The most common power electronic driver for BLDC motors is a three-phase H-bridge inverter. The motor winding current is commutated based on position sensor feedback or a sensorless algorithm. BLDC motors are driven by 120-degree trapezoidal control, where only two windings are conducting at a time. The winding current is controlled by single-pole switches (soft choppers), with each switch of the three-phase inverter conducting for 120 electrical cycles at this point. The winding current of the BLDC motor at any given time is calculated using the motor electrical model given in Equation 1.
Where V is the voltage applied to the two conductive windings, R is the resistance of the inter-line motor winding, L is the inductance of the inter-line motor winding, and E is the back electromotive force (electromagnetic field) between the lines.
Equation 1 shows that the instantaneous winding current depends on the back electromotive force (EMF), motor resistance, inductance, and applied voltage. The motor's back EMF is proportional to its angular velocity; therefore, under stall conditions (zero speed), the back EMF is zero. This means that when the motor stops, the steady-state current in the motor windings is limited only by the motor resistance. When the motor saturates under high (over) current, the inductance decreases, and the current rises even faster than the nominal current level.
Consider an example of a BLDC motor with a rated power of 400W, a rated DC voltage of 220V, and a rated RMS winding current of 3.6A. The winding resistance of the motor is approximately 6Ω. Therefore, the stall current = V/R = 220V/6Ω = 36.67A. This means that if we do not have proper current-limiting protection, the rated current of the inverter stage must be 36.67A.
If the motor drive system is allowed to carry stall current:
• The inverter stage must be rated to carry stall current, which makes the inverter stage bulky and expensive.
Allowing motor windings to carry stall current for extended periods can cause the motor to overheat. This can lead to winding burnout. Furthermore, permanent magnets may demagnetize due to high temperatures or high demagnetizing currents.
If we are designing a motor drive system for rated current, we need appropriate winding overcurrent protection to protect the inverter stage and the motor. The first step to achieve winding overcurrent protection is to detect the winding current.
Ideally, we could measure the three-phase winding current by connecting a current sensor in series with all phases or by placing a current sensor in all inverter branches. Alternatively, we could sense the currents of two phases and determine the third phase current by setting the algebraic sum of all phase currents to zero.
During the trapezoidal control of a BLDC motor, for each 60-degree electrical commutation cycle, only two inverter branches are active and supply power to the motor; a third inverter branch remains in a high-impedance state by simultaneously closing the high-side and low-side switches. Only two stages are active at any given time. This means we can measure the winding current by sensing the DC bus current. We can place a low-cost sense resistor at the DC bus return point to sense the motor current, as shown in Figure 1.
For a unipolar two-quadrant drive, pulse width modulation (PWM) is applied only to the high-side switch of one active arm. During the entire 60-degree electrical commutation, the low-side switch of the other active leg remains open.
Consider a commutation period where phases A and B are active. When the top switch is open, both phase windings are energized. When both the top and bottom switches are open, the DC bus current is the same as the winding current. When the top PWM is low, the top switch is closed, and the winding current freewheels through the diode of Q2. During this freewheeling period (top switch closed and bottom switch open), the winding current does not flow through the DC bus; therefore, the DC bus current is zero. The current does not increase during the freewheeling period but decreases. This demonstrates that the DC bus current measurement is sufficient to provide winding overcurrent protection.
From my explanation so far, we can see that motor winding current can be controlled by sensing the DC bus current. We can achieve peak current limiting control by sensing the DC bus current and designing the inverter for the nominal motor current, rather than over-designing for the stall current. For low-inductance BLDC motors (typically from a few microhenries to tens of millihenries), the higher winding resistance-to-inductance ratio results in a higher winding current rise rate. Current limiting protection must be fast (well below 1µs) and operate in every PWM cycle to avoid any short current spikes.
