Starting three-phase motors in single-phase mode. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Electric motors
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Starting three-phase electric motors (EM) in single-phase mode, especially powerful ones (3 ... 5 kW), often becomes a problem even for professional electricians. To start the 380/220 V electric motor from a single-phase 220 V network, they mainly use a classic “triangle” connection scheme, which in all cases cannot ensure reliable starting of the electric motor even without load, especially when the engine is powerful and the consumer is far from the transformer substation.
If the windings are switched to a "star", the ED easily accelerates even with a 2-fold reduced capacitor capacitance. However, the shaft power with a "star" is only 1/6 of the nameplate power in three-phase mode. How to start the engine "star", and then switch it to "delta"? Yu. Borodaty has already written about this more than once [1]. He published the schematic, but said nothing about the switching device. After all, it should be compact, reliable, simple and safe. The author of this article had a chance to test his idea.
A method is proposed for starting an EM by switching from a "star" to a "triangle" using a magnetic starter (CM), where there are three pairs of normally closed contacts and three pairs of normally open ones. These are reverse starters, they are used on drilling machines and other mechanisms where right and left rotation of the ED is needed. The author took the KM from a PME-041 type drilling machine and connected it to the PNVS button according to the diagram in fig. 1. The button is designed to start single-phase EMs with a contact in the middle (in the diagram - K2), which opens after the "Start" button is released.
On the PNVS, between the plastic holder of the closing movable contacts K1-KZ (Fig. 2), a cardboard gasket 1 mm thick must be placed under the contact plate K1, so that when turned on, contacts K2 and KZ are first closed and voltage is applied to the KM starter coil. In this case, the switching sequence is observed: the starter switches the motor to the "star" circuit, and then voltage is applied to the power contacts through K1 PNVS, the motor accelerates, and after releasing the "Start" button (opening K2), the KM coil is de-energized, the motor switches to the "triangle" . The starter control coil does not have to be 220 V.
The author had to put on NOV through a 5,6 kOhm resistor. For start-up, electrolytic capacitors 150 + 150 μFx250 V type K50-12 were used; 150 + 30 uFx350 V. They are now the most affordable and quite reliable. A capacitance of 660 uF is enough to start an electric motor with a power of 4 kW, 3000 rpm.
References:
- Bearded Yu. Inclusion of a three-phase motor in a single-phase network, facilitating start-up / Electrician. - 2000. - No. 8. - S. 15.
- Bearded Yu. On the inclusion of a three-phase motor in a single-phase network, facilitating the start / Electrician. - 2002. - No. 4. - S. 13.
Author: Yu.A. Sytnik
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