Can Variable Frequency Motors Be Started Directly? (I)

Can variable frequency motors be started directly? (I)

When starting, a variable frequency drive gradually increases the frequency and voltage, resulting in a low and smooth starting current. If you start directly without a variable frequency drive, you lose these protections. While you might want to eliminate the variable frequency drive, this is not worth the cost. The problems of direct starting are particularly pronounced with high-power motors. The variable frequency drive's inherent function is not just soft starting; it also provides protection, such as overload and phase loss.

It is generally not recommended to start a variable frequency motor directly from a commercial power source.

There are five reasons:

1. Excessive starting current

2. Impact on the power grid and equipment

3. Impact on the mechanical transmission system

4. Heat dissipation issues (especially at low speeds)

5. Loss of the variable frequency drive's protective functions

• 1. Excessive starting current

Variable frequency motors are designed for use with a variable frequency drive. When connected directly to a commercial power source (e.g., 380V/50Hz), they start at full voltage like a normal motor.

The starting current is very high, often 5-7 times the rated current or even higher (depending on the motor design). This stands in stark contrast to the controllable low starting current of the VFD during startup (typically limited to 1.2-1.5 times the rated current).

• 2. Impact on the Power Grid and Equipment

Large starting currents can cause significant voltage drops in the power grid, potentially affecting the normal operation of other equipment on the same grid.

This can cause severe electrical and thermal stresses on the motor's windings and terminals, as well as upstream switches, contactors, and protective devices (circuit breakers, fuses), shortening their lifespan or increasing the risk of failure.

• 3. Impact on the Mechanical Transmission System

During direct starting, the motor generates maximum starting torque (typically exceeding twice the rated torque) at near-zero speed.

This enormous torque is suddenly applied to mechanical transmission components such as couplings, gearboxes, belts, chains, and loads (such as pumps, fan impellers, and conveyor belts), causing severe mechanical shock. This can result in:
1. Damage or deformation of couplings, keys, and pins.
2. Gear tooth breakage.
3. Belt/chain breakage or increased slippage.

4. Damage to load-bearing equipment (such as pump shafts or fan blades).
5. Loose foundation or mounting bolts.