Machine Design #03: Choosing a Motor for Machine Design – Part 1

This article gathers the background knowledge and the selection principles for the motors most commonly encountered in machine design, including the popular motor types and the application characteristics of each. It is written as a systematic set of engineering notes, meant to support reference lookups and the mechanical design process.

Choosing between motor types – the basics of motor selection

The fundamental difference between AC/DC and Servo/Stepper
How an AC motor driven from an alternating-current supply works
Overview of the commercial power supplies on the market
Torque characteristics of AC motors
Environmental resistance of AC motors
Representative makers and products for AC motors
How a DC motor driven from a direct-current supply works
Securing a DC supply
Torque characteristics of DC motors
Environmental resistance of DC motors
The induction motor – the workhorse of industry
The advantages of a high-efficiency brushless DC motor
Representative makers and products

A perspective on selecting and classifying motors in mechanical design

Understanding the basic parameters: torque and rotational speed
The concept of inertia and its link to responsiveness
Accurate load calculation and the selection procedure
The critical role of the motor driver
Driver for a brushed DC motor
Driver for a brushless DC motor
Driver for a stepping motor
Driver for an AC servo motor (servo amplifier)

1. Choosing between motor types – the basics of motor selection

The fundamental difference between AC/DC and Servo/Stepper

When selecting a motor, engineers often run into two questions: "AC or DC?" and "stepping motor or servo motor?" The important thing here is to understand that these classifications rest on different axes of evaluation.

First, the split between an "AC motor" and a "DC motor" is based on the type of power source used to run the motor. An AC motor runs on the alternating current from a wall socket, while a DC motor runs on direct current from a source such as a battery or an accumulator. This is the most basic classification, and it ties directly to the physical construction and electrical characteristics of the motor.

The split between a "stepping motor" and a "servo motor", on the other hand, is mainly based on the control method and the intended purpose. A stepping motor rotates through defined angles in response to pulse signals, using relatively simple open-loop control to reach an accurate position. A servo motor, by contrast, is a system that uses closed-loop control with feedback from an encoder, allowing position and speed to be controlled with high accuracy and high speed.

These classifications are not fully independent; they can be combined. For example, the servo motor in common use today is the AC servo motor, which runs on an AC supply. Because of this, a design engineer should first define what the equipment needs — continuous rotation or accurate positioning — and decide the control method from there. Next, look at the available power source, which then efficiently narrows down the motor options.

How an AC motor driven from an alternating-current supply works

"AC motor" is the general term for motors that run on the alternating current (AC) supplied from a domestic or industrial socket. This commercial power supply is electricity delivered by the power utilities, and its specifications differ considerably depending on where the machine is used.

Overview of the commercial power supply The commercial power supply in Japan is classified mainly as follows:

Single-phase 100 V: the most common supply in homes.
Single-phase 200 V: used for household appliances that draw more power, such as air conditioners and induction (IH) cooktops.
Three-phase 200 V: the standard supply for industrial equipment, such as production equipment in factories, which demands high power.

To decide whether the power supply for the machine you are designing is compatible with the supply required to run an AC motor, the first step is to check the environment where the machine will be installed (home, office, factory, and so on) and the type of power that is available there.

The strength of an AC motor is that it can be used directly with this commercial supply, or through simple auxiliary devices, while also being simple in construction and highly durable.

However, when designing a product for export, take particular care: the voltage and frequency can differ from country to country and region to region.

Torque characteristics of AC motors AC motors, and induction motors in particular, are characterised by stable torque near their rated speed. At start-up the motor produces a certain level of torque (the starting torque). As the rotational speed rises, torque also rises to a peak, then falls slightly and stabilises as it approaches the rated speed. This characteristic changes with the number of poles of the motor. As a rule, a motor with few poles (such as a 2-pole motor) is a high-speed, low-torque type, while a motor with many poles (such as a 6-pole motor) is a low-speed, high-torque type.

Environmental resistance of AC motors AC motors have no wear-prone parts such as brushes and have a robust construction, so on the whole their environmental resistance is very good. Induction motors in particular can run stably even in dusty environments such as factories, or in places with a certain amount of temperature variation. There are also many models with a high protection rating (per the IP standard) to strengthen resistance to dust and water. A motor rated IP65, for example, can fully block the ingress of dust and withstand water jets from any direction, which makes it suitable for outdoor equipment that may come into contact with water, or for the wash-down stages of food-industry machinery.

Representative makers and products for AC motors When selecting an AC motor, it is common to look at products from makers with a strong track record and reputation.

Oriental Motor Co., Ltd.: a company with a large share of the small AC motor market. Its "KIIS Series", optimised for continuous operation such as conveyors, is widely used across many FA (factory automation) devices. URL: https://www.orientalmotor.co.jp/ja/products/ac-motors/induction/

Sumitomo Heavy Industries, Ltd.: a strong track record in large industrial geared motors. It offers standout products such as the "Hyponic® gearmotor", characterised by high efficiency and low noise, and the "Prest® NEO Gearmotor", which focuses on ease of use. URL: https://cyclo.shi.co.jp/

How a DC motor driven from a direct-current supply works

A DC motor is a motor that runs on a direct-current (DC) source such as a battery or an accumulator. By changing the applied voltage you can easily control the rotational speed, so this type of motor is widely used in equipment that needs speed adjustment or in portable devices.

Securing a DC supply When deciding whether the machine you are designing needs a DC supply, you do not have to assume battery operation. In practice, choosing a DC motor is very common even in an environment where an AC supply is available. In such cases, a DC supply is usually created from the AC supply by one of the following methods:

Using an AC adapter: the simplest way to obtain the low-voltage DC (such as DC 12 V or DC 24 V) used by electronic devices, taking it from a household AC 100 V socket.
A built-in switching power supply: widely used in the control cabinets of industrial equipment to create a stable DC supply from AC 200 V.

In this way, even in an environment that uses an AC supply, building a DC-motor system through an AC/DC converter has become one of the standard methods in machine design — a way to exploit advantages such as easy speed adjustment, high responsiveness, or a smaller motor.

Torque characteristics of DC motors The torque characteristics of a DC motor differ considerably depending on whether brushes are used.

Brushed DC motor: produces the greatest torque at start-up or at low speed, and tends to fall off linearly as the rotational speed rises.
Brushless DC motor: thanks to electronic control, it can produce stable, flat torque held across a very wide speed range, from low to high.

Environmental resistance of DC motors A brushed DC motor has a drawback: the brushes and commutator are in direct contact and are therefore prone to wear, which makes them weak against dust and moisture. When such contaminants get in, they can accelerate wear or cause conduction faults, so when used in a harsh environment, protective measures such as a shielding cover are essential.

A brushless DC motor, by contrast, has no mechanical contact parts, so it is easy to seal the interior completely and it can achieve high resistance to dust and water (for example, an IP67 rating). This makes it well suited to outdoor equipment such as drones, or to equipment in the medical and food fields that needs frequent wash-down cleaning.

Understanding these detailed differences between AC and DC motors helps reduce mistakes when selecting the right motor type.

The induction motor – the workhorse of industry

The induction motor is one of the most widely used AC motors. Because of its simplicity and rugged construction, it is even called "the workhorse of industry". The word induction refers to the very principle by which this motor rotates.

The rotation principle of the induction motor can be explained through a physical phenomenon known as "Arago's disc". When you spin a magnet above an aluminium or copper disc, the disc is dragged along and rotates behind the magnet.

Mapped onto a motor, it plays out as follows:

When alternating current flows through the windings of the stator (the stationary part), a rotating magnetic field is created (playing the role of the spinning magnet).
This rotating field passes through the rotor (the rotating part), which is a conductive body (usually a squirrel cage of aluminium).
By the law of electromagnetic induction (Fleming's right-hand rule), an induced current is generated in the rotor.
With current present, the rotor experiences a force within the magnetic field (Fleming's left-hand rule) and begins to rotate in the direction of the stator's rotating field.

The single biggest feature of the induction motor is that it transmits rotational force by electromagnetic induction, with no mechanical contact at all. Thanks to this principle, the motor does not need wear-prone parts such as brushes, which gives it high reliability and easy maintenance.

The advantages of a high-efficiency brushless DC motor

The brushless DC motor is seen as a symbol of how motor technology has advanced in recent years, and it is gradually replacing the traditional brushed DC motor in many devices. Its advantages are wide-ranging.

The single biggest advantage is a long life with almost no maintenance. Because there are no brushes to wear out mechanically, the motor needs no periodic replacement, which markedly raises the reliability of the equipment. In addition, because there is no sparking or electrical noise from brush contact, the motor is also suitable for use in precision equipment or clean environments.

Another important advantage is very high energy efficiency. Because there is no loss from brush friction, the motor consumes less power for the same output. This is especially useful for battery-powered equipment such as cordless machines and drones (extending flight time), as well as for improving the energy efficiency of home appliances.

In this way, the brushless DC motor brings together superior characteristics in efficiency, reliability, and effectiveness, making it an indispensable choice in modern mechanical design.

Representative makers and products for brushless DC motors Brushless DC motors are produced by many makers across many fields, from small precision devices to large industrial equipment.

Nidec Corporation (Japan): the global market leader in spindle motors for hard-disk drives (HDDs), and, building on that technology, a developer of many compact, precise BLDC motors. URL: https://www.nidec.com/jp/

MinebeaMitsumi Inc. (Japan): has a rich brushless DC motor lineup, from small and thin types to high-power types, widely used in digital AV equipment, computer peripherals, and automotive electronics. URL: https://product.minebeamitsumi.com/product/category/rotary/brushless/

2. A perspective on selecting and classifying motors in mechanical design

Understanding the basic parameters: torque and rotational speed

The two most basic figures in motor selection are torque and rotational speed. Understanding these two parameters correctly is an important first step toward choosing the right motor.

Torque: the force the motor applies to rotate its shaft, measured in newton-metres (Nm). In design you need to distinguish between the rated torque (the force the motor can sustain continuously) and the peak torque (the force the motor can produce for an instant at start-up or rapid acceleration). To drive the equipment, the torque must be enough to overcome resisting forces such as friction and gravity.

Rotational speed: the number of revolutions per minute, measured in rpm (revolutions per minute). In many motor types, torque and rotational speed have an inverse relationship; that is, when you want to spin fast, the motor finds it harder to produce large torque. So motor selection has to ensure that the motor supplies enough torque at the operating speed the machine requires.

The concept of inertia and its link to responsiveness

Inertia (in Japanese, the moment of inertia) is a figure that expresses how hard an object is to turn — or its ability to keep its state of rotation. This concept is especially important in drive systems that call for fast acceleration and deceleration or accurate positioning.

When selecting a motor, you need to consider both the moment of inertia of the motor's rotor and the moment of inertia of the load the motor drives (the whole machine mechanism). The ratio between these two values is called the inertia ratio.

If the inertia ratio is too large, the motor struggles to control the load as intended, leading to vibration, longer settling times, and similar problems. It is like trying to swing a hammer that is too heavy: the hammer is hard to guide accurately to the point you want to strike.

For a servo motor in particular, which demands high responsiveness, keeping the inertia ratio within the maker's recommended value (typically 10 times or less) is a mandatory condition for stable control.

Accurate load calculation and the selection procedure

Motor selection should not rest on gut feel or experience alone; it should follow a systematic procedure to avoid mistakes. This procedure has four main steps:

Define the technical requirements First, define concretely how the equipment operates (travel distance, speed, timing), the mass and type of the object to be moved, and the operating environment.

Calculate the load Calculate the force needed to meet the above requirements, including:

Continuous load torque: the force needed to overcome friction and gravity.
Acceleration torque: the force needed to accelerate the object, proportional to the moment of inertia described earlier. This calculation is especially important for equipment that moves.

Preliminary motor selection Based on the calculation results (the required peak torque and the maximum rotational speed), consult the maker's catalogue and draw up a list of candidate motors.

Final check and confirmation Confirm the inertia ratio, check whether the temperature during continuous operation stays within the permitted limit, and thereby select the most optimal motor for the application.

The critical role of the motor driver

A motor will not run as intended simply by being connected directly to a power source. To draw out the full capability of the motor and carry out accurate control, a motor driver — an electronic circuit — is indispensable.

The functions required of a motor driver change considerably with the motor type, so selecting the motor together with its matching driver is extremely important.

Driver for a brushed DC motor The driver for a brushed DC motor is relatively simple. Its main role is to adjust the voltage supplied to the motor to control its speed, usually done with pulse-width modulation (PWM). In addition, by using an H-bridge circuit to change the direction of the current, the driver can reverse the direction of rotation (forward and reverse).

Driver for a brushless DC motor For a brushless DC motor, the driver is not merely a control device but an essential part that makes the motor turn at all. The driver replaces the role of the brushes and commutator with an electronic circuit: based on rotor-position information received from Hall sensors or the like, it switches the current supplied to the three windings at exactly the right moments — a function called electronic commutation. Without this function, a brushless DC motor cannot turn.

Driver for a stepping motor The driver for a stepping motor interprets the pulse signals from the upper controller and generates a sequence of signals that switch (energise) the windings on and off in a set order. Today, most drivers also have a microstepping function, which changes the current supplied to the windings in small increments. This lets the motor rotate more smoothly than the basic step angle, reducing vibration and achieving smoother motion.

Driver for an AC servo motor (servo amplifier) In a servo-motor system, the driver is called a servo amplifier, and it is the most complex and highest-function type. Its central function is to process the extremely fast feedback signals from the encoder — up to tens of thousands or millions of pulses per second — calculate the minute error against the commanded value, and continuously adjust the output to the motor in real time to drive that error toward zero. Alongside these advanced calculations (for example, PID control), the driver also has a gain-tuning function to optimise the responsiveness of the system to match the mechanical characteristics of the equipment.

End of Part 1.

A note from MINATA

MINATA shares these notes as reference material from a practical engineering point of view. If there is anything you would like to discuss further — about drawings, standards, order conditions, or how to read a term across Japanese and Vietnamese — feel free to get in touch and we can review it together.

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