Can 50 Hz rated equipment operate on 60 Hz grid and vice versa?
Globalization affects various areas of our lives. Variable speed drive systems are no exception. It happens that components manufactured in a “50 Hz country” will be used in a “60 Hz country” or the other way round. A frequent question in this context is: Can 50 Hz rated equipment operate on 60 Hz grid and vice versa? Let’s have a brief look!
Power frequency worldwide
The nominal frequency in the electric power grid is also called power frequency. Two values of power frequency dominate the world: 50 Hz and 60 Hz.
It does not mean that you won’t find other frequencies, e.g. 25 Hz ¹ or 16.7 Hz ². However, those frequencies are specific for certain application or region and do not constitute a wide-scale power transmission system for general use.
¹ e.g. local power generation using a 25 Hz rated turbine-generator
² e.g. 16.7 Hz railway network system in Austria, Germany, Switzerland, Norway and Sweden
Equipment design with respect to rated frequency
How is the equipment designed with regards to the rated frequency? It depends a lot on the type of equipment and its use. For example, household consumers typically have a universal design matching both 50 Hz and 60 Hz grid. The power supply of your notebook can seamlessly work in 50 Hz network or 60 Hz network (you might need a socket adapter when going abroad but you certainly do not change the power supply due to that).
In large variable speed drive systems the situation looks different. The design is to large extend project-specific (tailor made). Nobody is dimensioning the system components for dual rating unless there is an extremely good reason for it.
Such reason could be e.g. a full load test [1] or string test in a facility having different rated frequency than at the location of final installation.
Can 50 Hz equipment operate on 60 Hz grid?
Now back to our original question: Can 50 Hz rated equipment operate on 60 Hz grid? Yes, under some circumstances. In following section we look at selected drive system components.
Main transformer
A converter transformer designed for 50 Hz frequency can principally operate in a 60 Hz network. Important is to consider the ratio of nominal voltage and nominal frequency (V/f). The ratio is proportional to the flux in the magnetic circuit of the transformer. Lower flux means lower utilization of the material but otherwise does not impose any risk. On the other hand, flux exceeding the design value may cause transformer saturation.
Remember: Always evaluate both voltage and frequency!
For example, transformer designed for 11 kV/50 Hz shall work fine in 11 kV/60 Hz grid. However, transformer designed for 10 kV/50 Hz may not be suitable for operation in 13.8 kV/60 Hz grid. First of all, the nominal voltage is higher and thus increased insulation level may be required. Second, the magnetic flux with 13.8 kV/60 Hz supply is 15% higher than with 10 kV/50 Hz. That can be enough to draw the transformer into saturation, unless it is designed with reduced flux density at 50 Hz.
Even though 50 Hz transformer may be suitable to operate on a 60 Hz network, some things shall be kept in mind:
- Transformer impedance (voltage) at 60 Hz is 20% higher → larger voltage drop at load
- Iron losses are reduced in case the nominal voltage remains the same
- Load losses increase by few % (skin effect)
- Rated power may be marginally reduced (2-5%)
Is there any design margin for the flux?
Most transformers are designed to operate quite close to the knee point of the B/H characteristic. The reason is simply a competitive design. According to IEC 60076-1, the transformer at no load condition shall be capable of continuous operation at V/Hz of 110% of the rated V/Hz. So there is a very little margin there. However, this clause is more a reserve for the actual voltage which, at no load, may be slightly higher than the nominal supply voltage.
Warning: Transformer designed to operate within a 60 Hz system will not automatically work at 50 Hz unless it is either specially designed for it or the 50 Hz grid has about 20% lower nominal voltage to avoid saturation.
Current transformer (CT)
Instrument transformers for current measurement may be designed such that they fit for 50 Hz as well as 60 Hz system. However, the feasibility shall be clarified with the manufacturer. If the CT is designed primarily for 50 Hz, it may still work well at 60 Hz but the accuracy class may eventually be impacted.
Voltage transformer (VT)
Here the situation is similar like for the main transformer. The nominal frequency shall be evaluated together with the nominal voltage to assess the risk of saturation. In addition, the measurement accuracy may be different at 50 Hz and 60 Hz.
Fix-speed motor
A fix-speed (direct on-line) motor will have approx. 20% higher speed when supplied from 60 Hz source instead of 50 Hz supply. There is impact on the electrical design (iron and winding losses) but also on the mechanical design (design integrity at higher speed, distance from critical speed etc). Small auxiliary motors may be designed more universal supporting both 50 Hz and 60 Hz and eventually also a range of input voltages (e.g. 400…480 V). The larger the power the less likely is such universal design.
It shall be checked whether the change in speed is acceptable for the load (e.g. impact on the hydraulic characteristic of a pump).
Variable frequency drive
VFD can principally work in a 50 Hz and 60 Hz system, at least when thinking about the main power hardware. The auxiliary system (motors for cooling fans or pumps, auxiliary transformers etc) might need an adaptation.
In case of a diode rectifier (DFE) the power part is normally exact same for 50 Hz grid and 60 Hz grid. When the VFD consists of an active front end (AFE), the hardware is normally also the same. However, there can eventually be certain power derating at 60 Hz due to higher switching frequency of AFE semiconductors.
Power cables
Same power cable can be used for 50 Hz and 60 Hz system. The current loadability at 60 Hz is slightly reduced due to skin effect.
Summary
Consumer electronics is often designed for a range of frequencies, e.g. 45…65 Hz so that it works fine in 50 Hz network as well as 60 Hz grid. Main reasons are unification of the product, compatibility and convenience for the user.
On the other hand, large industrial equipment, such as drive system components, are tailor made and do not inherently cover both 50 Hz and 60 Hz.
- For components with a magnetic circuit the frequency shall be evaluated together with the voltage as one main concern is the saturation. Therefore, the V/Hz ratio shall be checked.
- Transformer designed for 50 Hz frequency can usually operate at 60 Hz without any major problems. There can, however, still be some impact on the performance (e.g. accuracy class of a CT).
- Transformer designed for 60 Hz frequency does not automatically work at 50 Hz. Unless it is specifically designed for dual frequency or wider frequency range, there is a risk of saturation.
- Fix-speed (DOL) motor runs at higher rpm when supplied with 60 Hz. Besides small impact on the electrical design, the mechanical feasibility shall be checked.
- VFD main power part is suitable for either 50 Hz or 60 Hz and no design change is normally required. Active front end drives might have small power derating when operating at 60 Hz grid due to higher switching frequency. The auxiliary circuits of the VFD contain auxiliary transformers and motors where previously mentioned considerations apply.
⇒ Operation of equipment designed for 50 Hz on a 60 Hz supply is less of a problem compared to opposite situation.
References
[1] VFD testing, MB Drive Services, October 2020, available online, https://mb-drive-services.com/vfd_testing/
[2] What information belongs on a nameplate?, MB Drive Services, December 2021, available online, https://mb-drive-services.com/what-information-belongs-on-a-nameplate/
[3] SFC as grid inertie, MB Drive Services, November 2022, available online, https://mb-drive-services.com/sfc-as-grid-intertie/
[4] Can you operate a 60Hz Transformer at 50Hz?, Hammond Power Solutions, https://americas.hammondpowersolutions.com/en/resources/faq/general/can-you-operate-a-60hz-transformer-at-50-hz
