Why Low Voltage Isn't Suitable for Unlimited Torque in Electric Motors
In the realm of electric engines and motors, the decision to use a low voltage is often misunderstood, leading to inquiries about achieving unlimited torque. This article aims to clarify the relationship between voltage, current, and torque in electric motors, and explain why low voltage is not a viable solution.
Understanding Torque and Its Determinants
The torque of an electric motor is fundamentally determined by its power and speed. Contrary to what some may believe, the torque is not directly influenced by voltage. Instead, the relationship between torque, current, and voltage is explained by Ohm's law. Torque is proportional to current (T ∝ I), while voltage plays a role in the motor's speed (voltage is related to the motor's speed, speed ∝ V) but not directly in generating torque.
Torque and Power: The Core Relationship
Motor torque can be increased by increasing the current, which in turn increases the power delivered to the motor. However, increasing the current beyond a certain point is not advisable due to the significant increase in Ohmic losses. These losses are calculated as P_loss I^2R, where I is the current and R is the resistance in the motor's windings. Higher current leads to higher resistance, which in turn causes more heat, leading to potential damage to the motor and inefficiency.
High Voltage and Increased RPM for Maximum Torque
A better approach to achieving high torque is to operate the motor at a high voltage and high RPM, then use a step-down gearbox to reduce the speed and increase the torque. This method allows for more efficient operation of the motor and minimizes the current required, thereby reducing losses.
Shunt-Field and Series-Field Motor Concepts
Another relevant concept in motor design is the use of series-field and shunt-field windings. A series-connected field winding provides high torque at low speeds but can generate excessive current if not kept within controlled RPM ranges. This can result in mechanical failures, as seen in electric cars without proper traction control, and in diesel-electric locomotives where shear pins are installed to prevent damage.
Practical Considerations: Voltage and Current Tradeoffs
The fundamental tradeoff in electric motors is between voltage and current. Lower voltages require higher currents to produce the same amount of power, leading to larger and more expensive wire gauges and heavier motor designs. High current can also be dangerous, potentially causing wiring to melt or burn, thus making the solution impractical and potentially dangerous.
Conclusion
In summary, while there are theoretical and practical considerations such as series and shunt-field windings that can provide high torque at low speeds, utilizing low voltage is not a viable solution for unlimited torque. The relationship between torque, current, and voltage must be carefully managed to ensure optimal performance, safety, and efficiency in electric motors.
Understanding these principles can help engineers and designers create more efficient and reliable electric engines, ultimately leading to better performance and a more sustainable future.