Do I need a soft starter? 2

[CentauriX]

I am presently on a project which requires the installation of a DC motor onto an existing structure. The motor will be speed-controlled by an axial potentiometer on a dancer roller. The machine at the other end will be starting and stopping rather frequently. Will I need a soft starter for my motor?

 

[rhatcher]

A 'Soft Starter' is associated with the starting of AC motors. For your DC motor, the AC/DC converter, or DC Drive, will presumably ramp the motor up based on input from the dancer roll.

The isssue that comes to mind for me with respect to frequent starting and stopping of the DC motor is cooling. Depending upon the duty cycle for your machine, a self cooled motor may not be adequate, you may need a blower cooled or separately ventilated motor.

 

[jbartos]

Suggestion: Please provide more info:
1. DC motor kind
2. HP Size
3. Voltage
4. Controls
5. What kind of project it is
6. Whatever other may be to your concern

 

[zappit]

Outer Loop or Trim Control from a Dancer usually provides approx. 10 % of the Line Speed Pickup. A Master reference signal from a PLC /Drive Controller Master/ or even just a Speed Pot sets the desired Drive rate. Soft Control does not apply here. The accel/decel characteristics can be effectively tuned to achieve the ramping characteristics you need. There are a number of other parameters you need possibly consider that are associated with the characteristics of the motor (J bar etc.) There are a number of publications out that lay out DC Drive theory well.
In todays times however, many folks are considering Vector controlled AC drives in place of DC. Many Winder applications are now commisioning of these drives and they are finding that the percentage of Speed regulation and response is quite adequate for their needs. I have seen several Allen Bradley Force drives installed and not only work well, but last.

 

[jbartos]

Suggestion: The DC motor size has a significant impact on the dc motor control. Visit

http://www.sprint-electric.com/fram2.htm

for differences in dancer rollers:

http://www.boomasia.com/edan-r.htm

http://www.megtec.com/products/dsa.htm

etc. for more info
It appears that the higher horsepower DC motor may require smoother (softer) start than small rugged servo motor.

 

[jbartos]

Small encore: Traditionally, the AC motors have soft starts and full voltage across the line (FVL) starts. Therefore, terms "soft start" and "FVL start" are frequently used. However, the DC motors have to have arrangements to limit the armature current. This could be viewed as if the DC motors, except the small ones, were soft started, which is somehow tacitly assumed.
Reference:
Gordon R. Slemon, "Magnetoelectric Devices, Transducers, Transformers, and Machines," John Wiley & Sons, Inc., 1966,
page 292 states:
"It is only the very small machines with relatively high armature resistance that rated voltage can be applied suddenly to the armature at standstill without exceeding the maximum transient current that the commutator can switch without damage. For larger machines, either the rate of change of the armature voltage ea must be limited, or additional resistance must be added to the armature circuit."