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LED
2014.03.02, 02:46 AM
Hi

Can anyone explain what the drive frequency does?
In know that the lower the freq the higher the torque, but i would like to know what it is exactly.
Since this is a DC motor I would think you would apply DC voltage to it. Why the Freq in the signal?

Thank you

Jshwaa
2015.01.02, 11:46 PM
Hi

Can anyone explain what the drive frequency does?
In know that the lower the freq the higher the torque, but i would like to know what it is exactly.
Since this is a DC motor I would think you would apply DC voltage to it. Why the Freq in the signal?

Thank you

The frequency is so that you can apply a variable voltage to the motor.

If you want 5V to a motor, and you have a 10V source, you turn the power on for half the time, then off for half the time, really fast. This is called 'pulse width modulation'.

The frequency can vary within a band and not have much effect on the motor's performance, but if the frequency is too high, the inductance of the motor windings becomes significant, and you'll net less current through the motor. If the frequency is too low, then the motor's rotation becomes less fluent and more jerky. The xmod PWM frequency was between 300 and 400 Hz, but anywhere from 300 to 1000 Hz is good for mini-z motors.

Mike Keely
2015.01.03, 10:54 AM
So are the Hz something that you change to help with the torque of the motor or does it change the top end HP? If so how have you changed it to help what aspect of the motor? I have not bought a brushless car yet but I would like to know how the motor is adjustable. Thanks MK

LED
2015.01.03, 01:32 PM
@ Mike
This is for brushed motors regarding the ICS setting.


The frequency is so that you can apply a variable voltage to the motor.

If you want 5V to a motor, and you have a 10V source, you turn the power on for half the time, then off for half the time, really fast. This is called 'pulse width modulation'.

The frequency can vary within a band and not have much effect on the motor's performance, but if the frequency is too high, the inductance of the motor windings becomes significant, and you'll net less current through the motor. If the frequency is too low, then the motor's rotation becomes less fluent and more jerky. The xmod PWM frequency was between 300 and 400 Hz, but anywhere from 300 to 1000 Hz is good for mini-z motors.

Thx for your answer but I don't understand why this is relevant for the mini-Z. I understand the modulating as you explain it as a way to regulate speed. However the mini-z speed is regulated by the voltage itself. So my question is why there is a frequency in the DC voltage and not just the steady line.
Now I haven't measured the Z's motor voltage with a scope. So I'm wondering if this frequentie is a sinus going from positive to negative, or rather a sinus staying at the positive side.

TheSteve
2015.01.03, 02:14 PM
Without the PWM the speed control would only provide an on or off function to the motor, or one speed.
The drive frequency/PWM does not change the top speed as power is fully on at full throttle like the batteries and motor were directly connected.

The PWM signal is a square wave. It is used to vary the speed as it is very efficient. If you wanted to vary the voltage in a linear fashion the speed control would create tons of heat.

Without getting into the technical component of PWM(such as inductance) in the most basic sense a higher drive frequency will generally make a car feel less punchy and smoother to drive. If you lower the frequency you should feel like the car has a little more punch out of the corners.

Mike Keely
2015.01.03, 11:48 PM
Is there a point were you can not feel the difference in the higher frequencies? Is there a point were the Hz can be set to low?

LED
2015.01.04, 12:16 PM
Without the PWM the speed control would only provide an on or off function to the motor, or one speed.
The drive frequency/PWM does not change the top speed as power is fully on at full throttle like the batteries and motor were directly connected.

The PWM signal is a square wave. It is used to vary the speed as it is very efficient. If you wanted to vary the voltage in a linear fashion the speed control would create tons of heat.

Without getting into the technical component of PWM(such as inductance) in the most basic sense a higher drive frequency will generally make a car feel less punchy and smoother to drive. If you lower the frequency you should feel like the car has a little more punch out of the corners.

I'm still thinking about this. I said the mini-z was controlled by voltage, but I might be mistaken. Because a voltmeter would see it as an average voltage. I would need a scope to check, wich I dont have.

But regardless of that, how does the PWM exactly work?
For instance is it a constant 1000 Hz and when you give more throttle the voltage pulse lasts longer, and the gap in between pulses gets shorter?
Is this then the frequency you can change in the ICS settings?

Or does simply the pulse last longer but also the gap? But that would then mean the frequency also changes? And what is then the ICS setting?

Thx again for the answers

Jshwaa
2015.01.04, 09:46 PM
I'm still thinking about this. I said the mini-z was controlled by voltage, but I might be mistaken. Because a voltmeter would see it as an average voltage. I would need a scope to check, wich I dont have.

But regardless of that, how does the PWM exactly work?
For instance is it a constant 1000 Hz and when you give more throttle the voltage pulse lasts longer, and the gap in between pulses gets shorter?
Is this then the frequency you can change in the ICS settings?

Or does simply the pulse last longer but also the gap? But that would then mean the frequency also changes? And what is then the ICS setting?

Thx again for the answers

Well, first off, let's just say that we may be getting our wires crossed between two subjects. The PWM frequency on brushless motors is a little more critical than it is on 'brushed' motors, as the commutation is controlled by how fast the 'brushless' ESC is phasing the voltage to the motor in an attempt to provide starting torque. If you don't get it right, you will get the 'cogging' phenomenon people refer to.

As far as 'brushed' motors go, I defer to the experts that have designed xmod and mini-z boards to know what frequency works best, but frequency is just one aspect of 'pulse width modulation'. I found xmod PWM to be around 350Hz, give or take, but I would not want to go below 300 Hz, or above 1KHz, because if I go below 300Hz, then the on-off-on-off states of the H-bridge FET's becomes apparent in the torque output of the motor. You can actually feel the motor more or less fibrillate at the frequency, rather than rotate smoothly.

If you go too high in frequency, again, the inductance of the motor becomes significant, and you begin creating reactive power which is just the motor and the FET's exchanging energy, and less torque output.

With 'brushed' motors, there is a sweet spot, where going above or below will not net any bonus output power.

The reason for the 'settings' on the brushless ESC's is because there is no position feedback between the motor and controller on hobby brushless ESC's, so the ESC is actually blindly switching the phases in an attempt to produce starting torque. The operator is expected to adjust the settings to tune that in, but some starting torque loads may be outside the band of settings allowed, so you either get 'cogging' or slow acceleration. Finding the 'sweet spot' with brushless can be difficult without hall-effect sensor feedback, which requires at least 3 more wires to the motor.

Oh, and to answer your question, hopefully, in PWM, the frequency doesn't change, just the on time with respect to the overall time. In other words...

Let's say you have a 1KHz PWM, and your batteries are supplying 6V. Your frequency is going to have an overall period of 1ms (0.001 s). If you want 3V to the motor, you must turn it on for 500us (0.5ms or 0.0005 s) and off for 500us (0.5ms or 0.0005 s). If you want 1V to the motor, you turn it on for 166.67us (0.166ms ir 0.0001667 s) and off for 833.33us (0.8333ms or 0.0008333 s). Thus the frequency never changes because the on and off times always add up to 1ms.