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Choosing Your Motor

 Resources - Choosing Your Motor


How To Choose An Electric Motor

When selecting a motor there are a number of different requirements to consider to ensure you get the ideal solution to power your product. The information below should offer some guidance in how to do this.

With the guide below covering aspects of motor design such as torque, duty cycle, thermal limits, and efficiency, we hope to give you a starting point when selecting your motor. However, if you require some further guidance, please contact our sales team by email at or by phone on +44(0)1202 512575


One of the most important factors for supplying the correct motor gearbox is ultimately defined by the customer: Power Requirement. This power requirement is the mechanical power required by the application (Watts). The watts are determined by the speed and torque needed for the application, and is not always straightforward for the customer to determine in complex applications. Measuring or calculating these application requirements is the critical first step in motor and gearbox selection and optimisation of the customer application.



Torque is the force multiplied by distance, with common units being Nm, lb-ft and oz-in (conversion table displayed right).

With the torque required defined, understanding the speed requirement is critical. 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 calculated is a good indication of motor gearbox size, but it is only an indication as the same motor output watts can be archived by low speed with high torque or conversely high speed with low torque.

Given Unit   Required Unit
ncm gcm lb-in lb-ft oz-in
Nm 1 10.97x103 8.85 0.738 141.6
gcm 981x10-6 1 8.68x10-3 72x10-6 13.89x10-3
lb-in 0.113 1152.125 1 0.0833 16
lb-ft 1.356 13.825x103 12 1 192
oz-in 7.062x10-3 72.007 0.0625 5.21x10-3 1



Power (Watts) = Torque x RPM x 0.10472

Torque = Power (Watts) / (RPM x 0.10472)

RPM = Power (Watts) / (Torque x 0.10472)

1 Electrical Horsepower (HpE) = 746.000w

1 Metric Horsepower (HpM) = 735.499w

1 Mechanical Horsepower (Hpl) = 745.670w


F = 9/5C + 32

(Celsius to Farenheit)

C = 5/9(F - 32)

(Farenheit to Celsius) 



Torque Speed Graphs display information from the motor gearbox supplier of the performance of the product and are an important tool in application optimisation. They display the subtleties of themotor or motor and gearbox performance with the torque output displayed at all speeds, probably down close to stall.

With these simple motor gearbox torque speed graphs, and the torque and speed determined for the application it is possible to see if the selected motor gearbox can drive the application at all speeds required, and how much torque is still available to the application if needed. With this graph it is also possible to determine the current (amps) requirement for the application so as to aid the selection of drive control and motor protection (as shown below).

Parvalux generally publish 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 a selection of Dynamometers.

Example: The graph shown is for a PM63 DC motor with a GB9 worm wheel gearbox.It can be determined that if the application requires 30Nm, then the output speed from the gearbox would be 37RPM and require 11Amps. Should the load increase to 50Nm then the speed would decrease to 33RPM and require 17Amps



S1 Continuous Duty Cycle The motor works at a constant load for a long enough time to reach temperature equilibrium
S2 Short Time Duty Cycle The motor works at a constant load, but not long enough to reach temperature equilibrium. The rest periods are long enough for the motor to reach ambient temperature
S3 Intermittent Periodic Cycle Sequential, identical run and rest cycleswith constant load. Temperature equilibrium is never reached. Starting current has little effect on temperature rise
S4 Intermittent Periodic Duty with Starting Sequential, identical start, run, and rest cycles with constant load. Temperature equilibrium is not reached, but starting current affects temperature rise.
S5 Intermittent Periodic Duty with Electric Braking Sequential, identical cycles of starting and running at a constant load and running with no load. No rest periods
S6 Continuous Operation with Intermittent Load Sequential, identical cycles of starting and running at a constant load and running with no load. No rest periods
S7 Continuous Operation with Electric Braking Sequential, identical cycles of starting and running at a constant load and electric braking. No rest periods
S8 Continuous Operation with Periodic Changes in Load and Speed Sequential, identical cycles run at constant load and given speed, then run at other constant loads and speeds. No rest periods



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 optimisation of the application 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:

Continuus Duty Cycle (S1)

The thermal rating of the gearbox can be calculated using the following formula:

Approx. Thermal Rating (W) = ((Final RPM x Torque (Nm)) / 9.55) x (1/n) - 1

n = efficiency of the gearbox (available on request)

Intermittent Duty Cycle

For intermittent duty the thermal rating of the gearbox (see table) is increasd by multiplying the appropriate gearbox thermal rating by the factor x:

x = √(100% / Duty Cycle %)

Gearbox Type Thermal Rating (Watts)
Composite Bronze
S 20 25
M 38 45
MB, MF 40 48
L, LH, LB, LF, LHB, LS, LSH 60 72
G, GH 100 -
SS 25 30
MM 45 54
MBM 47 58
SIW 28 38
MIW 50 65



Efficiency of both the motor and the gearbox and the combined efficiency is a topic in its own right, but essentially normally more than the performance variables need to be included in the selection process. Parvalux has great experience in comparing the different motor and gearbox technologies and assessing the trade-off between them in a given application. Parvalux’s experience in supplying many industries is unprecedented and quickly allows discussions on the correct technology for the application. For OEMs requiring 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 in the table shown.

This example is certainly subjective, written in context to a battery operated winch and not exhaustive, but demonstrates other factors when considering efficiency. There are also only two technology combinations considered although many others motor/gearbox technolog 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 many technologies we can offer will save you time when selecting the most appropriate motor and gearbox technology for your application

Technology Type PMDC Motor & Worm Wheel Gearbox
Pros Cons
Simple Components Lower Efficiency
Many Ratios won't Back-Drive
Simple Controller
Simple Maintenance
Low Cost
Technology Type Brushless Motor & Epicyclic Gearbox
Pros Cons
Higher Efficiency Complex Components
Extended Battery Life Electronic Controller
Quiet May Back-Drive
May Need Brake
Greater Cost