ACC Motor Upgrade Specification

Design change of ACC fan motor to enhance vacuum conditions.

The Project

An Air Cooled Condenser (ACC) of twin fan design, located on a waste to energy plant was unable to operate with a fan speed greater than 80% of its design value.

The fans were being driven by variable speed drives with the maximum current limits set in line with the current rating of the motors.

At 85% of the maximum speed, the measured current reached the set limits in the variable speed drives for the motors. The speed range of the fans was therefore limited to a maximum of 80% to prevent exceeding the current limits. This restriction in fan speed was preventing the ACC from delivering the desired duty, resulting in a reduced rate of heat rejection by the ACC. This reduced performance left the ACC unable to achieve the specified vacuum at its design ambient temperature. As the ambient temperature rose the vacuum achieved was less than required. The reduced vacuum affected the output from the steam turbine, reducing the power the client had available for export and by default the revenue generated.

Scope of Engineering Works Our Approach The Results

Our client asked us to investigate the reason for the fan drive speed limit and to advise on what could be done to achieve 100% fan speed.

The data sheets for the fans and the motor/gearbox sets were analysed, the fan curves indicating that the power required to achieve 100% fan speed would be 52.5 kW.

The gearbox efficiency was listed as 98%, therefore giving a required motor power of 53.57 kW.

The manufacturers specification called for a 55 kW motor to be fitted, with a power factor of 0.86 at 100% Load and 0.81 at 75% load. There was no indication of power factor expected at any speed other than nominal 1480 rpm.

The load on the motor at full speed was expected to be 53.57kW with the motor rated at 55.0 kW. This did not give any spare capacity which often is needed for any variations in operating conditions such as ambient air temperature, atmospheric pressure and the effect on power factor of the motor being driven by a variable speed drive.

The fan data sheets gave the absorbed power for 90% speed as 40 kW, which clearly would have resulted in a much lower current than was actually being measured at 80% speed.

A monitoring unit was fitted to the fan motors and the data collected showed that the motor was running with a power factor of approx. 0.6 when operating at 80% speed and approx. 60% power.

This clearly identified that the high current was due to a poor power factor on the motor when operating at reduced power and reduced speed and not an overload condition.

Unfortunately it was not possible to use the variable speed drive to correct the power factor, so a new larger 75kW motor would be required.

This motor would have a higher current capability allowing it to operate at the required power with the poor power factor.

In order to keep capital costs to a minimum, the replacement motor would need to be compatible with the existing gearbox. The original motor fitted was a flange mounted 55kW 250 frame unit, unfortunately standard 75 kW motors are 300 framed units.

We specified a 75kW motor with a 250 frame flange mounting to fit the existing gearboxes.

We issued the client with a detailed report on the findings of the investigation and provided a full specification for the replacement motors, enabling suitable replacements to be ordered in advance of the next scheduled shutdown.

We also recommended that the variable speed drive unit be set up by a specialist to ensure that it would be compatible with the proposed new motors. This would enable the current draw to be mapped over the full speed range and a suitable current limit set to prevent the gearboxes from being over loaded.