Differences between Electrically Coupled and Mechanically Coupled UPS technologies

What does a UPS do?

Whether electrically coupled or mechanically coupled, a UPS sits in the power chain between the energy source and equipment requiring power. When a UPS is operating it continually conditions the power supply to the equipment.

Its other function is to ensure the power supply is not interrupted – for example in the event of a power cut. Therefore, any UPS is also connected to a short-term energy store (often a battery).

In the event of a power utility outage, power is initially sourced from the short-term energy store to ensure equipment remains powered.

This is maintained while a longer-term alternative power source is made available, often in the form of a back-up power generator. 

When the longer-term power source is available (e.g. the generator starts up and begins generating power) the load is seamlessly transferred to this alternative power source. This ensures continuous supply of electricity to vital equipment until the utility power comes back on.

Upon restoration of the utility power service, the process is automatically reversed.

Figure 1. Generic UPS, Utility and Standby Generator arrangement

What are the key differences between electrically coupled and mechanically coupled coupled UPS?

What happens inside a UPS can be fundamentally defined as being either mechanically coupled or electrically coupled.

Features of an Electrically Coupled UPS

Electrically coupled UPS are the most commonly deployed.

An electrically coupled UPS connects the energy store with the rest of the system using an electrical connection via a DC link. 

Power semiconductors are used to control the energy flow for both discharging and re-charging. The DC link allows either batteries or kinetic energy devices to be used as the short-term power source.

In the electrically coupled UPS, an electrical inverter (which converts DC to AC) is connected to the short term energy store. Power flow towards the load is controlled by this inverter.

Figure 2: Electrically-coupled UPS Block Diagram

Benefits of EC UPS

1. Partial load efficiency is typically better reaching around 3% difference at 25% load. This can sometimes reduce energy consumption and running costs.
2. Greater flexibility for system expansion and configuration. The mechanically coupled UPS tends to have a 1:1 relationship between engine and UPS power and often, they cannot be operated independently of one another.
3. Energy store options allow ride-through autonomy to be selected between seconds and minutes in conjunction with optimising other design criteria.
4. Lower maintenance requirements and free of annual maintenance altogether, leading to improved availability and lower operating costs.
5. Standby generation and UPS can be configured differently to suit the electrical requirements and physical constraints.

Features of a Mechanically-coupled UPS

A mechanically coupled UPS connects the energy store with the UPS using a mechanical drive shaft for both charging and discharging. 

In a mechanically coupled UPS an electrical machine is directly connected to an energy store which has two independently rotating elements, all turning on a common horizontal shaft. 

The two elements of the energy store rotate at different speeds, one at the synchronous speed for the operating frequency of the system (50 or 60Hz) and the other at a much higher speed in order to store energy.

In emergency mode, the energy is extracted from the higher speed element by controlled braking and transferred to the load mechanically.

Figure 3. Mechanically coupled UPS with standby generator

Mechanically coupled UPS benefits 

1. Peak efficiency in on-line mode is typically better than some electrically coupled UPS. 
2. Loads requiring high peak currents can be better handled than for some electrically coupled UPS. 
3. The equipment may be operated in a more rugged environment than certain electrically coupled UPS. 
4. The equipment is more suitable than certain electrically coupled UPS for high power applications.

Limitations of mechanically coupled UPS

1. The mechanically coupled UPS consists of large mechanical elements which can only be configured in one particular manner and all in the same location on a single driveshaft.
2. The engine is not usually available as a separate source to the UPS which limits flexibility during maintenance and repair.
3. The single shaft arrangement constitutes a single point of failure for the UPS and standby energy store.
4. The system uses a series of nested bearings and their overhaul usually needs to take place off site, taking considerable time and expense.
5. The physics of an induction coupling energy store significantly restricts the useable ride-through time. The engine must start up as soon as any utility perturbation is detected. It must get up to full speed often with the aid of adjustments to the normal engine setup.