2 speed motor from contactor to inverter

Thread Starter

ashish.rawatfbd

Joined Jan 16, 2020
3
Hello Everyone ..

I have a dual speed motor 1.9kw/6.5kw Star/Star 975rpm/1455rpm .. Now the present condition is that there is a 6 core cable connected to motor and that cable 3 core goes on one contactor(1 speed) and another 3 core is connected with the second contactor (2 speed),
first the small speed get start then there is a timer and change to second speed .... Now I am planning to remove all these contactors and just install a 3 phase inverter.

What I think is that I should just use the 2 high speed 3 cores and connect it to the inverter output , and just modify the acceleration and deacceleration time in inverter as per my requirement. ( and Just remove the 3 cores of the 1 low speed from the motor terminals )

Now my question is that I have never done that before..I have installed many inverters and motors but never modify like this so I am confuse.
Please clear me whether I am right or dual speed motor is something different and cannot be connected with the inverter.
 

Thread Starter

ashish.rawatfbd

Joined Jan 16, 2020
3
Same reasons for which every motors nowadays are connected to inverters on machines... giving a soft start , tracking the faults by getting alarms , reducing the MD
 

MaxHeadRoom

Joined Jul 18, 2013
20,703
Same reasons for which every motors nowadays are connected to inverters on machines... giving a soft start , tracking the faults by getting alarms , reducing the MD
By invertor, I assume your are referring to a VFD?
You can actually get solid state electronic 3 phase soft starters without going to the full expense of a VFD.
Max.
 

vu2nan

Joined Sep 11, 2014
148
Hi Ashish,

Your 2 speed motor has a pair of exclusive windings / contactors and a timer to change over from low speed to high.

You may run the motor with its high speed winding connected to a VFD, having a contactor for safety at its input, after programming the required speed, time duration etc., leaving its low speed winding disconnected. I guess there should be no problems in doing that.

However, the motor power at the lower speed would now be 6.5/1.5 = 4.3 kW (V/f is constant up to the rated speed with a VFD) and the corresponding torque 4.3 kg m (the same as that at the rated speed since the torque is constant up to the rated speed with a VFD).

In the case of the low speed winding the motor power would be 1.9 kW and the corresponding torque 1.9 kg m.

You are effectively saving on a contactor and spending on a VFD with added energy expenses at low speed (4.3 kW instead of 1.9 kW)!

In the existing system you are starting with its 1.9 kW winding and changing over to its 6.5 kW winding after 67% of its rated speed has already been attained. Thus, as it is, it roughly approximates star-delta starting.

It appears that there is nothing to be gained by making the change. You be the judge!

Regards,

Nandu.
 
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