Motor Control Term Definitions

Many advanced decoders have various motor speed tuning features such Speed Step modes, Speed Tables, Back EMF (BEMF), Torque Compensation, Kick Start, Start Voltage, Mid Voltage and Max Voltage. There is a subtopic that talks about what these feature are and the CV used to adjust them. For each CV, the name in Bold is the NMRA standards name. I include other popular names after that to tie it together.

Typically the factory settings normally provide good performance for most locomotives in H O-Scale.

Start Voltage - CV2 (Also known as Vmin & Vstart):

This is the amount of voltage sent to the motor when first starting up. We set CV2 so the locomotive is almost able to maintain movement at speed step 1.

Torque compensation kick rate - CV116:

How frequently the motor is ‘kicked’ at slow speed to keep it turning smoothly.

Torque compensation kick strength - CV117:

How hard the motor is ‘kicked’ at slow speed to keep it turning smoothly.

Maximum Voltage - CV5 (Also known as Vmax, Vfull, Maximum Speed):

Determines the maximum speed that the locomotive will move. This allows you to operate your locomotives in a prototypical speed range. If your locomotive runs too fast you can use CV5 to lower its maximum speed. Setting CV5 to 255 uses the maximum possible voltage to run the motor when full speed is requested. Set CV5 to a smaller value to reduce the top speed. A value of 128 will yield approximately 1⁄2 full voltage to the motor at top speed. 192 will provide about 3⁄4 full voltage. All speeds from the middle speed step to the maximum will be proportionally reduced (see diagram). If CV5 is set to 0 the decoder will use 255 for maximum speed. Always make sure CV5 is greater than CV6 to avoid erratic operation.

Middle Voltage - CV6 (Also known as Vmid & Vhalf):

CV6 determines how the motor responds through its middle speed ranges to advancement of the throttle. If you set CV6 lower than half the maximum speed you’ll have smaller increases in motor speed through the lower speed ranges. Then, as you hit the upper speed ranges there will be larger increases between speed steps. If you set Vstart larger than 0 you’ll will most likely want to raise Vmid so a reasonable slope is maintained in the ‘speed curve’.

Reverse trim (also forward trim) - CV95: Values from 1-127 make decoder run faster in reverse than forward. 1 is one speed step faster in reverse, 2 is two steps faster, etc. Values from 129-255 make decoder run faster in forward than reverse. 129 is one speed step faster in forward, 130 is 2 speed steps faster, etc. 0 and 128 add nothing to either direction .