Nuts & Volts

20 August 2010, 0856

Ok I figured I would bulk this into one main post. Edit me if I am a little bit off on my claims here.

The idea behind this set-up is to use two electric motors, a compatible controller, and some extra relays/contactors to perform the switching. This idea is basically a 2 speed electric transmission. First volt equates to speed/rpm and amps equate to torque. So on acceleration (low speed) you have the two motors connected in series this way both motors see the same amount of amps, but half voltage to each. Then you switch the motors to parallel so that the amps are halved between the motors and the voltage is equal. Series provides more torque to take off and parallel provides a higher operating speed.

So lets make an example to fully understand the whole situation. You have a 72V battery system, one controller, and two motors. You wire up contactors so that you can change the wiring of the motors (ill post a diagram later). Ok so you start the motors in series and pull the throttle so that 100 amps is drawn. That means at max speed the each motor will see 36V (based on equal motor resistances). But each motor will be getting 100 amps each so that is 200amps worth of torque. The power draw from the battery is at 7200 watts. When you reach the maximum speed of the motors at 36V you switch the motors to parallel setup. This allows both motors to see 72V, which will double the operating speed of the motors. But the amps will be halved and each motor will get 50 amps. The battery will be supplying 72V and 100amp (or 7200W) in both motor configurations.

series: motor has 36V x 100A = 3600W ------ torque=2, speed=1, power=2

parallel: motor has 72V x 50A = 3600W ------ torque=1, speed=2, power=2

Advantages: More torque without pulling more amps, saves your battery, faster acceleration, low gear for hill climbing

Disadvantage: Complexity (some), cost of two motors, need compatible motors, anymore?????

Only question I have, is it possible to series parallel switch two AC motors or brushless motors?

The idea behind this set-up is to use two electric motors, a compatible controller, and some extra relays/contactors to perform the switching. This idea is basically a 2 speed electric transmission. First volt equates to speed/rpm and amps equate to torque. So on acceleration (low speed) you have the two motors connected in series this way both motors see the same amount of amps, but half voltage to each. Then you switch the motors to parallel so that the amps are halved between the motors and the voltage is equal. Series provides more torque to take off and parallel provides a higher operating speed.

So lets make an example to fully understand the whole situation. You have a 72V battery system, one controller, and two motors. You wire up contactors so that you can change the wiring of the motors (ill post a diagram later). Ok so you start the motors in series and pull the throttle so that 100 amps is drawn. That means at max speed the each motor will see 36V (based on equal motor resistances). But each motor will be getting 100 amps each so that is 200amps worth of torque. The power draw from the battery is at 7200 watts. When you reach the maximum speed of the motors at 36V you switch the motors to parallel setup. This allows both motors to see 72V, which will double the operating speed of the motors. But the amps will be halved and each motor will get 50 amps. The battery will be supplying 72V and 100amp (or 7200W) in both motor configurations.

series: motor has 36V x 100A = 3600W ------ torque=2, speed=1, power=2

parallel: motor has 72V x 50A = 3600W ------ torque=1, speed=2, power=2

Advantages: More torque without pulling more amps, saves your battery, faster acceleration, low gear for hill climbing

Disadvantage: Complexity (some), cost of two motors, need compatible motors, anymore?????

Only question I have, is it possible to series parallel switch two AC motors or brushless motors?