[Episode 11] Learning And Growing: It’s Not Just About Turning the Motor!

Summary of the Previous Episodes

Last time, Ichinose was confused and flustered by an email from Sales.

Sugiken taught him the importance of immediately confirming anything he didn’t understand. He also learned that gaining knowledge breeds questions, and having questions leads to even more knowledge. He resolved to strive even harder toward becoming a super engineer.

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.

Motor Driving States and Protective Operations

A motor doesn’t just rotate; it can be in all kinds of other states—it can apply a braking torque, and it can be stopped, for instance. And even while turning, it may be affected from outside. When the motor may be affected by external forces, it may be necessary to protect the motor and the motor circuitry. And so, in order to develop a motor driver, it’s not enough just to get the motor running—a design that enables control of other states as well is essential.

Here I’ll introduce driving control of motor drivers.

Contents of Episode 11

• ・Control States
• ・Driving States
• ・Protective Operations

Control States

In addition to operation to rotate the motor, a motor driver enters various control states according to the purpose of driving. I’ll explain three of these control states: when all the power transistors are turned off, when only the low-side or high-side transistors are on, and the fixed state.

The diagrams below show the on/off states of power transistors in each of these control states. Diagrams also show changes in the winding voltages and winding currents when the motor is not rotating (non-rotating state) and when it is rotating due to inertia or for some other reason (rotating state).

All-off
This is the state in which all the power transistors are turned off. It is also called the open state, free-running, or Hi-Z (high impedance) state. Because all transistors are turned off, it can also be said to be a state with no control.

At this time there are no currents in the windings, so that a torque does not occur, and the rotor is in a free state. If the rotor is rotating due to inertia or an external force, an induced voltage appears across the winding terminals. The state in which this induced voltage can be observed is for example utilized in sensorless control, which is one method of controlling a brushless motor.

Only low- (or high-) side on
In this state, all the power transistors on either the high side or the low side are turned on. This state may also be called the low-side short, low-side short-circuit, high-side short, or high-side short-circuit state.

In this state, if the rotor rotates and induced voltages occur, currents flow in windings. The currents cause a torque that acts in the direction opposite the direction of rotation of the rotor. And so, this control state is sometimes also called a short braking state. If you want to quickly stop a rotating motor, instead of the all-off state, you should put the motor into the low-side on or high-side on state.

Fixed state
In order to fix the direction of the magnetic field, a state is used in which a voltage is applied across wires of your choosing. In the diagram below, the power supply voltage is applied to the U phase, and the W phase is fixed at ground level to pass a current.

You would use this state, with the winding magnetic field direction fixed, if you wanted to hold the rotor at some desired position.

Driving States

I just explained motor driving states involving power transistor on and off switching. Now I will talk about the states a motor is in when the motor driver is controlling the motor to make it rotate. For example, if a constant voltage is being applied and the motor is rotating a fan while the fan is affected by an external airflow, the state of the motor changes depending on the direction and strength of the airflow. In such a case, we can have any of the following five states.

• ・Loaded state
• ・No-load state
• ・Regenerating state
• ・Reverse-rotation state
• ・Locked state

I’ll explain these five states based on the conditions shown below (motor driving method, fan wind direction, external force).

Loaded state: An airflow or other external force is not present, or is small.
This is a state in which, with a certain voltage applied, the motor is rotating within the range of rotation rates and torques that are possible for the motor; you could say it is a state in which the load is normal. The directions of the applied voltage and the current are the same, and a positive power supply current flows. The S-T characteristic graph shown below indicates the range.

No-load state: A forward-direction outside airflow and the no-load rotation rate are balanced.
In general, the no-load state is a state in which the motor is turning without a fan or other load, and even if a load (such as a fan, in this case) is rotating due to an outside force, a no-load state is possible. In such a case, essentially no current flows. In the S-T characteristic shown below, this state corresponds to the point indicated.

Regenerating state: A forward-direction external airflow is stronger than the no-load state.
In this state, due to an external force or for some other reason, the motor is rotating at a rotation rate at least as great as the maximum rotation rate possible for the motor at a certain applied voltage (no-load rotation rate). The induced voltage is greater than the applied voltage, and the current direction is reversed. In this case, there is a negative power supply current, so that current returns to the power supply. You could call this a power-generating state, but if the power supply circuits cannot accept the generated power, then the circuits may fail, so you need to keep this in mind. The motor torque becomes negative, acting in the direction to stop rotation. The regenerating state occurs mainly under synchronous rectification driving control. In the S-T characteristic, it occurs in the range shown below.

Reverse-rotation state: A reverse-direction external airflow is greater than the generated torque.
This is a state in which the torque produced by the motor cannot overcome a reverse-direction external force, so that the motor is rotating in the direction opposite the direction in which it should rotate. The direction of the induced voltage is the reverse direction, and as the winding currents increase, the motor torque and the external force become balanced. However, unexpectedly large currents can occur, so you need to exercise caution. The S-T characteristic is shown below.

Locked state
Apart from states in which the motor is affected by an airflow as I just described, there are states in which a motor is locked in place by some external force and so is not rotating. In the S-T characteristic, this is the position at which the rotation rate is zero.

So we can summarize the five driving states I’ve just described on a single S-T characteristic graph as follows.

Protective Operations

Protective operations are operations that a motor driver performs in order to avoid malfunctions and dangerous situations.

There are a number of things that can cause malfunctions in motors. For example, certain voltages, currents, or temperatures in the electrical circuits of the motor driver can cause malfunctions or abnormal behavior. And because a motor is a rotating machine, if it rotates rapidly, the resulting centrifugal forces can cause damage. The motor driver detects such states and performs protective operations so that malfunctions do not occur.

One thing you should keep in mind about the protective operations of motor drivers is that it is not really possible to provide protection from all abnormal situations. For example, if the power supply voltage applied to the control IC exceeds the IC withstand voltage, it is difficult to avoid IC failure through a protective operation. In such cases, a Zener diode or other external component must be used for protection.

Normally when an abnormal state has ended, the protective operation is canceled.

In the protective operations relating to voltages, temperatures, and structures (high-speed rotation) in the above table, a hysteresis amount is provided for the threshold values at which an anomaly is detected, so as to prevent frequent repetition of operation and cancellation cycles. For example, if temperature protection is to be performed at 90°C, the cancellation temperature is set to a value somewhat lower than 90°C (say, 85°C).

You need to be careful when cancelling overcurrent protection. When an overcurrent is being detected using the voltage difference across a shunt resistor, as in the circuit diagram above, if power transistors are turned off as part of a protective operation, a current no longer flows in the shunt resistor. This means that during the protective operation, changes in the current flowing in windings can no longer be detected. And so in an overcurrent protective operation that uses a shunt resistor at the position shown in the diagram, you should cancel the protection after a certain length of time, or else after each PWM period.

In addition, you can detect problems with Hall signals, or detect the failure of the motor to rotate as intended as in the two methods indicated below, and perform an operation to stop the motor driving control. These actions may be handled as protective operations during malfunctions.

• ・Open circuit detection (discontinuity of windings, copper traces)
• ・Lock detection (rotor rotation failure)

Protective operations are a vital function for securing safety of the product. In addition to the protective operations I’ve described, a motor driver may be provided with appropriate protection functions as necessary.