D R . T A R E K A . T U T U N J I

P H I L A D E L P H I A U N I V E R S I T Y , J O R D A N

2 0 1 4

Motor Selection Criteria REV II

Actuators

Actuators are the muscle behind a mechatronics system that accepts a control command (i.e. electrical signal) and produces a change in the physical system by generating force, motion, heat, flow, etc.

Actuators Selection

Affects the system’s dynamic behavior

Dominates the power needs

Affects the coupling mechanisms of the system

DC vs. AC Motors

DC Motors AC Motors

Advantages Disadvantages

Simple Speed Control Not applicable for low speeds

Simple Position Control Poor Positioning

Simple Drive Circuit

Disadvantage Advantage

High maintenance Reliable

Low Cost

High Power

Motor Requirements

The motor must be capable of matching the power requirements of the driven load. Therefore, the motor power available should be enough to cope with

the anticipated demands of the load.

The motor must have enough torque available on start-up to overcome the static friction, accelerate the load up to the working speed, and be able to handle the maximum overload.

Some systems also therefore require a “soft start”

whereby the motor torque is gradually increased to allow the load to accelerate gently.

Motor Requirements

The operating speed of the motor will be fixed by the point at which the torque supplied by the motor is just balanced by the torque requirements of the load.

At any other condition, the motor and load will be either accelerating or decelerating.

Correct matching of a motor to a driven machine can only be confidently accomplished if both the motor and the load torque–speed characteristics are known.

The motor torque–speed characteristics are usually provided by the supplier.

Motor Requirements

Fans and blowers have a torque–speed characteristic that increases parabolically from zero as the speed increases. Such machines do not, therefore, need much motor torque to enable

them to start.

High inertia devices like machine tool drives, rolling mills, and electric lifts require a large torque on start-up to overcome the inertia. Once motion is established the torque requirements tend to decrease with increasing speed. The series-wound dc motors are ideal for these types of loads.

General Objective

Get the best performance for the best price

Find the smallest motor that fulfills the requirements

Matching objectives

Match the motor’s torque with the load torque as close as possible

Match the motor’s inertia with the load inertia as close as possible

Find a motor that matches or exceeds the required speed

Motor Selection Steps

1. Determine the drive mechanism

2. Set specifications

3. Calculate the load torque and inertia

4. Select the motor

1. Determine the Drive Mechanism

Linear or rotational motion Choose type for linear motion

Ball screw, belt pulley, conveyer belt

Choose power transmission Gears

Determine the dimensions and mass for the load and all parts Check shaft dimensions – select couplings Check mechanical components for speed and acceleration

limitations

Mechanical Components

For linear movements: Conveyor Leadscrew Rack-Pinion Linear Actuator

For speed transmissions:

Gear Belt Drive Chain Drive

For other purposes: Coupling Brake Encoder

Drive Mechanisms

2. Define Specifications

Velocity

Acceleration

Motion profile

Total mass

Friction coefficients

Positioning distance

Resolution

Accuracy

Power

Frequency response

Operating environment

Temperature considerations

Linear or rotary motion

Vertical or horizontal

Load variation

Jerk limitation

Thrust load

3. Calculate inertia and torque

Calculate inertia of all moving components Determine inertia reflected to motor Determine velocity, acceleration at motor shaft Calculate acceleration torque at motor shaft Calculate constant torque at motor shaft Calculate total acceleration and RMS torque at motor

shaft

Velocity Profile

Inertia and Torque

4. Select Motor

Select the motor type: DC or AC In general, use AC for constant speed and high torque and DC for speed and

position control For DC: Servo, Stepper, or brushless

Select the specific motor to use Use Performance (torque-speed) curves supplied by the manufacturers

The basic selection criteria are:

The motor’s rated speed must be equal to or exceed the application’s maximum speed

The motor’s intermittent torque must be equal or exceed the load’s maximum (intermittent) torque

The motor’s rated torque must be equal to or exceed the load’s RMS torque The ratio of load inertia to rotor inertia should be equal to or less than 6:1

DC Motor Selection Example

Objective

Motion Profile

Load Calculations

Load Speed

Torque Calculations

Torque Calculations

Motor Requirements

Torque-Speed Curves

Increasing Drive Voltage

Torque-Speed Curves

Ok Failure

Holding Brake

The effect of a holding brake is important for vertical linear applications.

The main purpose of a holding brake is to relief the motor from maintaining the holding torque during standstill periods in a vertical linear motion application.

The holding brake adds inertia to the motor load, therefore increases torque and power requirements during acceleration and deceleration

Vertical Applications

Load Inertia and Torque Calculations: Basic Equations

Basic Calculations

The motor selection is based on speed, inertia and torque comparisons. The calculation of mechanical devices is used to determine these parameters.

Even the most complex mechanical devices are calculated based on three “basic” components:

Solid Cylinder

Hollow Cylinder

Solid Rectangular

Solid Cylinder

Hollow Cylinder

Rectangular Block

Basic Equations

Basic Equations

Basic Equations

Basic Equations

Disc

Chain Drive

Gears

Conveyer Belt

Lead Screw

Rack and Pinion

Load Calculations

Motor Load Transmission

Example: Servo Motor Selection

Example: AC Motor Selection

Summary

Motors are selected in order to provide the appropriate motion to the plant under control

Motor selection is an important part of the mechatronics design process

This selection affects important issues, such as dynamic behavior and power consumption

The selection process can divided into five steps:

References

Wilfred Ross, Copperhill technologies applications, 2007

Oriental Motor General Catalogue 2012/2013