The brushless dc (BLDC) motor is taking over ever more applications formerly consigned to ac and brush-type dc motors. The reason is that electronic rather than mechanical formats are more efficient, quieter (acoustically as well as in regard to EMI) and require less maintenance.
The traditional dc motor became widely available in the second half of the 19th century. It was based on a brush-commutator mechanism that simultaneously got the electrical energy into the spinning rotor and periodically reversed polarity of that current so the motor could actually spin. This arrangement provided a viable source of rotary motion until it was replaced by the Tesla-Westinghouse induction motor. The dc-brushed motor lived on in certain applications such as automotive starters and elevators, brushes notwithstanding, in applications using battery power or characterized by the need for smooth speed control.
The brushless dc motor offers significant advantages though its implementation requires a theoretical leap into the high-tech world of electronic control. Traditional dc motors supply alternating current to windings in the rotor by means of a spinning commutator mounted on the rotor shaft. In contrast, the rotor in a dc brushless motor typically contains just a permanent magnet. Alternating current is instead supplied to windings in that stator. That way brushes are not required.
External commutation is a familiar concept that had long been used in the ac synchronous motor. In this arrangement the commutation is supplied by the utility alternating current, a sine wave, so there is no need for a mechanical commutator as part of the motor.
In the dc brushless motor, commutation is also external. It comes from an electronic module, usually residing adjacent and cable-connected to the motor, which can be single phase or constructed for two, three or more phase operation.
The electronic commutator delivers voltage pulses to the stator windings as required to generate a rotating magnetic field. Hall effect sensors in the stator inform the controller of the position of the stator relative to the rotor so current pulses flow through the windings in the correct sequence.
Whereas the brushless dc motor is relatively simple compared to other types of rotary machinery, dedicated electronic control is needed to make it turn. Despite its complexity, the large number of units currently manufactured has been accompanied by economies of scale, so that the cost of the motor together with the controller is not excessive.
With this discussion in mind, one might wonder if there is a way to determine the technology of motor without disassembling it and examining the rotor and stator. With a few exceptions, the National Electrical Code requires motors to have an attached nameplate that contains this information as well as other specifications. This practice helps assure selection of the correct motor for a given application. It also aids in designing and installing branch-circuit wiring and auxiliary equipment in a manner that will not be hazardous.
The NEC specifies that motor nameplates contain the following information:
1) Manufacturer’s name.
2) Rated volts and full-load current. For a multispeed motor, full-load current for each speed, except shaded-pole and permanent-split capacitor motors where amperes are required only for maximum speed.
3) Rated frequency and number of phases if an ac motor.
4) Rated full-load speed.
5) Rated temperature rise or the insulation system class and rated ambient temperature.
6) Time rating. The time rating is to be 5, 15, 30, or 60 minutes or continuous.
7) Rated horsepower if 1/8 hp or more. For a multispeed motor 1/8 hp or more, rated horsepower for each speed, except shaded-pole and permanent-split capacitor motors 1/8 hp or more where rated horsepower is required only for maximum speed. Motors of arc welders are not required to be marked with the horsepower rating.
8) Code letter or locked-rotor amperes if an alternating-current motor rated ½ hp or more. On polyphase wound-rotor motors, the code letter is to be omitted.
9) Design letter for Design B, C or D motors.
10) Secondary volts and full-load current if a wound-rotor induction motor.
11) Field current and voltage for dc excited synchronous motors.
12) Winding – straight, shunt, stabilized shunt, compound, or series, if a dc motor. Markings aren’t required on fractional horsepower dc motors seven inches or less in diameter.
13) A motor provided with a thermal protector is to be marked “thermally protected”. Thermally protected motors rated 100 W or less are permitted to use the abbreviated marking “T.P.”
14) A motor complying with Section 430.32(B)(4) is to be marked “Impedance Protected.” Impedance-protected motors rated 100 W or less are permitted to use the abbreviated marking “Z.P.”
15) Motors equipped with electrically powered condensation prevention heaters are to be marked with the rated heater voltage, number of phases, and the rated power in watts.
There are additional requirements regarding locked-rotor indicating code letters, multispeed motors, single-speed motors, dual-voltage motors, 50/60 Hz motors, part-winding motors and torque motors. Additionally, distinctions are made between factory-wired and non-factory-wired motors. All of this is rather complex, but the information is needed if a motor installation is to be correctly designed and constructed.