Motor Stall Current : LUSENET : Model Railroading : One Thread

Larry, Thanks for your interesting webpages.

I have a question regarding your information on motor stall currents.

Were your tests conducted with the armature stalled or the model slipping because whilst I accept that stalled armature current will be the highest current likely with a major transmission failure this is very rare in diesels (can't say the sme for steam but not all that frequent!)

Therefore in normal operation the maximum current a decoder should see is when loco is operating up to stall condition on the track ie wheels slipping, this is the safety mechanism for the motor as well, otherwise stalled armature results in very short motor life.

Please consider!


-- Bruce Norton (, May 14, 1998


Bruce--you've got things a little backwards with respect to stall currents, but first let me explain the tests. Yes they were done with a stalled armature and a stabilized power suppy so there was no sag in current as the unit was stalled. I stalled the morot by grabbing the flywheel. Now, most manufacturers use the full stall current as their maximum rating. Some refer to maximum continuous draw which is lower than the stall value. The closest a motor gets to the meausred stall current is when it is first starting up and creeping along. Think about it, at that point the armature is essentially stalled! There is little or no back current being generated to offset the draw and it's essentially the same as when you grab a flywheel. Consequently you yard switchers that creep arond the layout are placing a greater load on the decoder than a fast fregth zipping along at high speed.


-- Larry Puckett (, May 16, 1998.

Both you guys are correct here, it all depends on how one looks at it and the type of drive being used to power the motor.

But non the less Full Stall or Locked Rotor current draw is what it should be. This is also sometimes refered to as erroring on the safe side.

When a motor has a Locked Rotor, it is pure restance. Once it starts to rotate it becomes a combo of resistance and inductance, no matter the case it produces rotational torque at this point.

Now in relation to the type of drive used to power the motor. As an example a veriable DC supply, here just before the motor starts to rotate, the resistance of the coils will = the restance of a Locked Rotor, but the supply voltage will be low thus will = low current draw. In the case of say a DCC decoder, this drive is basically a PWM drive. Here the motor is still in a Locked Rotor state. The pulses are at full V+, thus the current will = full Locked Rotor current at max rated V+. But only at the tops of the pulses.

So what this means is with DCC PWM drive on the motor. As soon as you ask the motor to rotate, yes it does see full Stall Current, or Locked Rotor current. But only during the peek of the pulses, and the duration of the pulse.

Now knowing this, and the fact that once the motor starts to rotate, it also starts to convert resistance to inductance where is the point of convergents from current draw related to rotational torque as related to full stall current. In other words Locked Rotor Current = X at 12 volt DC. Where does a Rotating Rotor = X with PWM.

This can actually be calculated, but is based on motor characteristics such as resistance, inductance, ect and current RPMS. Also on the PWM frequency of the drive, ect. But one thing can be assured, that just before the motor starts to rotate, it will see Locked Rotor Current rated at V+ during the peak pulse duration.

Also on a Side Note:

Full Stall or Locked Rotor current relates to just that on both the motor specs and Decoder specs, when specified.

Wheel slip relates to the maximum contiuous current specs. of a motor or Decoder contiuous specs. Not the Locked Rotor specs.

Again if in doubt always error on the safe side: Go with the Full Stall also known as Locked Rotor specs.

-- Don Crano (, June 07, 1998.

To Don Crano,

Thanks for setting us both straight!

It is important to cover locked rotor as there are always some in clubs who will grab a loco to prevent an accident and hey presto locked rotor condition.

-- Bruce Norton (, June 08, 1998.

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