Elmo HARmonica 104, Bldc commutation policy, Mechanical and electrical motion Figures are missing

HARSFEN0602ElmoHARmonicaSoftwareManual PRELIMINARYDRAFT

In the stepper commutation policy, the windings field is set to point at the desired rotor position. The commutating device doesn't have to know where the rotor is – it just assumes that the rotor will come to rest at the field position.

The stepper commutation has the advantages of simplicity and reliability. The main drawback is that normally θ << 90o, thus to generate a given torque large currents are

required.

At the steady state, the motor torque is zero, and indifferent to the motor current. The sensitivity of the motor torque to deviation of the rotor angle is maximal.

The large sensitivity of the torque to the rotor angle generates a fast, but oscillatory position feedback.

9.1.3The BLDC commutation policy

104

In the stepper commutation policy, the windings field is set to point 90oaway from the rotor position. The commutating device has to know where the rotor is in order to keep the field

direction 90oaway.

The BLDC commutation has the advantage of maximum torque per given motor current, and of smooth, controllable torque. The BLDC policy involves much real time calculation and it requires a rotor position sensor. The commutation dependence in the sensor decreases motor reliability.

The torque is not sensitive to the rotor angle.

The BLDC commutation is ideal for servo applications.

9.2Mechanical and electrical motion Figures are missing

Most of the brushless motors has two or three phases (windings). The figure below shows a two phased linear motor and a three phased linear motor.

Figure 3: Two phased linear motor

Figure 4: Three phased linear motor

The Harmonica suits only three phased motors.

When the rotor travels, the coils of the 3-phased motor are powered in the sequence A-B-C- A-B-C... and so on. Although the moving part (rotor) travels continuously, it sees the windings in a repetitive, cyclical pattern. When the rotor passes from a location over the A coil to the next A coil, it covers an electrical cycle.

"Rolling" the linear motor of Figure 4 so that its right hand C phase rolls towards the left hand A phase makes a rotary three-phase motor.

Figure 5: Three phase rotary motor

The motor of Figure 5 has two coil sets (and also two magnetic pole pairs in the rotor). The rotor follows two electrical cycles in each mechanical revolution.

For rotary motors, normally the number of coil sets equals the number of magnet pole pairs, so the term "number of pole pairs" is commonly used as substitute to "number of electrical cycles in a shaft revolution".

You can read the electrical and the mechanical angle of the motor using the following commands:

Command Description