How To Choose An Electric Motor | Parvalux Electric Motors

Power requirement

Power requirement is one of the most important factors when specifying a motor gearbox and is the mechanical power required by the application, measured in watts or horsepower. Power is 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 the requirements of the application is the critical first step in motor and gearbox selection and optimization of the customer product.

Power (Watts) = Torque x RPM x 0.10472	1 Electrical Horsepower (HpE) = 746.000w
Torque = Power (Watts) / (RPM x 0.10472)	1 Metric Horsepower (HpM) = 735.499w
RPM = Power (Watts) / (Torque x 0.10472)	1 Mechanical Horsepower (Hpl) = 745.670w
Celsius to Fahrenheit F = 9/5C + 32	Fahrenheit to Celsius C = 5/9(F – 32)

Torque

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

With the required torque defined, understanding the speed requirement is also critical. The speed is defined as rad/s – the equations shown below take care of this and use the more common RPM.

The mechanical power calculated provides a good indication of the motor gearbox size needed but it is only an indication because the same motor output watts can be achieved in ‘low speed with high torque’ or ‘high speed with low torque’ configurations.

Given Unit	ncm	gcm	lb-in	lb-ft	oz-in
Nm	1	10.97³	8.85	0.738	141.6
gcm	981×10^-6	1	8.68×10^-3	72×10^-6	13.89×10^-3
lb-in	0.113	1152.13	1	0.0833	16
lb-ft	1.356	13.825×10³	12	1	192
oz-in	7.062×10^-3	72.007	0.0625	5.21×10^-3	1

Torque Speed Graphs

Torque Speed Graphs display information from the motor gearbox supplier of the performance of the product and are an important tool when choosing an electric motor. They display the subtleties of the motor 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, aiding the selection of drive control and motor protection (as shown). 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 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, the output speed from the gearbox would be 37RPM and require 11Amps. Should the load increase to 50Nm, the speed would decrease to 33RPM and require 17Amps.

Gearbox Thermal Limits

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

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 in 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:

Continuous 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 increased by
multiplying the appropriate gearbox thermal rating by the factor x:

x = √(100% / Duty Cycle %)

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. Rest periods allow the motor to reach ambient temperature
S3	Intermittent Periodic Cycle	Sequential, identical run and rest cycles with 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 Load & Speed Changes	Sequential, identical cycles run at constant load and given speed, then run at other constant loads and speeds. No rest periods

Efficiency

When choosing an electric motor, efficiency is an essential factor. 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.

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

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 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, a winch may need to self-sustain (not move when the power is 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 a permanent DC motor & worm wheel gearbox. Parvalux’s experience with the many technologies we can offer will save you time when choosing an electric motor and gearbox technology for your application.

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