[Episode 8] The First Meeting! Lessons Learned in a Real Setting

Summary of the Previous Episodes

The results of Ichinose’s studies are beginning to show in his questions. Having learned from Teacher Sugiken that “not to leave things one doesn’t understand and to have a keen attitude on learning is important for engineers,” Ichinose and Ninomiya have once again begun their journey to becoming super engineers…

Character Introduction

• Ichinose (the protagonist) is a new engineer. He has been aiming to become an engineer since he was in middle school, and finally joined ROHM. He is so passionate that he forgets to eat and sleep while studying on motor drivers. Currently, only Ichinose can see Dora and Tako.
• Ninomiya is in the same year as Ichinose. Her grades are always at the top. She has a strong personality, but she is also a hard worker and has a high opinion of Ichinose. She is in secret a big fan of Sugiken.
• Teacher Sugiken is a super engineer at ROHM. He is usually kind, but he is passionate and takes pride in his work as an engineer. Sugiken used to be able to see Dora and Tako, but now he can’t.
• Dora is a motor driver fairy who loves people who are passionate about motors. He has a crush on Tako, but is always at the mercy of the insensitive Tako.
• Tako is a motor fairy and childhood friend of Dora. She is knowledgeable about motors, and her knowledge surpasses that of Dora. Although she is a reliable older sister, she is insensitive when it comes to love, and is unaware of Dora’s feelings.

Required Performance of Brushless Motors: Efficiency

Up until my seventh talk, I was discussing the principles of operation of brushless motors and driving circuitry for such motors, and was explaining brushless motor characteristics while showing you actual waveforms. From this point on, I’ll be drawing on this earlier information to explain the performance of brushless motors.

Contents of Episode 8

• ・What Performance Is Required of Brushless Motors?
• ・Efficiency of Brushless Motors
• ・Losses in a Brushless Motor

What Performance Is Required of Brushless Motors?

Brushless motors are used in various products and industries, among them household appliances, automobiles, and industrial equipment. Three basic types of performance sought from such motors are efficiency, quietness, and reliability. First, let’s briefly explain these three performance characteristics.

Efficiency

It is said that motors account for half of the electric power used around the world. So of course we’re going to want those motors to be highly efficient. Various kinds of machinery incorporate many different parts and components, but just by improving the efficiency of the motors that are included in these components, we can suppress power consumption by the entire machines.

Also, keep in mind that motor efficiency is greatly affected not only by the motor structure, but also by motor control (see Episode 7, “Advance Angle Control of a Brushless Motor”).

Quietness

Motors are used as a source of mechanical power to move all kinds of objects, and normally, it is required that the objects remain quiet. The quietness of a motor concerns not only any noises produced by the motor itself, but also the ability to suppress any noises that occur when the object is moved.　The low-pitched buzzing sounds from air conditioners or refrigerators at night, or the rattling sounds from a washing machine, can be unpleasant. In the next Episode 9, I will explain the mechanism that causes such noise.

Reliability

Motor reliability affects product lifetime and the occurrence of initial defects.

The next article explains how to ensure reliability from both mechanical and electrical perspectives.

Efficiency of Brushless Motors

What exactly is the efficiency of a motor? It’s a measure that indicates how much output is generated for a given amount of input power. In other words, it’s the ratio of the output to the input power, as in the following equation.

What is motor output?

What is the “output” in that equation? The output of a motor is the output power obtained by multiplying the motor rotation rate, the output torque, and a constant. It’s calculated as shown below.

In general, output is defined as the movement distance times the force. When expressed for the case of a rotating body like a motor, it becomes:

Circumferential length (２πｒ) × rotation rate N(S) × force (F)

In this equation, we can substitute the motor torque (T = rF) for the product rF, and with N(s) as the rotation rate (where N is the number of rotations per second), and the remaining 2π/60 set as the constant (α), we obtain α × T × N (where the value of α is 0.1047).

And what is motor input?

So what then is the “input” in the equation? The motor input is the product of the applied voltage and the current. This input is equal to the sum of the motor output and losses in the motor. When a motor generates an output, this output is accompanied by losses, and so the input that is required includes these losses. If these losses can be kept small, the motor efficiency can be improved. So let’s think about motor losses.

Losses in a Brushless Motor

Losses that affect motor efficiency can be broadly divided into losses that occur in the motor mechanism, and losses that occur in the circuitry.

In the motor mechanism (the motor body), there are three losses: copper loss, iron loss, and mechanical loss. I’ll first explain these three types of loss.

<Copper loss>
Copper loss occurs because of electrical resistance when a current flows in the windings. Because copper wire is mainly used in windings, it is called the copper loss. The winding specifications (number of windings, copper wire diameter) are related to the resistance value of windings. Also, the current that is needed when causing the motor to generate a torque also varies depending on the winding specifications, and so the copper loss is different depending on the motor specifications and the output state.

<Iron loss>
Iron loss occurs when magnetic flux passes through the iron plates that form the motor. The iron loss is the sum of two losses that are called the eddy current loss and the hysteresis loss.

Eddy current loss is a resistive loss caused by the eddy currents that appear when the amount of magnetic flux within an iron plate changes. These eddy currents appear in a direction that tends to reduce the change in magnetic flux, and so a loss occurs.

Hysteresis loss is a power loss that occurs when the magnetization state in an iron plate is inverted. Once an iron plate is magnetized by an external magnetic field, the magnetization remains even after the external magnetic field disappears. This is called residual magnetic flux, and a graph of its characteristic is called a hysteresis curve. When, with residual magnetic flux present, an external magnetic field is applied with polarity in the opposite direction to magnetize the iron plate, an extra amount of power corresponding to the residual magnetic flux is necessary, and so this too results in a loss.

<Mechanical loss>
This is mainly the loss that occurs due to friction in bearings.

In addition to these losses that occur in structural parts of a motor, there are losses that occur within the circuitry. These are losses in the motor driving circuitry, and they consist of power consumption in the driving circuit and losses in the power transistors.

<Control circuit power>
This is the power that is consumed by the control circuit, and refers to the power used by the controller IC and level shifter, Hall elements, and other components (the level shifter also includes power to drive the power transistors). These losses differ from the losses in the motor mechanism in that the power consumed is essentially constant and unrelated to the motor characteristics or the output state (but the power used to drive the power transistors may depend on the winding currents).

<Power transistor loss>
This is the loss occurring in what are called power transistors (MOSFET transistors or the like) that supply power to the motor windings. This loss is related to the transistor drain-source voltage Vds and to the drain current Id, and consists of three losses, explained below: switching loss, on-resistance loss, and diode loss.

Switching loss: This loss occurs when transistors are switched on and off. During switching operation, Vds and Id change as shown below. Time is required for these state transitions, and there are differences in the timing of the voltage and current transitions, so that power consumption occurs because of simultaneous voltage and current conditions (a current flows while a voltage is being applied).

On-resistance loss: This loss occurs due to resistance while a MOSFET is turned on. It is determined by the on-resistance Ron and by Id.

Diode loss: This is a loss that occurs while a current is flowing in the parasitic diode of a MOSFET; it is determined by the diode voltage VF and by Id.

Methods of measuring motor losses

The losses I’ve just described are measured by connecting measurement instruments such as those shown below.

・Set the desired rotation rate and torque for which you want to measure losses, and then use two wattmeters (shown below) to measure the motor input and the input to the circuitry.
The difference between the motor input and the motor output is taken to be the loss in the motor mechanism; the difference between the circuit input and the motor input is the circuit loss.

*Calculate the breakdown of the motor mechanism losses.

・Copper loss: Calculated from the winding currents and the winding resistance values

・Mechanical loss: With motor driving by the motor driver stopped, use an external force to turn the motor, and calculate the mechanical loss from the torque and rotation rate when it is turned.

・Iron loss: The iron loss is what remains when the copper loss and the mechanical loss are subtracted from the motor mechanical loss.

*When determining the details of circuit losses as well, you’ll need to separate the power input into the power supplied to the control circuit and the power supplied to the power transistors, and then connect wattmeters to each to perform measurements.

In motor design, the following methods are being studied as means of reducing the losses I’ve been talking about. In addition, new motors are being developed with emphasis on reducing losses. One example is motors with smaller winding currents (and so reduced copper losses and power transistor losses), achieved by increasing the magnetic flux density in the permanent magnets.