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Protecting motor and driven load: let’s torque sense

07 February 2016

I recently read an article implying that motor current can be used as an alternative to motor power to protect mechanical drive trains, writes Malcolm Greenhill.

The Unipower HPL500 digital motor load monitor from Charter Controls protects machinery by monitoring the electrical power of the motor driving it

After clarification from the author, I was still a bit concerned that, while the advice in the article may protect the motor, it may also leave the machine itself vulnerable to damage. 

Motor power is the only purely electronic measurement of motor load which is linear with, and proportional to, the mechanical load on the motor. An alternative is torque measurement; however this requires a speed input from an electro-mechanical sensor and can be significantly more expensive.

Motor current is essentially flat line from idle load to 50-60 percent of full motor load and fluctuates with site voltage.  Above 50-60 percent of full motor load, depending on the motor efficiency, motor current increases at an almost exponential rate with an increase in mechanical load. It is nigh on impossible to establish a reliable and repeatable trip point on an exponential curve. Take this example for a motor rated 10A full load current:

Using the power equation: 1.73 x I x V x Cos f (I=current; V= voltage)

At idling: 1.73 x 5 x 415 x 0.15 = 538W
At approximately 50-60 percent load: 1.73 x 5 x 415 x 0.5 = 1,794.8W
At approximately 100 percent load: 1.73 x 10 x 415 x 0.8 = 5,743.6W

In the first two examples the current has not changed; Cos f changes and the power increases correspondingly. In the third example the motor is running at its rated current and approximate motor plate Cos f. Modern motors can run at Cos f of between 0.85 and 0.9. This exacerbates the problem because the current increase comes later in the motor's power band.

Over-sizing motors
Current may be suitable for a crude motor load application until you consider the common practice of over-sizing motors. In the article I referred to above, it was stated that up to 80 percent of pump motors could be oversized by as much as 10 to 15 percent. Engineers may be doing this to be ‘on the safe side’ or may be incorporating a ‘non-overloading characteristic’. 

If a motor is over-sized for the task, it may never reach the 50-60 percent of load at which its current starts to increase; and you won’t know until the motor has already done the damage. Motor load monitors are the only electronic devices suitable to monitor mechanical systems and installations when a mechanical problem occurs.

Mechanical devices such as clutches, ball-detent torque limiters and shear-pins are often installed externally and open to the elements. They require regular inspection and maintenance to give constant and reliable protection. Motor load monitors, on the other hand, are located in the motor control panel and are truly ‘fit-and-forget’ devices.

Malcolm Greenhill is managing director, Charter Controls

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