Rated torque above rated speed for different load types 2

[HanyHamed]

Hi,
rated motor torque obove rated speed is reduced by the rlation of (rated speed/running speed).
my direct question is:
(is this relation is true for PUMP load?)

 

[itsmoked]

Heck no!
It goes up quickly!

 

[HanyHamed]

i am talking about the motor torque de-rating, i know that the pump load is square to speed.

my question in another form:

for constant load nature, and quadratic load nature, how much reduction in AVAILABLE motor torque at for example 75 Hz of normal 50Hz rated motor frequency?

 

[laundry]

Hi DriveMaster
Increasingthe speed from 50HZ to57HZ
Increasing the speed of the inverter will continue to raise the output frequency although the voltage supplied to thr motor cannot increase, being limited to the level of the incoming line supply. The motor will increase in speed responding to the rising frequency.Now the available shaft torque will fall away as the squar of the effective voltage reduces.Example a10% increase in frequency above 50Hz would normally require a motor supply increase of 10% to maintain design torque;So this 10% deficiency in voltage will result in a 19% shortfall in torque. In this case the motor is essentially aconstant KW machine, since reducing torque while increasing speed results in a constant power charicteristic

Barry.

 

[electricpete]

I agree with laundry.

There are two different torques to talk about:

Max steady state torque goes as 1/N above rated speed. This is required to keep power (current) from exceeding rated based on thermal limits.

Available breakdown torque (applicable to momentary overload) goes as 1/N^2 above rated speed. This is due to T~(volts/hz)^2.

So above rated speed, available breakdown torque decreases faster than max steady state torque.

So if breakdown torque was 200% at base speed and max steady state torque was 100%, than at 200% speed, breakdown torque would be the same as max steady state torque (1/4*200%=1/2*100%) and there is no momentary overload capability at all. In practice I believe speed increase is often limited to around 150%.

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