We all know that drives can be used to save energy by controlling motor
speed, but did you also know that other in-built control functions such
as PID loop algorithms can also be exploited to gain even more energy
savings. Automotive wheel maker, Magnetto-Topy has used the extra
functionality of its drives to considerable advantage
Steel wheel maker, Magnetto-Topy is achieving unprecedented energy
savings by applying closed loop control to an existing variable speed
drive installation. The original installation saved the company some
£23,000 per year in energy. But an additional 50% energy saving has now
been achieved by applying closed loop flow control, using water
temperature as the control variable. A PID controller is inserted within
the control loop, which is tuned to ensure stable and accurate control
with good response times. Because the PID controller is built into the
drive, the benefits of closed loop control can be realised without the
additional expense of external control equipment.
Magnetto-Topy use four drives, rated from 37 to 55kW, on water pumps
within four cooling towers that supply water to the wheel making process.
Before installing variable speed drives, the company used star-delta
switching which turned the pumps on or off. Thus, the pumps were left
running at full speed on the 50Hz supply. With drives fitted, the company
can reduce the running speed on a 55kW pump to 35Hz, resulting in a 53%
energy saving.
Countering legionalla colonisation
A further assessment showed that by applying closed loop control, the
energy consumption could be reduced by an additional 50%. But
Magnetto-Topy was cautious about further slowing the pumps, because of
the risk presented by legionella bacteria, which can spawn in cooling
towers. To prevent this, a continuous water flow is essential. Along with
water flow, the water temperature needs to be maintained at a maximum of
20º C to prevent the growth of legionella bacteria. Changing the water
flow rate can also affect the water temperature.
The system was tuned for these worst-case conditions, which normally only
occur during the summer, when the water temperature reaches its 20ºC
maximum. But during the winter months when the water temperature is
naturally cooler, there is an opportunity to slow the pumps down even
further. This is where PID control comes in.
A temperature transducer is placed into the water tank and this signals a
running at 40Hz. As the temperature drops to 13ºC and below, the drive
runs at 25Hz. As the temperature of the water rises, the pump speeds up
and allows more water to cascade over the cooling tower.
The result is that with PID control, the energy bill has been halved yet
again, says Phil Smith, maintenance service administrator. PID control
is a compensatory control method. Without it we have to cater for the
worse case which means running pumps at 40Hz - the summer setting.