DOL starting of a motor with a fluid coupling.

The conventional approach: star-delta starting

A star-delta starter is generally employed for a cage motor to reduce the value of the starting current kick. In star mode the voltage drops to 57% of the full delta value of 100%. So if the motor's starting current kick is 6.0 pu (600%), star mode brings it down to 6 × 0.57 = 3.42 pu.

The catch is torque. A motor's torque is proportional to the square of the voltage, so in star mode the starting torque becomes 0.57² = 0.333, or 33.3% — one third of the delta value. A motor that produces 1.60 pu starting torque in delta delivers only 1.6 × 0.333 = 0.53 pu in star.

Why that creates a second problem

With only one third of the starting torque, the load takes roughly three times as long to accelerate compared with delta mode. Because the motor coupled to the load accelerates slowly, it reaches 70% speed only after a long period, so the motor current decays very slowly.

After accelerating the motor and load for 10 to 15 seconds in star mode, the connection is switched to delta. This switching causes a large current kick — about 1.73 times the current at the moment of switching, multiplied by the transient surge. If the motor is at around 30% of synchronous speed at that point, the kick can be as high as 5.0 pu. So star-delta starting trades the original 6.0 pu kick for a 3.42 pu kick at start and a fresh kick of about 5.0 pu later on.

It is also common practice to oversize the motor when star-delta starting is used, so that even with one third of the starting torque the motor can still accelerate the load to at least 70% speed within 10–15 seconds. Oversizing means a higher initial cost and a recurring loss of energy.

DOL starting with a fluid coupling

Now consider direct-on-line (DOL) starting with a fluid coupling in the drive system:

• The fluid coupling lets the motor start on no load at full voltage.
• The motor draws a starting current kick of 6.0 pu, but because it starts on no load it reaches about 85% speed within 2 seconds — the speed at which efficiency and power factor are far better — so the motor current decays from 6.0 pu to 3.0 pu very fast. This is a big advantage.
• Full starting torque is available to the motor for its own start, and peak (pull-out) torque is available for accelerating the load.
• The motor experiences only one starting current kick, compared with two in star-delta starting.
• Accounting for acceleration time and current value, the heat generated in the motor and supply system is 5–7 times higher with star-delta starting than with the DOL-plus-fluid-coupling combination.

When the supply authority insists on star-delta

Where a client will not accept DOL starting and still uses a fluid coupling, the star-mode duration must be restricted to less than 2 seconds. In such cases the motor protection relay in the starter must be set to a higher value to tolerate two switching kicks (typically 3.99 and 5.0 pu) and to accept starting current for a longer duration.

Using star-delta starting together with a fluid coupling is not technically justified — but where the electricity board is adamant, the 2-second restriction is applied. Otherwise a star-delta starter is simply not required.

No-load starting, full starting torque for self-acceleration, peak motor torque for load acceleration, fast decay of starting current to 3.0 pu within 1–2 seconds, low stresses on the motor and system, and smooth load acceleration are all good enough on their own to justify using a fluid coupling with DOL starting.