How to determine the size of standby generator?

Question

I have 5 elevators in one building, 4 units are identical with motor 15kW, and one unit is 11kW. The power supply is 3-phase, 380VAC, 60Hz. I want to know how to calculate the size or the capacity of the standby generator if:-
Case 1: All 5 units elevator should be operative by standby generator when power outages
Case 2: Only one elevator to be operative during power outage driven by standby generator.

Answer 1

Generator sizing becomes more difficult when electric motors are involved, due to their large starting demand. Typically, there are two motor styles: CODE G requires 3 times running watts to start, and CODE L requires 5 to 6 times running watts to start.

It is not practical to think that all loads will start at the same time. Nor is it practical to size a generator using all starting watts for all selected motors, as some motor loads will already be running (or OFF), when others are ready to start. As you can see, it can be difficult to exactly size a generator for a group of specific loads, as they are constantly changing in their "ON" or "OFF" cycle. Always try to limit your load selections to the bare essential items.

TO FIND:
1 PHASE KW = V * A * PF / 1000
3 PHASE KW = V * A * 1.73 * 0.8 / 1000
1 PHASE KVA = V * A * 1000
3 PHASE KVA = V * A * 1.73 / 1000
MOTOR - REQD 1 PHASE KW = ( MOTOR HP) * (EFF)
MOTOR - REQD 3 PHASE KW = (HP) * (0.746) * (EFF)
MOTOR - REQD 1 PHASE KVA = (MOTOR HP) * (EFF) * (0.8)
MOTOR - REQD 3 PHASE KVA = (HP) * (0.746) * (EFF) * (0.8)
1 PHASE PF = 3 PHASE PF = KW / KVA
1 PHASE AMPS = (KW * 1000) / (VOLTS * PF) = KVA * 1000 / VOLTS
3 PHASE AMPS = (KW * 1000) / (1.73 * VOLTS * PF) = (KVA * 1000) / (1.73 * VOLTS)

REQUIRED PRIME MOVER (ENGINE) HORSE POWER
KW / (GENERATOR EFF. X .746)

FREQUENCY (HERTZ)
NUMBER OF POLES X R.P.M / 120

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EXAMPLES IN GENERATOR SIZING FOR LARGE ELECTRIC MOTORS

All AC electric motors require large amounts of electric power to start up, due to it's starting winding. After approximately 3/4 - 1-1/2 seconds, the starting winding drops out, and the running winding continues, at a much lower power demand. A generator must be sized to handle the load of the starting winding.

Sizing a generator for three phase motor starting LRA :
The formula is: Motor HP x KVA per HP x (1000 / (Volts x 1.73)) = LRA.
Given the size of your machines, estimate KVA/HP at approximately 6.
Therefore, for 11 kW ... LRA = 134.6 = 135 amps. Generator must produce this current at 380 V, so (1.73 * V * A * 0.8 / 1000) = KW = 71.
Similarly, for 15 kW .... LRA = 184 A and KW = 97.

BOTH SINGLE PHASE AND THREE PHASE MOTOR SIZING EXAMPLES ARE BASED ON INFORMATION IN NATIONAL ELECTRICAL CODE HANDBOOK, ARTICLE 430.7 AND IS INTENDED FOR MAXIMUM VOLTAGE DIP OF 25% UPON INITIAL MOTOR START-UP. IF 35% VOLTAGE DIP IS ALLOWABLE, REDUCE LRA, THEREFORE KW SIZE, BY 25% (LRA or KW X .75). IT IS NOT RECOMMENDED TO CHOOSE ANY GENERATOR KW SIZING THAT YIELDS MORE THAN 35% VOLTAGE DIP.

Starting more than one motor, at one time: make a list of all motors, totaling all running watts. Calculate the starting demand when the largest motor is started while all others are running. Add the largest motor starting watts, to the running watts of all smaller motors already running.

Answer 2

An electric motor draws more at startup than running under load. Your 15k motors will probably draw 20k then. Simplify things and consider the 11k as a 15k. I would use a 20k generator to run one and a 100k. to run all of them. Then add in capacity for lights et. and a safety buffer and get a 120kw to reliably do everything. They will need to be capable of 440 volts and 3 phase.

Answer 3

The 5 sewing machines are going to be a draw, and a heavy one depending on what fabric or action is being done. Squirrel fans are best. The ballast on the lights is going to have to be good.