**Drive and Motor Sizing Made Easy**

Size your drive and motor in three easy steps:

- Determine the application requirements
- Size the motor to meet the application
- Size the drive to meet the motor and the application

**Determine the Application Requirements**

What are the torque requirements?

- Motor torque (not power) is usually the decisive factor
- Torque requirement establishes current requirement

- Continuous torque requirements
- Variable torque vs. constant torque

- Intermittent (peak) torque requirements
- Starting torque
- Acceleration torque

- Maximum speed
- Minimum speed

**Torque, what is it?**

A measure of the effect of a force applied at a distance to an axis.

- Torque is a force that tends to rotate or turn things
- Torque(lb-ft) = Force (lb) x Radius (ft)

**Variable Torque**

Variable torque changes as the operating speed changes.

- Fans
- Centrifugal pumps
- Centrifugal blowers
- Mixers (material dependent)

**Constant Torque**

Constant torque remains the same as the speed changes.

- Conveyers
- Positive displacement pumps
- Extruders
- Crushers
- Mixers (material dependent)
- Rotary kilns
- Hoists
- Elevators

**Constant vs. Variable Torque**

Why should I care? Isn’t all torque the same?

Yes. But ….

- Motor current is proportional to torque
- Motor heating is proportional to current
- In the case of a TEFC motor, cooling is proportional to speed

Result — a TEFC motor's ability to thermally handle torque varies with speed

A constant torque load often requires a larger TEFC motor than that required for an equivalent variable torque load.

- Alternative is a separately driven fan, TEBC motor

**Intermittent Torque**

Intermittent torque is torque that is required for a relatively short period of time. Examples:

- Torque to breakaway the load and start motion
- Friction

- Torque to accelerate the load
- Inertia

**Speed**

With direct mechanical drive, motor speed is determined by mechanical speed and physical dimensions.

Speed (RPM) = v (ft/min) / (r (ft) x 2 x pi)

**Speed Example**

Speed (RPM) = v(ft/min) / (r(ft) x 2 x pi)

Speed (RPM) = 750(ft/min) / (1 (ft) x 2 x pi) = 119 (RPM)

**Power**

Power is the product of torque times speed.

- Power (HP) = Torque (lb-ft) x Speed (RPM) / 5252
- For our example:
- Torque = 100 lb-ft
- Speed = 119 RPM
- Power = 100 (lb-ft) x 119 (RPM) / 5252 = 2.3 HP

HP |
Base Speed |
Rated Torque |

3 | 1790 | 9 |

5 | 1790 | 15 |

7.5 | 1790 | 22 |

10 | 1790 | 29 |

15 | 1790 | 44 |

30 | 1790 | 88 |

40 | 1790 | 117 |

**Motor Sizing**

In our example:

- Torque = 100 lb-ft
- Speed = 119 RPM
- Power = 100 x 119 / 5252 = 2.3 HP

- 3 HP @ 1790 RPM?
- 40 HP @ 1790 RPM?

A motor only develops its nameplate power at its nameplate speed. At a reduced speed it develops a proportionately reduced power.

**Motor Sizing**

What if we add a gear box?

- Torque at motor = torque / gear ratio
- Speed at motor = speed x gear ratio

Now what motor do we pick?

**Intermittent Torque**

Torque for Acceleration

- Torque = Inertia x Acceleration rate
- If you know:
- Inertia (WK2) in lb-ft2
- Acceleration time in sec.
- Change in motor speed in RPM

- Then:
- Torque = WK2 (lb-ft2) x Speed(RPM) / (Accel time(sec.) x 307.6)

**Intermittent Torque**

Torque for Acceleration

- Assume for our example:
- Total WK2 = 1.2 lb-ft2
- Includes 100 lb load, drum, 15:1 gear box and motor

- Change in speed is 1790 RPM
- If accel time is 10 seconds
- Accel Torque = 1.2(lb-ft2) x 1790(RPM) / (10(sec.) x 307.6)
- Accel Torque = 0.7(lb-ft)
- Total torque = 6.7 + 0.7 = 7.4 lb-ft; less than rated motor torque

- If accel time = 1 second
- Accel Torque = 1.2(lb-ft2) x 1790(RPM) / (1(sec.) x 307.6)
- Accel Torque = 7.0 (lb-ft)
- Total torque = 6.7 + 7.0 = 13.7 lb-ft, 150% of rated motor torque

- Total WK2 = 1.2 lb-ft2

**Pick A Drive**

- Assume for our example:
- Motor is 3 HP, 1790 rpm, 4.2 FLA, 9 lb-ft
- Torque to lift load and accel in 10 s is 7.4 lb-ft
- Max current is less than 4.2 amps
- Use 3 HP normal duty drive, 4.9 amps, with 110%

O.L. (5.4 amps peak)

- Torque to lift load and accel in 1 s is 13.7 lb-ft
- Max current is about 6.4 amps
- Use 3 HP heavy duty drive, 5.6 amps, with 150%

O.L. (8.4 amps peak)

**Special Cases**

Intermittent torque is required for a relatively long time.

- Large inertias
- Results in long accel time, several minutes
- Drive and motor sized for acceleration torque
- Examples
- Centrifuges
- Kilns

- Long periods of breakaway torqu
- Mixer starting with product

**Watch the limits**

- Limits that can come in to play
- Torque
- AC Motors have max torque limits, about 200% (Drive limits motor to about 70% of motor’s rated breakdown torque)

- Speed
- Limited by maximum safe mechanical speed
- Limited by maximum drive frequency
- Limited by reduced maximum torque above base speed

(Constant HP operation)

- Current
- Limited by inverter
- Full speed motor current rises when line voltage is low

- Regenerative (braking) torque
- If less than 10% flux braking may be good enough
- If more than 10% but intermittent, such has stopping only, use brake chopper and resistor
- If more than 10% and continuous, consider a regenerative drive

- Torque