Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

About motor speed control

Status
Not open for further replies.

mahaju

Full Member level 2
Joined
Mar 17, 2007
Messages
125
Helped
7
Reputation
14
Reaction score
2
Trophy points
1,298
Activity points
2,252
What exactly is softstart?
 

VVV

Advanced Member level 5
Joined
Nov 26, 2004
Messages
1,582
Helped
384
Reputation
768
Reaction score
88
Trophy points
1,328
Activity points
19,971
It means various things to various people.
However, in principle is a way of applying a slowly-varying voltage to a circuit in order to reduce the starting current, overshoot, etc.

I assume you are referring to a motor. When you first power it up, it is not moving, therefore the back EMF is zero and hence the entire supply voltage is applied simply across the DC resistance of the motor (armature). Since that resistance is low, the curent is very high, until the motor accelerates and the back EMF is significant.

Look at the equation;

Va=La*dIa/dt+Ra*Ia-Eo

Va is the armature voltage, Ra, La are the resistance and inductance of the motor, Eo is the back EMF. Clearly, when Eo is zero (when the motor is stalled or not yet spinning) the armature current Ia is very large (the armature time constant is much smaller than the time it takes the motor to accelerate and so the current rises rapidly to the value Ia=Va/Ra)

The solution is then to apply an armature voltage Va that starts off from a low value, such that the current is maintained within reasonable limits. Then, as the motor accelerates and the EMF Eo builds up, the Va voltage is slowly increased, but maintaining the Ia current within a safe limit.
This is then soft-start: applyt he voltage slowly, as opposed to a hard-start, where the full voltage is applied full-blast.
 

    mahaju

    Points: 2
    Helpful Answer Positive Rating

mahaju

Full Member level 2
Joined
Mar 17, 2007
Messages
125
Helped
7
Reputation
14
Reaction score
2
Trophy points
1,298
Activity points
2,252
Thank you very much for that reply
If it is possible could you please direct me to some web site where I can find a circuit that will help me accomplish soft start?
Thank you
 

Status
Not open for further replies.

Similar threads

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Top