Which balancing method is performed with the machine under operating conditions?

• A. Field balancing
• B. Static balancing
• C. Dynamic balancing
• D. Thermal balancing

Answer: (C)

Balancing rotating equipment aims to remove forces that cause vibration when the shaft spins. If you balance with the rotor at rest, you can fix the gravity-related unbalance (static unbalance), but that doesn’t address how the unbalance behaves once the rotor is actually turning. Dynamic balancing, on the other hand, is done while the rotor is in motion, and often involves balancing in two planes to correct both static and dynamic unbalance. This approach directly measures how the unbalance shows up at operating speed, including phase information, so the adjustments you make (adding or removing weight at specific locations) reduce vibration during real operation. That is why this method is associated with performing balancing under operating conditions—the balance you achieve reflects how the machine behaves while it’s running. The other methods either require the rotor to be stationary (static) or refer to balancing done in a field or lab setup that doesn’t capture running-speed dynamics in the same way.