Description

Speed Control Applications

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 and load. It then calculates an error signal and drives the actuator in the increase- or decrease-fuel direction to correct prime mover speed and load.

Figure 4-1 shows the Electrically Powered Governor in more detail. The speed control is housed in a die cast aluminum enclosure.

The Electrically Powered Governor 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). The controller compares the two inputs and changes the actuator position if an error exists. Gain and stability adjustments in the controller 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 must be set before idle speed. Speed sensor output is a voltage proportional to magnetic pickup frequency. The frequency range of the control 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 actuator current or position.

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 droop possible one phase of the PTs (potential transformers) and CTs (current transformers) are used. 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.

There is a failed speed sensor detector 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. This circuit then moves the actuator to the minimum-fuel position and stops the prime mover, preventing overspeed.

Actuator

As shown in Figure 4-2, the actuator is mechanically simple. It has specially designed rotor and stator shapes which provide reliable effective performance. The rotary design gives 35 degrees shaft rotation to low mass, low friction fuel controls. The magnetic circuit, when powered by the speed control, applies torque in the increase-fuel direction. Two preloaded internal return springs supply shaft torque in the decrease-fuel direction. The preload can be factory reduced to compensate for some external linkage forces acting in the decrease-fuel direction.

Applications Using a Ramp Generator

The Ramp Generator slows the speed change between idle and rated speeds. It has no effect on steady state speeds. Once set it provides a constant speed change per second by biasing the speed reference when changing from idle to rated and vice versa. The accel and decel pots control the rate of change. Acceleration and deceleration times depend on accel and decel pot settings and the difference between idle and rated speeds.

Paralleled Generator Applications

These single phase droop EPGs have droop paralleling capabilities using one phase of the PT’s and CT’s. These controls droop off of current cosine phase (current amplitude times the cosine of the phase difference of the PT and CT). Droop operation is required when paralleling with an infinite bus or units not having compatible electric governors.

Troubleshooting

Even though governor faults cause improper prime mover operation, improper prime mover operation can be also caused by other items, such as low fuel pressure. When the prime mover stops working properly find out which part is defective. Do this by:

Substituting, if available, a part that works for the one suspected of causing the problem. Simplifying the system. 

Remove options and observe performance after each removal. 

Testing the parts suspected of causing the problem. Follow the manufacturer’s instructions or set up input and operating conditions which produce known outputs.

To test the EPG, use Chapter 2 to verify the installation is correct and perform the installation check. Those checks are the best way to test the EPG. Step 6.2 of Chapter 2, Preset Rated Speed, is the best test of the EPG’s ability to control speed. It requires the use of a signal generator with an isolated output. If appropriate, do the Checks for Paralleled Generator Applications, also in Chapter 2.

Do the installation checks described in the previous paragraph first. Then check the following:

If the prime mover is stable at some speeds or power outputs but oscillates at others, the linkage may not be compatible with the fuel control. Refer to Linkage Compatibility under Actuator Mounting and Linkage in Chapter 2. If the prime mover oscillates at low frequency (about 1 Hz) and Gain and Stability Adjustments, Chapter are correct, then friction in the linkage may be the cause. Disconnect the actuator from the fuel control. Manually stroke the fuel control linkage from stop to stop as if the actuator were moving it. The linkage must move freely without friction and without backlash. Lubricate or replace linkage or fuel control parts as required. If the prime mover is unstable only when load sharing make sure: The current and potential transformers are connected correctly. The voltage regulator is not intermittent or otherwise faulty. If the fuse or breaker opens after the prime mover has been running, the speed control may have been damaged. Turn governor power off and remove all wires from terminals 1, 2, 3, and 

Measure the resistance from terminal 1 to 2 and from 3 to 4, if either is less than 100 ohms the speed control must be replaced. If the control is not damaged, then high voltage spikes from the battery or battery charger may be the problem. Verify that the battery connections are correct. Provide separate wires from the speed control to the battery terminals as shown in the top of Figure 2-10. If the prime mover oscillates when cold and stabilizes when warm, turn the gain pot slightly ccw. Turn the stability pot slightly cw if required to maintain stability.

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