Our dependence upon electronic control systems is growing, from the factory floor to the kitchen. Meanwhile, parameters such as efficiency, reliability, ruggedness, noise, size, and cost are increasingly at the forefront of designers' minds as they search for optimum motor solutions. Reaching these solutions, however, often involves choosing between the relatively expensive and bulky brushless dc (BLDC) motors (Fig. 1) and their brush-type counterparts. When that happens, the lower-cost, smaller, brush-type motors have the advantage at the lower end of the applications spectrum.
But BLDCs have myriad benefits, too. They enjoy longer life, higher efficiency, low EMI, low RFI, lower temperature rise, excellent instantaneous speed variation, and low audible noise. Qualities such as these have granted these devices a place at the head of most designers' stables of motor options. This place can only be guaranteed, though, by recent advances in MOSFET and surface-mount technology. Now, lower overall power consumption and cost have opened up a whole range of new applications. Yet while BLDCs are finding increased popularity, gaps still exist in user knowledge of what exactly these motors are capable of and how best to apply them.
The basic circuit diagram for a BLDC control circuit is shown in Figure 2. Typically, Hall-effect sensors detect the rotor position and send this information to the motor-control logic. The motor-control logic interprets this message and turns on the appropriate motor windings through the three-phase MOSFET drive. The current-sense resistor picks up the current seen by the windings and feeds this data to the current limit. If the current is in excess of a set amount, the current limit sends a signal to the motor-control logic that momentarily shuts the motor down. This usually takes place upon starting the motor.
Once the motor has reached its speed, its back EMF keeps its current below the set point. Also, the current limit is no longer in effect. A 45- or 50-line, 0- to 5-V tachometer signal is produced inside the motor. In turn, it's presented as an output to be used for speed control. The motor controller also accepts three additional inputs, namely start, direction, and pulse-width-modulation control. Supply and ground motor voltage, as well as supply and ground 5-V logic voltages, are to be provided to the motor.
It's important to point out that the BLDC motor has the same speed-and-current-versus-torque relationships as conventional, brush-type dc motors. But the inside spin of a brush-type motor means it has less inertia, by a factor of three. This gives it much faster acceleration overall. Combined with their lower cost (see the table), brush-type motors have two key advantages—but that's changing fast. (The motor described in the table is of the outside-rotor configuration.)
The high cost and size of BLDC motor systems were mostly attributable to the extra heatsinking required to cool the inefficient bipolar transistors used as the drive outputs. These transistors are inefficient due to their high emitter-collector voltage drop, which causes as much as 3 V to be lost across these devices. For example, only 21 V would actually be available across a 24-V motor's windings.
Thanks to advances in MOSFET and surface-mount technology, the scene has changed dramatically. The use of MOSFETs in lieu of bipolar transistors isn't new. Designers have long recognized that their relatively low resistance from drain to source (RDS) enables the motor windings to see more of the actual supply voltage. As the RDS continues to fall from tens of milliohms into the single-milliohm range, though, the MOSFET implementation's advantages become even clearer.
In the case of the same 24-V motor, the voltage drops across the MOSFETs could be as low as 0.5 V, permitting 23.5 V to be delivered to the motor windings. This reduces the required run current, allowing a smaller power supply to be used. Consequently, overall system cost and size are reduced.
This alone is enough to broaden BLDC motor technology's potential range of applications. But the ability to surface-mount MOSFETs onto a pc board, without the need for additional heatsinking in many cases, also has had a dramatic effect. Along with savings attributable to the elimination of the through-hole process, the all-surface-mount board is smaller, as well. And, it can accommodate more circuits and components. In standard BLDC fashion, this board can then be mounted within the motor package. Circuits could be provided for the actual motor driver, current limiting, tachometer output signaling, pulse-width modulation, motor enable/disable, and direction control.
These circuits would augment the motor's inherent low-noise (both electrical and acoustic), long-life, high-efficiency features. The devices, then, can broaden their already tight grip in applications such as copiers, printers, and home appliances. The FCC and CISPR bodies have come down heavily on the amount of EMI/RFI allowable in these environments, making BLDCs the only viable solution.
BLDCs have many advantages and potential applications. But design engineers are still raising a plethora of questions about what they're actually capable of and how best to implement them. Here are some of the most frequently asked questions:
Compared to a brush-type motor, what kind of life can a brushless dc motor expect? There are no brushes to wear out, so the BLDC motor commonly has three to five times the life of a conventional, brush-type motor.
What's the maximum speed at which BLDCs can be used? For the outside-rotor, 58- and 98-mm OD BLDC motors, a maximum speed of 3000 rpm is recommended. Higher speeds can be achieved, but dynamic balancing would be required.
What's the minimum usable speed? For the outside-rotor, 58- and 98-mm OD BLDC motors, a minimum speed of 300 rpm is suggested. At lower speeds, cogging torque becomes an issue, and efficiency is affected.
What type of speed control is available? Pulse-width modulation is the most common, but 0- to 5-V, 0- to 20-mA, and variable-frequency controls also are available.
Can the motor produce a real-time tachometer signal? The brushless motor can generally produce a 36- to 100-line/revolution tachometer signal. Higher resolution would require a separate tachometer or encoder.
Can the BLDC motor pass EMI regulatory requirements, such as FCC or CISPR? Most BLDC motors meet these requirements—dBA levels of less than 20 are normal. And, levels of less than 10 dBA are achievable for motors with output-power requirements of less than 20 W. It's important to note that these devices are proliferating. As such, the need to meet EMI requirements is becoming more and more a demand in the industry. This is necessary to avoid interference with data-exchange systems involving microprocessors and similar devices.
In continuous-speed applications, the outside-rotor BLDC motor offers excellent instantaneous speed variation. This is due to its high rotor inertia, which mechanically integrates or dampens the effects of instantaneous load variation. Speed variations of less than 0.25% are typical, even under open-loop conditions. Closing the loop on the brushless dc motor has yielded speed variations of less than 0.1%.
Audible noise is a newer design criterion that designers now face. The use of a brushless dc motor can assist the designer in this regard. In office equipment, noise specifications of less than 35 to 60 dBA are not uncommon. This is the noise-level range found in an ordinary library or office. The 58-mm brushless motor's noise output is less than 35 dBA, while the 98-mm brushless frame contributes less than 55 dBA. The noise is measured with the motor under load and without any external cover.