Use Variable Speed Drives to Save Energy

The only reason to specify a variable speed drive in a control application is because you expect your control system to vary the motor's speed, right?

Wrong!

According to Akseli Savolainen at control-compent supplier ABB, variable-speed drives are a green technology as well. That is, users can save energy costs and reduce greenhouse-gas emissions by using variable-speed drives to match motor output to process demands.

'So much energy is wasted by inefficient constant speed and mechanical control mechanisms,' according to Savolainen, 'that every industrialized nation around the world could make several power stations redundant simply by using AC drives and high-efficiency motors.'

Controlling motor speed makes matching process demand and machine output possible. Control of a pumping system provides a good example. Rather than controlling flow in the normal way via a throttle valve, a variable speed drive can be more efficient. Suppose the flow rate is to be half maximum, reducing the speed by reducing the motor speed reduces energy consumption by a factor of eight. The throttle valve, which reduces flow by introducing an impedance, actually increases the motor's load, increasing energy use.

Pump operation provides an example of another way to save energy with variable-speed drives. The traditional way to maintain liquid level in a tank is by turning a pump on when the level reaches a low set point and off when it reaches the high set point. With a variable speed drive, one can control pump output to very nearly match flow requirements. Since startup current can be as much as 7 to 8 times full-load value, slowly varying the motor's speed as requirements change can save considerable energy compared to duty-cycle control.