Driving 3-phase motor from 2 amplifier phases - Page 1
Forum section: electronics / projects

author: Circlotron
author_rank: Super Contributor
timestamp: December 27, 2018, 03:45:29 am
content: Got a 3-phase permanent magnet synchronous motor. One wire tied to ground and the other two are fed with sine waves 120 deg apart. The motor won’t rotate, it only jiggles back and forth a bit. If the windings from ground to each phase are fed equal sinusoidal voltages 120 deg apart then shouldn’t the third winding that goes between the driven points be getting the same voltage and the right phase displacement?

author: Circlotron
author_rank: Super Contributor
timestamp: December 27, 2018, 04:05:46 am
content: Looks like it's more complicated than I thought...Putting it another way, say we had a 3-phase supply that is completely floating. If we ground one of the motor wires and look at the first ungrounded wire with a scope, what is the relative phase of the second ungrounded wire to the first, and to ground?

author: soldar
author_rank: Super Contributor
timestamp: December 27, 2018, 08:48:11 am
content: Quote from: Circlotron on December 27, 2018, 03:45:29 amAll this is extremely confused and confusing. You have power with two phases and neutral? Where did the third phase go?Can you post a clear schematic of the whole thing?

author: Circlotron
author_rank: Super Contributor
timestamp: December 27, 2018, 09:13:56 am
content: Looks like the phase difference between the two amplifier outputs has to be 60 deg. That way at least there is equal voltages applied across each motor winding. Still won't run!

author: Circlotron
author_rank: Super Contributor
timestamp: December 27, 2018, 10:11:49 am
content: Well, eventually got it to run!Maxed out at 120Hz, 30V peak, 2400rpm. (6 pole motor)The amplifier was *very* sensitive to volts/Hz. When the volts was right for the Hz the current would be lowest. Slightly off and the amp (2x LM3875) would go into current limiting and the motor would growl. What was really interesting though is the phase difference between the two amplifier outputs. In theory at least, it should have been 60 deg all the way, but in fact the optimum varied from 50 to 70 deg. Dialed it back and forth a degree at a time to find the smoothest running.Next step is to get a proper electronic speed control like used on drones etc. Not sure whether to go for sensor type or sensorless. The motor does have 3 hall effect position sensors, just I don't know which would work better.

author: fsr
author_rank: Regular Contributor
timestamp: December 27, 2018, 10:36:36 am
content: But a 3 phase motor needs 3 AC phases at 120 degrees of each other. Otherwise the electric company would save a wire, if they could. Even if you did made the motor turn, it won't have the same characteristics. Maybe stalls at the first opportunity.

author: Circlotron
author_rank: Super Contributor
timestamp: December 27, 2018, 10:48:51 am
content: Quote from: fsr on December 27, 2018, 10:36:36 amAnd so this one does.Between channel A and gnd.Between channel B and gnd.Between channel A and B.It is no different to running a motor off a transformer with a delta secondary and earthing one of the phases.

author: soldar
author_rank: Super Contributor
timestamp: December 27, 2018, 11:17:16 am
content: Quote from: fsr on December 27, 2018, 10:36:36 amNope. Yes but no. Think about it.Take a three phase generator connected in triangle (delta). Now remove or just interrupt one of the windings. You still have three phases fully working... except that now two windings are doing the work of three and can be overloaded. But all three wires (phases) have the correct voltage and phase.It would not work the same in a star connected generator with neutral where if you interrupt one winding then that phase is gone.Using an amplifier like the OP is using a delta configuration and so it works. Note there is no neutral.

author: fsr
author_rank: Regular Contributor
timestamp: December 27, 2018, 07:10:36 pm
content: Oh, i see. But, if the output waves are at 60 degree of each other, there won't be 120 degrees between phases. That doesn't introduce problems?

author: soldar
author_rank: Super Contributor
timestamp: December 27, 2018, 07:24:05 pm
content: They should correctly be spaced 120º so that you get 360º as you go around.

author: Benta
author_rank: Super Contributor
timestamp: December 27, 2018, 07:29:22 pm
content: It will work, but your control scheme needs refining.Running one phase at ground and the other two phases at -60 and +60 degrees is fine, this will in fact give you 3 x 120 degrees at the motor leads.But a characteristic of PMSMs is that they have no feedback, and you need a concept of starting it. This is usually done by increasing voltage and frequency from zero up to running speed. You also need to place the motor in a certain starting position first, this is normally done by energizing one pair of phases to get it positioned.

author: soldar
author_rank: Super Contributor
timestamp: December 27, 2018, 07:42:48 pm
content: Quote from: Benta on December 27, 2018, 07:29:22 pmTaking one phase as reference that means the other one is at 120º. If you make that -120º then the rotation of the phases changes direction and the motor will rotate in the opposite direction. It is the same as just electrically switching both phases.

author: Benta
author_rank: Super Contributor
timestamp: December 27, 2018, 07:49:44 pm
content: Quote from: soldar on December 27, 2018, 07:42:48 pmThis is semantics and we mean the same thing.One option is running one amplifier directly with no phase shift and the other at 120 degrees.The other is running one at -60 and the other at +60 degrees, which is easier to do.The end result is the same.

author: soldar
author_rank: Super Contributor
timestamp: December 27, 2018, 08:01:42 pm
content: Quote from: Benta on December 27, 2018, 07:49:44 pmNo phase shift with respect to what? What is your reference? I am using one phase as reference. Using a point equidistant from two phases is ... strange. I have never seen it done in a three phase system to say one phase is -60º, another is +60º, and the third is 180º. I suppose you could do that but it just seems unusual. Normally you would take one phase as reference.

author: Circlotron
author_rank: Super Contributor
timestamp: December 27, 2018, 08:27:06 pm
content: Quote from: Benta on December 27, 2018, 07:29:22 pmThat’s exactly what I thought initially too. But if we are speaking with reference to the first driven phase then the second has to be 60 deg different. If it is 120 deg different then the voltages across the windings are unequal for one thing. I know 60 deg sounds ridiculous. Had to sim a three phase source and earth one corner and measure around it. Hard to get ones head around it. I’ll post the sim later on.

author: Benta
author_rank: Super Contributor
timestamp: December 27, 2018, 08:29:46 pm
content: I look forward to seeing your sim, this sounds strange.Soldar, stop trolling.

author: Dave
author_rank: Super Contributor
timestamp: December 27, 2018, 08:44:15 pm
content: Mentally shift the whole phasor diagram so that point b sits in the origin. This is now ground.Vector ba is now sitting at 30°, vector bc is at 90°. They are 60° apart.So, you drive points a and c with voltage sines that are 60° apart and you have your balanced 3-phase signal.That's it. That's all there is to it.

author: jmelson
author_rank: Super Contributor
timestamp: December 27, 2018, 09:09:16 pm
content: A permanent magnet motor cannot be started by just applying constant-frequency 3-phase power to it. You need to apply a very low frequency, low voltage drive, and then ramp up the frequency and voltage simultaneously, so the motor can stay in sync with the AC excitation.An induction motor can start with constant frequency excitation because the rotor magnetism can slip and stay in sync with the stator field.Jon

author: Kalin
author_rank: Regular Contributor
timestamp: December 28, 2018, 12:35:42 am
content: Is this the same concept as a synchronous 3 phase motor? If so does it actually need a ramp in frequency or could it be done in discrete steps say 10-20-30hz.Sent from my SM-G965W using Tapatalk

author: duak
author_rank: Super Contributor
timestamp: December 28, 2018, 01:47:23 am
content: Kalin, I developed the analog and power sections of an ultraprecision BLDC motor system. Until the DSP portion with an encoder was completed, for testing I drove the motor with arbitrary frequencies. My experience is that a smooth ramp is required and the rate depends on the motor and its load. The motors I worked with might tolerate a 1 or 2 Hz jump, but not much more. When DC currents were applied to the motor, you could turn the motor shaft back and forth and it felt just like a spring. If you let it go, it would oscillate like a balance wheel in a clock. So while the rotor in the motor tends to follow the magnetic field generated by the windings, if they get out of sync, there's no net torque on the rotor and it slows down to a stop.Cheers,

author: Kalin
author_rank: Regular Contributor
timestamp: December 28, 2018, 01:51:43 am
content: Quote from: duak on December 28, 2018, 01:47:23 amThanks for the insight. I work with VFD controlled motors alot but 1hp is a very small motor to me. Are BLDC motors typically permanent magnet motors or squirrel cage?Sent from my SM-G965W using Tapatalk

author: Psi
author_rank: Super Contributor
timestamp: December 28, 2018, 01:58:43 am
content: Look at brushless DC speed controller schematics.They're more or less a MCU and 6 mosfets

author: Circlotron
author_rank: Super Contributor
timestamp: December 28, 2018, 02:59:46 am
content: Quote from: Benta on December 27, 2018, 08:29:46 pmThis shows that my three voltage sources are 120 deg apart with respect to neutral.

author: Circlotron
author_rank: Super Contributor
timestamp: December 28, 2018, 03:01:30 am
content: And with the ground shifted to the B phase, showing the difference in phase between B-A and B-C is 60 deg.

author: Dave
author_rank: Super Contributor
timestamp: December 28, 2018, 01:46:20 pm
content: Quote from: Kalin on December 28, 2018, 01:51:43 amMotors with squirrel cages are induction motors. BLDC motors always have permanent magnets inside.

author: Benta
author_rank: Super Contributor
timestamp: December 28, 2018, 03:39:37 pm
content: Quote from: Dave on December 28, 2018, 01:46:20 pmNot necessarily. Larger types may have field windings. Those are rare, though.

author: Dave
author_rank: Super Contributor
timestamp: December 28, 2018, 05:19:00 pm
content: Quote from: Benta on December 28, 2018, 03:39:37 pmCould you please provide an example of such motor?To my understanding, you can't have a field winding on the rotor without having some sort of brushes in place to transfer the current onto the spinning rotor, therefore it would not be a brushless motor anymore (BLDC).

author: Benta
author_rank: Super Contributor
timestamp: December 28, 2018, 05:36:57 pm
content: The "brushless" part relates to commutation. Slip rings is another story.It's a question of the classification of the motor.Some automotive drives used this experimentally to cut cost, neodymium or cobalt-samarium magnets aren't cheap in the needed size.

author: Zero999
author_rank: Super Contributor
timestamp: December 28, 2018, 05:58:31 pm
content: Slip rings require brushes, so are not brushless.There is really no such thing as brushless DC motor.

author: jbb
author_rank: Super Contributor
timestamp: December 28, 2018, 06:45:35 pm
content: I’m not sure if you’re using amplifiers to drive the motor for testing or for some kind of low noise operation, but I have an efficiency concern; I expect that the power factor on your two amplifiers could be quite bad under some conditions. This could lead to unexpectedly high dissipation in a Class B amp.