Starting of low-voltage motors

It should be possible, just my personal opinion

It’s a bit uncertain; it would be better to use a soft starter, as that provides extra protection. This is just my personal suggestion for your reference

Sure, there’s no problem at all. The transformer is sufficient to power the motor. I have repaired a 250KW motor of this type, which was used in refrigeration systems; it started without any load, and then a load was applied afterward, with autotransformer step-down starting being used during that process. Once starting was complete, it switched to normal power supply.

What is the voltage drop at startup? Could someone calculate it in more detail?

If it’s too wasteful to use a transformer to power just one motor, then why not use variable-frequency starting?

From a testing perspective, it is entirely feasible to use a 315KW transformer to power a 250KW motor, but from an engineering application standpoint, the safety factor cannot be guaranteed! For loads that are started frequently, the service life of the transformer will be reduced exponentially! Moreover, the service life of the transformer is directly related to the motor’s starting method, the number of starts, and the multiple of the starting current. Based on the 3 transformer accidents I handled some time ago, it can be seen that transformers manufactured in recent years, due to the soaring price of copper, are far inferior to similar transformers from the year 2000 in terms of weight and insulation lifespan! Therefore, placing orders deserves great attention!

It’s a bit unbelievable to use a 315kVA transformer to start a 250kW motor, but since someone has already done it, let’s give it a try!

A transformer paired with a motor is used in this situation: there was originally a 1250KVA transformer in the plant, and its capacity was already fully utilized; it was not possible to add any more load. Now, however, it is necessary to install another 250KW device. There are several options: First, one can use a 250KW 10KV high-voltage motor directly, eliminating the need for a 315 transformer. Moreover, the basic electricity cost would be calculated based on 250 KW, resulting in a monthly savings of (315–250) * 28 = 1820 yuan. However, the low-voltage motor has already been purchased and the equipment foundation has also been poured, so this solution is no longer viable. II. Using 315 and 1250 in parallel is an option, but the capacity difference between the two transformers is too large, making it difficult to achieve impedance matching. The manager said it’s definitely not feasible, so this approach has to be abandoned. III. Therefore, the only option is to use 315 to drive 250 directly; using an inverter is too expensive and not practical. Moreover, manufacturers of autotransformer starting cabinets have already produced such devices, so there is only one way left. However, the main problem is that there is a distance of 100 meters from the distribution room to the motor; therefore, when the motor starts, the voltage drop across the transformer combined with the voltage drop in the wiring is estimated to exceed 30%. This may result in difficulty in starting the motor, or even an inability to start it at all. As for what causes the reduced service life of the transformer, that’s not something I need to worry about; I just need to ensure that the motor can start properly. Colleagues, if you have any good suggestions, I would appreciate it if you could share them for reference. Thank you! I will do my best to give you extra points.

Try to do the calculations: the rated current of a 315KVA transformer is 315*1.5=470A. The starting current of the motor when it starts without load is taken as 4 times that value, while for an autotransformer, 30% is used as a factor; thus, 250*2*4*30%=600A. The rated current of the motor, on the other hand, is 250*2=500A, which exceeds the rated current of the transformer! My knowledge is limited; I’m not sure if this is correct

Since you insist on doing this, after reviewing the description of your project, I understand! To ensure the reliable start-up of the motor and the safe operation of the transformer, it is necessary to find solutions in the motor’s start-up control circuit. The specific approach is: a 15% voltage drop on the motor side at startup ； 1.5 times the motor’s rated current ； Startup time is greater than 50 seconds ； The autotransformer starting cabinet is designed to operate less than 3 times per hour, and a compensation capacitor is connected in parallel on the motor side; its maximum capacity shall not exceed 30% of the motor’s capacity, with the specific value to be determined based on the actual conditions of the project

A step-down start using an autotransformer is feasible, as is soft starting; both options work

It’s a bit confusing – my facility uses 280KW motors, but Tiansheng soft starters with a capacity of 320KW are being used. The startup time is a little over 1 minute, with the current remaining at 1800A. The current after startup is 260A. The rated current of this motor is 500A. Furthermore, when starting your 250KW asynchronous motor using an autotransformer for voltage reduction, can it start completely under no-load conditions? Also, there isn’t just one 250KW motor per unit of equipment! It’s better to use a larger transformer.