10-07-2005, 09:26 AM
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| | | Join Date: Oct 2005 Location: Canada
Posts: 10
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Originally Posted by najnielkp hello all. i have a dc gearmotor ,1/8 hp, 24v dc that i am using with a pwm controller. i need the motor to stop instantly .i have seen other applications
use large wire wound resistors? how does this work? |
In part it is a function of the controller.
Commonly in PWM the motor has one side connected to the supply and th eother is switched on and off with varying on to off ratios to give the speed control. To stop, they often just stop pulsing and so the motor just spins down.
To brake, what you do is to essentially short circuit the otor - usually connecting both sides of the motor to ground. But a motor is actually a generator in disguise, so the spinning motor generates a voltage (known as back-EMF) which needs to be handled - hence the big resistors. These are used to limit the current (and thus avoid burning out the motor) whilst still absorbing the energy and bringing the motor to a fast stop.
To hold the motor locked is another issue. This is usually done by running PWM at 50% duty cycle onto BOTH sides of the motor at the same time but in opposite phase. THus the motor veryt rapidly tries to go one way, then the other. Essentially remaining stopped if the PWM is going fast enough, or if the PWM frequency is a little low the motor will hunt backwards and forwards.
Does that help?  |
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