Motor Slip
| % Slip = | (ns - n)/ns | x 100 |
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 |
Electrical Formulas
To Find
Alternating 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
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)
6356 x Mechanical Efficiency of Fan
Or
HP =
Volume (cfm) x Pressure (lb. Per sq. ft.)
3300 x Mechanical Efficiency of Fan
Or
HP =
Volume (cfm) x Pressure (lb. Per sq. in.)
229 x Mechanical Efficiency of Fan
Accelerating Torque
ACCELERATING TORQUE =
WK2N (in lb.ft.)
308t
Where:
N
=
Change in RPM
W
=
Weight in Lbs.
K
=
Radius of gyration
t
=
Time of acceleration (secs.)
WK2
=
Equivalent Inertia
308
=
Constant of proportionality
| To Find | Alternating Current | |
| Single-Phase | Three-Phase | |
| Amperes when horsepower is known | HP x 746 | HP x 746 |
| Amperes when kilowatts are known | Kw x 1000 | Kw x 1000 |
| Amperes when kva are known | Kva x 1000 | Kva x 1000 |
| Kilowatts | I x E x pf | 1.73 x I x E x pf |
| Kva | I x E | 1.73 x I x E |
| Horsepower = (Output) | I x E x Eff x pf | 1.73 x I x E x Eff x pff |
I = Amperes; E = Volts; Eff = Efficiency; pf = Power Factor; Kva = Kilovolt-amperes; Kw = Kilowatts
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)
6356 x Mechanical Efficiency of Fan
Or
HP =
Volume (cfm) x Pressure (lb. Per sq. ft.)
3300 x Mechanical Efficiency of Fan
Or
HP =
Volume (cfm) x Pressure (lb. Per sq. in.)
229 x Mechanical Efficiency of Fan
Accelerating Torque
ACCELERATING TORQUE =
WK2N (in lb.ft.)
308t
Where:
N
=
Change in RPM
W
=
Weight in Lbs.
K
=
Radius of gyration
t
=
Time of acceleration (secs.)
WK2
=
Equivalent Inertia
308
=
Constant of proportionality
radius x 2 x rpm x lb. or 2 TM
| HP = | radius x 2 | = | TN |
HP =
W x S
33,000 x E
| 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. |
| HP = | Volume (cfm) x Head (inches of water) |
| HP = | Volume (cfm) x Pressure (lb. Per sq. ft.) |
| HP = | Volume (cfm) x Pressure (lb. Per sq. in.) |
| ACCELERATING TORQUE = | WK2N (in lb.ft.) |
Where:
| |
|
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