Motor Start/Stop - Soft and Hard

Hard start/stop - contacts close, motor starts "hard" and comes up to speed immediately

Soft start/stop - a series of contacts (within the starter cabinet) close in secession (or through the use of a VFD) and the motor ramps up slowly and comes up to speed slowly.

KJK;

You are right on. This is exactly what is ment by soft start. I have a router that uses a soft start . You don't get that jerk as You would when using a fast start machine. Really works nice especially when doing delicate inlay work.

oilcan13

Allen Bradley soft starts are just gated paired SCR's that slowly ramp up to speed by slowly varying the gating until full phase current can flow. I don't believe this soft start ever converts the incoming power to DC (semi-half weave rectification in the SCR does occur, but both the high and low swing of the phase is passed by the paired arrangement) as a VFD does. This arrangement will not allow a single phase input to power a three phase load as some VFD's can. I know - the theory is basically the same as a VFD.

What I would like to know is why a solid state soft start that handles a given HP (KW) is usually twice the cost of a VFD with the same ratings. Who buys a soft start instead of a VFD?

Soft stop - power removed, motor coasts to a stop.

Hard stop - while running at full speed a large steel rod penetrates the motors fan shroud becoming lodged btween the armature and the rotor.

I guess it's all relative...

starting and stopping when you are hard, same as starting and stopping when it is soft.

I guess....

Hard Start/Stop: When the motor is hardwired directly to start/ stop switches.

Soft Start/stop: When you are starting/ stopping the motor via DCS, PLC or any other control systems. These system commands are routed through relays and MCCs to start or stop the motor.

In the case where a user is billed on a peak demand kW tariff, it could be a lot of money each year.

In the case where lighting is fed from the same supply, the difference between lights dimming slightly and going completely off while the motor runs up.

Hello PW

Not sure about the money saving. I believe peak demand tariffs use maximum demand averaged over 20 minutes or so (I can't remember exact time), not instantaneous, so choce of starting method makes very little difference to the electric bill.

Cheers.........Codey

With soft start the motor comes up to speed slowly and so inrush current can be lower than with a plain contact. This can allow a motor to start on when powered by a generator that would not handle the inrush current. A hard start can also be a start relay-capacitor combination that will start a motor with more of a load on it than normal. -- JHF

While the answer provided by KJK/USA (comment # 1) makes more sense in terms of the use of soft starter but, in reality, the answer provided by Gkishorenair (comment # 5) is more correct.

The hard start/stop and soft start/stop are all about the sort of start/stop signals applied to the motor starter.

Hard start/stop: When the motor start/stop operation is done by using open/close contact with applying the electrical signal (generally voltage) directly to the starter contactor through the start stop mechanism (either by HOA or DCS or PLC system), it is called hard start/stop.

Soft start/stop: When the motor start/stop operation is done by using instrument signal (either from DCS or PLC system) applied to the digital relay (such as E3 Plus, Multilin etc.) of the motor starter through the Devicenet or Modbus or other sort of communication protocol, it is called soft start/stop. A soft start/stop doesn't really need of using soft starter or VFD.

The above definitions are written exclusively from my experience and I didn't see such definitions written anywhere but this is the way I found they are being used in many projects. However, the terminology may differ from project to project or industry to industry.

- MS

What you are saying is correct. But the terms also apply as I said in my post. -- JHF