a resting state to its operational speed. In the case of brushless motors, they possess a high starting torque, and suffer less wear than brush motors.

The motor’s starter type may be:

1. Direct: the starting current is normal; therefore, the motor receives a current that is three to seven times the nominal current. The starting torque will be higher than the rated torque.
2. Star-triangle: used for limiting the intensity at start-up. For this purpose, the current and torque are reduced up to three times when compared with the nominal current.
3. Autotransformer: delivers a voltage that is lower than nominal to the motor, which will be between 30% and 70%. The current and torque will be the square of the input voltage.
4. Soft starter: the motor receives a reduced voltage that will progressively increase up to the rated voltage.
5. Variable speed drive or variable frequency drive: it is the best start-up method, since it allows for smoother transitions and a precise speed control.

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Furthermore, depending on the application, you should look for a motor with a specific type of motion (step motors, for example) and with the ability to vary the rotational direction and speed.

Lastly, among the manufacturer’s specifications in the motor’s spec sheet, you will have to pay special attention to two items: the torque and the maximum performance current or rated current. The latter, which is measured in amperes (A), is the maximum current that can flow continuously through the motor.

Do you need help with your selection? CLR, Compañía Levantina de Reductores has the necessary experience to assist you with selecting the electric motors that best suit the requirements of your project, no matter which industry your company belongs to.