Hi!

What do you think about this circuit for controlling a 12V DC motor. The motor is normally current controlled, this will not work for a voltage controlled motor.

 

The 40Ω resistor is short circuited.
With the pot at either end of its travel one or other of the zeners will be connected directly across the 18V supply and will be destroyed.

 

The motor is normally current controlled, this will not work for a voltage controlled motor.

I am assuming this is a wound/series field DC motor?

 

I am assuming this is a wound/series field DC motor?

Pretty much.

 

The 16V zener is connected directly across the 18V supply so the 3A fuse or the zener will immediately blow.

 

Is the motor under load when started?
If so, what is the nature of it.?
Have you considered a ramp start?

 

I think You need to start over from the beginning.

Start by stating what the application is,
then why You want to control the speed of the Motor, and what the Motor is powering.

State exactly how You would like the Motor to behave,
under every circumstance that it will be required to operate under.

Do You need precise speed-control ?
Do You need Ramp-Up and Ramp-Down speed-control ?
Do You need Motor-Braking ?
( It appears that You don't need Reversing capability )
What is the "Locked-Rotor-Current" ? ( actually measured with a Meter ) or DC-Ohms of the Motor.

There is no such difference as a Current-Controlled-Motor, vs, a Voltage-Controlled-Motor.
The Speed-Controlling-Circuitry may be Voltage, or Current based, or both.
.
.
.

 

If it IS a series field motor, speed control is very hard to accomplish.
If precise RPM is required a shunt field or PM field motor is preferred.

 

There is no precise control required. Its just a motor for a pump or door. All I need it to do is start, work smoothly on a constant speed and then turn off fast. No rump of the speed needed. No motor braking needed. No reversing needed. Normally I control them with a pot and nothing else.

 

All I need it to do is start, work smoothly on a constant speed and then turn off fast. No rump of the speed needed.

As I already mentioned, a series motor RPM varies quite drastically with change of load, off load they essentially operate at a run-away speed. RPM control is usually done with some form of optical RPM (slot) sensor.