VFD question

mdim said:

No!

I was very likely to stop just with the above answer, but let me continue a little. With soft starter you can gradually increase to nominal speed(50/60 Hz) whatever is the line frequency. With VFD you can run the motor at the frequency you want continuously or you can change it continuously to adjust (in your case) output flow or pressure or whatever you would like to control on your pump.

Click to expand...

iraiam said:

mdim is correct, they are not the same thing.
There is such a thing as a soft start. It's not a full blown VFD, but only a soft start that ramps up to line frequency/voltage on start. I have a couple at work on 75HP motors, the application does not need full capabilities a VFD can provide, so a considerably less expensive soft start was used instead of a large VFD that could cost 5 thousand dollars or more.

Click to expand...

1. A Soft Starter changes VOLTAGE only, and since torque varies as the square of the applied voltage, torque is reduced, then since current follows torque, current is reduced.
2. Frequency through a Soft Starter is always the same as line frequency.
3. If you reduce voltage/torque/current with a soft starter for too long, the motor will never finish accelerating and stall.
4. So a Soft Starter can ONLY be used in machines that will end up running at full speed. If you need to change speed, it cannot work.
5. A 3 phase Soft Starter gives off heat at about 4-1/2W per running load amp, so if enclosed, you must have adequate ventilation. But if you can’t ventilate, you typically use a “bypass contactor” once it is done ramping, since the motor will be running Across-the-Line (DOL) at that point anyway. This avoids having to deal with all that excess heat. Some Soft Starters include that, some do not but it can always be added.
6. There are cheap versions of Soft Starters that only use SCRs on 2 of the 3 phases to reduce costs; the 3rd pole is a piece of bus bar. These put the motor at risk and cause added motor stress so they should be avoided if possible.

1. A VFD converts AC to DC, then recreates a “pseudo (fake) AC output to the motor.
2. In doing so, if can create any combination of voltage and frequency it is commanded to do.
3. AC induction motors are designed to produce a given amount of torque at a particular ratio of voltage and frequency (+-10%), ie 460V 60Hz is a V/Hz ratio of 7.67:1, coincidentally 380V 50Hz is 7.6:1, almost the same and in fact close enough to be interchangeable.
4. So by maintaining that V/Hz ratio the motor is designed for, a VFD allows a motor to provide full torque at any speed.
5. Because of that, a VFD can accelerate a motor FASTER than a Soft Starter, or with less current without having to be concerned for the length of time.
6. But a VFD is, for the most part, 5-10x more expensive than a Soft Starter, so if a machine does not need to be changing speed, a Soft Starter is more economical.
7. It is very difficult to safely bypass a VFD while the motor is running, so the heat created by a VFD, roughly 3% of the load, must be dealt with either by ventilation or by using a sealed enclosure and an Air Conditioner.

rootboy said:

And most (some?) soft starters come with mechanical contacts that bypass the VFD after the starter has done its job so that there are no additional VFD losses.

Click to expand...

jraef said:

But a VFD is, for the most part, 5-10x more expensive than a Soft Starter, so if a machine does not need to be changing speed, a Soft Starter is more economical.
[/LIST]

Click to expand...

AustralIan said:

Please send me part numbers of soft starters 10x cheaper than a VFD.

Click to expand...

A VFD can be setup to be more efficient than a soft start+DOL even at line frequency, when the load is less than the nominal load.

Click to expand...