Hi,

It`s true that the rated capacity of an ONAN transformer could be derated to 115 % to 120% of the rated capacity by forced-cooling of the transfo. Does it hold true for an indution motor ?

Regards.

Bob

Hi Bob, my feeling on this is that at full load the Induction motor has got the full effect of the shaft mounted cooling fan so I would say no. The forced induction on motors is impotant when you are running an induction motor at high torque and low rpm. The transformer on the other hand is relying on convection so you cn acheive greater ratings by enhanced cooling. Just my opinion though.

bob,
your statement is not perfect. I would write "Sometimes the rated capacity of an ONAN transformer could be uprated to 115% to 120% of the rated capacity by forced-cooling of the transfo."
Sometimes the induction is too high, it is not possible to increase maximum continuous power for a trafo.

Regarding motors, the situation is similar. If the induction motor is near saturation of magnetic circuits, a decrease of temperature cannot give any benefits in terms of maximum power.
Instead, if the motor has a poor cooling system, and induction is low, and thermal transmission by conduction from wires to the motor frame is very good, it is possible to increase maximum power. Please note that motors must survive also when there is some filth on the frame. If you increase thermal transfer between surface and air, you have to warranty a certain cleanness on surfaces.

Regards
Mario

Hi bob

There are two major issues with the motor rating.

• Maximum or transient torque (or power)
• Maximum Continuous power.

The peak torque is determined by the flux in the iron. This in turn is limited by the quality and cross section of the iron. Too much flux will lead to saturation and associated problems. Cooling will have little or no effect on the flux so the peak torque will not be affected.
The maximum continuous power rating of the motor is determined by the maximum temperature of the windings. This is a function of the losses in the motor and the thermal resistance of the motor (ability of the motor to get rid of heat). If you increase the cooling capacity of the motor, you will increase the ability of the motor to get rid of losses and therefore you can increase the losses in the motor by increasing the continuous load. Reducing the ambient temperature also reduces the operating temperature of the windings enabling you to increase the continuous load.

If you increase the cooling and do not increase the load, then the windings will operate at a lower temperature and this will extend the life of the insulation. A common rule of thumb is that for every ten degrees C that you reduce the operating temperature, you double the life of the insulation.

So, yes, increasing the cooling will enable you to increase the continuous load, but will have no impact on the peak load. Note the copper loss is proportional to the current squared, so you do need to increase the cooling significantly to have much impact.

Best regards,

Marke,

I notice that motor saturation is often quoted. Information I have been provided by manufacturers indicates saturated and unsaturated values for equivalent circuit parameters as a function of slip. I know that in a transformer sub-synchronous operation increases the core hysteriesis losses and can lead to vibration and heating resulting in damage. I have also been told by VSD manufacturers that for low speed operation drives are often oversized due to saturation issues. Can you tell me what effects result from saturation in induction motors and under what conditions does this become a problem?

Many Thanks

Matt

Hello Matt

As you increase the flux in the iron, the "inductance" changes. There is a maximum flux density, beyond which, the iron reaches saturation.
If you start with a magnetic circuit comprising a coil on an iron core and apply an AC voltage to that core, you can control the flux at a given frequency by controlling the voltage across the coil. As you increase the voltage, the current will increase. Beyond a certain voltage, the current will begin to increase rapidly. This is due to the iron core becoming saturated and as the flux increases, the inductance reduces. This in turn increases the flux even further and reduces the inductance even further etc.
You can achieve the same characteristic by keeping the voltage constant and reducing the frequency. The important issue is to keep the V/Hz ratio constant for a given magnetic circuit and coil.
If the iron begins to saturate, you get a rapid increase in magnetizing current, plus a rapid increase in iron loss, plus a rapid increase in harmonic currents.

The net result is, saturate the iron in a motor, and it will get hotter, reducing it's life.
If you reduce the frequency, you must reduce the voltage proportionally. This applies to 60Hz motor operating on 50Hz etc.

Best regards,