Phase Current

Phase current is the current flowing through the motor windings. In VESC motor controllers, phase current is regulated directly by the motor control algorithm (FOC or BLDC) and is the primary current responsible for producing motor torque.

Because torque in most electric motors is approximately proportional to phase current, increasing phase current increases the torque the motor can produce. This makes phase current one of the most important factors affecting acceleration, hill-climbing ability, braking force, and overall motor performance.

Unlike battery current, which measures the current flowing from the battery to the controller, phase current measures the current circulating between the controller and the motor. Since the controller acts as a DC-to-AC power converter, phase current can often be significantly higher than battery current.

⚠️ Phase Current is not the same as battery current. Many users assume that a motor drawing 200A of phase current is also drawing 200A from the battery. In reality, the controller can convert lower-voltage, higher-current power at the motor while drawing a much lower current from the battery.

⚠️ Motor controllers are limited by phase current, not by battery current. Motor controllers have to deal with the full phase current at all times, regardless what the battery current is. Therefore it is impossible to tell how much battery current a motor controller can handle as the relation between battery current and phase current depends on the operating point of the motor. At low speed most current circulates between the power stage and motor and at higher speed more and more current flows from the battery through the power stage to the motor. However, regardless of speed, the power stage always sees the full phase current.

⚠️ Motor and controller losses are proportional to the square of the phase current. This means that doubling the phase current will generate four times more heat in the motor and controller. Therefore it is very important to choose appropriate gearing for your system. Insufficient gearing where the motor always operates at low speed will cause excessive heating in the motor and controller even at low power levels.

At low motor speeds:

• Phase current can be much higher than battery current
• High torque can be produced with relatively modest battery current

At higher motor speeds:

• Phase current and battery current become increasingly similar
• The controller’s ability to multiply current decreases
• Battery current becomes a larger factor in determining available power

Phase current limits should be selected based on:

• Motor current rating
• Motor cooling capability
• Controller current rating
• Desired torque output
• Application requirements

Setting phase current too high can result in:

• Excessive motor heating
• Reduced motor lifespan
• Controller overheating
• Over-current faults
• Demagnetisation of motor magnets in extreme cases

Example

A system may be configured with:

• Battery Current Max: 50A
• Motor Current Max: 200A

When accelerating from low speed, the controller may draw approximately 50A from the battery while supplying up to 200A of phase current to the motor. This allows the motor to produce high torque without requiring the battery to deliver the same current.

⚠️ The maximum allowable phase current is limited by both the motor and controller. Always refer to the controller datasheet and motor specifications when selecting current limits.

In simple terms: phase current is the current flowing through the motor windings and is the primary factor that determines motor torque.