Motor drives

The idea

The last link in the chain is a set of switches arranged so the motor gets the right voltage, polarity and timing. PWM sets the speed. The arrangement of switches sets the direction, and for brushless motors, which coil is energised.

Brushed DC: the H-bridge

One MOSFET with PWM (a single-switch chopper) controls speed, but only in one direction. To reverse the motor you need to flip the polarity across it. Four switches in an H do that:

Switches on	Result
S1 + S4	Forward: current left → right through the motor
S2 + S3	Reverse: current right → left
S3 + S4	Brake: the motor’s terminals are shorted, so its own back-EMF slows it fast
S1 + S3 or S2 + S4	Never. This connects +V straight to 0 V through two switches (shoot-through), which destroys them. Drivers add dead time to prevent it

The driver must also carry the motor’s continuous current, and should limit it so a stalled motor doesn’t overheat.

Brushless (BLDC, PMSM): the three-phase inverter

Three half-bridges, one per motor phase (U, V, W), six switches in total. Switching them in sequence makes a rotating magnetic field that drags the magnet rotor round. With no brushes, there’s less wear and higher efficiency.

$P_{in,motor}=\sqrt{3}{V}_L{I}_{phase,RMS}\cos \phi V_L=\frac{V_{DC}}{\sqrt{2}}$

$I_{phase,RMS}=\frac{P_{out}}{\sqrt{3}{V}_L\cos \phi \eta }{I}_{D,RMS}=\frac{I_{phase,RMS}}{\sqrt{2}}$

• $\cos \phi$ is the power factor, typically 0.92–1. $\eta$ is the efficiency, typically 90–95%.
• The last formula has a $\sqrt{2}$ because each transistor carries its phase’s current only half the time, and RMS scales with the square root of the on-time ($\sqrt{1/2}=1/\sqrt{2}$). That $I_{D,RMS}$ is what goes into the MOSFET power loss calculation.

Worked example

A 500 W brushless motor runs from a 48 V battery, with $\cos \phi =0.95$ and $\eta =0.92$. Find the phase current and the current through each transistor.

Answer

$V_L=48/\sqrt{2}=33.9$ V $I_{phase}=\frac{500}{1.732\times 33.9\times 0.95\times 0.92}=\frac{500}{51.4}=9.73$ A $I_{D,RMS}=9.73/1.414=6.88$ A per transistor

Stepper motors

Type	Coils	Driver
Bipolar	Two separate coils	Two H-bridges, to reverse the current in each coil
Unipolar	Centre-tapped coils	Simple low-side switches that pull each half-coil to ground in turn

Both use PWM current chopping: the driver measures the coil current and switches off the instant it reaches the rated limit.

Check yourself

1. Which pair of switches in an H-bridge must never be on together, and why?

Two on the same side (S1 + S3, or S2 + S4). They short the supply to ground.

2. How many H-bridges does a bipolar stepper need?

Two, one per coil.