Questions about direct motor starting
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At what power level or below can a motor be started directly? What is the relationship between this and the transformer?
The specifications state clearly: For motors that start frequently: it must be greater than 90% of the rated voltage； Motors that are not started frequently: it should not be lower than 85% of the rated voltage.
It seems that for us, those in the 30–55 range can start operating directly; for higher values, it’s sufficient to refer to the capacity of the power grid and transformers
For our engineering projects, we usually start the machines directly if their power is 37 KW or less.
Generally, motors below 22kw are started directly, while those above that capacity use star-delta or variable-frequency starting.
In our factory, motors with a capacity of 55 KW or less are generally started in a direct start manner. Since the voltage in our factory is slightly high (400V), the impact on the power grid during startup – in terms of a drop in grid voltage – is not particularly significant
The motor starting method is determined based on the transformer’s capacity; generally, if the load exceeds 20% of the transformer’s capacity, voltage reduction starting for the motor must be considered.
The motor starting method is determined based on the transformer’s capacity; generally, if the load exceeds 20% of the transformer’s capacity, voltage reduction starting for the motor must be considered.
The issue of direct motor starting is primarily considered in terms of the impact it has on the power grid during motor startup. In larger power distribution networks, the power levels are relatively high; some motors even have a capacity of 90 KW. In grids with smaller capacity, direct starting is generally used for motors with a capacity of 45 KW or less.
For those below 22KW in our case, direct starting is used, mainly for oil pumps.
Of course, it depends on the transformer capacity. Generally, as long as it does not exceed 20% of the transformer’s capacity (provided by the design institute), full startup is possible. But nowadays, transformers have much higher load capacity than before, especially dry-type transformers, which have an even greater overload capacity. The power of motors that can be started directly can be increased accordingly; I haven’t conducted any experiments on this, so it depends on the specific circumstances. This is some of my experience; I hope everyone can offer their feedback. This post was last edited by chinahbzq on 2009-2-25 15:07.]
Generally, it’s below 18.5 KW; it depends on whether your transformer can handle it!
There is no fixed type; there are also those that can start directly with 185KW. It mainly depends on the allowable capacity of the electrical components upstream of the motor, such as transformers, cables, various switches, contactors, etc. Mechanical issues such as startup frequency and torque also need to be considered ； When being strict, the impact on surrounding electrical systems (such as magnetic fields) must be considered.