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jit_singh_tara
Joined: 22 Dec 2006
Posts: 110
Helped: 6
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 02 Feb 2008 9:42 controlling dc motor speed |
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hi , i have a dc motor of 24V 3 amp , how can i scale down its rpm to 1 rpm but i dont want to reduce its torque.
1.if i reduce voltage to 1 or 2 volts the rpm reduces but motor can be stopped by hand .
2. if i try using pwm , then what is the principle behind this.
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umery2k75
Joined: 19 Apr 2006
Posts: 378
Helped: 24
Location: Pakistan
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02 Feb 2008 10:29 Re: controlling dc motor speed |
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About the PWM:
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It's like if you have a 24V motor, try running it at around 12V.The motor speed would be half, this means speed is directly proportional to potential difference of Armature Voltage, not considering the field current in this case.
Suppose you want to run your motor at half the full rated speed at 24V.The PWM will switches on and off the applied voltage of 24V very fast enough, so that it's average output would become somewhat 12V and your motor would run at half the speed.The motor only notices the average voltage appearing across it.
If the switching is fast enough, the motor doesn't notice it, it only notices the average effect.When doing PWM controlling, keep in mind that a motor is a low pass device. The reason is that a motor is mainly a large inductor. It is not capable of passing high frequency energy, and hence will not perform well using high frequencies. Reasonably low frequencies are required, and then PWM techniques will work
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FvM
Joined: 22 Jan 2008
Posts: 5148
Helped: 766
Location: Bochum, Germany
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02 Feb 2008 16:38 Re: controlling dc motor speed |
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Hello,
your second question has been answered verbosely, I'll try the first.
Every motor has a specific characteristic showing speed dependence on torque. For permanent magnet DC motors, this is simply a drop proportial to torque. It is a simple function of basic motor parameters, among others winding resistance. Cause it's a linear constant, a gradient, the effect is stronger at low speed. To overcome it, you have basically two options:
1. Using a speed controller with speed sensor
2. Using an electronic compensation of armature winding resistance
The latter can't be perfect, but may be sufficient for many application. It effectively introduces an negative resistance in series with the motor, increasing the motor voltage when torque and motor current increases.
Regards,
Frank
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