Scenario (Early 2000):

Originally facilities were powered by 480 volts, 3 phase, 50hz (generation voltage 480V / 13.8kv and (underground transmission) then 13.8kV/480V for distribution) from diesel power generator(s), and due to more power demand, and frequent failure of diesel engines, the commercial power source introduced in to the facilities, as 13.8kv, 3phase, 60hz, (480volts, 3phase, 60hz)

Due to lack of budget, client desired to use the same equipment with 60hz new power source except critical equipments, tools and facilities.

Action had been taken:
Replaced all lighting ballasts with 60hz, since it was failed frequently.

All induction motors (over 100 motors) were rewound with next wire size than original wire size, there was enough space on stator (slot) to accommodate the new wire size and reduced the bamboo size.

The reason was, those motors with original (50hz) was failed and followed the same for the rest of the motors and till now working without failure, the site is located 5500ft above the sea level and ambient temperate not exceeds 25 deg centigrade even in summer.

As well the same (step up & step down) transformers were used but in reverse order with few modification of protection system, because of all switchgear and existing setup.

Since, then no failure had been occur and saved millions of dollar and time.

In quality and safety concern,
Product or service produced meet the quality expectation.
Satisfies a real need and reliable and safe.

Question:
Is this right method or procedure and safe to use the electrical power equipments?

Edited: 08 September 2012 at 02:56 PM by jagansarah

since the speed of the motor is basically controlled by the frequency of the supply a 50Hz motor will run faster at 60Hz but without any real harm full effects to the motor.

It will increase the speed, reduce the locked rotor torque which will increase the current and running temperature.

Motor manufacturers can give to the maximum speed for a motor but what you have to ask is,,,,,, is it safe to operate the machine, pump, etc that the motor is driving at an increased speed?

Thank you for your quick response Mr. Dave,

Q1. How to convert the 50hz induction motor to 60hz motors and wise versa, what are the recommended solution (such as stator winding, cooling fan, etc.)?
Q2. Is it safe to operate the 50hz motor in 60hz, since shaft connected load is constant (e.g. pump, fan, etc),
Q3. Can step up transformer used as step down transformer and wise versa, and is it recommended to use 50hz (name plate) transformer in 60hz (distribution transformers in industries, less than 10MVA), what are the cause effect and risk factors?

Q1. Not sure!

Q2. Be careful. If it is connected to a fan (which is not changed) then the fan will be spinning up to 20% faster with the 60Hz supply than originally it was with the 50Hz supply, and since power for a fan can rise as the cube of speed, you could draw up to 70% more power with that same fan. Probably it will not be this much more because the slip will increase due to the additional power output, but fan and pump loads on such induction machines need careful consideration.

Q3. Yes, but you should expect the transformer to generate more heat when used at 60Hz than when it is used at its nameplate 50Hz, due to eddy current and hysteresis losses in the core. It would be worth derating the full power to compensate. Incidentally, if you were to use a 60Hz transformer at 50Hz, that would NOT be safe if you kept the voltage the same since there would be a risk of core saturation and high overcurrents (even off load) if voltage happened to be at the upper end of the expected tolerance region.

Andrew

-------------------------
Dr. Andrew Roscoe

http://personal.strath.ac.uk/andrew.j.roscoe

Q3. Can step up transformer used as step down transformer and wise versa, and is it recommended to use 50hz (name plate) transformer in 60hz (distribution transformers in industries, less than 10MVA), what are the cause effect and risk factors?

The difference between 50Hz and 60 Hz is obvious. For the same transformer to be used either in step up or step down mode. consideration is to be made on the vector group. Generally a neutral is required on the load end.

Regards
Chris Chew

Q1. To operate induction motor on different frequency....... with some inefficiency!
Induction motors use an iron core and require flux in the iron to operate. In order to achieve the commercial goals of smallest size and lowest price at best efficiency, induction motors are designed to operate at a high level of flux in the iron. The flux is determined by the turns, voltage and frequency. In a modern motor, if the flux is increased by a small amount, the iron losses increase and the iron tends towards saturation. At saturation, the inductance begins to fall and the current increases further. To reduce the flux at a given voltage and frequency, the turns on the stator are increased. This reduces the Iron loss, but a longer length of thinner wire is used and the copper loss increases. Design becomes a balancing act between copper loss and iron loss and so the design is optimized for a given voltage and frequency.

If the voltage applied to the motor is held constant and the frequency is increased, the inductive reactant increases and so the flux reduces. This effectively reduces the maximum torque capacity of the motor and so the motor power rating at the higher frequency remains the same.
If the voltage applied to the motor is held constant and the frequency is reduced, the current will increase and in theory, the torque will also increase. The motor should be able to deliver the same power also, BUT the flux in the iron is now too high resulting in excessive iron loss, and the motor will fail prematurely. Above a very low frequency, (5 - 10Hz) the impedance of the magnetizing circuit of the motor is primarily inductive and so in order to keep the flux within limits, it is important to keep a linear V/F ratio (Voltage to Frequency ratio). If the frequency is reduced by 10%, the voltage must also be reduced by 10%. Because the flux in the iron remains the same, the torque capacity remains the same and so the power rating of the motor also drops by 10%.