I was given a UP FEF brass locomotive as a gift for helping out a friend. It is DC and I have since converted my layout to DCC. I have been told that to convert this loco to DCC, I will have to replace the open motor with a can motor. It was also implied that I should get a motor with back-emf since this will give me smoother operation. HELP - both with motor type and with motor acquisition as I don’t know enough about the problem to really solve it.
Ron K.
Replacing the motor may not be necessary, depending on a number of factors that can’t be answered without knowing more about exactly what you have.
You need to find someone who can help you measure the current draw of the motor that is presently in the model to determine if it exceeds the rating of most decoders (greater than 1 Amp); how difficult it is going to be to isolate the motor’s brushes from the frame; and whether or not the locomotive runs smoothly enough to justify a decoder installation.
In some cases, it may cost more to replace and upgrade the motor than the locomotive is worth in the long run.
I think back emf is a function of the decoder you choose. Not the motor.(I think)
9 times out of 10 the amp draw of an open frame motor (depending on the make and how old it is) will exceed the rating of even the best decoders. Of all the old open frame motorsI’ve had, the only one that didn’t go over 1 amp was a Mehano motor that was in a very old Bachmann diesel. I successfully converted it to DCC (had to break off a grounding lug on the motor to isolate it) but eventually had to replace the motor anyway to get the kind of operation I wanted.
Every DC motor generates back EMF, which is why the current draw is relatively constant as the voltage and motor RPM increase. The back EMF feature of a DCC decoder involves electronically detecting back EMF that is there regardless.
The one key to installing DCC in an older loco is being able to isolate both motor brushes from the LOCOMOTIVE frame, not necessarily the motor frame. In many cases, this is no more complex than breaking a solder joint (the brushes are isolated, but the manufacturer soldered one connecting ‘ear’ to the motor frame.) Others can be isolated by slipping a piece of wire insulation over the uninsulated end of the wishbone-shaped brush tension spring. An extreme case might require installing a sheet of insulation (paper) between the motor and the locomotive frame, and using plastic rather than metal screws for mounting.
Open-frame motors draw more current than more modern can motors, which will require using a decoder with sufficient capacity. Fortunately, the tender of your FEF is big enough to accept almost any decoder, and massive enough to heat sink even a high-current model.
Chuck (modeling Central Japan in September, 1964 - with open-frame motors)
So your saying BEMF is like electro/mechanical resistance produced by the motor? And then the decoder reads this value and tries to keep it constant?
All this info is correct. But go with what Jeff W. says. Replace the motor (this is HO right?) with a Mashima or sagami, or even Kato Atlas or Proto 2000 motor. any of those will give you smooth operation. you will probably have to ‘invent’ some kind of mount, but as long as you keep the drive shafts (for lack of a better term) as level as you can and the motor and flywheels from touching anything (so as to not restrict movement or short out the loco) you should be fine.
Back EMF is a voltage produced by the motor while it is acting like a generator. EMF stands for Electro Motive Force, physics speak for Voltage. DCC powers the motor with pulses, between the pulses the decoder can read the voltage from the motor, which is proportional to the speed of the motor. So, it doesn’t know the real speed, but it knows if it slows a lot or a little, and can adjust the input to comp
There are several characteristics of open frame motors that are a concern for DCC users. Many of them have erratic BEMF pulses that can be harmful to many decoders - irregardless of the decoder’s current rating. Those ratings are designed to be used as a guide for maximum STALL current. Back-current pulses created as the motor rotates can produce spikes that can damage the decoder - a common problem with open frame motors used in older brass locomotives.
Stall current is another factor to check; it is safer to check stall current with an input voltage of 13.6 ~ 14V as this is more common for DCC max voltages than 12 Volts.
Visually inspect the motor while it is running. Look for heavy arcing of the brushes and commutator at low, medium and high speeds. Clean the brushes and commutator with a suitable electric contact cleaner and retest. You may need to service the motor (the aforementioned cleaning, lubrication, replacement of worn brushes, etc) to achieve suitable results.
As mentioned before, not all open frame motors are an issue; many DCC users operate open frame motors in their locos without incident. But careful inspection and testing on DC is strongly recommended prior to installing a DCC decoder. Can motors are commonly recommended because they typically draw significantly less current, are quiet, have better low speed torque and provide cleaner pulses for optimum BEMF operation.