Inside a motor controller: three-phase bridge, transistors, current measurement
From the outside, a controller looks like a box with thick wires on one side, smaller wires on the other, and a heatsink. Inside, you'll find two distinct sections: a power section that handles hundreds of amps, and a control section that calculates what the power needs to do. This guide opens up the box, without mentioning any specific products.
The essentials
- The core of a controller consists of a six-switch electronic bridge, with two switches per motor phase.
- These switches, typically MOSFET transistors, open and close tens of thousands of times per second.
- The controller continuously measures currents, voltage, and temperatures to regulate its operation and provide protection.
- Its actual limit is almost always related to heat: specifically, the amount of heat it can dissipate.
The three-phase bridge
Each phase of the motor is connected to two switches: one to the positive terminal of the battery (the "high" switch), and the other to the negative terminal (the "low" switch). With three phases, this means six switches: this is the three-phase bridge, or inverter.
By closing the top switch of a phase, it is connected to the positive terminal; by closing the bottom switch, it is connected to the negative terminal. By rapidly alternating between the two, with a variable proportion of time, the average voltage can be set to any value between zero and the battery voltage. This technique is called Pulse Width Modulation (PWM). The motor windings smooth out these pulses, and the current flowing through them becomes nearly sinusoidal.
Never close both switches of the same phase at the same time; this would cause a direct short circuit from the battery.
Power transistors
In scooter, bicycle, or light motorcycle controllers, the switches are generally MOSFET transistors. When closed, they behave like a very low resistance; when open, they block the battery voltage. For the higher voltages found in electric cars, IGBTs or silicon carbide transistors are more commonly used.
To allow for higher current flow, it is common to place multiple transistors in parallel on each channel. Their maximum voltage rating must significantly exceed the voltage of a fully charged battery, in order to withstand voltage spikes during switching and braking.
Where does the heat come from?
A transistor can overheat in two ways:
- In conduction mode: the current flows through its small resistance. This loss increases with the square of the current: doubling the current multiplies the heat by four;
- During switching: During the brief moment when it transitions from open to closed, it experiences both voltage and current. The more frequently the switching occurs, the more significant this loss becomes.
That's why a controller advertised for a very high current rating will only sustain that current for a few seconds if it's not properly cooled.
Grid timers and downtime
A MOSFET is controlled by its gate, which must be charged and discharged very quickly for it to switch properly. The microcontroller does not have the power to do this directly; specialized circuits, called gate drivers, handle this task. They also manage the high-side switch, whose reference voltage "floats" with the phase.
Between the opening of one switch and the closing of another on the same phase, there is a short delay during which both are open: this is the dead time. If it is too short, it risks causing a short circuit during the transition; if it is too long, it distorts the voltage and adds losses. Its adjustment depends on the transistors and their drivers.
Measuring current
For vector control, as with the protections, the controller must know the current flowing through the phases very precisely. Two methods are dominant:
- Shunts: These are very small, precision resistors placed in the current path, and their low voltage is measured
- Hall-effect current sensors, which measure the magnetic field created by the current without being in its path, are more common in applications with high currents.
The measurement must be taken at the correct point in the switching cycle, away from the switching instants where it would be disrupted.
Capacitors for the bus system
The bridge draws a pulsed current from the battery, at the switching frequency. The battery cables, which are too long and too inductive, cannot deliver these pulses. Capacitors placed as close as possible to the bridge act as a reservoir: they provide the current peaks and smooth the current seen by the battery. They also generate heat, and their selection is crucial for the lifespan of the controller.
This is also why a high-voltage controller needs a pre-charge circuit or a spark-proof connector: when connected, empty capacitors draw a large surge of current.
The control system
A microcontroller executes the motor control command in each clock cycle: it reads the currents, calculates the rotor position, performs regulation, and generates the control signals for the power bridge. Surrounding it, you will find:
- The inputs for the throttle, brake, and motor sensors;
- Communication links with the display, another controller, or a tuning computer;
- The measurements include battery voltage and temperatures (of the transistors and often the motor)
- A low-voltage power supply is drawn from the battery to power the electronics and, sometimes, accessories.
Protective gear
| Protection | What it monitors | What it does |
|---|---|---|
| Overcurrent | Phase or battery current exceeding an absolute limit | It cuts the connection in just a few microseconds |
| Temperature | Transistors and motor | Gradually reduces the current, then cuts it off |
| Undervoltage | Battery too low | Reduces power to protect the battery |
| Overvoltage | Increasing tension, especially during electric braking | Limits braking, then cuts power |
| Driver errors | Control voltage, short circuit of a transistor | Stop the bridge and report the fault |
Cooling: A critical limitation of a controller
The transistors are mounted on a surface that conducts their heat to the enclosure, and then to the air or the frame. The quality of this thermal path determines the current that a controller can handle over time. Two controllers with the same transistors can have very different performance depending on their thermal design and how they are mounted within the machine.
Workshop tip
Always mount the controller on a metal surface that dissipates heat, ensuring good contact. A controller enclosed in foam or a plastic bag will never maintain accurate readings.
Frequently Asked Questions
Why is a controller making a whistling sound?
The current chopping process causes slight vibrations in the motor windings and some components. If the chopping frequency falls within the audible range, a whistling sound can be heard. This is normal as long as it remains subtle.
Is a fried controller repairable?
Sometimes, the failure is limited to just a few transistors. However, a power surge often damages the gate drivers or the circuit traces; the repair is only reliable if you understand the reason for the failure.
Why do we talk about "peak" and "continuous" current?
The actual limit is thermal. A controller can handle a high current for a few seconds, while its components heat up, but a lower current indefinitely. Both figures are useful, as long as you know which one you're looking at.