How low can the frequency dip , before appliance motors start to have problems ? At work , we set underfrequency relays for 59.3 hz but that is not a good indicator of what I'm asking . My question relates more to the use of portable generators and the stability of the frequency .
Question about frequency
- Thread starter djohns6
- Start date
- Status
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
There is no simple answer to that.
If an appliance uses only resistive heating elements, such as a toaster or simple oven, the answer is never. Frequency is relatively unimportant to a purely resistive load. In fact it could even be 25Hz, or even DC. Ever notice that an incandescent light bulb does not say AC or DC?
If the appliance has an SMPS (Switch Mode Power Supply) to run electronics, most likely it was designed for use anywhere in the world, so 50Hz -10% would be typical as a lower threshold. Below that and the little transformer that is part of the SMPS would start to over heat.
Linear power supplies, found in many older TV sets and radios, would be frequency sensitive and likely over heat with low frequency. They will typically have a limit printed on the nameplate data.
If the appliance has a motor, then it will depend a lot on loading. Since mechanical HP (shaft kW) is a function of speed and torque, if your speed drops, so does the shaft HP. If the load is essentially shed with a drop in speed, i.e. a centrifugal pump on a washing machine, then probably the overall effectiveness of it's primary task may suffer, but no harm to the machine. But f the loading does not diminish with speed, such as the drum of a dryer, then the motor will more easily overload.
broadgage
Senior Member
- Location
- London, England
A comutator motor wont know the difference.
An induction motor will alter speed roughly in proportion to the change in frequency.
Low frequency will reduce the speed and therefore reduce the HP supplied to the load, this might be expected to reduce the amps used, but in practice does not because at a lower frequency the magnetising current and losses tend to increase.
However in practice small changes in frequency have almost no effect.
Here in the UK, the legal limits of grid frequency are from 49.5 to 50.5 cycles.
Good generating plant normally supplies from 49 to 51 cycles and does not appear to cause any problems.
Cheap portable generators can vary by as much as 5%, from 47.5 to 52.5 cycles, and in practice seem to work most appliances just fine.
Standby generating plant is often set to start on quite small drops in frequency, but that is because a drop in grid frequency often suggests iminent grid collapse, not because the low frequency is a problem in itself.
An induction motor will alter speed roughly in proportion to the change in frequency.
Low frequency will reduce the speed and therefore reduce the HP supplied to the load, this might be expected to reduce the amps used, but in practice does not because at a lower frequency the magnetising current and losses tend to increase.
However in practice small changes in frequency have almost no effect.
Here in the UK, the legal limits of grid frequency are from 49.5 to 50.5 cycles.
Good generating plant normally supplies from 49 to 51 cycles and does not appear to cause any problems.
Cheap portable generators can vary by as much as 5%, from 47.5 to 52.5 cycles, and in practice seem to work most appliances just fine.
Standby generating plant is often set to start on quite small drops in frequency, but that is because a drop in grid frequency often suggests iminent grid collapse, not because the low frequency is a problem in itself.
- Status