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Locking a 300V BLDC motor for encoder calibration

Dave2024

May 18, 2024
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Hi how’s it going, I was wondering if someone with a bit more knowledge on 3 phase brushless DC motors could help me out with “locking” my motor? See I’m getting ready to remove the hall sensor on a 300v DC spindle motor from out of a CNC mill and upgrade it to use an encoder instead. The end result is to turn the motor into a servo for more precise speed and even be able to use the spindle to do tapping operations. Part of installing the encoder is that the motor must be locked or more like the phase 1 wire be energized and the phase 2 wire be grounded but leaving the phase 3 wire completely disconnected. This should make the motor rotate to a fixed angle and be locked. Then with the motor locked im suppose to send the encoder the zero command which will tell it to store the position the motors at as zero and save it in its non-volatile memory.

What I’m wondering is since I’m doing this all at home and only have the skills of a hobby enthusiast is it going to be ok to use a 4S (14.8V) battery to energize the two wires on the motor and lock it or am I risking burning the pole windings up on the 300V BLDC motor? Or would be the opposite and 14.8V even be enough to move the motor at all? See I don’t have a variable power supply to work with or anything here at my place. I just don’t want to burn the motor windings up or anything.

Anyways if someone could confirm hooking a 4s(14.8v) lipo battery up to a 300V BLDC motor is a good idea to lock it? Thanks
 

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Bluejets

Oct 5, 2014
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Use ohms law to calculate the current.
Brushless motors are essentially induction motors and as such rely on the inductive properties to limit the current.
Pump in DC at whatever voltage and the resulting current could fry the windings.
A 4s lipo can provide currents of varying amounts as shown on the casing(but not here sadly) that can be up to 60 times the capacity.
 

Dave2024

May 18, 2024
4
Joined
May 18, 2024
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Use ohms law to calculate the current.
Brushless motors are essentially induction motors and as such rely on the inductive properties to limit the current.
Pump in DC at whatever voltage and the resulting current could fry the windings.
A 4s lipo can provide currents of varying amounts as shown on the casing(but not here sadly) that can be up to 60 times the capacity.


Ok so on the casing here im seeing 300V, 1100W, 4.7A. So what's the ohms law formula to calculate the voltage and current ill need to lock it at say 1/3 the max power?
 

Bluejets

Oct 5, 2014
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Induction calculations are unnecessary............use a basic multimeter as you are injecting dc.
Measure the winding resistance and work out the value of current using ohms law.
If it overrates the current ability of the winding, forget it.

This IS brushless motor??...not stepper?
Just at one stage you were talking cnc.
 

Dave2024

May 18, 2024
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May 18, 2024
Messages
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Induction calculations are unnecessary............use a basic multimeter as you are injecting dc.
Measure the winding resistance and work out the value of current using ohms law.
If it overrates the current ability of the winding, forget it.

This IS brushless motor??...not stepper?
Just at one stage you were talking cnc.
Ok so your saying I should take a multimeter and get a reading of the continuity on one of the phase wires and then that should give me a safe current to use?

And yes its a spindle motor out of a CNC mill that was controlled with PWM and I'm converting it to use an encoder rather than a hall sensor so essentially becoming a stepper servo when done
 

Bluejets

Oct 5, 2014
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If you get the resistance of the winding you can calculate the expected dc current that will flow (ohms law given it is DC) with whatever power source (voltage) you are injecting.
Be aware your power source will not have infinite capacity and will definitely drop it's voltage level under load.
If that current is within safe current value for the winding then you are good to go.
Otherwise you will need to create some type of current limit.
Whether or not that safe limit will lock the rotor is another story.
 

Dave2024

May 18, 2024
4
Joined
May 18, 2024
Messages
4
Use ohms law to calculate the current.
Brushless motors are essentially induction motors and as such rely on the inductive properties to limit the current.
Pump in DC at whatever voltage and the resulting current could fry the windings.
A 4s lipo can provide currents of varying amounts as shown on the casing(but not here sadly) that can be up to 60 times the capacity.
Ok well hey thanks for helping me out with this Bluejets. So I've came across a power supply and a couple multimeters. On one of the multimeters I connected to analyze the current from the PS and the other I have reading the voltage. They seem to be fairly accurate except the meter reading the voltage is 2X on its display what the PS is putting out. Anyways what's a good current to start with as not too fry any windings on the motor like the one I've got here? Thanks
 

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Minder

Apr 24, 2015
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To turn a BLDC motor into a servo you require an encoder with the 3 commutation tracks, You also need to know the pole count of the motor in order to order the correct commutation spacing, this is easy to find out by shorting the 3 conductors turn one revolution and measuring the 'bumps,/ rev.
To align it, you need to back-feed the motor , ~ 200rpm's or so.
And use a double beam 'scope to sync the encoder com pulses to the generated winding outputs.
I have done several over the years for CNC usage.
 

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