In this article, we will discuss “How to read the Speed Torque Curve”. For understanding the speed torque curve we consider a compressor is driven by an electric motor for easy understanding. The speed-torque curve for other driven equipment’s such as pumps, fan, blowers will also follow the same below rules only.
Normally the choice of an electric motor as a drive for the compressor is being favoured due to lower capital, installation cost and reliable electric power supply when compared with other drive equipment such as the turbine.
To read about Compressor Performance Curve Click Here
To read about Pump Performance Curve Click Here
Important of Speed Torque Curve
- The speed torque of the motor(driver) is not matched with the compressor speed torque curve(driven equipment) then the motor(driver) cannot able to drive the compressor.
- By using Speed torque curve, we will ensure the compressor never trip during the voltage dip.
Hence more care shall be given to read the speed torque curve of the compressor (driven) and motor (driver) is essential. In order to avoid improper selection of driver.
Electric Motor Speed Torque Curve
With reference to the below figure, the motor draws high currents during startup, as a result, the line voltage dip. The magnitude of which voltage dip is depended upon the size of the electrical system. In cases where motor driving the compressor is the largest in the plant, voltage dip can be 20% ~ 25% or more. This reduced voltage reduces motor torque by the square of voltage. Hence matching the motor torque under worst voltage condition to the compressor torque should be ensured. Make sure the motor torque is always higher than the compressor torque.
Each motor has a maximum torque. A load above this torque means that the motor does not have the power to rotate. With a normal load, the motor works significantly under its maximum torque. However, the starting phase of the motor involves an extra load. These characteristics of the motor are usually presented in a torque curve.
As we see from the above motor speed-torque curve for an induction motor. When the motor starts the torque is high is called starting torque T(s). After that, the motor is accelerated hence the torque first drops a little is called minimum Torque T(min) and then rise to its max. value before dropping is called maximum torque T(max).
Compressor Speed Torque Curve
Compressor torque comprises of friction, inertia and thermodynamic power. Care shall to taken for preparing the compressor speed torque curve, sometimes the inertia factor will be neglected. Then this will leads to incorrect motor (driver) selection.
In order to examine the speed torque curve of compressor and motor, the curve of compressor and motor, the compressor speed torque curve is superimposed on the motor speed torque curve (refer below figure). As we seen from the curve the motor speed torque curve at 80% voltage shall be minimum of 25% higher than the compressor curve. It is because during voltage dip also the motor can able to overcome the torque required of the compressor to drive it.
The intersection of “motor speed torque curve” and “compressor speed torque curve” is representing the normal operating point of the compressor. Refer below figure.
As we said at the beginning of the article selection of suitable driver (ie., motor) for any driven equipment (ie., compressor, pump, fan, etc.,) Speed torque curve is one of the basic checkpoints of proper selection of equipment. An improper selection of a driver will lead to wasting of both money and time. Because failure due to speed torque curve will be identified during the commissioning stage.
This problem can be overcome by knowing “How to read the Speed Torque Curve”.
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