Comparison of Starting Current

It should work, if you accelerate gradually, say over a period of 30 seconds or so. If you know the rotational inertia of the fan, you can calculate this more precisely.

Hi Tornado

Thanks for your reply . Wil definitely check with the prolonged acceleration time.

The load characteristic of usual blowers is well suited for softstarter starting, therefore unless a variable final (=top of ramp) speed is required, a VFD is only lost money (both investment and energy losses, as well as device lifetime).

Make sure the softstarter is automatically by-passed when the full speed is reached and use a softstarter with 3 controlled phases (some cheap ones don't feature that). If you really want to save money you can opt for the "inside delta" configuration (see the softstarter manuals if allowed) but be aware that doing so you increase the risk of wiring error as many electricians are not used to that configuration.

The power of the motor, the start duration and the number of starts per hour will allow you to determine the size of the softstarter.

I would be a little concerned for using a VFD rated 208A on a motor with a FLC of 210A, but otherwise, no problem using a VFD.

If it is starting now using Star-Delta, then for sure a solid state soft starter will work fine as well, as long as, per the previous post, variable speed is not needed. But if your blower uses mechanical dampers of any sort to alter air flow, then disable those and use the VFD, it will save significant energy in comparison.

If, however, the reason for considering using the VFD as a soft starter is because even using star-delta starting, it creates too much voltage drop on the source, then a VFD might be the only solution. A VFD is the only way to accelerate the motor at FLC, provided acceleration time is irrelevant, because with a VFD, I can ramp the motor over 30 seconds, or 30 hours, or 30 days, or 30 months if I need to in order to maintain the maximum current at FLC.

I would be concerned with your running load. 208 amps VFD on a 210 rated motor is cutting it very close. Fan loads typically draw all their power when running at rated speed. You may be into a questionable operating point. (Is that 208 amps heavy duty (150% OL for 60 seconds), or normal duty with 110% OL for 30 seconds? Be sure you can handle your normal running load plus I recommend a minimum of 10-15% head room for a fan on normal duty rating.) Check the VFD ratings carefully - it is a game the manufacturers play. A new energy efficient motor for VFD service may give you a relatively short payback.

Hi there,

Any idea as to why a VFD is being pushed as a starter for that motor. Has someone been suggesting that they will save money?

As I read it, that has not been discussed.

Only cost saving will be if you currently block the airflow to reduce flow.

The issue appears to be starting current.

(Guess it dims the lights?)

Hi Guys

Thanks for your comments. A vfd isbeing used because they need variable speed ( so soft starter is not at all an option ) . And its a 110 KW ND drive ( 120 % rated FLA for 60 seconds ). Dampers are being used in the system.

And also the main concern here was the starting current actually as at present with their star delta starter thir normal running current is around 110 amps. So i dont think that the normal operation load should pose any problem with the section of the VFD.

• ... A vfd is being used because they need variable speed ( so soft starter is not at all an option ) . And its a 110 KW ND drive ( 120 % rated FLA for 60 seconds ). Dampers are being used in the system.

OK, that's established, so let's get off of the soft starter issue then.

• And also the main concern here was the starting current actually as at present with their star delta starter thir normal running current is around 110 amps. So i dont think that the normal operation load should pose any problem with the section of the VFD.

OK, then as I said earlier, there is NO starting current issue using a VFD. If necessary, you can accelerate that motor with 100% of the motor rated full load current. No other starting method is capable of that beside a VFD, so if you already need the VFD, your concerns about starting current are over and done with.

Personally, I never size a VFD for the "normal running current", because it is presumptive, and presumptive is not a good engineering practice. But people do what people do for the reasons they see as most important, despite it being a bad engineering practice. Some of the worst disasters known to man have been the result of such decisions, yet it has yet to stop them from being made.

Dear Mr.stoned,

The VFD has low STARTING TORQUE. THIS ASPECT SHOULD BE TAKEN CARE.

While the blower is starting, it pushes the air, and torque demand will vary as the square of the speed. Pl.note and take care of this point.

DHAYANANDHAN.S

VFD is capable of providing any torque the motor is capable of handling, assuming VFD is sized for the application.

Absolutely right.

Keep in mind that with a VFD the delivered output current can be higher than the input current (especially during starts), it depends on how you dimension the drive. Also a VFD can start the ramp with literally no voltage (but don't touch terminals as there's no galvanic separation, even when the motor is not running).

Softstarter handle voltage ramps but obviously cannot deliver variable frequency.

It's important to understand the difference between VSD and softstarters.

The starting current duration is what is limited in a star delta starter.At the time of changing over to delta a jump in starting current will be noticed .It only controls the duration of starting current flow With a high inertia load motor takes longer duration to accelerate and reach the rated RPM. A VFD can ensure a decent start .However if the motor is going to run at full speed continuously it will be costly.Then a soft starter is better option.However the minimum starting ramp will ensure a staring current unlike a VFD which can ensure no starting current.But why do they want to change over unless it disturbs his system voltage or maximum demand.

In many cases softstarting is chosen to simply reduce the stress the equipment even if a direct start would be possible from a power supply POV. Large motors of several hundred kW are sometimes started directly

Softstarting is OK as long as torque requirements can be met and that there's no need to vary speed after the start. VSD starting can overcome torque limitations (of course the motor must allow it, also from a thermal overload POV).

Good softstarters include a motor overload protection function and, if the bypass is external, some have connections for the bypass contactor in order to be able to protect the motor against overload also when the bypass is closed (in such case the current is measured, if using an external bypass without special connections or external CTs you need a separate thermal relay unless you use a circuit breaker with motor protection release (also called trip unit)).

Softstarters can be used for very long starts (if running at set current limit the ramp time won't matter), it's all about the dimensioning of the softstarter and the motor.

It's also possible to use a single VFD with synchronizing option, or softstarter, to start several (preferably identical) motors in sequence while after its start each motor is connected directly using a contactor but it's a cabling mess and requires very careful design (like those cases where one synchronizing system is used for different generators). Technically possible but not recommended.