Motor Frequency

Let me start the ball rolling by concentrating on frequencies.

If the supply is 50 Hz and requirement is 60 Hz

Effect on motor –

Theory:

Induction motors use an iron core and require flux in the iron to operate. These are designed to operate at a high level of magnetic flux to reduce the size and increase efficiency. The flux is function of no of turns, voltage and frequency. If the flux is increased the iron losses increase and the iron tends towards saturation then the inductance begins to fall and the current increases further. To reduce the flux at a given V and number of turns must be increased. This will reduce the Iron loss, but a longer length of thinner wire will increase copper. The motor are designed (as per maxima minima principle) when at a given load, frequency and voltage copper loss and iron losses are balanced. Hence the design is optimised for a given voltage and frequency.

If the voltage applied to the motor is held constant and the frequency is increased, the inductive reactance 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 is constant and frequency is reduced, the current increases and theoretical torque increases. The motor should be able to deliver the same power but the flux in the iron has increased (excessive iron loss) , and the motor may fail prematurely. If the frequency is not very low (say above 10Hz) the impedance of the magentising circuit of the motor is primarily inductive and so in order to keep the flux within limits, a linear V/F ratio must be maintained. If the frequency is reduced by 10%, the voltage must also be reduced by 10%. Since the flux remains the same, the torque capacity remains same and thus the power rating of the motor also drops by same %.

Provided the voltage is dropped by the same proportion as the frequency, it is OK to run a 60Hz motor on 50Hz. The speed will be reduced by the reduction in frequency and the power capacity will also reduce by the reduction in frequency.

Maintaining V/f (voltage / frequency) will maintain same magnetic flux. However due to 17% reduction in frequency (50/60)

The motor turns 17% slower.

The internal current goes up by 17% to maintain the same flux.

The power (watt) goes down by 17 %

The mechanical cooling is less, because of 17% less rotation of fan.

The result is a higher current, than designed and the insulation of the electrical wiring deteriorate much quicker, unable to dissipate the heat, which after sometime results in a burn-out.

Load is Compressor/Pump

The compressor / Pump will run at the reduced speed (50/60 ie at 83% of the designed speed).

If the pump is centrifugal

As per the pump law

Flow is proportional to speed ie Q a f

Head a f ²

Power a f ³

Thus the changing frequency from 60 to 50 will reduce flow to 83%, Head to 63% and Power to 58%

Pump is positive displacement or rotary (eg External/ Internal gear, Lobe, Progressive cavity, screw, Flexible tube, vane etc):

Flow is proportional to speed ie Q a f

Head is constant (or defined by the load) eg orifice – if flow rate varies, the pressure required by pump will vary.

Power a f

the flow rate will be 83% of the rated flow.

Power requirement will be 83% of the rated flow.

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