Electrical System
Description
The MU2 electrical system is comprised of two 24VDC batteries, two 28VDC engine driven generators and two AC inverters, one main and one standby. An external power receptacle behind the cabin door is also available to connect an external GPU for engine starts and standby power.
When the battery switch is ON, batteries are connected to the main DC bus. See the Electrical Distribution diagram further below for reference. The main bus is comprised of left and right distribution buses which are always cross tied via a main bus tie circuit breaker (CB) located in the aft electronics enclosure behind the cabin door. The main bus distributes power to left and right feeder load buses. Further, each feeder load bus is separated into two regions by isolation diodes where the upstream portion of each feeder bus is called LH/RH feeder 1 and the downstream portion LH/RH feeder 2.
Each feeder bus is connected to the main bus by a feeder relay, which is energized through two CBs on the aft end of the main fuse panel (see illustration below). Pulling a CB for a feeder will isolate/disable the upstream portion of the feeder, that is feeder 1, while the downstream portion of the feeder, feeder 2 receives power from the opposite feeder 2 bus through a feeder bus tie CB. To fully isolate an entire feeder side (feeder 1 and 2), you would need to pull the feeder CB for a given side and then also the bus tie CB on the main fuse panel.
The feeder buses distribute DC power to other DC buses as illustrated below, i.e. the LH/RH radio buses, the LH/RH overhead buses.
AC electrical power is supplied by two single phase inverters, a main and a standby, each of which receive DC power input from the the LH/RH feeder 1 buses respectively . Each inverter system is independent of the other. The main inverter is powered via the left side feeder 1 load bus, while the standby inverter system is powered via right side feeder 1 load bus. During engine starts, the AC buses are automatically disconnected from the system; however, a special AC power distribution sub-system is activated only during the engine start cycle to power the torque and fuel flow gauges.
After engine start, when the left and right DC generator switches are ON, the generators are connected to the main bus. The electrical load for each generator and the monitored bus voltage are indicated by their respective voltammeters. DC power is distributed from the main buses to the load buses, then through cockpit circuit breaker panels and finally on to to the various aircraft components.
Separate left hand (LH) and right hand (RH) radio buses are connected to the load buses through dedicated radio master switches located on the circuit breaker panel to the left of the pilot seat. These switches must be on to utilize the Radios and GPS. Overhead panel buses are powered by the load buses through circuit breakers.
From an operational perspective, the electrical system is fully automatic, requiring only the switching of the two generator switches and two radio master switches to ON after engine starts.
Electrical System Controls & Indicators
The electrical system is controlled and monitored via controls located on the left side of the pilot switchpanel.
A master key connects the batteries to the main and starter busses. Two DC generator switches energize relays that connect the generators to the main bus after engine start. In the event of an unusual voltage spike from the generators for some reason, the voltage regulator will disconnect the generator from the bus to protect equipment. The RESET position of the DC generator switch may be employed to restore normal generator operation after such an occurrence. Toggling the DC gen switches to the RESET position before setting the DC switches to ON is an acceptable practice.
The inverter switch controls electronic switchgear to connect either the main or standby inverter to their respective AC buses.
The MASTER SWITCH to the left of the generator is used only in case of emergency to reduce any chance of electrical spark causing a hazard. With the switch in the emergency position, generators and batteries are disconnected from the electrical buses and only the fire detection system and cockpit flood light receive power from the battery.
Two voltammeters above the Generator control panel indicate electrical voltage and load on their respective LH/RH buses
Distribution Diagram
