Hi,
In a scenario where there is no power grid available and an electric Generator is needed to power motors.

I have been told that if one wants to run a 20 kw motor from a Generator .The Generator needs to at least produce 60 kw because when a motor is first started it requires three times the power. That is to say at start up the motor requires 3 x 20 kw, hence the need for a 60 kw Generator . There after the motor requires only 20 kw so is only using 20 kw discharged from generator and the rest is wasted.

Is this correct ?

Secondly if one connects the 20 kw motor via a variable frequency drive. Then the motor would be started up gradually using the VFD so would not require 60 kw generator and the same 20 kw motor could be powered by 20 kw - 25 kw producing Generator.

Regards

Zahid

 

True a VFD will soft start a motor as it is programmable, there is also soft start electronic contactors that will do it.
Max.

 

Thank you.

I have just looked up soft starter on the net.

So a combination of soft starter and changing the pully size on the motor (22kw /30HP) to run the pully connected machine at half speed would be a good solution ?

[The machine is only required to run at a constant speed @ 50% ] this is other reason vfd was used.

The same time reducing the Generator size from say 70 kw to 25kw ?

There by saving cost on Generator , fuel used, VFD .

Are my assumptions correct ?

Regards

Zahid

 

If you don't need the variable speed control offered via a VFD, then the soft starter may be the better choice together with the reduction..
It would be cheaper than the VFD.
Max.

 

The 3x may just be a rule of thumb. You don;t necessarily waste power if over-sized. Quiescent fuel consumption might be higher.

What is the power factor of the motor and how heavily loaded is it? Will it start under load?

 

If you don't need the variable speed control offered via a VFD, then the soft starter may be the better choice together with the reduction..
It would be cheaper than the VFD.
Max.

A few years ago, the difference in price between a soft starter and a VFD was in the range of about 15%. If that's still the case, I'd think that a VFD would be a better investment, since it gives you far more flexibility than a simple soft starter.

 

If you are talking about 3 phase then using motors that can be configured for star delta starting would reduce the problem.

Les.

 

If you are talking about 3 phase then using motors that can be configured for star delta starting would reduce the problem.

Les.

thank you.
It would appear that certainly when there is no load , starting the 3 phase motor in a star with a timer switching to delta would remove the problem of "inrush" surge required.

 

Is there a way of starting or a starter where the inrush could be temporarily stored to kickstart the motor just in case there is some load ?

Zahid

 

Put a much larger flywheel on the generator. This should prevent the engine from stalling but you may have to do something to any overcurrent protection circuit. I think the alternator should stand the overload for the short time it takes for the motor to start. If there was a clutch between the motor and the load you could use a smaller motor to spin the main motor up to speed before connecting the main motor.

Les.

 

Put a much larger flywheel on the generator. This should prevent the engine from stalling but you may have to do something to any overcurrent protection circuit. I think the alternator should stand the overload for the short time it takes for the motor to start. If there was a clutch between the motor and the load you could use a smaller motor to spin the main motor up to speed before connecting the main motor.

Les.

The Generator is 180 kw it doesn't stall. There are three motors running 22 kw -25kw each. The 6 litre engine running @ 2200 rpm is very thirsty.
I want to be able to use a 90 kw generator to run these motors. I want to overcome the problem of inrush surge required for the first 1-2 seconds. vfd, soft starter, star delta are ways to reduce the inrush required by the 3 phase motors.
One of the motors starts with no load at all , star delta might do the job.
If there is some load will the soft starter do the job of the motor not requiring the inrush if started at gradual speed. Will the load correlate to the required inrush so even with soft starter there will be a in rush. Will the inrush be greatly reduced with the soft starter. These are the questions I am looking at. If there was a way to store the inrush required by the motor and can grab it when needed from that source rather than from generator and with combination of soft starter , I think I can reduce size of generator from 90 to 180 kw. That is the objective anyway.

Thank you
Zahid

 

The problem with a soft starter, often they have no way of graduating the start period as does a VFD.
And as @cmartinez mentions there is little difference in cost.
Max.

 

In the literature at the link it is stated
"
The inrush current is restricted to 200% of the motors full load current (FLC) "
Also
"Three adjustments can be made on the

• Initial start voltage (initial torque).
• Soft start ramp time (ramp up).
• Soft stop ramp time (ramp down). "

I think this is very close to what I need.

Just one CLARIFICATION PLEASE
We put an amperage meter reader on the motor wires to measure amperage drawn by the motors to try and measure load and I believe it was reading around 5 amps.

Is 5 amps likely to be correct value for the 20 kw 30 three phase motor ?


https://www.motorsoftstart.com/products/stb-basic-soft-starter-6a

 

On a 3phase motor there will initially be very high decreasing current until up to the rated rpm. It is not unusual to see very low current with no load.
It all depends on the inertia of the rotor, disregarding any load.
I guess the soft starters have changed since I used them.
Max.

 

On a 3phase motor there will initially be very high decreasing current until up to the rated rpm. It is not unusual to see very low current with no load.
It all depends on the inertia of the rotor, disregarding any load.
I guess the soft starters have changed since I used them.
Max.

Thank you for the intro to "soft start"
Your words mean I get it.
Just need to find out which model of soft starter.
I guess I can find out from the motor name plate for Amps and corresponding model of soft starter.

 

I once worked with a soft-start device from "Safco", which had an adjustable ramp up time and could also be set to ramp up based on current. It also had overload and short circuit protection built in. So devices with those features available do exist, or at least did exist.

As for motor inrush current, many generators specify a higher current motor starting rating.