Introduction of Motor Sizing and Selection Process # 1

The Motor Sizing and Selection Process

he motor sizing and selection process is based on the calculation of torque and inertia imposed by the

mechanical set up plus the speed and acceleration required by the application. The selected motor must

be able to safely drive the mechanical set up by providing sufficient torque and velocity.

Once the requirements have been established, it is easy to look either at the torque vs. speed curves or motor

specs and choose the right motor.

The sizing process involves the following steps:

1. Establishment of motion objectives

2. Selection of mechanical components

3. Definition of a load (duty) cycle

4. Load calculation

1. Establishment of motion objectives

A written outlining of the motion control application will help to establish the necessary parameters needed for

the next steps.

1. Required positioning accuracy ?
2. Required position repeatability ?
3. Required velocity accuracy ?
4. Linear or rotary application ?
5. If linear application: Horizontal or vertical application?
6. Thermal considerations – Ambient temperature ?
7. What motor technologies are best suited for the application

2. Selection of mechanical components

The engineer must decide which mechanical components are required for the application. For instance, a

linear application may require a leadscrew or a conveyor. For speed transmission a gear or a belt drive may

be used.

1. Direct Drive ?
2. Special application or standard mechanical devices ?
3. If linear application: Use of linear motor or leadscrew, conveyor, etc. ?
4. Reducer required – Gearbox, belt drive, etc. ?
5. Check shaft dimensions – select couplings
6. Check mechanical components for speed and acceleration limitations

3. Definition of a load (duty) cycle

The engineer must define the maximum velocity, maximum acceleration, duty cycle time, acceleration and

deceleration ramps, dwell time, etc., specific to the application.

1. Define critical move parameters such as velocity, acceleration rate
2. Triangular, trapezoidal or other motion profile ?
3. If linear application: Make sure the duty cycle does not exceed the travel range of linear motion device.
4. Jerk Limitation required ?
5. Consideration of thrust load ?
6. Does the load change during the duty cycle ?
7. Holding brake applied during zero velocity ?

4. Load calculation

The load is defined by the torque that is required to drive the mechanical set up. The amount of torque is

determined by the inertia “reflected” from the mechanical set up to the motor and the acceleration at the

motor shaft.

1. Calculate inertia of all moving components
2. Determine inertia reflected to motor
3. Determine velocity, acceleration at motor shaft
4. Calculate acceleration torque at motor shaft
5. Determine non-inertial forces such as gravity, friction, pre-load forces, etc.
6. Calculate constant torque at motor shaft
7. Calculate total acceleration and RMS (continuous over duty cycle) torque at motor shaft

5. Motor Selection

The motor must be able to provide the torque required by the mechanical set up plus the torque inflicted by

its own rotor. Each motor has its specific rotor inertia, which contributes to the torque of the entire motion

system. When selecting a motor the engineer needs to recalculate the load torque for each individual motor.

1. Decide the motor technology to use (DC brush, DC brushless, stepper, etc.)
2. Select a motor/drive combination
3. Does motor support the required maximum velocity ? If no, select next motor/drive.
4. Use rotor inertia to calculate system (motor plus mechanical components) acceleration (peak) and RMS torque
5. Does motor’s rated torque support the system’s RMS torque? If no, select next motor/drive.
6. Does motor’s intermittent torque support the system’s peak torque? If no, select next motor/drive.
7. Does the motor’s performance curve (torque over speed) support the torque and speed requirements? If no, select next motor/drive.
8. If the ratio of load over rotor inertia exceeds a certain range (for servo motors 6:1) consider the use of a gearbox or increase the transmission ratio of the existing gearbox. Servo motors should not be operated over a ratio of 10:1.

As mentioned earlier, a modest oversizing of the motor of up to 20% is

absolutely acceptable. The oversizing factor should be implemented during

the torque requirement checks. In this case it also acceptable to use a higher

factor for the acceleration (peak) torque.

The motor selection process as described also explains the popularity of motor

sizing programs. The process of recalculating the torque requirements for each

individual motor/drive combination can be extremely time consuming

considering the vast amount of motors available in the industry. The goal of

motor sizing is to find the optimum motor for the application and that can only

be accomplished with sufficient choices available, i.e. with a great number of

applicable motors.

The following flow chart demonstrates the motor sizing and selection process:

Picture : Motor Sizing and Selection Flow Chart