Author Topic: Why using pwm for controlling motors?  (Read 2005 times)

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Offline little_carlos

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Why using pwm for controlling motors?
« on: July 19, 2014, 12:17:16 am »
During these days, I´ve been wondering about why would someone use pwm for controlling a motor?
Why not just using a potentiometer as a voltage divider, conecting it to the base of a transistor and just controlling the voltage?
is it more precise with pwm or why people use it?

Offline David_AVD

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Re: Why using pwm for controlling motors?
« Reply #1 on: July 19, 2014, 12:21:44 am »
Simply reducing the steady DC power to a motor will see the torque drop off disproportionately.  The motor is also likely to stall at low voltages.

PWM offers an improvement in these areas.  The PWM driver will also run much cooler than an equivalent linear driver.

Offline Rudane

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Re: Why using pwm for controlling motors?
« Reply #2 on: July 19, 2014, 12:52:38 am »
PWM also offers you a way to digitally (precisely) control the motor speed. Digital pots are low power and usually very course adjustment.
Voltage appears across and current flows through.

Offline XFDDesign

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Re: Why using pwm for controlling motors?
« Reply #3 on: July 19, 2014, 01:12:24 am »
Think about the losses involved with using a pot. Suppose you have a 1HP/750W motor that runs on 100VDC (because why not). This comes out to 7.5A of current. You want to run it at 50%? Suppose then you need to drop it to 50VDC. Suppose for the sake of this example, the effective resistance of the motor is 13.33Ohm (this is a really gross approximation). Your potentiometer would have to be about 13.33 Ohm too. The input power is 375W. Power to the motor is 187.5W. Power burnt in your potentiometer is also 187.5W. This is a pretty poor efficiency - 50%.

Now, we go to PWM. We use FETs or IGBTs to switch the connection of the motor in such a way that it's either on 100%, or 0% (off).

Suppose we only use one FET with an RDSon of 0.1Ohm.

100V input, 13.3333Ohm of the motor, 0.1Ohm of the fet. Power delivered to the motor is 738.9W, power lost in the fet? 0.075W. Our delivered efficiency while on is 99.99%.

Now we PWM the thing, 50% of the time, the motor is on. 50% of the time, it's off.

Let's call the time when it's on "t1" which is t1 = 0.5 T (T is the period of the frequency)
The time t2 = (1-0.5)T (the off time)

This means, that we can average the "power" delivery:
Pavg = 738.9W * t1/T + 0*t2/T
Pavg = 738.9 * t1/T
Since duty cycle is what is recovered from t1/T
D = t1/T
Pavg = 738.9W*D
For 50% on? Pavg = 370W
25%? 184.7W

The value add? We're not wasting power with the potentiometer. This is _very_ efficient.
« Last Edit: July 19, 2014, 02:49:48 pm by XFDDesign »

Offline hurricanehenry

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Re: Why using pwm for controlling motors?
« Reply #4 on: July 19, 2014, 08:00:25 am »
Great explanation. I enjoyed learning about it too.

Offline edy

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Re: Why using pwm for controlling motors?
« Reply #5 on: July 19, 2014, 01:04:20 pm »
It seems like the same reasons why some voltage step down converters also use PWM and then average/smooth out the voltage through filters rather than just voltage dividers with resistor to step things down.

Almost forgot.... The OP didn't mention type of motor, like brushed versus brushless, which cannot be run simply on single source potted DC anyways. There has to be precise timing of PWM on several coils as the drive speeds up and down otherwise it just sits there doing nothing. Think hard drive motors and model airplane.
« Last Edit: July 19, 2014, 01:20:16 pm by edy »
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