4. Brushless Motors Flashcards
What are some of the drawbacks of brushed motors?
- Wear
- Wasted energy
- Arcing
- Radio and TV interference, fire or explosion
Book 4 page 52
What is the tradeoff which is reached when a brushless motor is used as compared to a motor with commutator and brushes?
Increased reliability at the expense of the extra complexity required to drive the motor
Book 4 page 54
What is a Hall-effect sensor and how would it be used in a BLDC motor?
Semiconductor device that converts magnetic flux density to a voltage
Mounted close to the rotor and detects the varying magnetic field as the rotor rotates, thus stator excitation is based on present position of the rotor
Book 4 page 55
How is it possible that speed and torque are not related in a BLDC motor?
Speed is fixed by the pulse rate
Torque is varied by controlling the voltage of the pulses, hence the current in the stator windings
Book 4 page 55
Why are the number of poles which can be used in a permanent-magnet stepper motor restricted?
The stator becomes more complicated to manufacture
Windings have to be smaller in size, and inevitably in magnetic field strength, reducing the achievable torque
The driver circuit uses more transistors and becomes increasingly complex
Book 4 page 55
Which type of stepper motor uses a permanent-magnet rotor, and which uses a soft iron rotor?
Permanent-magnet - Can-stack motor
Soft iron - Variable-reluctance motor
Book 4 page 59
Which stepper motors require a means of switching the direction of current flow to achieve movement in both directions and why?
Permanent magnet and hybrid motors
Variable-reluctance uses soft iron so doesn’t require current to flow in any particular direction
Book 4 page 62
What would stepper motors be used for?
Precise positioning by moving a rotor in discrete steps
Book 4 page 63
Which stepper motor achieves the smallest step size?
Hybrid
Book 4 page 64
How can you create unipolar drive in hybrid and permanent-magnet stepper motors?
Centre-tapped windings
Book 4 page 63
What techniques are available which can increase the resolution of a stepper motor?
Half stepping and microstepping where rotor positions are used other than the normal steps where poles exactly align
Book 4 page 64
