This DC series wound motor controller is based on standard Pulse Width Modulation (PWM) techniques and it uses MOSFET switches.
The design of the controller is based on the most simple type that can be used for this purpose.
The switching of high inductive loads (motor) will result in severe voltage “spikes” that can damage the MOSFET switches. Protection for the MOSFETs must therefore be provided in the form of “fly-back” diodes and “snubbers”.
The Block Diagram of the controller is shown below:
The capacitor bank (20 x 820uF) needs to be charged slowly after “switch-on”. Relay R1 switches a 33 ohm resistor in series with the capacitors when the ignition switch is on. After about 2 seconds the capacitors will be charged to the battery voltage and the Contactor switch may be swithced to on. If a voltage is applied from the Accelerator to the PWM Controller, PWM pulses are provided to the gates of the MOSFETs. The MOSFETs switch the motor accordingly. The “fly-back” diode across the motor protects the MOSFETs against the voltage spikes generated by the motor inductance.
However, in practice there are always stray inductance present due to the wire length between components. Due to the very high switching currents through the MOSFETs when the PWM ratio is low, the stray inductance may cause voltage spikes which could damage the MOSFETs. Therefore, in order to avoid these unwanted spikes, the conductors indicated as thick red conductors in the block diagram, should be kept as short as possible in any layout of this controller.
Further protection for the MOSFETs has been provided i.e. current sensor and temperature sensor.
The IRFP4668pbf Mosfet has a 200V limit and very low R-on characteristic. Depending on the battery bank voltage and the “spike” situation, other Mosfets may be considered, e.g. IXFK150N30P3 or IXTK120N65X2 etc.
The detail of the controller design is given in the follow-on sub-menu pages.