reliance - formulas & calculations.pdf

8/10/2019 Reliance - Formulas & Calculations.pdf

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12/23/2014 Reliance - Formulas & Calculations

http://www.reliance.com/mtr/flaclcmn.htm 2/5

ns= 120 x f

P f =

P x ns120

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

nsx 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

PF = power factor as a decimal

Equivalent Inertia

In mechanical systems, all rotating parts do not usually operate at the same speed. Thus,

we need to determine the "equivalent inertia" of each moving part at a particular speed


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WK2EQ= 3200 lb.ft.2

The total WK2equivalent is that WK2seen by the prime mover at its speed.

Electrical Formulas

To FindAlternating Current

Single-Phase Three-Phase

Amperes when horsepower is known HP x 746

E x Eff x pf

HP x 746

1.73 x E x Eff x pf

Amperes when kilowatts are known Kw x 1000

E x pf

Kw x 1000

1.73 x E x pf

Amperes when kva are known Kva x 1000

E

Kva x 1000

1.73 x E

Kilowatts I x E x pf

1000

1.73 x I x E x pf

1000

Kva I x E

1000

1.73 x I x E

1000

Horsepower = (Output) I x E x Eff x pf

746

1.73 x I x E x Eff x pff

746

I = Amperes E = Volts Eff = Efficiency pf = Power Factor Kva = Kilovolt-amperes

Kw = Kilowatts

Locked Rotor Current (IL) From Nameplate Data

Three Phase: IL= 577 x HP x KVA/HP

ESee: KVA/HP Chart

Single Phase: IL

=1000 x HP x KVA/HP

E

EXAMPLE:Motor nameplate indicates 10 HP, 3 Phase, 460 Volts, Code F.

IL=577 x 10 x (5.6 or 6.29)

460

IL= 70.25 or 78.9 Amperes (possible range)

Effect Of Line Voltage On Locked Rotor Current (IL) (Approx.)

IL@ ELINE= IL@ EN/Px ELINE
http://-/?-


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EN/P

EXAMPLE:Motor has a locked rotor current (inrush of 100 Amperes (IL) at the rated

nameplate voltage (EN/P) of 230 volts.

What is ILwith 245 volts (ELINE) applied to this motor?

IL@ 245 V. = 100 x 254V/230V

IL@ 245V. = 107 Amperes

Basic Horsepower Calculations

Horsepower is work done per unit of time. One HP equals 33,000 ft-lb of work per

minute. When work is done by a source of torque (T) to produce (M) rotations about an

axis, the work done is:

radius x 2 x rpm x lb. or 2 TM

When rotation is at the rate N rpm, the HP delivered is:

HP =radius x 2 x rpm x lb.

33,000 =

TN

5,250

For vertical or hoisting motion:

HP = W x S

33,000 x E

Where:

W = total weight in lbs. to be raised by motor

S = hoisting speed in feet per minute

E = overall mechanical efficiency of hoist and gearing. For purposes of estimating

E = .65 for eff. of hoist and connected gear.

For fans and blowers:

HP = Volume (cfm) x Head (inches of water)

reliance - formulas & calculations.pdf

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