Induction motors

The nominal power in kW (Pn) of a motor indicates its rated equivalent mechanical power output. The apparent power in kVA (Pa) supplied to the motor is a function of the output, the motor efficiency and the power factor. [math]\displaystyle{ Pa=\frac{Pn}{n\cos\phi} }[/math]

Current demand

The full-load current Ia supplied to the motor is given by the following formulae:

• 3-phase motor: Ia = Pn x 1,000 / (√3 x U x η x cosφ)
• 1-phase motor: Ia = Pn x 1,000 / (U x η x cosφ)

where
Ia: current demand (in amps)
Pn: nominal power (in kW)
U: voltage between phases for 3-phase motors and voltage between the terminals for single-phase motors (in volts). A single-phase motor may be connected phase-to-neutral or phase-to-phase.
η: per-unit efficiency, i.e. output kW / input kW
cosφ: power factor, i.e. kW input / kVA input

Subtransient current and protection setting

• Subtransient current peak value can be very high ; typical value is about 12 to 15 times the rms rated value Inm. Sometimes this value can reach 25 times Inm.
  
• Schneider Electric circuit-breakers, contactors and thermal relays are designed to withstand motor starts with very high subtransient current (subtransient peak value can be up to 19 times the rms rated value Inm).
  
• If unexpected tripping of the overcurrent protection occurs during starting, this means the starting current exceeds the normal limits. As a result, some maximum switchgear withstands can be reached, life time can be reduced and even some devices can be destroyed. In order to avoid such a situation, oversizing of the switchgear must be considered.
  
• Schneider Electric switchgears are designed to ensure the protection of motor starters against short-circuits. According to the risk, tables show the combination of circuit-breaker, contactor and thermal relay to obtain type 1 or type 2 coordination (see chapter N).

Motor starting current

Although high efficiency motors can be found on the market, in practice their starting currents are roughly the same as some of standard motors.
The use of start-delta starter, static soft start unit or variable speed drive allows to reduce the value of the starting current (Example : 4 Ia instead of 7.5 Ia).

Compensation of reactive-power (kvar) supplied to induction motors

It is generally advantageous for technical and financial reasons to reduce the current supplied to induction motors. This can be achieved by using capacitors without affecting the power output of the motors.
The application of this principle to the operation of induction motors is generally referred to as “power-factor improvement” or “power-factor correction”.
As discussed in chapter L, the apparent power (kVA) supplied to an induction motor can be significantly reduced by the use of shunt-connected capacitors. Reduction of input kVA means a corresponding reduction of input current (since the voltage remains constant).
Compensation of reactive-power is particularly advised for motors that operate for long periods at reduced power.

As noted above [math]\displaystyle{ \mbox{cos}\phi= \frac{\mbox{kW input}}{\mbox{kVA input}} }[/math]  so that a kVA input reduction in kVA input will increase (i.e. improve) the value of cosφ
The current supplied to the motor, after power-factor correction, is given by:[math]\displaystyle{ \mbox{I} = \mbox{Ia}\frac{\mbox{cos}\phi}{\mbox{cos}\phi^'} }[/math]
where cos φ is the power factor before compensation and cos φ^' is the power factor after compensation, Ia being the original current.

Figure A4 below shows, in function of motor rated power, standard motor current values for several voltage supplies.

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kW	hp	230V	380 - 415V	400V	440- 480 V	500V	690V
		A	A	A	A	A	A
0.18 0.25 0.37	- - -	1.0 1.5 1.9	- - -	0.6 0.85 1.1	- - -	0.48 0.68 0.88	0.35 0.49 0.64
- 0.55 -	1/2 - 3/4	- 2.6 -	1.3 - 1.8	- 1.5 -	1.1 - 1.6	-  1.2 -	- 0.87 -
- 0.75 1.1	1 - -	- 3.3 4.7	2.3 - -	- 1.9 2.7	2.1 - -	- 1.5 2.2	- 1.1 1.6
- - 1.5	1-1/2 2 -	- - 6.3	3.3 4.3 -	- - 3.6	3.0 3.4 -	- - 2.9	- - 2.1
2.2 - 3.0	- 3 -	8.5 - 11.3	- 6.1 -	4.9 - 6.5	- 4.8 -	3.9 -  5.2	2.8 - 3.8
3.7 4 5.5	- - -	- 15 20	- 9.7 -	- 8.5 11.5	- 7.6 -	- 6.8 9.2	- 4.9 6.7
- - 7.5	7-1/2 10 -	- - 27	14.0 18.0 -	- - 15.5	11.0 14.0 -	- - 12.4	- - 8.9
11 - -	- 15 20	38.0 - -	- 27.0 34.0	22.0 - -	- 21.0 27.0	17.6 - -	12.8 - -
15 18.5 -	- - 25	51 61 -	- - 44	39 35 -	- - 34	23 28 -	17 21 -
22 - -	- 30 40	72 - -	- 51 66	41 - -	- 40 52	33 - -	24 - -
30 37 -	- - 50	96 115 -	- - 83	55 66 -	- - 65	44 53 -	32 39 -
- 45 55	60 - -	- 140 169	103 - -	- 80 97	77 - -	- 64 78	- 47 57
- - 75	75 100 -	- - 230	128 165 -	- - 132	96 124 -	- - 106	- - 77
90 - 110	- 125 -	278 - 340	- 208 -	160 - 195	- 156	128 - 156	93 - 113
- 132 -	150 - 200	- 400 -	240 - 320	- 230 -	180 - 240	- 184 -	- 134 -
150 160 185	- - -	- 487 -	- - -	- 280 -	- - -	- 224 -	- 162 -
- 200 220	250 - -	- 609 -	403 - -	- 350 -	302 - -	- 280 -	- 203 -
- 250 280	300 - -	- 748 -	482 - -	- 430 -	361 - -	- 344 -	- 250 -
- - 300	350 400 -	- - -	560 636 -	- - -	414 474 -	- - -	- - -
315 - 335	- 540 -	940 - -	-  - -	540 - -	-  515 -	432 - -	313 - -
355 - 375	- 500 -	1061 - -	- 786 -	610 - -	- 590 -	488 - -	354 - -
400 425 450	- - -	1200 -  -	- - -	690 - -	- - -	552 - -	400 - -
475 500 530	- -  -	- 1478 -	- - -	- 850 -	- - -	- 680 -	- 493 -
560 600 630	- -  -	1652 - 1844	- -  -	950 - 1060	- -  -	760 - 848	551 - 615
670 710  750	- - -	- 2070 -	-  - -	- 1190 -	- -  -	- 952 -	- 690 -
800 850 900	- - -	2340 - 2640	- - -	1346 -  1518	- -  -	1076 - 1214	780 - 880
950 1000	- -	- 2910	- -	- 1673	- -	- 1339	- 970

Fig.A4: Rated operational power and currents

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