Why Your Motor Stumbles at Startup: Understanding Voltage Drops

When you start an electric motor, you might notice a momentary drop in voltage. This is a common occurrence, and it’s caused by several factors.

When you start an electric motor, it requires a surge of current to overcome the inertia and start rotating. This sudden surge of current causes a voltage drop in the system, which can be particularly noticeable if you’re running on a low-voltage system.

In this article, we’ll take a closer look at why voltage drops during motor starting and explore some of the related concepts.

Table of Contents

Signs of Voltage Drop During Motor Starting

There are a few signs that can indicate that the voltage is dropping during motor starting:

Slow or delayed startup: If the motor takes longer than usual to start, or seems to be struggling to get going, it could be a sign that the voltage is dropping.
Flickering lights: If the lights in the same electrical circuit as the motor flicker or dim when the motor starts, this could also be a sign of voltage drop.
Overheating: If the motor gets hot quickly during startup, or if it overheats and shuts down, this could be due to a voltage drop causing excess current to flow through the motor.
Unusual noise: If the motor makes a strange noise during startup, such as a buzzing or humming sound, this could be a sign that the voltage is dropping and the motor is struggling to start.

To confirm whether voltage drop is the issue, it’s best to use a multimeter to measure the voltage at the motor terminals during startup.

If the voltage is significantly lower than the rated voltage of the motor, then voltage drop is likely the cause of the issues.

For more information about The impact of voltage variation on motors read my article here.

Common Causes of Voltage Drop During Motor Starting

The voltage drop is caused by several factors, including:

The inrush current: When the motor starts, it requires a surge of current to overcome the inertia and start rotating. This current is known as the inrush current and can be up to seven times higher than the rated current of the motor. This sudden surge of current causes a voltage drop in the system.
The impedance of the motor: The impedance of the motor increases as the speed of the rotor increases. This increase in impedance can cause a voltage drop in the system.
The length of the cable: The longer the cable, the greater the resistance, which can cause a voltage drop in the system. Check out my article here for in-depth information about How Distance Impact the Voltage.
The quality of the power supply: If the power supply is unstable, it can cause voltage fluctuations that can affect the performance of the motor.

How can you prevent voltage drop during motor starting?

There are several ways to prevent voltage drop during motor starting, including:

Using a soft starter: A soft starter is a device that gradually increases the voltage to the motor, reducing the inrush current and preventing voltage drop. Read my article about Motor Starting Methods, for more information.
Using a variable frequency drive (VFD): A VFD is a device that regulates the frequency and voltage of the power supplied to the motor, reducing the inrush current and preventing voltage drop.
Using a motor with a higher starting torque: A motor with a higher starting torque can overcome the inertia more easily, reducing the inrush current and preventing voltage drop.

Voltage drop during motor starting can have several effects on both the motor and other loads connected to the same electrical system.

Effects on the motor:

Reduced torque: A voltage drop can cause a reduction in the torque delivered by the motor. This can result in slower acceleration, longer start-up times, and reduced overall performance.
Increased heat: When a motor experiences a voltage drop, the current passing through it can increase, leading to an increase in heat production. This can cause the motor to overheat, leading to premature failure or reduced lifespan. Want to learn more? check out my in-depth article How Ambient Temperature Affect Electric Motor.
Damage to windings: A voltage drop can cause a decrease in the voltage applied to the motor windings. This can result in damage to the windings, including overheating and insulation breakdown.

Effects on other loads:

Dimming lights: When a motor starts, it can cause a temporary voltage drop that can affect other loads on the same electrical system. For example, the lights may dim or flicker during motor starting.
Damage to other equipment: A voltage drop can also cause damage to other equipment connected to the same electrical system. For example, a computer or other sensitive electronic device may shut down or experience damage due to the drop in voltage.

To minimize the effects of voltage drop during motor starting, it’s important to ensure that the electrical system is properly designed and maintained.

This may involve using voltage regulators, capacitors, or other equipment to stabilize the voltage and protect both the motor and other loads.

In addition, it’s important to choose motors and other equipment that are properly rated for the application and that can handle the starting current without causing excessive voltage drop.

Conclusion

Voltage drop during motor starting is a common occurrence that can be caused by several factors, including the inrush current, the impedance of the motor, the length of the cable, and the quality of the power supply.

To prevent voltage drop, you can use a soft starter, a VFD, or a motor with a higher starting torque.

By understanding the factors that cause voltage drop, you can take steps to ensure that your electric motor starts smoothly and operates efficiently.

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