Calculation of star delta motor

Google star-delta starters and look for suppliers?

To answer your question, you must to know the voltage you use. And choose contactor we must depend on the supplier. now I'll give you a general imformation for your question.

• 200V-240V 3phase
• 30Hp= 22Kw
• Full load current of the motor 93A
• Rated current of contactor delta MCD 65A
• Overload relay 85-125A
• Rated current of contactor for starting MCY 40A
• Rated current of circuit breaker 175A
• Cross section of the cable 38mm2
• 380V-440V 3phase
• 30Hp= 22Kw
• Full load current of the motor 46.5A
• Rated current of contactor delta MCD 40A
• Overload relay 34-50A
• Rated current of contactor for starting MCY 32A
• Rated current of circuit breaker 100A
• Cross section of the cable 14mm2

Best regards

Hello haivot,

May You give me Your personal mail, please. The information about the motor control is very interesting of me and I think Your knowledge will be very helpfull for me!

My mail is d_jelev@yahoo.com

See You!

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You Must!

For her Beautiful Truth.

Very good explanation + diagram. Everything in EU standards (including symbols).

For USA Schematics are drawn a little bit different, Voltages also are different.

To give an electricians a full schematics you need also give Delta/Star (or Wye in USA) control circuit diagram with timer in (a) hard wired control system or (b) ladder logic for PLC system.

If time for start is long (the time delay to switch to Delta for continuous run), Start (Y) contactor should be rated for more than 33% of FLA.

Sometime you have to check for others aspects, e.g. the mechanical load characteristic.

this i could understand in a moment but r u sure the ol relay would be 34-50a, n if it is then where to put it , at the delta one or the main contactor. please mail me if u can Vicky2k4in@yahoo.co.in

Actually, you bring up a good point. In that diagram, the OL relay would need to be sized for 58% of the FLA, so in the example shown where she states the FLC would be 46A, the OL relay F2 should be sized to protect at a level of 26.7A, which would be too low for the OL relay range suggested.

Note: 1. Diagram shows 3 fuses (F1) but in Calculations listed are CB (Circuit Breakers) so different sybbols should be in Power Ckt -3ph supply.

2. There is no Control Circuit diagram (how K contractors coils are to be en-de-energized and there is no time delay relay for switching from Y do D. Of course if PLC is a controller, ladder diagram is necessary.

Anyway - the answer is almost perfect and with good examples.

Simpler than all that.

Each main power contactor may be rated for 58% of the motor rating, but of course, larger is OK. The shorting (star) contactor can be 33% of the motor FLA.

But really, you should abandon star-delta starting all together and use solid state starting. Star-delta is bad for the motor, load and power supply because of switching transients when you transition from star to delta. The only advantage it has is that it is cheap, but even then, only if you ignore the long term costs of the other issues.

If you are going to go with the star - delta configuration, the simple way is to go on the internet to one of the major motor starter manufacturers and you can get a starter for this purpose already designed for the horsepower you require, install it, adjust the start duration relay, check rotation and you are done.

haivot's circuit is for Wound Rotor.

Different for Sqirrel Cage.

Jraef's post is best advice. Motor and driven plant are not subject to massive shocks of Star/Delta.

It's the only arrangement I have seen for squirrel cage. I have never seen a wound-rotor diagram.

She showed a right diagram. U-V-W // X-Y-Z are EU symbols for ends /or terminals of Square Cage Induction 3-phase motor stator windings.

In the USA they would be:

T1-T2-T3 // T4-T5-T6

No, sorry, that is not a wound rotor diagram, she posted a correct star-delta IEC diagram for open transition.

You did not indicate the operating voltage nor the motor rpm. However, the information given here are for standard 1500rpm motors with normal inner and outer surface cooling, operating at 0.84pf and 92% efficiecy:

At 220v, motor full load current - 71A

At 380-400v, motor current is 41A

1. Thermal overload relay to be set at 58% of rated motor current.

2. Both line and delta contactors should be rated at not lower than 0.58 x rated motor current

3. Star contactor to be rated at 0.33 x rated motor current

4. Contactors are selected by utilization categories and as such knowing the rated current may not suffice. Whatever current rating you arrive at for any contactor, always ensure that it satisfies the utilization category of the required operation. For example,

AC-1 category is for Non-inductive or slightly inductive loads, resistance furnaces

AC-2 is Slip-ring motors: starting, switching off

AC-3 Squirrel-cage motors: starting, switching off runing motors. Contactors for this category may be used for occasional inching or plugging during a limited period such as for setting up a machine; however, during this limited time period, the number of operations must not exceed a total of five per minute or more than ten in a ten minute period.

Other categories for contactors include: AC-4, AC-5A, AC-5B, AC-6A, AC-6B, AC-7A, AC-7B, AC-8A and AC-8B.

5. In view of item (4) above, the contactors you need should be AC-3 duty rated at the desired voltage and at HP/KW that will translate to current not lower than those of 2 and 3 above respectively.

An Assignment for Electrical Engineering students:

The power circuit elements were exactly selected according to cited above 1-3 conditions.

PLC controlled their start and after a minute or so the star (Y -Wye) contactor got welded their contact. What was the reason?

If you don't mind, can you provide us with the circuit diagram. The arrangement described may not be the conventional or standard star-delta circuit, otherwise I do not see why the Wye contactor should be in circuit for that long. However, until details are provided a control logic problem could be assumed as it appears there was no transition from star to delta.

Answer could be deducted from # 6 & 2.

By the way You have made the v. good list of possible conditions /class AC-xx that could give students / practitioners something to think. Many of info-pieces are good, but there is no control circuitry diagrams. Some manufacturers (e.g. Allen Bradley -USA, Siemens -EU) show examples in their catalogs

Have you ever installed Y/Δ system for huge blowers?

Have a nice experience in CR4. It seems the initiator of this discussion already has gotten the answer!

Suitable contactors for mortor 30HP works Y/A:

• 380V-440V 3phase
• Motor 30Hp
• Full load current of the motor 46.5A
• Rated current of contactor delta MCD 40A
• Overload relay 34-50A
• Rated current of contactor MCY 32A

Regards....