3-phase questions

[BrianG]

Hello,

I have a delta-wired, 2-pole, 3-phase R/C brushless motor. It has 10 turns, 1668 rpm/v, rated for about 50,000rpm, ~1,000w, 70mm long, and 36mm in diameter. It's used for radio-controlled vehicles and I was just playing around with it trying to see how much power it could generate by spinning the shaft. I hooked it to a "transmission" so the motor will spin about 6X the speed of the shaft. I used a drill, spinning at about 1100 rpm.

When I tie all the phases together, it turns quite hard telling me it has some serious EMF brake capabilities. Seeing this, I figure it should be able to produce a healthy amount of current.

Below is a table of various loads, AC voltages, and AC currents when using just a single phase of the motor:

[b]RLoad  Vload  Current[/b]
short  -      10.6A
.33    2.4v   7.27A
1      2.78v  2.78A
5.6    2.9v   .518A
open   2.94v  -

Then, I built a 3-phase bridge rectifier using a couple 25A bridge units and a single 10,000uF 16v capacitor for filtering, and then re-ran the test.

[b]RLoad  Vload  Current[/b]
short  -      7.14A
.33    1.7v   5.15
1      2.0v   2.09A
5.6    2.4v   .428A
open   3.13v  -

This isn't really what I was expecting. I figured I'd have slightly higher voltages and higher currents since I'm rectifying all phases instead of taking AC measurements on a single phase. Also, when reading short-circuited DC values, I don't get near the EMF brake effect than if I short-circuit the phases directly. Do these values seem correct?

 

[Analog]

I'm not familiar with the type of motor you say. Sounds like a squirrel cage motor? If so, you need to connect capacitors to it to create a generator if I remember correctly.

 

[eblc1388]

It is of a type of motor design originally used in CDROM drive, beefed up for R/C operation.

The test results seems reasonable, considered you have dropped 1.4V or more(two diodes voltage drop of 0.7V each in each phase arm of the bridge rectifier) on each phase.

The voltage drop becomes very significant because of the low voltage involved.

 

[Hero999]

I doubt it's a squirrel cage motor because you wouldn't be able to use it as an alternator, it's either has a permanent magnet or wound rotor.

Where did you get this motor from?

 

[BrianG]

Thanks for answering!

I did account for the voltage drop of the rectifier, but I thought I would get more voltage and current from all thee phases vs a sine phase.

**broken link removed** is where I got my motor. As far as what it is, construction is quite simple: The outer can has all the coils wound on it. The rotor is simply a permanent magnet. There are three wires coming out, one for each phase.

 

[Nigel Goodwin]

It is of a type of motor design originally used in CDROM drive, beefed up for R/C operation.

They long pre-date CDROM's, VCR's used them long before that - but even earlier, reel to reel tape recorders did - back in the 1960's!!.

 

[hjames]

My guess is that the waveforms coming out of the motor aren't sine waves - this is probably playing weird tricks with your DMM.

Also, considering the low voltages that the motor operates at, you'll probably get a big benefit out of using low Vf schottky diodes. Plus, shorting the output (after the diodes) means that the motor allows a ~1.4V back EMF voltage before the diodes start conducting - which ends up meaning a lower current - which means a lower torque.

 

[Hero999]

No they date way further back than that. These sorts of motors have been around since 1888

https://en.wikipedia.org/wiki/Synchronous_motor#AC_motors

 

[Nigel Goodwin]

No they date way further back than that. These sorts of motors have been around since 1888

https://en.wikipedia.org/wiki/Synchronous_motor#AC_motors

I suggest you read this thread properly?, it's about DC Brushless Motors. I'm somewhat doubtful about the state of electronics back in 1888

 

[Hero999]

I know but the kind of motor described above is identical to a synchronous AC motor.

 

[BrianG]

My guess is that the waveforms coming out of the motor aren't sine waves - this is probably playing weird tricks with your DMM.

Also, considering the low voltages that the motor operates at, you'll probably get a big benefit out of using low Vf schottky diodes. Plus, shorting the output (after the diodes) means that the motor allows a ~1.4V back EMF voltage before the diodes start conducting - which ends up meaning a lower current - which means a lower torque.

Yeah, I don't have an o-scope to check, but I would think if I have a steady load and turning it consistently, I would think the waveform should be close to sinusoidal?

Yeah, the voltage is kinda low. I tried spinning it with my dremel tool, but the dremel heats up from the excessive load when I try to draw any kind of appreciable current. With the dremel and no load on the BL motor, I get over 11v AC or DC spinning at about 20,000 rpm at a guess (just from experience on how the motor sounds at various rpms). My drill only spins at ~1100 rpm, so I'm kinda limited.

This motor is generally suitable up to about 35,000 rpm. It's rated at 1668rpm/v unloaded. Loaded, the kv constant (rpm/v) is around 1500. So, it would take a little over 23v to get to that speed. It don't expect to get 23v by spinning the motor at 35krpm, but maybe 15-18v. I was just curious what kind of current I would be able to generate while still delivering, say, 12v. It was just a fun experiment.

Thanks for your answers!

 

[RODALCO]

Are you sure it is a 3 phase motor?
A 2 pole 3 phase motor will run a maximum speed of 3000 RPM at 50 Hz, 3600 RPM at 60 Hz.
Unless it is ex military where 400 Hz is used to make equipment more efficient and compact , then probably 24000 RPM is possible.
Not something of a selsyn alternator generator set ? not exactly sure how those work but i have read about them and these run at high speeds too.

 

[Nigel Goodwin]

Are you sure it is a 3 phase motor?
A 2 pole 3 phase motor will run a maximum speed of 3000 RPM at 50 Hz, 3600 RPM at 60 Hz.

Is it just me?.

IT'S A DC BRUSHLESS MOTOR!!!

The mains frequency has nothing to do with it, it runs from a battery, the speed is dependent on it's electronics.

 

[JimB]

Nigel,

No it is not just you, I feel confused.
The motor was originally stated to be a "brushless motor", but if you follow the link in one of BrianGs' posts...

**broken link removed** is where I got my motor. As far as what it is, construction is quite simple: The outer can has all the coils wound on it. The rotor is simply a permanent magnet. There are three wires coming out, one for each phase.

the motor appears to be a three phase motor with a separate controller which converts the DC to three phase at some frequency.

BrianG appears to be trying to use the bare motor without its controller, as a generator for some reason. Quiet why, I dont know.

JimB

 

[Hero999]

Exactly, it's a low voltage synchronous three phase motor. The driver electronis are a kind of inverter that converts the DC voltage to AC to power the motor.

 

[Nigel Goodwin]

Exactly, it's a low voltage synchronous three phase motor. The driver electronis are a kind of inverter that converts the DC voltage to AC to power the motor.

Exactly as the brushes do in brushed DC motor - there's really no such thing as a 'DC motor'.

'Inverter' isn't the right word either, the electronics simply switches the coils in the correct sequence, based on the position of the rotor.

 

[Hero999]

'Inverter' isn't the right word either, the electronics simply switches the coils in the correct sequence, based on the position of the rotor.

I'm not sure about this part, there's no mention of any feedback, like hall effect sensors.

 

[BrianG]

Sorry for the confusion guys!

Normally, you would use one of several **broken link removed** to drive the motor. They are not really inverters, they are more like variable frequency drives.

As to the reason why I was doing this; I was just curious. I had my **broken link removed** apart for cleaning and the motor was sitting on the bench. No real goal to this experiment, just wanted to see what it would do.

And there are no Hall-effect sensors; this is a sensorless motor. The controller reads the back-EMF pulses from the motor to determine rotor position.

 

[Sceadwian]

Hero... that's exactly what a brushless DC motor is. Everyone seems to be arguing over what to call the duck. It quacks, it's a Duck!

Tonnes of information on these kinds of motors and their variations can be found simply by looking up the Wikpedia entry "brushless DC motor"

If I'm not mistaken these are used in R/C aircraft frequently because they can generate a lot of torque for their size so can drive bigger props and the internals lend themselves well to air cooling coming directly from the prop.

 

[BrianG]

Yup, that's right. Most aircraft people tend to use "outrunner" types. These have the stationary part of the motor on the inside and contain the windings. The rotor, which is the outside bell, is the part that spins. From the little I understand about aircraft, props can only spin so fast before they stop doing their job. These outrunners spin at a slower rpm so there is no need for any gear reduction yet, still has lots of torque. They seem to be built much like a CDROM motor.

The brushless motors most of us "car guys" use are inrunners, like in the links I posted before. These spin at much higher rpms yet still have tons of torque. So much torque that we often break transmissions, driveshafts, differentials, etc. Many people take gas-powered vehicles and convert them to electric because the speed and torque are so high. And we usually disable the two-speed transmissions found in the gas cars because first gear is useless. Very fun.

There are some inrunners that use hall-effect sensors, but tend to be smaller that the sensorless counterparts, and can't spin as fast for some reason.