Factors to consider when selecting a Motor for a particular application

When selecting a motor for a specific industrial application, there are several factors and specifications that you should consider. We look at these factors that will guide you when choosing the appropriate motor for your industrial use.

The following are some of the key questions or factors that you should consider when choosing the suitable motor for a given application:

Will the motor start and will it accelerate fast enough?

The torque at zero speed, called the starting torque, is the torque the motor can deliver when rotation begins. For the system to be self-starting, the motor should be able to generate sufficient torque to overcome friction and any load torques. The acceleration of the motor and load at any instant is given by:

Where α is the angular acceleration in rad/sec², T_motor is the torque produced by the motor, T_load is the torque dissipated by the load and J is the total polar moment of inertia of the motor rotor and the load. The difference between motor and load torques determines the acceleration of the system. When the motor torque is equal to the load torque, the system is at steady state operating speed.

How much power does the load require?

The power rating is a very important specification for a motor. Knowing the power requirements of the load, a designer should choose a motor with adequate power based on the duty cycle.

What is the operating duty cycle?

When a motor is not operated continuously, the person selecting the motor should consider the operating cycle of the system. The duty cycle is the ratio of the time the motor is on with respect to the total elapsed time. If a load requires a low duty cycle, a lower-power motor maybe selected that can operate above rated levels but still perform adequately without overheating during repeated on-off cycles.

What power source is available?

Whether the motor is AC or DC might be a critical decision. Also, if battery power is to be used, the battery characteristics must match the load requirements.

What is the maximum speed the motor can produce?

The zero torque point on the torque-speed curve determines the maximum speed a motor can reach. The motor cannot deliver any torque to the load at this speed. When the motor is loaded, the maximum no-load speed cannot be achieved.

What is the load inertia?

Consider this equation:

For fast dynamic response, it is desirable to have low motor rotor and load inertia J. When the load inertia is large, the only way to achieve high acceleration is to size the motor so it can produce much larger torques than the load requires under steady state conditions.

Is accurate position or speed control required?

In the cases of angular position at discrete locations and incremental motion, a stepper motor is a good choice. A stepper motor is easily rotated to and held at discrete positions. It can also be rotated at a wide range of speeds by controlling the step rate. The stepper motor can be operated with open loop control, where no sensor feedback is needed. However, if you attempt to drive a stepper motor at too fast a step rate or if the load torque is too large, the stepper motor may slip and not execute the number of steps expected. Hence, a feedback sensor such as an encoder might be included with stepper motor to check if the motor has achieved the desired motion.

For some complex motion requirements, where precise position or speed profiles are required e.g. in industrial automation applications where machines need to perform prescribed programmed motion, a servomotor maybe the best choice. A servomotor is a DC or AC or brushless DC motor combined with a position sensing device e.g. a digital encoder. The servomotor is driven by a programmable controller that processes the sensor input and generates amplified voltages and currents to the motor to achieve specified motion profiles. This is a closed loop control, since it includes sensor feedback. A servomotor is normally expansive than a stepper motor, but It can have much faster and smoother response than a stepper motor.

For small-scale robotics and hobby projects, an RC servo is a good option. An RC servo is a small DC motor with integral potentiometer to sense shaft angle and feedback electronics to provide position control directed by PWM input signal. The pulse width of the PWM signal dictates the motor position within a limited range of motion. When pulses of a certain width are sent to the motor, the shaft turns to and holds the corresponding position until the pulse width is changed. RC servos can be modified to operate in continuous rotation in either direction at varying speeds.

Is it necessary to reverse the motor?

Some motors are not reversible due to their construction and control electronics. Therefore, you should be careful when selecting a motor for an application that requires rotation in two directions.

Is a transmission or gearbox required?

Often loads require low speeds and large torques. Since motors have better performance at high speed and low torque, a speed-reducing transmission (gearbox or belt drive) is often needed to match the motor output to the load requirements. The term gearbox motor is used to refer to a motor-gearbox assembly sold as a single package.

Are there any size and weight restrictions?

Since some motors can be large and heavy, designers need to be aware of this early enough in the design phase.

Is the motor torque-speed curve well matched to the load?

If the load has a well-defined torque-speed relation, called a load line, it is recommended to select a motor with similar torque-speed characteristics. If you do so, the motor torque in this case, will match the load torque over a large range of speeds, and the speed can be controlled easily by making small changes in voltage to the motor.

For a given motor torque-speed curve and load line, what will be operating speed be?

Consider the graphs below:

From the figure above, for a given motor torque-speed curve and a well-defined load line, the system settles at a fixed speed operating point. Moreover, the operating point is self-regulating. At lower speeds, the motor torque exceeds the load torque and the system accelerates toward the operating point, but at higher speeds, the load torque exceeds the motor torque, reducing the speed toward the operating point. The operating speed can be actively changed by adjusting the voltage supplied to the motor, which in turn changes the torque speed characteristics of the motor.