Question:
Up to which power can motors be connected directly to a constant-voltage-constant-frequency system and which conditions must be observed for a Y-delta start?
Answer:
The permissible voltage dip in the mains is decisive for the permissible motor power when connected directly to the mains.
The starting current of electrical machines at a constant-voltage-constant-frequency system can be up to ten times the rated current which may cause the mains voltage to drop unpermissibly. For the public mains
in Germany technical connection conditions (Technische Anschlußbedingungen TAB) apply which specify limit values. These conditions are issued by VEDW (association of utility companies) and can be obtained from the utility companies.
An excerpt from the
German utility company eon-energie AG specifies:
>>The start of motors must not cause disturbing voltage changes in the mains.
This condition is generally fulfilled for occasionally starting motors, if - AC motors with an apparent power of not more than 1.7 kVA or
- Three-phase motors with an apparent power of not more than 5.2 kVA or
- for higher apparent powers, motors with a starting current of not more than 60 A are used.
- For motors with occasional start (max. 10 times daily) and with higher starting currents than 60 A, the planner or installer agrees necessary measures to avoid disturbing voltage changes with the utlity company unless a non-objection in terms of possible disturbing mains perturbations was proved.
- For motors which can cause disturbing mains perturbations due to heavy starting, frequent switching or oscillating current consumption, such as lifts, saws and cutters with a starting current of more than 30 A, the planner or installer agrees the necessary measures for the reduction of the mains perturbations with the utility company.<<
For industrial mains with their own medium voltage supply and low-voltage transformer the operator of the industrial mains (industrial company, end customer) specifies the mains voltage dip which is tolerable, depending on the utility company (voltage dip on medium voltage level) and the distance of the motor from the point of supply (low-voltage transformer).
Moreover,
Y-delta starters, soft starters, inverters, slipring rotors can be used.
For Y-delta starters it must be observed that the motors for 400V are in delta connection and all winding ends are brought out. Motors with voltage specifications D 230 V/ Y 400 V are not suitable for Y-delta starter.
