6 replies to this topic

[shady]

I need a full comparison between soft starters and variable speed drive.

[jraef]

That is like asking for a comparison of Bicycles and Motorcycles. They both do a similar thing but in totally different ways, and one does something completely different that the other. But here goes anyway.



Everything you need to know...

VFD can do everything a soft starter can do.
Soft Starter cost is 1/2 to 1/10 that of a VFD, depending on size / voltage.

VFD can change the speed of the motor.
Soft Starter cannot.

[jOmega]

Everything you need to know...
• VFD can do everything a soft starter can do.
• Soft Starter cost is 1/2 to 1/10 that of a VFD, depending on size / voltage.
• VFD can change the speed of the motor.
• Soft Starter cannot.

JR / Shady,

Here are some things you might also want to consider.

1. In a given power rating (i.e. 100 kW vs 100 kW), the Soft Starter can source more current to the motor.

2. In a given power rating, the Soft Starter supports the development of more torque at the motor shaft on
starting, than can the VFD (assuming that LRT is greater than 1.5 x Motor Rated Torque).

3. Soft Starter can operate the motor at RMS Fundamental mains (line) voltage at mains (line) frequency;
the most VFDs cannot. (Why is this important ?????)

4. Soft Starter can transfer a motor to the mains and recover a motor from the mains with the operation of a
simple contactor; VFD cannot. Requires complex additional control circuitry and additional components
and cost.

5. A Soft Starter CAN vary the speed of an AC induction motor by manipulating the slip of the motor by varying the voltage; similar to adjusting the rotor secondary resistance of a Wound Rotor motor.
HOWEVER: motor must be sized to accommodate additional heating consequent thereto, and SS must be designed to permit such operation.

Be that as it may, the VFD is the BETTER choice for variable speed applications.

6. A VFD, upsized to source the requisite current, can produce more starting torque from the motor than can a Soft Starter ! (Upsizing a Soft Starter won't do it.)
Think about it.

7. Heat build-up in motor is less with VFD than with Soft Starter; ergo, successive restarts with VFD can be performed; while with Soft Starter ..... considerable dwell time in Power Off State must elapse before any restart attempt is made (Soft Start and Motor thermal Limitations)

Kind regards,
jΩ

[jraef]

Good points all, that's why I started off saying that it is not really a fair comparison.

I will only add the point #2 can be overcome by up-sizing the VFD, and I would caution anyone trying to vary speed of an AC motor using a soft starter and an over sized motor as in point 5. That is a dangerously tricky endeavor not to be undertaken lightly.

[jOmega]

I will only add the point #2 can be overcome by up-sizing the VFD,

JR... in point #2, I intentionally stated : "In a given power rating" ... viz-a-viz .... 100 kW Soft Start vs. 100 kW VFD (with 150% C.L.).

see point #6.


Your point about NOT using the Soft Start for a variable voltage controller is well taken. I've always thought it to be a less than intelligent approach to obtaining variable speed control of an asynchronous motor. But, in over 46+ years in the business, I've seen it done more than once because it was 'cheap and dirty'. I think the last time I saw it applied was on a center winder application. And within its limitations, it worked reasonably well. But they sure didn't get 10 year life out of the motor. <g>

Cheers.

jΩ

[jraef]

Sorry, missed your "given power range" qualifier.

But they sure didn't get 10 year life out of the motor.

LOL, undoubtedly designed by the motor salesman!

[marke]

A few more points to ponder:

1. Life expectancy : A VFD will typically last for 7 years due to the life expectancy of the electrolytic capacitors. After seven years, the technology has moved on and it is difficult to buy spares. A well designed and speced soft starter will last for twenty years because it does not use power electrolytic capacitors and the SCR technology does not change.
2. Efficiency : The VFD has a power dissipation of 3 - 5 times that of a soft starter. This impacts on the operating efficiency of the installation, heat build up in cabinets and switchrooms etc.
3. Cost : The VFD is typically 3 - 5 times the cost of a soft starter.
4. Harmonics : The VFD has a continuos high harmonic current draw from the supply. The soft starter has a burst at start only. (Execept for the energy saving soft starters)

Best regards,