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I'm somewhat familiar with Tesla's electric vehicles and that they use AC motors, but I'm completely unfamiliar with EVs that use DC motors. I know that in a Tesla an inverter is required to convert DC from the battery to AC for the motor, but since a battery always supplies DC, do batteries supply a DC motor with power directly in EVs that use a DC motor instead? Or what sits in between?

Also, if no inverter is required, which type of component is commonly used to regulate the frequency and amplitude of the DC motor in EVs?

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In a DC motor, the frequency of the (also required) AC current is created mechanically, by the turning of the rotor and the work of the commutator.

The most energy-preserving way to control the torque of a DC motor, and thus, the speed of the drive, is to adjust the voltage. Today this is done by using a buck converter.

You may want to look up pre-1960ies electrical locomotives for information about how it was done before power electronics.

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  • \$\begingroup\$ *brushed DC motor. Not that there really is a true motor with no brushes to begin with which truly runs off DC \$\endgroup\$
    – DKNguyen
    May 18, 2020 at 4:37
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Both AC and DC motorized vehicles use a combination buck/boost converter. Which mode it is in depends on the gas pedal position, and how fast you move it, and the 'Mode' setting.

Typical battery voltage when charged is 200 volts (Toyota Prius), and is enough for driving in slow traffic. Stomp on the gas pedal with the 'Mode' set to 'Insane' and it boost the motor voltage to 500 volts, AC or DC. Right now these are common values that may change with time and technology.

Many of these hybrid or EV cars also have a front panel 'Mode' control to adjust how the car reacts to gas pedal motion. The 'Economy' setting conserves the most power and gas, if it is a hybrid. At the top of the list is the 'Insane' setting which turns the car into a rocket. No attempt is made to conserve fuel or the batteries.

The Porsche 918 has 750 HP, with a 150 HP electric motor on the front and rear axle. The rest is from an instant on/off 450 HP gas engine. Base price is $1.25 million USD.

The AC motor versions have a DC to 3 phase converter, to drive a variable frequency motor of up to 150 HP so far. Both AC and DC motors use special alloy (NIB) permanent magnets so they have tremendous torque even at low speeds.

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A DC motor can be controlled with an H-bridge constructed from MOSFET's or IGBT's. The way to think of it is that looking from the battery to the motor is a synchronous buck regulator. Looking from the motor toward the battery, it is a synchronous boost regulator.

schematic

simulate this circuit – Schematic created using CircuitLab

When the motor is spinning forwards, M4 is on, M3 is off, and M1 and M2 are the switching elements of the synchronous buck regulator.

In this configuration, the effective applied voltage to the motor is:

Vapp = Vbatt * D

  • Vapp is effective applied voltage.
  • Vbatt is the battery voltage.
  • D is the buck duty cycle.

If the applied voltage is less than the back EMF, then M1 and M2 act like a boost regulator, boosting the back EMF up so that current flows into the battery even though the battery voltage is higher than the EMF. In this situation, the motor torque will try to slow the motor's speed. The torque will operate in the opposite direction to the rotation of the motor.

If the applied voltage is greater than the back EMF, then the FET's plus the winding inductance make a buck converter, and current will flow into the motor. In this case, the motor torque will attempt to make the motor spin faster. The torque will operate in the same direction as the motor rotation.

In order to operate in reverse, M2 can be on, M1 off, and M3 and M4 can be switched in complimentary fashion. Everything is exactly the same in reverse. Just that the current flows in the other direction through the winding.

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