Overspeeding

Overspeeding is a common problem encountered by most of us when trying to fit a railroad into a small area. The primary cause, of course, is making the downgrades too steep, but the problem can be intensified by other factors.

First, You need to understand some peculiarities of the brush type DC motors used on most locomotives. All of these motors are generators as well as motors. Whenever they are running, they generate a voltage which is dependant on the motor's rotational speed and directly opposes the incoming voltage which is trying to push current through the motor. This generated "back" EMF (Electromotive Force -- a fancy word for voltage) opposes the applied voltage of your power supply to limit the amount of current through the motor. When the motor needs more current to produce more torque, as when pulling a load up a hill, it slows down slightly. The back EMF reduces along with the speed thereby reducing the amount it opposes the incoming voltage, which can then force more current to flow through the motor to produce the required torque. That is, of course, unless there is resistance in the wiring, or worse, a resistor was purposely inserted in the path to attempt to decrease the speed or acceleration. Unfortunately, you cannot compensate for the problem by merely increasing the supply voltage. It will still cause the train to slow or stall when trying to navigate a curve or pull a grade.

The DC motor has an inherent braking ability known as "dynamic braking". However, the selection of too small of a wire size during railroad construction or using inexpensive power supplies can disable or limit this feature. Either one can allow the voltage to increase as the motor current is reduced by the rotational speed increase on downgrades. This allows the speed to increase. If the input voltage remained constant, the voltage generated by the motor would exceed the incoming voltage and the motor would actually assume the function of powering headlamps, smoke units, sound boards, etc. Most people (including some electrical engineers) are unaware of the extra torque required to rotate a generator while it is generating power to run such devices. This load on the motor has a tremendous braking effect substantially reducing the overspeeding. Dynamic braking, by the way, is used on most diesel locomotives and many industrial DC motors.

Another cure for overspeeding is our Magic Brakeman. Click Here to go to the Magic Brakeman page.


	Overspeeding -- Causes and Fixes Overspeeding is a common problem encountered by most of us when trying to fit a railroad into a small area. The primary cause, of course, is making the downgrades too steep, but the problem can be intensified by other factors. First, You need to understand some peculiarities of the brush type DC motors used on most locomotives. All of these motors are generators as well as motors. Whenever they are running, they generate a voltage which is dependant on the motor's rotational speed and directly opposes the incoming voltage which is trying to push current through the motor. This generated "back" EMF (Electromotive Force -- a fancy word for voltage) opposes the applied voltage of your power supply to limit the amount of current through the motor. When the motor needs more current to produce more torque, as when pulling a load up a hill, it slows down slightly. The back EMF reduces along with the speed thereby reducing the amount it opposes the incoming voltage, which can then force more current to flow through the motor to produce the required torque. That is, of course, unless there is resistance in the wiring, or worse, a resistor was purposely inserted in the path to attempt to decrease the speed or acceleration. Unfortunately, you cannot compensate for the problem by merely increasing the supply voltage. It will still cause the train to slow or stall when trying to navigate a curve or pull a grade. The DC motor has an inherent braking ability known as "dynamic braking". However, the selection of too small of a wire size during railroad construction or using inexpensive power supplies can disable or limit this feature. Either one can allow the voltage to increase as the motor current is reduced by the rotational speed increase on downgrades. This allows the speed to increase. If the input voltage remained constant, the voltage generated by the motor would exceed the incoming voltage and the motor would actually assume the function of powering headlamps, smoke units, sound boards, etc. Most people (including some electrical engineers) are unaware of the extra torque required to rotate a generator while it is generating power to run such devices. This load on the motor has a tremendous braking effect substantially reducing the overspeeding. Dynamic braking, by the way, is used on most diesel locomotives and many industrial DC motors. Another cure for overspeeding is our Magic Brakeman. Click Here to go to the Magic Brakeman page.

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