Locking BLDC motor rotor for encoder zeroing calibration

Question

I know the title of this post is a real tong twister but my brain is in much worse condition as far as what's goin on here. Basically I have a motor that I'm installing an encoder onto so it can be converted from a PWM controlled to Step/Dir driven. In the procedure of what's required to calibrate the newly installed encoder is that the motor must be energized on phase 1 and phase 2 locking it into position. With the phase's energized a set zero command then needs to be sent to the encoder that will make it become calibrated.

In the manual to the AMT31 Series encoder I have it says this:

The AMT31 series encoder requires minimal setup time for brushless DC (BLDC) motor applications. Installation can be completed with either a PC with the AMT ViewpointTM application installed, an AMT-OTZ-1 zero alignment module, or any 5V serial interface. The following steps explain the proper commutation alignment procedure for the AMT31 encoder.

1. Ensure AMT31 encoder is set for correct pole count. To verify or change settings use the AMT ViewpointTM software.

2. Mount encoder following AMT Assembly procedure.

3. Use the motor manufacturer’s documentation to determine the correct motor phase to energize for alignment. Energized phase will coincide with the rising edge of the AMT31 encoders ‘U’ signal. This typically means energizing phase 1 by applying positive voltage to the wire labeled ‘phase 1’, and grounding the wire labeled ‘phase 2’. The third wire always remains unconnected.

4. Using a power supply, energize the two wires found previously. This will lock the rotor into a fixed position.

5. Connect AMT31 encoder to an AMT-OTZ-1 zero alignment module, the AMT ViewpointTM application, or any suitable 5V serial interface.

6. Use any of the connected devices to issue an alignment command to the encoder. This will digitally set the rising edge of ‘U’ and the ‘Z’ index to the current angular position.

7. Remove power from motor phase windings; connect motor and encoder to proper motor driver.

8. If the above is done correctly your AMT31 encoder is now ready for operation. As a verification of alignment you may power the encoder, and use an oscilloscope to monitor phase 1 of the motor and the ‘U’ channel of the encoder as you hand spin the motor. If alignment is correct, the square wave generated on the ‘U’ channel will overlap perfectly with the sine wave generated by phase 1 of the motor. If alignment does not match, recheck motor documentation and retry alignment procedure.

On step 3 I don't know what voltage and current to energize the phases to and don't want to burn them up. How do I determine how much power I should be putting on the phases?

• Jens: You just need a DC current strong enough to hold the rotor in the angle, that corresponds to the phase. If you cannot move the rotor at all by hand or it turns back to the same position if left alone, the current is sufficient. There may be multiple such positions on a full 360° turn. Chose the one, where you want to receive the index mark impulse later.

Answer 1

Adding to @Jens comment, you will be safe with any current less than the motor's rated current, but not at the motor's rated voltage. Take the following steps:

1. Find the motor's rated current.
2. Find (or measure) the phase-to-phase resistance. You can use an ohmmeter to measure the resistance between the wire labeled "phase 1" and "phase 2".
3. Apply a DC voltage that will result in a current about 25%-50% of the rated current. This will probably only be a few volts or even less. If you have a benchtop power supply with a current limit, you can set the current this way.
4. When you apply power, the rotor should jump to its proper position. Then you can set the encoder.

Two warnings:

1. It is important that you choose the correct phases and that the power supply polarity is correct. Otherwise the rotor will be in the wrong position.
2. Do not disconnect the power supply from the motor while it has current in the windings. Instead, turn down the power supply voltage to zero before disconnecting.