starter since all of the windings used during starting are in  series with the source. While acting as a starter, the unit  makes no practical use of its shunt field. A source of 24  volts and 1,500 peak amperes is usually required for starting. When operating as a generator, the shunt, compensating,  and commutating windings are used. The C field is used  only for starting purposes. The shunt field is connected in  the conventional voltage control circuit for the generator.  Compensating and commutating or interpole windings  provide almost sparkless commutation from no load to full  load.  Figure 5-18  illustrates the external circuit of a starter  generator with an undercurrent controller. This unit controls  the starter generator when it is used as a starter. Its purpose is  to assure positive action of the starter and to keep it operating  until the engine is rotating fast enough to sustain combustion.  The control block of the undercurrent controller contains  two relays. One is the motor relay that controls the input  to the starter; the other, the undercurrent relay, controls the  operation of the motor relay.  The sequence of operation for the starting system is discussed  in the following paragraphs.  [Figure 5-18]  To start an engine  equipped with an undercurrent relay, it is first necessary to  close the engine master switch. This completes the circuit  from the aircraft’s bus to the start switch, to the fuel valves,  and to the throttle relay. Energizing the throttle relay starts  the fuel pumps and completing the fuel valve circuit gives  the necessary fuel pressure for starting the engine. As the  battery and start switch is turned on, three relays close: the  motor relay, ignition relay, and battery cutout relay. The motor  relay closes the circuit from the power source to the starter  motor; the ignition relay closes the circuit to the ignition units;  the battery cutout relay disconnects the battery. Opening the  battery circuit is necessary because the heavy drain of the  starter motor would damage the battery. Closing the motor  relay allows a very high current to flow to the motor. Since  this current flows through the coil of the undercurrent relay, it  closes. Closing the undercurrent relay completes a circuit from  the positive bus to the motor relay coil, ignition relay coil, and  battery cutout relay coil. The start switch is allowed to return  to its normal off position, and all units continue to operate.  Figure 5-18.  Starter generator circuit. 5-12 To external power receptacle B G C M C B A E Advance Retard To fuel  pumps On Off To fuel valve Battery on Start Emergency stop Engine master switch Battery and start switch Battery cutout relay Throttle switch Throttle relay Motor relay Under current relay Throttle relay lock Ignition relay Ignition vibrator Starter  generator B-4 relay