Torque Shop: EVs produce sound to warn other road users of their approach
Sign up now: Get ST's newsletters delivered to your inbox
Whirl you were driving: The EV's onboard inverter makes noises when altering the current going to the motor.
PHOTO: ST FILE
Why do some electric vehicles (EVs) make whirling noises when driving at low speed?
Electric and hybrid vehicles sold in the European Union and the United States have been required to have an acoustic vehicle alerting system for a while now.
The system makes a sound when the vehicle is driving at low speed and when reversing. This is meant to warn other road users that an EV is approaching, although the exact activation speed and sound requirement vary according to jurisdictions.
Otherwise, the only sounds emitted by a moving EV would be the noise from rubber tyres rolling on the tarmac.
There are other sources within an EV where the somewhat eerie sounds come from. Most of the noises are produced by the inverters, which are major components of an EV’s power-control module.
Electrical energy for EVs is stored in a battery pack which supplies DC (direct current). The energy goes through an inverter to convert it into AC (alternating current) required by the vehicle’s motor.
The inverter also controls motor speed by varying the frequency of the AC, based on the driver’s accelerator-pedal input. The whine that one hears from an EV is emitted by the inverters during the current conversion and when the alternating frequency varies. It is most apparent when moving off from rest.
One hears a similar whine or high-frequency hum from MRT trains that is most audible when they are departing a station. Power for the trains supplied by the third rail is 750V DC and, therefore, the inverters need to convert DC to AC while also providing variable frequency for the AC traction motors.
The other main source of the typical EV whine is the electric motor. Mechanical noise within a motor is virtually non-existent, but some sounds result when the rotor – which is separated from the magnetic-field producing stator by mere millimetres of air – spins as fast as 12,000rpm.
EV sounds were much more pronounced in early models, but these days – with the latest components technology and noise-isolation designs and materials – the whine is barely noticeable.
In some countries, though not in Singapore, EVs are required by local legislation to produce an artificially generated and sufficiently audible sound, as a warning to pedestrians of an approaching EV.
