ESSENTIAL FORMULAS FOR MOTOR ENGINEERING

Essential Formulas for Motor Engineering

Motor Engineering is a vital part of the Engineering sector. There is no industry where a motor is not used; from utility to machinery keeping it running is mandatory. 

Due to the use of huge motors, a large portion of everyday work time also consumed this fellow; and need to face new and critical problems every time.

Yes, friends, to solve problems sometimes a quick and short-cut formula is very effective. That is the main reason today's presentation is on a very easy and basic formula related to motors which we call an Essential Formula for Motor engineering.

We know, You who are engaged with motor engineering know these formulas very well, but sometimes memory is lost. Hope this will help like a friend.

Ohm's Law in Ectricity

Ohm's Law is the very basic formula for electrical engineering. Everybody knows this formula, here all of this formula is in a circle for easy understanding.

Three Phase Values

Just memories the value 1.732 which comes from the square root of 3; multiply with this value to get three values from a single phase. Follow the following table for some common values.

For 208 volts x 1.732	use 360
For 230 volts x 1.732	use 398
For 240 volts x 1.732	use 416
For 415 volts x 1.732	use 719
For 440 volts x 1.732	use 762
For 460 volts x 1.732	use 797
For 480 volts x 1.732	use 831

  

AC/DC Formulas

Horse Power, Kilowatts, KVA related some formula for AC-Alternating Current and DC- Direct Current when one term is known and other terms need to fiend out.

To Find	Direct Current	AC 1 phase	AC 3 phase
Amps when Horsepower is Known	HP x 746 E x Eff	HP x 746 E x Eff x PF	HP x 746 1.73 x E x Eff x PF
Amps when Kilowatts is known	kW x 1000 E	kW x 1000 E x PF	kW x 1000 1.73 x E x PF
Amps when kVA is known		kVA x 1000 E	kVA x 1000 1.73 x E
Kilowatts	I x E 1000	I x E x PF 1000	I x E x 1.73 x PF 1000
Kilovolt-Amps		I x E 1000	I x E x 1.73 1000
Horsepower (output)	I x E x Eff 746	I x E x Eff x PF 746	I x E x Eff x 1.73 x PF 746

AC Efficiency and Power Factor Formulas

We need some formula to fiend out Motor Efficiency and Power factor for single phase or three phase motors. Follow the following table formula.

To Find	Single Phase	Three Phase
Efficiency	746 x HP E x I x PF	746 x HP E x I x PF x 1.732
Power Factor	Input Watts V x A	Input Watts E x I x PF x 1.732

Motor Application Formulas

There are four formulas related to motor horsepower, kilowatts, torque in pound-foot, or newton-meter. These formulae we are calling the Motor Application Formula. 

Horsepower =   Torque (lb-ft) x RPM                              5252	Kilowatts =           Torque (N-m) x RPM                                     9550
Torque (lb-ft) =    Horsepower x 5252                                  RPM	Torque (N-m) =          Kilowatts x 9550                              RPM

IEC-Style Contactors and Starters Selection Guide: 

Average Amperage for Given Horsepower Motor Ratings

The table below provides the average full-load currents of squirrel cage motors based on NEC (National Electrical Code) Tables 430-148, 149, and 150. 

These values are given only as a guide - they may vary depending on the type of motor and manufacturer. Refer to the actual motor nameplate for full-load current values.

HP	110 to 120 VAC			220 to 240 VAC			440 to 480 VAC
	Single Phase	Two Phase	Three Phase	Single Phase	Two Phase	Three Phase	Single Phase	Two Phase	Three Phase
1/10	3.0	-	-	1.5	-	-	-	-	-
1/8	3.8	-	-	1.9	-	-	-	-	-
1/6	4.4	-	-	2.2	-	-	-	-	-
1/4	5.8	-	-	2.9	-	-	-	-	-
1/3	7.2	-	-	3.6	-	-	-	-	-
1/2	9.8	4.0	4.4	4.9	2.0	2.2	2.5	1.0	1.1
3/4	13.8	4.8	6.4	6.9	2.4	3.2	3.5	1.2	1.6
1	16.0	6.4	8.4	8.0	3.2	4.2	4.0	1.6	2.1
1 1/2	20.0	9.0	12.0	10.0	4.5	6.0	5.0	2.3	3.0
2	24.0	11.8	13.6	12.0	5.9	6.8	6.0	3.0	3.4
3	34.0	16.6	19.2	17.0	8.3	9.6	8.5	4.2	4.8
5	56.0	26.4	30.4	28.0	13.2	15.2	14.0	6.6	7.6
7 1/2	80.0	38.0	44.0	40.0	19.0	22.0	21.0	9.0	11.0
10	100.0	48.0	56.0	50.0	24.0	28.0	26.0	12.0	14.0
15	135.0	72.0	84.0	68.0	36.0	42.0	34.0	18.0	21.0
20	-	94.0	108.0	88.0	47.0	54.0	44.0	23.0	27.0
25	-	118.0	136.0	110.0	59.0	68.0	55.0	29.0	34.0
30	-	138.0	160.0	136.0	69.0	80.0	68.0	35.0	40.0
40	-	180.0	208.0	176.0	90.0	104.0	88.0	45.0	52.0
50	-	226.0	260.0	216.0	113.0	130.0	108.0	56.0	65.0
60	-	-	-	-	133.0	154.0	-	67.0	77.0
75	-	-	-	-	166.0	192.0	-	83.0	96.0
100		-			218.0	248.0	-	109.0	124.0
125	-	-	-	-	-	312.0	-	135.0	156.0
150	-	-	-	-	-	360.0	-	156.0	180.0
200	-	-	-	-	-	480.0	-	208.0	240.0
250	-	-	-	-	-	602.0	-	-	302.0
300	-	-	-	-	-	-	-	-	361.0
350	-	-	-	-	-	-	-	-	414.0
400	-	-	-	-	-	-	-	-	477.0
500	-	-	-	-	-	-	-	-	590.0