Inverter drives | Eight lesser known facts

Inverter drives have been a key technology for industrial engineers for many years but Matt Handley of Mitsubishi Electric thinks they can be underappreciated. Here he highlights the first eight from a list of twenty five of the interesting facts about them.

1. Inverter drives save energy. Especially on variable torque loads such as fresh water pumps or cooling fans. On these load types, affinity laws describe the relationship between speed and other variables. The power-speed relationship is also referred to as the ‘cube law’. Controlling the flow by reducing the speed means that for a 20% reduction in speed up to 50% of the energy can be saved. Modern drives feature energy optimisation functions, automatically adjusting on the fly to maximise energy savings. Often the biggest saving comes if a drive can be used to stop a motor altogether or reduce its speed to a very slow idle over a large part of its duty cycle.

2. Pumps are often run at full speed, with the flow being controlled using a mechanical restrictor valve. It is far more energy efficient to fit a drive and vary the pump’s speed. The same can be said of fans and louvers. In recent years HVAC engineers have started fitting drives to their many pumps and fans, with most achieving significant energy savings immediately.

3. Inverter drives can be set up to run automatic profiles. For instance, you may want a centrifuge to start slowly and accelerate gently to a first set speed, then quickly ramp up to full speed and hold this for a set time before decelerating fairly quickly to a very slow speed (which assists the tipping out of the contents), then finally stop altogether.

4. The example in (3) is a time based motion profile but speed changes can also be implemented with other signals, such as a load change, a moving object breaking a light beam, a temperature variation or a safety alert.

5. Motors, their gearboxes and the equipment they are driving can be subjected to shock loads on start up. A drive can provide a soft starting solution, which ensures for example, a heavily loaded conveyor or a mixer in a viscous liquid comes up to speed relatively slowly. (The term ‘relatively slowly’ can cover a range from a fraction of a second to several minutes, depending on the application).

6. Inverter drives can start loads that are already spinning, even in the wrong direction. A typical example would be a large tunnel ventilation fan which is turning slowly in the natural air flow; a drive can be set up to sense this speed and match it before engaging.

7. Drives benefit from tuning after installation. Some drives are fitted but never tuned so may provide little by way of user benefits. Some get only basic tuning, others get a thorough tuning. The best are regularly retuned and this is usually done automatically during normal operations.

8. If used with a feedback device such as an encoder, a drive can often perform at a level of precision that rivals servos.

Go to 999 Inverters for Mitsubishi Electric’s complete AC variable speed inverter drive line-up