Difference between revisions of "Phasing a.k.a. phase search"

Revision as of 17:07, 17 May 2016

Construction of synchronous motor: center part is the rotor (in this case two magnetic poles), and outer part the stator (3 phase windings).

Unlike DC motors, AC & BLDC & linear motors (synchronous motors) require electrical commutation. This means that sinusoidal waveforms fed to windings (stator) must all times run in synchronism with motor magnets (rotor). As rotor is the moving part, drive must track it's motion in order to commutate current to correct winding phases.

Suggested reading How Brushless Motors Work.

Phasing methods

Normally optimal angle difference between the magnetic field generated by windings and magnets is 90 degrees. So to keep commutation angle at 90 degrees, drive must first know the initial position of rotor. After power-up of drive, drive does not know the angle of rotor and it must be found out before starting servo control. This can be usually achieved by using one of the following methods:

1. By driving current to windings and letting motor to settle to certain magnetic angle
2. By utilizing absolute feedback device, such as Hall sensors or resolver, to give all needed information directly

Method 1 (sensorless)

The first method induces small motion during drive power-up as windings pull nearest magnets towards them. The maximum amount of motion is +/-90 degrees of magnetic pitch, i.e. for 4 pole motors the physical motion angle stays within +/-45 degrees from the initial angle. This method typically consumes few seconds of time before motor is operational. This method has pros and cons:

Pros

• No any kind of absolute position sensor needed (i.e. Hall sensors)
• Less wiring
• Works with all motors

Cons

• Motor needs to move freely in both directions from the start position
• External conditions may lead to imperfect phase angle which leads to reduction of torque or servo stability. Such conditions are:
  • High torque applied to motor while phasing is in progress (such as vertical axis)
  • Blocked motion (motor can't freely rotate)

Method 2 (sensor based)

In the second method, a commutation sensor (typically Hall sensor) gives the needed magnetic angle info directly to drive and no motion is needed to determine the rotor angle. This method allows instant power-up of motor without any induced motion or delay.

Pros

• Instant power up of motor
• No motion from start position needed
• Insensitive to external conditions

Cons

• More wiring and more parts that may malfunction
• Not well standardized in industry (hall sensors may not be similarly aligned in different motors, causing difficulties when utilizing them with servo drive)

When setting up a servo motor, it is usually recommended to use method 1 (not enabling drive's Hall sensor support). Enable Hall sensors as last step of tuning procedure if possible to avoid confusion if Hall sensors are not working properly.

Troubleshooting

In ideal case, when phasing has properly finished, motor torque sensitivity is at maximum, meaning that any amount of torque produces maximum amount of torque obtainable from the motor. Relation between current and torque is expressed as T=I*Kt, where T is torque, I is current, and Kt is torque constant of motor (for example 2 Nm/A which means it produces 2Nm torque at 1 A current).

That equation applies only if phasing finished properly. If it finishes improperly, it becomes T=S*I*Kt, where S is a scaler value between -1.0 to 1.0 (in properly phased motor this is 1.0, so it can be neglected). For example, in bad phasing S could be 0.7 which means that motor outputs only 70% of it's torque at any given current, so motor may seem weaker than it actually is. Or it could be -0.9 which means that it produces 90% torque in inverted direction. That would typically mean instant runaway and tracking error of motor.

Failure reasons and testing methods

Method 1 (sensorless)

• Significant amount of external torque is applied to motor while it is performing phasing. Note: method 1 works ideally if motor can turn freely without load, and any external load will make S to deviate from 1.0. However, normally when external load is small or moderate, S stays above 0.9 which still gives near perfect outcome.
• To verify this, disconnect all motor load during phasing and then re-attach the load to motor and compare behavior with the test case where load is connected also during phasing. If there is significant difference between the tests, then it indicates variation in S.

Method 2 (sensor based)

• Sensor based phasing go wrong if commutation sensor is improperly wired, or not outputting signals that are expected by drive. This can yield randomness in initialization (S may seem random after each power on).
• To verify if sensored method works, try starting up drive after motor shaft has been set at different angle on each test, and then try drive the motor. I.e. first power on it at angle 0°, then 5°, then 10°, then 15° and perhaps few random angles too. If motor stays stable and operates properly after each power on at any angle, then phasing is working correctly.

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