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RESOURCES

CHOOSING YOUR MOTOR 

When choosing the right motor gearbox for an application, one of the most important factors is defined by the customer: power requirement.
The power requirement is the mechanical power required by the application (measured in watts) and it's determined by the speed and torque needed for the application. However, it's not always straightforward for the customer to determine this figure when the application is complex. Measuring or calculating these application requirements is the critical first step in motor and gearbox selection, to make sure it's optimised for the customer's application.

Torque is the force multiplied by distance, with common units being Nm, lb-ft and oz-in (conversion table displayed below). Once the torque required is defined, understanding the speed requirement is critical as the mechanical output power is a function of torque and speed. The speed is defined as rad/s, but the equations shown below take care of this and use the more common RPM. The mechanical watts figure calculated is a good indication of motor gearbox size but it is only an indication, as the same motor output watts could be arrived at by selecting low speed with high torque or conversely, high speed with low torque.

Torque Speed Graphs display information from the motor gearbox supplier about the performance of the product and are an important tool to ensure optimisation with the customer's specific application. They display the subtleties of the motor or motor / gearbox performance with the torque output displayed at all speeds, often down somewhere close to the stall speed. With these simple motor gearbox torque speed graphs and the torque and speed determined for the application, it's possible to see whether the selected motor gearbox is able to drive the application at all the speeds required and how much torque is still available to the application. With this graph it is also possible to determine the current (measured in amps) requirement for the application to aid the selection of drive control and motor protection (as shown below). Parvalux generally publishes motor gearbox data in table form due to the sheer number of combinations and variants possible but Torque Speed curves are produced in-house on our dynamometers.

Duty Cycles for electric motors range from S1-S8 and this describes their type of use: continuous use, where the application is using the motor gearbox continually, or some form of intermittent use. The power rating of the motor and gearbox can very often be increased if it is being used intermittently. Ultimately this allows the optimisation of the customer application by the use of smaller motor gearbox when intermittently used.


Gearbox Thermal Limits are a limiting factor when using motor gearboxes in excess of the normal continuous S1 duty cycle. This continuous duty cycle is most likely the duty cycle displayed in the Torque Speed graph but Parvalux has many years’ experience optimising to meet the needs of a range of applications and has comprehensive data on our gearboxes for intermittent use. Approximate Parvalux gearbox rating limits can be calculated for both continuous and intermittent duty cycle by using information below:


Efficiency of both the motor and the gearbox and their combined efficiency is a topic in its own right but essentially, it's normal for more than just the performance variables to be included in the selection process. Parvalux has deep experience in comparing the different motor and gearbox technologies available and assessing the trade-off between them in a customer's application. Parvalux’s experience supplying a wide range of industries enables our customers to have informed discussions on the best technology for the application. For OEMs that require large volumes, ‘Technology Rigs’ can be constructed using different technologies to empirically compare data in the application but many non-performance variables inevitably need to be added to the selection process when seeking ultimate efficiencies (costs, marketing benefits, system complexity etc). 

A very simplified example of the ‘trade-offs’ when considering efficiency is displayed in a battery operated winch example, below.

The example above is subjective, written in context to a battery operated winch and not exhaustive but demonstrates some of the other factors to be included when considering efficiency. In this example we've only considered two technology combinations, although many other motor/gearbox technology combinations could be compared. When comparing technology groups for ultimate efficiency you may wish to select a brushless motor with epicyclic gearbox, which in general, would give you the best efficiency but at additional cost and complexity. For example, being a winch it may need to self-sustain (not move when the power if off but the load is still applied), which a worm-wheel gearbox can achieve due to its inefficiencies. The epicyclic gearbox may well not self-sustain due to its increased efficiency, therefore requiring extra items such as a mechanical brake and all the control complexities that go with it. The cost of the additional brake, its control, along with the PWM controller for the brushless motor, can be considerable when compared to the simplicity of Permanent DC Motor & Worm Wheel Gearbox.

Parvalux’s experience with the full range of technologies means we can save you time when selecting the most appropriate motor and gearbox technology for your application.