if VFD MOTOR is ok with standard Soft Start Controller 3

[DAUGAVA]

I have Standard Duty Soft Start Drive. However, the submittal for the project specifies VFD Duty motor.
I do not see any problem with Standard Control (RVSS).
Do I miss anything?

 

[waross]

This may be a "boiler plate" spec.
A soft starter does not subject a motor to the same high voltage transients that a VFD may. That said, a VFD duty motor will be better able to withstand any over voltage transient than a non VFD rated motor.
The engineer spec'ing the project may have a reason for wanting VFD motors even for non VFD drives.

Bill
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"Why not the best?"
Jimmy Carter

 

[jraef]

I agree with Bill that the specifier may have had other reasons, but in specific response to your question, there is no need for an Inverter Duty motor to be used in a soft starter application, a solid state soft starter is not an inverter.

"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[Skogsgurra]

I have come across applications where standard VFDs have been used for soft starting. The reason seems to be that standard VFDs are (or were) cheaper and easier to buy, reduced inventory if VFDs were used in other parts of the plant or machine and also reduced the learning needed.

Bypassing the VFD after start made it possible to use 'undersized' VFDs, which made the VFD choice easier.

It is possible that the specs were written to cover such cases. Better check with the project manager.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[electricpete]

I guess (?) the difference is that inverters (most vfd's) use PWM method which involves switching thousands of times per second while soft-starter uses firing angle control which involves switching once per cycle. The high switch rate devices of PWM need very quick transition from one state to another (to minimize losses among other things), which corresponds to high dv/dt. Lower switching rate of firing-angle-controlled device allows us to use lower-transition rate devices with lower dv/dt. It that roughly correct?

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(2B)+(2B)' ?

 

[jraef]

Yes Pete, that's essentially correct. To further the idea of the reason for "Inverter Duty" motors, it also has to do with "standing waves" that can be created in the leads running from the inverter to the motor as a result of capacitive coupling between the conductors themselves and that high switching rate / dv/dt issue. peak voltage spikes can exceed 2X line peak voltages and do so constantly, which eventually causes phase-to-phase shorts in the end turns of the stator windings. So Inverter Duty motors will have a higher voltage rating on the magnet wire compared to "standard" magnet wire. Then they throw in more mass so cooling at lower speeds is not as much of an issue, and/or separately powered cooling blowers and you get a nice expensive motor package, but one that survives the abuse often inadvertently heaped onto motors when VFDs are used.

"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[LionelHutz]

There are certain loads that don't work well with a soft-starter. Typically, the concern is very high inertia loads.

There are certain VFD motors that have a modified speed-torque curve that don't play well with soft-starter. In these cases, the locked-rotor and pull-up torque is sacrificed to give more breakdown torque.

These days, most standard motors are built with the same wire insulation regardless of being VFD rated or not. However, I have seen crappy "VFD Rated" motors fail. So, in the end, a "VFD Rated" tag on an otherwise standard induction motor really means very little. There are motor lines specifically built for VFD use. The BlackMAX motor would be one example of this.