Variable speed drives – how to protect these vital operations components

Variable speed drives (VSDs) form an integral part of mines today; it is an energy-optimised solution integrated into various mechanical components prevalent in today’s mining operations. For example, VSDs offer significant energy savings when integrated into mines’ cooling systems which are particularly beneficial in times when operators are under increasing pressure to optimise energy usage.

However, as with many other components, VSDs’ operational effectiveness also undergoes tremendous strain when exposed to unplanned power outages such as loadshedding. This damage is further exacerbated by the quality of energy when the power does return. Often when grid power is restored it’s not a pure sine wave (unclean power) which damages equipment.

To prevent damage to VSDs and reduce downtime, double conversion online uninterruptible power supplies (UPSs) can be connected to the inlet power supply of the said VSD drives. A UPS will ensure that when a power outage occurs, the VSD will not experience loss of power. Also, a double conversion UPS can deliver a pure sine wave while mitigating potential damage to power-sensitive equipment.

How does a double conversion UPS protect a VSD from damage? It converts power twice through the following:

• Power comes in from the main grid in the form of an alternating current (AC) input which may contain voltage spikes, distortion, and other anomalies.
• This AC power is converted into direct current (DC) whereby voltage spikes and distortions are removed with the use of the rectifier and the DC power leaving the rectifier is then fed to an inverter as well typically used to charge a set of batteries.
• The DC is then “Inverted” back into AC that is tightly regulated by the UPS and thus all power leaving the UPS is clean as connected to power the VSD drive.

Even though there is an interruption in power during a load-shedding incident, the DC link, within the UPS is maintained by the battery and the output voltage is unaffected and continuous.

Here it’s important to understand that a UPS is a temporary power solution, providing energy until power from either the main grid or other energy resources such as generators is restored.