Brushless motors, or induction motors, operating on alternating current (AC) power sources are semi-synchronous. This means that the motor operates at multiples or integer fractions of speeds that are synchronous with the frequency of the AC current powering the motor. For 60-hertz AC power sources, the brushless motors operate at 3600 RPM (2-pole), 1800 RPM (4-pole), 1200 RPM (6-pole), and 900 RPM (8-pole).
Even though what is stated here about AC induction motors is correct, it is not applicable to B
ess DC (BLDC)
motors, and may cause some confusion. First off, the name. BLDC motors are not induction motors. The description above is correct for induction motors, but BLDC motors are permanent magnet 3-phase motors, and are neither induction, nor have a slip-speed. The rotor contains permanent magnets, and does not rely on induced magnetic fields in the rotor.
The speed of a BLDC motor is driven by a frequency, but that frequency is not the line-AC frequency. It is an artificially created frequency in the motor controller. There are no limits, per se, for this frequency, except for any mechanical limits on what the motor can spin.
.....one thing I've noticed with the other brushless drill drivers (Panasonic mostly) is that they have a split second worth of delay between pulling the trigger and something happening.
This delay is not a function of the motor or motor type, but of the soft-start nature of the PWM controller, as it is slowly ramping up power to get the motor to spin. As a matter of fact, it is typically more pronounced on non-BLDC motors because they lack the direct feedback at the winding-level. They can only see the external shaft RPM, which is less reliable at near-zero speed. The BLDC motor has internal sensors that tell the controller the exact rotational position of the rotor. (See below)
=============================================Back to the original topic.
BLDC motors are much more complex than brushed motors, and their respective controllers are more complex. The brushed motor PWM controller needs just a single power transistor for the output, and a single RPM sensor for feedback. The controller only needs to compare the set-speed with the feedback speed, which is a very simple circuit.
The BLDC motor, on the other hand, must include the added expense of the permanent magnets, plus at least 2 internal hall effect sensors to tell the controller the exact rotational position of the rotor.
The controller, instead of just 1 power transistor, it must have 6 power transistors. It doesn't just turn power on/off to the motor, but needs to artificially create a 3-phase, square wave, AC power to the motor from a DC supply. 3 transistors are required to control the 3 phases for positive voltage, and 3 more transistors to control the negative voltages.
The purpose of the brushes in a universal motor are for mechanical commutation (polarity reversal) for the windings. With a BLDC motor, this must be accomplished externally by the controller. This requires that the controller know the exact rotational position of the rotor (kind of like what the positions are on the hands of a clock). The controller uses the two sensors inside the motor to calculate the exact position, and it then uses this result to determine which of the 6 transistors must be turned on for that instant in time. And that isn't even to control the speed yet. That's just what's necessary to make the motor turn at any speed.
The speed control is a separate, but related circuit, which controls how often all this other switching is taking place based on the amount of power going into the transistors. Therefore, the controller for a BLDC motor is much more complex than a simple speed controller for a universal motor.