# Speed Of Brushless Motors Without Load



## Mike K2 (Jun 5, 2008)

I posted a question last week that didn't get an answer so I thought I'd try rephrasing it so it made more sense.

I'm thinking about using a brushless motor to power a small precision drill press, but am worried that the speed won't be consistent enough. When you guys run them in cars and boats, what happens when the wheels/propeller get in the air while you're at partial throttle? Does the RPM run wild while in the air or does the controller keep the speed close to the same? If it stays close to the same, I think it would work.

The size I'm interested in is about 200 watts, and about 1000 RPM per volt.

Any help would be appreciated.


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## hankster (Jan 1, 1998)

The RPM increases a lot with no load. All the controller does is provide a constant voltage and has no control over the RPM. Not sure what type of motor would provide the specs you are looking for. A 200 watt RC motor would be maybe 7K RPM per volt. I think the current Novak 5400 (5.4K per volt) motor is about 150 watts.


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## Mike K2 (Jun 5, 2008)

I was afraid of that. Thanks for the reply.
Do you know how these controllers get timing information from the motor? My understanding is that they provide AC current to the motor of a frequency that matches the motor speed, but how does it know?


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## hankster (Jan 1, 1998)

Timing information is not used for frequency. Many RC brushless motors are sensorless where there is no feedback at all. Sensored motors use the information mainly at low speeds for smooth RPM and to tell the controller which way the motor is turning when they start from zero RPM.

They are not very sophisticated. They just put out a frequency according to RPM requested and the controller doesn't really care what RPM the motor is actually turning.


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## Mike K2 (Jun 5, 2008)

Industrial sensorless motor controls use the back EMF from the motor to synchronize a 3-phase AC current with the rotational position of the motor at any given time. 

I hope you will forgive my lack of knowledge of these hobby motors, but I don't understand how the controller knows to increase the speed (frequency) when the load is reduced. Does it increase the frequency to try to keep the current constant?

If one of these little motors could be made to work for my application it would sure be nice. The motors are small and light enough that you mount one directly to the quill of the drill press and eliminate the splined shaft altogether.

As you have no doubt noticed, I'm hoping that there is some way to make this work. But like so many other things, it's looking like it's too good to be true!


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## harringBONE (Oct 15, 2001)

industrial controllers implement closed loop speed control. in the sensorless variation, the speed as well as position is calculated from the back emf, ie the voltage present on the windings that are not being energized. so knowing what speed it is currently running at it can calculate wheter to increase or decrease it's speed according to its set speed by increasing or decreasing the average voltage applied to the motor. in rc it is an open loop system and the average voltage applied to the motor is only controlled by the position of the throttle, ie 10% throttle = 10% average voltage applied to motor, so the speed more or less dictated by the load appled to the motor.


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## hankster (Jan 1, 1998)

As harringBONE said, the controller doesn't care what speed the motor is turning. If the controller is commanded to supply 50% voltage, it does that irrespectly of what the motor actually does.


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## Mike K2 (Jun 5, 2008)

Thanks to you both for sharing your expertise! 

It sounds like for my application the controller is more the problem than the motor.


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## Mike K2 (Jun 5, 2008)

Can anyone tell me what kind of signal the receiver sends to the controller when the operator wants to go faster? I would guess a simple voltage or resistance change, but I really don't know


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## harringBONE (Oct 15, 2001)

the receiver sends a pulse that corresponds to a throttle position, ie zero throttle is a pulse that is high for 1.25ms, while full throttle is 1.75ms. i assume you are looking at making your own speed control for your drill press. to maintain a constant speed under load you need to sense the speed, which can be obtained from one of the sensor wires, and adjust the width to maintain your speed setting.


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## Mike K2 (Jun 5, 2008)

Do these motors have sensor wires? I've only noticed the 3 power leads in the photos. 

The pulse width thing sounds a little complex for my limited electronics skills. I'm having trouble visualizing a control system for that.


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## DJ1978 (Sep 26, 2001)

Mike K2 said:


> I posted a question last week that didn't get an answer so I thought I'd try rephrasing it so it made more sense.
> 
> I'm thinking about using a brushless motor to power a small precision drill press, but am worried that the speed won't be consistent enough. When you guys run them in cars and boats, what happens when the wheels/propeller get in the air while you're at partial throttle? Does the RPM run wild while in the air or does the controller keep the speed close to the same? If it stays close to the same, I think it would work.
> 
> ...


How are you going to set this up? direct drive to the quill? or with gear reduction thru a belt system. The motors you are asking about turn a tremendous amount of RPM.
Smaller motors in the 25,000 rpm range. Bigger motors can spool up to over 50,000 easily.
1000 per volt is actually a VERY small motor. Not sure how much torque it would produce. 
You didn't say what your are going to be drilling or the drill sizes you normally use. 
What is wrong with the stock motor in the drill?

Check out motors from NEU.


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## Mike K2 (Jun 5, 2008)

I didn't know that bigger motors turned faster. What about this one:
http://cgi.ebay.com/E-flite-RC-Powe...770Kv-60A_W0QQitemZ130228903950QQcmdZViewItem
I would think that a motor like that would have lots more than I need for an 1/8" diameter drill. Am I missing something?


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## hankster (Jan 1, 1998)

That is 770 RPM per volt motor (unloaded). At it's recommended 12v range it would turn 9200 RPM (unloaded). Since there is no way to control the loaded RPM, there is no way to know what the RPM would be in use. You really need a motor/system that has RPM feedback so you can tightly control the RPM of the motor. RC hobby motors are not suitable for this use as none of them have RPM feedback.


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## kevinm (Jan 16, 2002)

The motor above says it's rated at 700 watts, which is almost 1 horsepower. And it draws 42 amps. Probably a bit much for a 1/8" drill press. The Kv range you want definitely sounds more like an airplane motor than a car motor. Airplane motors are built for torque and usually have more poles.

The Novak car motors (and many other brands with "Sensored" motors) DO have RPM feedback. They have an extra 6 wires connected to hall-effect sensors and a temperature sensor in the end of the motor. The "Sensorless" motors (i.e. anything for an airplane or from Castle) use the motor windings to sense (sorta) the speed. Novak's website has this article on it explaining the differences.
http://www.teamnovak.com/tech_info/brushless/sensor_vs_sensorless.html

Pretty much all of the opinions that have been posted on here (including _mine_) are based on what we _*think *_we know about R/C brushless systems. If you REALLY want an answer you can trust, I'd suggest sending emails to Novak and/or Castle tech support and asking your questions directly to the people who design the things.


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## hankster (Jan 1, 1998)

RC grade motor controllers DO NOT control the RPM of the motor and would not be suitable for his application. In other words, if the RPM slows down because of load, the motor controller will not apply more voltage to keep the RPM constant.


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## harringBONE (Oct 15, 2001)

kevinm said:


> Pretty much all of the opinions that have been posted on here (including _mine_) are based on what we _*think *_we know about R/C brushless systems. If you REALLY want an answer you can trust, I'd suggest sending emails to Novak and/or Castle tech support and asking your questions directly to the people who design the things.


fyi, i am a power electronics design engineer that has designed three phase brushless motors and drives and also hold patents on a brushless motor drive.

on current BL escs the senor wires are used only for determining when each FET turns on and is not used as a feedback for RPM regulation, as hank posted above.


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## hankster (Jan 1, 1998)

I am not an engineer or hold any patents, but I did stay at a Holiday Inn one time  I have worked in plant maintentance with industrial motor controllers that do monitor and control motor RPM. The advantage to sensored RC motors over sensorless is in low speed control, I would venture to say that at high speed there isn't much advantage to one over the other for RC use.... but I could be wrong there.


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## Mike K2 (Jun 5, 2008)

Let me see if I have this right:

The controller sends a voltage that is proportional to the length of the pulses from the receiver. The frequency of the power going to the motor is only a reflection of the frequency the back EMF from the motor and is completely independent of the pulse length. 

If that is true, then for my application I need a source of pulses whose duration can be adjusted to control motor speed and some kind of feedback that can be used to influence the pulse length to maintain speed when load changes occur.

I would guess that when the motor is pulled down by a load increase that the current draw goes up and the voltage goes down. Wouldn't there be a way to use the changing current or voltage as a source of feedback to adjust the pulse length and bring the speed back up?


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## hankster (Jan 1, 1998)

Or you could attach an RPM sensor and adjust the pulse width according to RPM. I would think that would be much simpler.


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## kevinm (Jan 16, 2002)

At the risk of asking an obvious question, wouldn't this be simpler?


http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=44505


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## Mike K2 (Jun 5, 2008)

No.

That Harbor Freight POS isn't capable of drilling a .013" hole in anything. The lack of precision precludes it. Small drills need high speed, excellent concentricity and good sensitivity.
Those little benchtop cheapies from China don't have any of that. I wish it were that easy.


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## gezer2u (Sep 25, 2001)

I have this Dremel. It will slow for a second but will come back up to speed. If you put it with this drill press and chuck, it might be what you are looking for.


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## Mike K2 (Jun 5, 2008)

kevinm and gezer2u, Thanks for the pointers, but I'm looking more for an alternative to machines like this:
http://www.jlindustrial.com/SDP-77000M/SEARCH:POPULAR/product.html

Almost $2,000.00!! The chuck alone is over $200.00. Tiny drills break very easily, and without lots of precision (expensive) they can be a pain in the neck.


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## gezer2u (Sep 25, 2001)

Mike K2 said:


> Tiny drills break very easily, and without lots of precision (expensive) they can be a pain in the neck.



After doing a search, I can believe that! Man, those things are BANK! :freak:

I saw this yesterday on Ebay. Who knows what it will end at. 

That exhaust my knowledge on the subject! :lol:

Hope you find what you need.


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