In the previous post, we briefly covered the basic function of an electric motor. This time we give a high-level overview of electric motor inverters.

High-level overview

Electric motor inverters, which are also commonly referred to as motor controllers in the robotics industry and electronic speed controllers (ESC) in the drone industry, are used to precisely control the delivery of electrical power to an electric motor. It is their job to make sure that each coil is energised at exactly the right time, in the right order and with the right amount of current.

Inverters typically have three main functions:

Convert a director current (DC) into alternating currents (AC)
Measure and adjust the size of the currents supplied to an electric motor
Precisely time the supply of currents so as to control speed and maximise the torque output

Inside a typical inverter are a collection of transistors, current sensors, filtering components and a microcontroller. The microcontroller takes in information about the desired motor speed and torque along with data about the motor shaft position. It then performs calculations to decide how long to switch each transistor on and off so as to produce the required AC outputs.

Some inverters are also able to take electrical energy produced by a motor when it is acting as a break and turn it back into DC for feeding back to the source. Inverters with this ability to regeneratively break are considered bidirectional power converters.

Deeper explanation

1. The DC to AC conversion process

To understand how an inverter converts DC to AC consider the H-Bridge circuit below.

We have a DC power source that is connected to a 10-ohm resistive load to represent a single phase of an electric motor with a current sensor in series. On either side with have a collection of transistors that can be switched on or off so as to direct a current through our load (motor) in either direction. By switching the transistors off and on again we can cause a current to flow back and forth through our motor, creating an alternating current.

The above was produced using Falstad. Note that a real inverter uses gate drivers, a higher switching frequency and an inductive load. See here for a slightly more realistic example Thanks to Oskar Weigl for pointing this out.

Note that depending on the power and switching frequency required different transistor technologies such as MOSFET and IGBT may be used.

However, simply switching the current flow direction back and forth is not enough and we would ideally like to be able to control the size of the current flow while also making it look more sinusoidal.

2. Alternating current measurement and adjustment

To control the size of the current supplied by an inverter, not just its direction, we can carefully adjust the length of time that each transistor is switched off and on. This process is called Pulse Width Modulation (PWM). If more torque is required from the motor then more current can be supplied by increasing the length of time each transistor remains on (its duty cycle) during its particular portion of the sinewave generation process.

The duty cycle is constantly adjusted in an inverter thousands of times per second by using a closed feedback loop where the measured current is constantly compared to the required current.

At Kite, we refer to the switching frequency as the frequency that the transistors are physically switching off and on.

Using PWM, we can now supply a smaller or larger current to an electric motor in either direction at any time. The next step is to also use PWM to help us better approximate a sine wave current output as shown in the image below.

The ‘stair stepping’ of the current output can then be smoothed by the addition of capacitors across the load and inductors in series with the load to form a low-pass filter.

A typical electric motor inverter will contain capacitors but not inductors. Instead, the electric motor itself is used as the inductor.

By producing a sine wave instead of square waves we can more efficiently produce torque in an electric motor.

At Kite, we refer to the electrical frequency as the frequency of the resulting sine wave that is produced by the inverter and seen by the motor. Note that the electrical frequency is often 10-30x lower than that of the switching frequency. i.e. A common switching frequency of an inverter may be around 9 kHz while the electrical frequency may only be 600 Hz.

3. Time supply of AC

The final job of an inverter is to adjust the electrical frequency supplied to a motor to:

Produce the required rotational speed
Make sure that the current supplied to each coil is at just the right time so as to efficiently produce the correct amount of torque

To achieve this a shaft position sensor typically supplies the inverter with information about the speed and position of the motor. This information is then used to calculate and then adjust the PWM duty cycle to create a higher or lower electrical frequency that is also matched to produce the required current.

If you take the above circuit and theory and copy it two more times so that you have three separate sinusoidal outputs (three phases) then you now have a basic electric motor inverter.

Conclusion

Electric motor inverters are electric power converters that use relatively simple electrical components (transistors & sensors) in combination with some clever computation to carefully adjust the current supplied to an electric motor so as to efficiently produce the required torque and speed.

Want to learn more about Kite Magnetics?

Get in touch by emailing Richard at richard@kitemagnetics.com