Basic Motor Formulas And Calculations

The formulas and calculations which appear below should be used for estimating purposes only. It is the responsibility of the customer to specify the required motor Hp, Torque, and accelerating time for his application. The salesman may wish to check the customers specified values with the formulas in this section, however, if there is serious doubt concerning the customers application or if the customer requires guaranteed motor/application performance, the Product Department Customer Service group should be contacted.
Rules Of Thumb (Approximation) motor formulas motor formulas motor formulas motor formulas motor formulas
At 1800 rpm, a motor develops a 3 lb.ft. per hp
At 1200 rpm, a motor develops a 4.5 lb.ft. per hp
At 575 volts, a 3-phase motor draws 1 amp per hp
At 460 volts, a 3-phase motor draws 1.25 amp per hp
At 230 volts a 3-phase motor draws 2.5 amp per hp
At 230 volts, a single-phase motor draws 5 amp per hp
At 115 volts, a single-phase motor draws 10 amp per hp
Mechanical Formulas motor formulas motor formulas
Torque in lb.ft. = HP x 5250 rpm HP = Torque x rpm 5250 rpm = 120 x Frequency No. of Poles
Temperature Conversion
Deg C = (Deg F - 32) x 5/9
Deg F = (Deg C x 9/5) + 32
High Inertia Loads
t = WK2 x rpm 308 x T av. WK2 = inertia in lb.ft.2t = accelerating time in sec.T = Av. accelerating torque lb.ft..
T = WK2 x rpm 308 x t
inertia reflected to motor = Load Inertia Load rpm Motor rpm 2
Synchronous Speed, Frequency And Number Of Poles Of AC Motors
ns = 120 x f P f = P x ns 120 P = 120 x f ns
Relation Between Horsepower, Torque, And Speed
HP = T x n 5250 T = 5250 HP n n = 5250 HP T
Motor Slip
% Slip = ns - n ns x 100

Code KVA/HP Code KVA/HP Code KVA/HP Code KVA/HP
A 0-3.14 F 5.0 -5.59 L 9.0-9.99 S 16.0-17.99
B 3.15-3.54 G 5.6 -6.29 M 10.0-11.19 T 18.0-19.99
C 3.55-3.99 H 6.3 -7.09 N 11.2-12.49 U 20.0-22.39
D 4.0 -4.49 I 7.1 -7.99 P 12.5-13.99 V 22.4 & Up
E 4.5 -4.99 K 8.0 -8.99 R 14.0-15.99
Symbols
I = current in amperes
E = voltage in volts
KW = power in kilowatts
KVA = apparent power in kilo-volt-amperes
HP = output power in horsepower
n = motor speed in revolutions per minute (RPM)
ns = synchronous speed in revolutions per minute (RPM)
P = number of poles
f = frequency in cycles per second (CPS)
T = torque in pound-feet
EFF = efficiency as a decimal