Unfortunately, the most robust and most economic squirrel cage induction motor has a characteristic making the motor start direct on the mains difficult or under various circumstances impossible. Especially during standstill and at low speed in the moment of starting they generate only a low torque, while also requiring the same time a very high current, which is a multiple of the rated current of the motor. So the driven working machine, the technological process or the upstream mains limit the use of this simple drive concept.

A filled mill for example, needs a very high torque at start-up, or even a special initial break-away torque, with a size possibly much higher than the nominal torque of the used motor. The high inertia of a filled mill leads additionally to a long ramp-up time, so that the high torque demand is required for a long duration even at low speed. If the technological process requires a start several times a day, then the thermal load on the drive components are quite high and may limit the number of start-ups.

In case of a high ratio of nominal motor power to the ability of the upstream mains a high overcurrent during starting leads to a large voltage dip, which may affect parallel consumers in their undisturbed operation. This is notably the case when single motors of high power are installed relative to the total mains power.

The construction as induction motor with slip rings allows in combination with an appropriate designed starter a starting torque in the magnitude of the breakdown torque of the motor, which is commonly between two and three times the corresponding nominal torque at a current equal or marginal higher than the nominal current.

Under these circumstances the use of slip ring motors are often useful. In contrast to drive applications with frequency converters where they must be highly oversized for high starting torque and producing losses during the whole operational time the high starting torque is slip ring motor immanent and the starters of slip ring motors are lossy only in the short starting phase. By changing the motor characteristic curve with the help of external resistors in the rotor circuit the motor itself produces only slightly higher losses, so that the frequency of starts is not thermally limited from motor side anymore.

General characteristic:

• Very high starting torque, two to three times nominal torque
• Low starting current at start, no or marginal higher than nominal current
• No limited acceleration time and number of starts
• No or very low torque impulses during start depending on starter technology
• High efficiency during continuous operation (no converter losses)
• No special environmental conditions
• No harmonic distortions
• No EMI measures necessary, no need of shielded connection cable