5448-906 WOODWARD

Speed Control 

The basic speed control components and connections are shown in Figure 1-1. 

There are no mechanical drive or hydraulic connections. All input power comes 

from the battery. The speed control compares the actual speed with the desired 

speed. It then calculates an error signal and drives the actuator in the increase or 

decrease fuel direction to correct prime mover speed. 

Figure 4-1 shows the Electrically Powered Governor in more detail. The speed 

control is housed in a die-cast aluminum enclosure. 

The EPG has two control loops. The speed loop ensures prime mover speed 

remains constant. The current loop ensures proper drive to the actuator. 

Speed Loop 

The speed loop controller has two inputs: the desired speed (speed reference 

signal) and the actual speed (the speed sensor signal). It compares the two and 

calculates an error signal which includes dynamic response considerations. Gain 

and stability adjustments tailor the governor’s response to the requirements of 

the specific prime mover. Rated speed is set by the rated speed pot and, if 

attached, a speed trim pot. The idle reference is controlled by an external idle 

speed pot. Rated speed should be set before idle speed. Speed sensor output is 

a voltage proportional to magnetic pickup frequency. The frequency range of the 

magnetic pickup is set by an internal resistor. The specific frequency range of a 

specific EPG Speed Control is indicated by the part number of the speed control. 

Current Loop 

The current loop error signal can be considered a command for the correct 

amount of actuator current. 

The actuator’s controller circuit compares actual current (from the current sensor 

circuit) to the desired current level (from the speed loop controller) and generates 

a current loop error signal. To make the current driver efficient, it is operated as a 

switch. Actuator current is changed by changing the duty cycle. The pulse width 

modulator converts the current loop error signal from a dc voltage to a switching 

signal. For this reason measurements of speed control output [3 (+) and 4 (–)] 

indicate only general conditions. Excessive currents are prevented from flowing 

through the actuator coil by the energy limiter. It prevents the actuator from 

overheating but allows enough current to keep the actuator at the maximum fuel 

position. 

The auxiliary input is jumpered except when a Load Sensor is added for 

paralleled generator applications. There is a failsafe circuit which senses MPU 

frequency and forces the pulse width modulator input to zero if the MPU 

frequency or voltage are below acceptable limits, as they would be if an MPU 

wire broke.