SLVA654B June   2014  – March 2019 DRV8301 , DRV8301-Q1 , DRV8302 , DRV8303 , DRV8307 , DRV8308 , DRV8312 , DRV8323R , DRV8332

 

  1.   Hardware design considerations for an efficient vacuum cleaner using a BLDC motor
    1.     Trademarks
    2. 1  Suction Principle
    3. 2  Brushless DC Motors (BLDC)
      1. 2.1 Construction of BLDC Motors
      2. 2.2 Working of the BLDC Motor
        1. 2.2.1 Types of Control
          1. 2.2.1.1 Sensor Control
          2. 2.2.1.2 Sensorless Control
            1. 2.2.1.2.1 Sensorless Control: Using Zero Crossing of the Back EMF Signal
          3. 2.2.1.3 Calculations
    4. 3  Microcontrollers
    5. 4  Gate Driver and MOSFETs
    6. 5  Isolation
    7. 6  Power Management (6 to 60-V DC Power Supply)
    8. 7  CAP and QEP interfaces
    9. 8  Enhanced Controller Area Network (eCAN)
    10. 9  High-Resolution and Synchronized ADCs
    11. 10 DRV8323R
    12. 11 Feedback Stage
      1. 11.1 Torque or Commutation Loop
      2. 11.2 Speed Loop
      3. 11.3 Position Loops
    13. 12 Conclusion
    14. 13 About the Author
    15. 14 References
  2.   Revision History

2.2.1.3 Calculations

Use Equation 2 to calculate the no-load speed.

Equation 2. No-load speed = V / KE × 1000 (RPM)

    where

  • KE = back-EMF constant (V/kRPM)

The KE constant can also be used to determine how fast a motor runs with a certain voltage applied to it. The higher the applied voltage for a motor with a given back EMF constant (KE), the faster the motor runs. Conversely the lower the applied voltage for a motor with a given back EMF constant (KE), the slower the motor runs.

To calculate the voltage required at the motor, use Equation 3.

Equation 3. Voltage at motor (V) = ([τL + τM] / Kτ × [Rθ]) + (KE × w)

    where

  • τL = load torque (oz-in)
  • τM = friction torque of motor (oz-in)
  • Kτ = Torque Constant (oz-in/A)
  • KE = back EMF constant (V/kRPM)
  • Rθ = Thermal resistance (Ω)
  • w = desired motor speed (kRPM)
  • Torque (τ) = Kτ × Current (I)

Use Equation 4 to calculate efficiency.

Equation 4. η = PO / PI
Equation 5. PO = ω × τ

    where

  • ω = angular velocity (rad/s)
  • τ = torque (Nm)
Equation 6. PI = rated voltage × rated current

Table 1 lists the specifications of the selected motor (part number DN4261-24-053).

Table 1. BLDC Motor Specifications

VALUE
Rated speed 4000 RPM
KE 3.72 V/kRPM
Kτ 5.027 oz-in/A
Rated torque 0.125 Nm = 17.7 oz-in
Weight 0.45 Kg
Body length 61 mm
Number of phases 3
Number of poles 8

To find voltage required at motor for different torques use Equation 7.

Equation 7. Voltage at motor (V) = ([(τL + τM) / Kτ] × Rθ) + (KE × w)

    where

  • Rθ = 2.6 Ω
  • τL neglects τM

Calculate the voltage to produce a torque of 15 oz-in at 3000 RPM using Equation 8.

Equation 8. V = ([15 / 5.027] × 2.6) + (3.72 × 3) = 18.91 V

Use Equation 9 to find the voltage used to produce a torque of 15 oz-in at 4000 RPM.

Equation 9. V = ([15 / 5.027] × 2.6) + (3.72 × 4) = 22.63 V

These equations can be used to calculate a similar voltage requirement for any given torque and speed.

The curve in Figure 6 is linear which provides better speed controllability.

As shown in Figure 7 the rated current of the motor is 3.54 A. The characteristic is linear.

speed_v_torque_slva654.gifFigure 6. Speed Versus Torque
current_v_torque_slva654.gifFigure 7. Current Versus Torque

Table 2. BLDC Motor Advantages

BLDC ADVANTAGE REASON
Linear speed-torque characteristics, better controllability Internal feedback. Permanent magnet design with feedback gives BLDC motors linear characteristics when compared to open-loop AC-induction motors or brush DC motors. Series DC motors have exponentially decreasing characteristics.
High starting torque Internal feedback gives higher starting torque. The torque produced at any instant in a BLDC is twice the torque produced in brush DC motor of the same rating, as two phases are on in every commutation step.
Adjustable speed With the Texas Instruments DRV8323R motor driver, smooth speed control is possible.
Higher efficiency A permanent magnet in the rotor reduces efficiency loss and increases the efficiency.
Better heat removal The heat generated in the stator is dissipated easily as it is outside of the rotor unlike the brush DC motor.
Noiseless Because of the absence of brushes the operation is noiseless.