Motor driver for brushless DC (BLDC) motors: Hall commutation, PWM speed control and current limits

01

Hall sensors at 60° or 120°: the commutation sequence the driver must follow

Microchip's AN885 explains that three Hall sensors embedded in the stator signal which rotor pole is passing; they may be placed at 60° or 120° phase shift to each other, and the motor manufacturer defines the commutation sequence that the controller must follow. In each step one winding is driven positive, one negative and the third is left unpowered. [1]

One Hall sensor changes state every 60 electrical degrees, so six steps complete an electrical cycle, and the number of electrical cycles per mechanical revolution equals the rotor's pole pairs; AN885 notes that rotors can vary from two to eight pole pairs. The Hall sensors need their own supply, which the note gives as 4 to 24 V at 5 to 15 mA, and their outputs are normally open collector, so a pull-up may be needed in the driver. [1]

Editorial calculation: a motor with 4 pole pairs gives 6 × 4 = 24 Hall transitions per revolution; at 4,000 r/min that is 4,000 ÷ 60 × 24 = 1,600 commutations per second, and an electrical frequency of 4,000 ÷ 60 × 4 ≈ 267 Hz. AN885's rule of thumb of a PWM frequency at least 10 times the motor's maximum frequency then calls for at least about 2.7 kHz. [1]

Give the driver supplier the motor's Hall spacing (60° or 120°), its commutation table, pole pairs, rated voltage and current, and the Hall sensor supply voltage.

02

PWM duty sets the speed; Hall timing or an encoder closes the loop

AN885 states that with the bridge switched fully on in sequence and the DC bus at the motor's rated voltage, the motor runs at rated speed; varying the PWM duty within each step lowers the average voltage and the speed. Limiting the duty also lets a driver with a bus voltage well above a motor's rating run that motor at its rated average voltage. [1]

For closed-loop speed control the note measures speed by timing between Hall transitions for low-cost, low-resolution needs, or fits an optical encoder for higher resolution and direction. [1]

Microchip's AN857 adds why start-up needs current limiting: at standstill only the small winding resistance and inductance oppose the current, and unlimited start-up current can strip gears, slip couplings or destroy the driver MOSFETs. It weighs PWM frequency against switching loss and current ripple and uses about 10 kHz in its example. [2] Oriental Motor's 60 W BMU driver, for comparison, sets acceleration and deceleration from 0.1 to 15.0 s by potentiometer and stores four speed settings. [4]

03

Sensorless back-EMF commutation and its low-speed limit

AN857 describes sensorless control as timing commutation from the back-EMF on the undriven winding, which removes the Hall sensors. It lists the drawbacks: the motor must turn fast enough to generate measurable back-EMF, abrupt load changes can make the loop lose lock, and back-EMF can be measured only near the ideal commutation rate for the applied voltage. [2]

AN857 concludes that sensorless control may be the better choice where low cost is a primary concern, low-speed operation is not required and the load is not expected to change rapidly. [2] Editorial note: for a CNC axis or any load that must hold torque at low speed or start under load, ask for a sensored (Hall or encoder) driver, and ask a sensorless driver's supplier for its minimum speed and start-up method.

04

Continuous and peak current, braking voltage, alarms and altitude derating

Fulling's BLD2405SC data shows the ratings a low-voltage BLDC driver is bought on: 10–30 V input, 5 A rms continuous and 25 A peak, TTL Hall feedback, natural cooling, 130 g, IP20, and 0–40 °C operation. [3]

An AC-input BLDC speed-control driver is rated differently. Oriental Motor's BMU pairing of a 60 W motor and driver runs from single-phase 100–120 V AC, gives 0.19 N·m rated torque over 80–4,000 r/min, and lists alarms for overcurrent, main circuit overheating, overvoltage, undervoltage, sensor error, overload and overspeed; the driver heat sink temperature rise is 50 °C or less after rated continuous operation. [4] Editorial calculation: 4,000 ÷ 80 = a 50 : 1 speed range.

05

Send the motor's Hall data, supply and duty with the enquiry

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Motor: [rated voltage; rated and peak current; pole pairs; Hall spacing 60° or 120°; Hall supply voltage]
Control: [sensored Hall; encoder; sensorless; minimum speed; start under load]
Supply: [DC voltage range or AC input]
Duty: [continuous current; peak current and time; acceleration time]
Braking: [regeneration; external resistor]
Interface: [analog speed input; digital I/O; UART; CAN; RS-485]
Site: [ambient temperature; altitude; IP rating]

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