Motor rotation speed - RC Groups
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Dec 20, 2017, 11:55 AM
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Motor rotation speed


Rotation speed of 3 phase motors is determined by frequency.

These small RC 3 phase motors define rotation speed as a function of KV. 500K, 500*5 volts and we get 2500 RPM. A KV value gets us to our RPM requirements but, does an ESC output vary frequency as well as voltage on these small 3 phase motors?
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Dec 20, 2017, 02:00 PM
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The rotation speed is determined by the supplied voltage and the load. The supply voltage will cause a large current and therefore a large torque. That torque will cause acceleration. As the motor speed increases, the back-emf of the motor will increase. The motor will continue to accelerate as long as the supply voltage is greater than the back-emf and the voltage drop due to the current in the windings.

In order to run properly, the rotational speed of the electromagnetic field in the motor has to be equal to the rotational speed of the rotor. The ESC therefore has to know the rotor position so it can change the current in the windings at the correct time. The faster the rotor is turning, the faster the ESC has to switch current in the windings (that is, the higher the frequency). But this change in frequency is a reaction to the change in rotor speed ... it doesn't determine the rotor speed.
Dec 20, 2017, 07:03 PM
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Quote:
Originally Posted by lhedrick
Rotation speed of 3 phase motors is determined by frequency.

These small RC 3 phase motors define rotation speed as a function of KV. 500K, 500*5 volts and we get 2500 RPM. A KV value gets us to our RPM requirements but, does an ESC output vary frequency as well as voltage on these small 3 phase motors?
Quote:
Originally Posted by learningrc
The rotation speed is determined by the supplied voltage and the load. The supply voltage will cause a large current and therefore a large torque. That torque will cause acceleration. As the motor speed increases, the back-emf of the motor will increase. The motor will continue to accelerate as long as the supply voltage is greater than the back-emf and the voltage drop due to the current in the windings.

In order to run properly, the rotational speed of the electromagnetic field in the motor has to be equal to the rotational speed of the rotor. The ESC therefore has to know the rotor position so it can change the current in the windings at the correct time. The faster the rotor is turning, the faster the ESC has to switch current in the windings (that is, the higher the frequency). But this change in frequency is a reaction to the change in rotor speed ... it doesn't determine the rotor speed.
Synchronous speed of an ac induction motor is not just determined by input power frequency alone. Pole count is also involved. The formula is expressed as:

n=f(2/p)60

n=shaft rotation in rpm
f=frequency in Hertz
p=pole count

But generally speaking what you have been told is correct for most of the hobby BLDC controllers but there are a few on the market now that employ Field Oriented Commutation(FOC). If I'm not mistaken this is a control type of a variable frequency drive(VFD) which would indeed control the speed of the motor by varying the input frequency and voltage.
Last edited by 1boho; Dec 20, 2017 at 07:49 PM.
Dec 20, 2017, 07:30 PM
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Thanks for the explanations. I didn't know the ESC used rotor possition sensing. Much cooler then the old 100 pound 3 phase motors I worked with back in the 70s.
Dec 20, 2017, 10:27 PM
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Quote:
Originally Posted by lhedrick
Thanks for the explanations. I didn't know the ESC used rotor possition sensing. Much cooler then the old 100 pound 3 phase motors I worked with back in the 70s.
3 phase induction motors and VFD drive technology is just as cool today if not cooler . They are what powering some of todays most popular EV's . The technology has made advancements in leaps and bounds in the last decade. If you haven't looked lately you might be surprised at some of the efficiencies they are extracting out of such power systems.
Jan 01, 2018, 10:48 AM
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Quote:
Originally Posted by 1boho
3 phase induction motors and VFD drive technology is just as cool today if not cooler . They are what powering some of todays most popular EV's . The technology has made advancements in leaps and bounds in the last decade. If you haven't looked lately you might be surprised at some of the efficiencies they are extracting out of such power systems.
I need to dig in and try to catch up a bit.

I know it not RC related but, itís still on the subject of induction and motors so I guess this is worth a comment.

Back in 2002 I was working in an old 10 story office building built back around 1910. While there, Otis was in to do major service on the elevator system. I asked one of the guys if they would replace the motors and he said, the motors were the originals, those will stay and they are so solid they will run for another 100 years. All they were going to do was clean them up and check the bearings. I assume they were 3 phase but I really donít know what they were.
Jan 01, 2018, 11:37 AM
I am a nice guy! Really!
Most elevator and hoist motors are wound rotor induction motors. With a wound rotor external resistance can be added to decrease the speed of the motor this allows very fine control of the speed.

For a point of clarification while our brushless DC motors do have three phase leads they are not three-phase motors the technical term is externally commutated DC motors.
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Jan 03, 2018, 08:01 PM
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Mike

I believe you are a nice guy

Thanks for the explanation

This forum helped me to build motors lamination by lamination years ago

That does not mean that I know very much about them

It just means I know how to throw the lamination off the magnetic chuck

while trying to thin it to 0 .2 mm , bit of a mess
Last edited by NX-687; Jan 03, 2018 at 08:13 PM.


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