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goelvaibhav
09-10-2009, 07:27 PM
Hi

I have an application as follows:

A DC Geared motor will rotate a huge flywheel (Moment of Inertia = 3kgm2). The max rpm is 50. The wheel needs to reach the max rpm in about 2 secs. It will rotate at its max speed for about 5 sec and then start decelerating. It has to stop in 20sec with its speed linearly decreasing.

I have successfully implemented this using a PWM circuit made up of a uC and transistor.

I am worried about the reliability of this technique, as in what all protection (starting current limitation, etc) is required so as the system can be operated for industrial purposes.

Regards,
Vaibhav

docel
09-12-2009, 03:32 AM
It is not a good idea to use Gears to drive a flywheel.
While 3Kgm2 isnt much, the dynamics of the flywheel is remarkable! You will need some dynamic braking to stop the wheel.

1. The initial torque will be high, therefore the starting current will also be high. The Power supply must be rated to consistently provide this high starting current.

2. Freewheeling DC motors are current generators. This current WILL FRY the drive electronics, unless precautions are taken. The usual damping circuits must be applied so that the generated back emf is fully absorbed and dissipated.

You have not specified the Motor type- is it Field coil or PM ?
Fly-wheel Diameter?
Deceleration is electronic or mechanical?

goelvaibhav
09-15-2009, 08:04 PM
If a geared motor is not used, what motor could provide the necessary starting torque. Also, I need to rotate the wheel at 50rpm, thus a gearbox is required.

Normally dynamic breaking is required to stop a wheel in a short period of time. I have to gradually decrease its speed, so if you calculate the back emf generated due to the deceleration, it wont be much.

As I mentioned I am using PWM to control the speed, your point is justified if the max rpm is reached and the motor is switched off, it will then produce back emf and high current as the motor armature resistance is low.

My motor is PM. The wheel is a spoked wheel with a diameter of 4ft.
Deceleration is electronic (PWM) and not via any clutch brake mechanism (mechanical).

What I actually wanted to ask was, How do people drive 12/24VDC motor in the industry. All the Industrial drives available are for higher voltages.