BLDC / AC Servo Motors

Introduction & Description | Construction & Operating Principle | Technical Data & Dimensions | Applications

Construction and Operating Principle

BRUSH LESS DC (BLDC) / AC SERVO MOTORS are a type of synchronous motor. This means the magnetic field generated by the stator and the magnetic filed generated by the rotor rotate at the same frequency. BRUSH LESS DC (BLDC) / AC SERVO MOTORS do not experience the ‘slip' that is normally seen in induction motors.

BRUSH LESS DC (BLDC) / AC SERVO MOTORS come in single-phase, 2 phase and 3-phase configurations. We manufacture 3 phase motors but input to the motor is 230 V single phase only (carrier voltage) out of which only 170 V single phase will be used to run the motor. (see table 'A').


The stator of our BRUSH LESS DC(BLDC) / AC SERVO MOTOR consists of stacked steel laminations with winding placed in the slots that are axially cut along the inner periphery. Traditionally, the stator resembles that of an induction motor; however, the windings are distributed in a different manner. Most BRUSH LESS DC(BLDC) / AC SERVO MOTOR have three stator winding connected in star fashion. Each of these windings is constructed with numerous coils interconnected to form a winding. One or more coils are placed in the slots and they are interconnected to make a winding. Each of these windings is distributed over the stator periphery to form an even numbers of poles. We offer all motors with 8 pole winding.

There are two types of stator windings variants:
trapezoidal and sinusoidal motor.
This differentiation is made on basis of the interconnection of coils in the stator windings to give the different types of back Electromotive Force. (EMF)

Circular Core with rectangular magnets inserted into the rotor core.


The rotor is made of permanent magnet and can vary from two to eight pole pairs with alternate North (N) and South (S) poles; we have 8 magnets on rotor and eight poles windings on Stator.
Based on the required magnetic filed density in the rotor, proper magnetic material is chosen to make the rotor. (see figure-1).

Circular core with rectangular magnets inserted into the rotor core

Unlike a brushed DC motor, the commutation of a BLDC / AC Servo is controlled electronically. To rotate the BLDC / AC Servo motor, the stator winding should be energized in a sequence. It is important to know the rotor position in order to understand which winding will be energized following the energizing sequence. Rotor position is sensed using Hall effect sensors embedded into the stator (see figure-2).

Figure-3 shows a transverse section of BLDC / AC Servo motor with a rotor that has alternate N and S permanent magnets. Hall sensors are embedded into the stationary part of the motor. Embedding the hall sensors into the stator is a complex process because any misalignment in these hall sensors, with respect to the rotor magnets will generate an error in determination of the rotor position. To simplify the process of mounting the hall sensors onto the rotor, in addition to the main rotor magnets. These are a scaled down replica version of the rotor. Therefore, whenever the rotor rotates, the hall sensor magnets give the same effects as the main magnets. the hall sensors are normally mounted on a PC board and fixed to the enclosure cap on the non-driving end. This enables users to adjust the complete assembly of Hall sensors, to align with the rotor magnets in order to achieve the best performance.

Based on the physical position of the hall sensors, there are two versions of output. The hall sensors may be at 60 or 120 phase shift to each other. We define the commutation sequence, which is followed when controlling the motor. (see Figure-4 & Table-B)

Sequence for Rotating motor

Sequence for rotating the motor in clockwise direction when viewed from non-driving end

Seq #
Hall Sensor Input
Active PWMs
Phase Current
1 0 0 1 PWM1(Q1) PWM4(Q4) DC+ Off DC-
2 0 0 0 PWM1(Q1) PWM2(Q2) DC+ DC- Off
3 1 0 0 PWM5(Q5) PWM2(Q2) Off DC- DC+
4 1 1 0 PWM5(Q5) PWM0(Q0) DC- Off DC+
5 1 1 1 PWM3(Q3) PWM0(Q0) DC- DC+ Off
6 0 1 1 PWM3(Q3) PWM4(Q4) Off DC+ DC-

Sequence for rotating the motor in counter-clockwise direction when viewed from non-driving end

Seq #
Hall Sensor Input
Active PWMs
Phase Current
1 0 0 1 PWM5(Q5) PWM2(Q2) Off DC- DC+
2 0 0 0 PWM1(Q1) PWM2(Q2) DC+ DC- Off
3 1 0 0 PWM1(Q1) PWM4(Q4) DC+ Off DC-
4 1 1 0 PWM3(Q3) PWM4(Q4) Off DC+ DC-
5 1 1 1 PWM3(Q3) PWM0(Q0) DC- DC+ Off
6 0 1 1 PWM5(Q5) PWM0(Q0) DC- Off DC+

Hall Sensor signal, BSCK EFM, output torque and phase current

Winding energizing Sequence with respect to the hall sensor

Theory of operation

(A) AC Servo motor used for Position Mode mainly, also with Torque Mode & Velocity Mode have magnets on rotor as per designed poles of stator, for example if stator has 8 poles winding, the rotor will have 8 magnets on its rotor. Here high frequency pulse is injected in the stator, i.e. in 2 phases out of 3 phase winding, to from + & - on the stator. This gives magnets fitted on the rotor a starting push, the position of stator winding is achieved by (3 hall sensors) encoder fitted on non drive end of rotor, the servo motor, which is driven by a servo drive detects the positioning of rotor and accordingly gives pulse in all two phases out of three, where 1 phase is floating, i .e. if 'R' & 'Y' are given + & - pulse respectively 'B' is floating, if 'B' & 'Y' are given + & - pulse respectively 'R' is floating. Similarly in case of 'B' & 'R' given + & - pulse respectively 'Y' is floating. See Figure-5.

The position of 3 phases is sensed by Hall Effect sensors, which gives position of stator winding excitement with pulse, which is sensed by hall effect sensors fitted at 120 degree, i.e. each at 60 degree from another. 8 poles motor will have weight small magnets fitted on dice to pass through (over) these sensors to give signal to panel for injecting pulse, i.e. supply.

(A) The above motor is also called BRUSH LESS DC (BLDC) / AC SERVO MOTOR in technical terminology because commutation of this is taken care by encoder as designed in AC Servo motors because 3 phase AC winding given on stator is also called 3 phase AC servo motor & because commutation is achieved by encoder. Therefore it is also called BRUSH LESS DC (BLDC) / AC SERVO MOTORS.

Torque / Speed Characteristics

Figure '6' shows an example of torque / speed characteristics. There are two torque parameters used to define a BLDC / SERVO motor, peak torque (TP) and rated torque (TR). (Refer to Table B + Figure B: “Typical Motor Technical Specification“ for a complete list of parameters).

During continuous operations, the motor can be loaded up to the rated torque remains constant for a speed range up to the rated speed. The motor can be run up to the maximum speed, which can be up to 150% of the rated speed. But the torque starts dropping.

Applications, that have frequent starts and stops and frequent reversals of rotation with load on the motor, demand more torque than the rated torque. This requirement comes for brief period, especially when the motor starts from a standstill and during acceleration. During this period, extra torque is required to overcome the inertia of the load and the rotor itself. The motor can deliver a higher torque, maximum up to the peak torque, as long as it follows the speed torque curve.

21st Century electric motor will be BLDC / AC Servo

Final Conclusion

To-date after 20 years, with increase in usage of software & hardware, the electrical motor technology has changed & automation has taken over in every sphere of manufacturing units. The concept of CNC is established. The automation of machines (automatic mechanical operations) is achieved by motors & controls i.e. BLDC / AC Servo motors & controls, the work of motor is now governed by 3 factors (1) Position Mode (2) Torque Mode (3) Velocity Mode.
Motors in the next decade will be operated in following manner. First we will explain its construction details.

CNC application of BLDC/AC SERVO MOTOR and its uses in Industrial / Agriculture etc.

Velocity Mode: It runs like ordinary DC motor or AC Motor (run in frequency drive) from zero to full speed, i.e. variable speed with very high torque i.e., nearly 20 times more than ordinary AC motors & DC motors unlike AC motor consume 27% less electricity at no load & 5 to 7% on full load current.

Position Mode: This mode is bifurcated into two types, you can give command to the motor to repeat the

On/Off (strokes) for continuous process maximum 600 strokes can be achieved in single minute with the accuracy of 0.0001 mm. In the same way you can program the motor forward / reverse and can also incorporate the timing required to do the same with 0.0001 mm tolerance.

Torque Mode: In this mode you can achieve torque higher than +ve rated in both mode used in above cases i.e., Position mode & Torque mode.

Solutions for problems of Clients / Users of CNC machines from our end

We are into manufacturing of AC Motors & DC Motors for 25 years and we for last three years are after Brush less DC and AC Servo and finally we have come up with solutions as under: All AC servo manufacturers are offering controls & motors having a monopoly by offering packages in such a way that no user can do anything without their controls. It cannot be replaced by other controllers.

Every user of SERVO / BLDC motors expect following 5 factors for them to operate without hindrance their machines fitted with motors & its controls. ( i.e. AC SERVO / BLDC).

(1) Transparency (2) Reliability (3) Availability (4) Technology (5) Flexibility

We have in all used 35 components in our control card, which can operate all type of AC servo motors manufactured in this with the help of encoder that we offer worth US $20*. The price of control card without IGBT & heat sink is mere US$ 40* .((*subject to change )

Control card carries the number for each component & its make value & price are available in all parts of world. We offer the program free of cost. Price of motors are equivalent to ordinary AC 3 phase motors. Further the frame size and mounting of AC motors are of entirely of alluminium die cast (made in Europe) flange mounted / foot mounted.

A user can also have customized options which we can offer to his suitability.

In short we offer AC Servo motor for all 3 mode of operation in standard AC frame sizes with controls (at the price of AC starters) & these controls can operate any AV Servo motors, at the same time our motor can replace any AC motors.For Education in colleges of Electrical / Electronic Engineering.

We have developed a machine called TURBO-10000, which can be fitted & run in Engineering colleges' laboratory, where a student can change rotor and run motor of 2kw capacity in laboratory AC motor, DC motor, Brush less motor and AC servo or SLR (Switch Reluctance Motor) with the help of same stator winding only rotor to be changed controls common for all 2 motors and can ascertain by himself load / torque / speed characteristic. In this 21st century we need to save energy by at least 25% & in future vehicles will also run with this system because BLDC/AC servo gives high torque speed & run at high RPM & can be operated by Solar Energy also.

Introduction & Description | Construction & Operating Principle | Technical Data & Dimensions | Applications