how to assess your motor

1. MOTOR ASSESSMENT

2. Motor Specification Age?
Rewinding?

3. LOAD ESTIMATION

4. Example:
Motor load profile – rubber mixing
Robin Kent, in Energy Management in
Plastics Processing (Third Edition), 2018
Jiri George Drobny, in Handbook of
Thermoplastic Elastomers (Second Edition),
2014

5. Load Estimation
■ Input power measurement
■ Line current and voltage measurement
■ Slip measurement

6. Load estimation
by input power measurement

7. Load estimation
by line current measurement
• Not recommended for load
below 50%
• Not taking PF into account

8. Load estimation
by slip measurement

9. RECOMMENDATION

10. Recommendation
■ Install variable speed drive
– Reduce power requirement by reducing speed at reduced load
– Increase power factor close to 1
■ Install soft starter
– Lower inrush current
– Control start-up torque

11. EFFICIENCY
MEASUREMENT

12. Efficiency definition

13. Motor losses
Motor losses
Fixed losses
(independent
of load)
Magnetic core
losses
(dependent on
voltage)
Friction and
windage losses
Variable losses
(dependent on
load)
Stator
resistance
losses (I²R)
Rotor
resistance
losses (I²R)
Miscellaneous
stray losses

14. Determine efficiency of motor
 no-load test to measure fixed loss
■ No-load test = running motor without load (shaft disconnected from load) using
rated voltage (V) and rated frequency (f)
– Measure no-load current (INL)
– Measure no-load power (PNL)
■ No-load power is fixed loss plus stator resistance loss
■ Stator resistance loss at no load: 𝑃𝑆,𝑁𝐿 = (𝐼𝑁𝐿)² ∙ 𝑅𝑆,𝑐𝑜𝑟𝑟
– Stator resistance: measured by disconnecting the AC connection and connect
a DC voltage between two phases, corrected at T = 120°C (assumed
temperature at normal operation)
𝑅𝑆,𝑐𝑜𝑟𝑟 = 𝑅𝑆,𝑎𝑚𝑏𝑖𝑒𝑛𝑡
235+120
235+ 𝑇𝑎𝑚𝑏𝑖𝑒𝑛𝑡
■ Fixed loss = No-load power – Stator resistance loss

15. Determine efficiency by full load
■ Poutput = Pfix.loss + PS.loss + PR.input + Pstray.loss
■ Stray losses: Table
■ Fixed loss is measured by no-load test
■ Stator resistance loss is calculated by this equation:
𝑃𝑆 = (𝐼)² ∙ 𝑅𝑠, 𝑐𝑜𝑟𝑟
■ Rotor power input is calculated by this equation:
𝑆𝑙𝑖𝑝𝐹𝐿 = 𝑆𝑠𝑦𝑛𝑐ℎ𝑟𝑜𝑛𝑜𝑢𝑠 − 𝑆𝐹𝐿
𝑃𝑅,𝑖𝑛𝑝𝑢𝑡 = 𝑃𝑜𝑢𝑡
(1−𝑠)
Motor
rating (HP)
Motor
rating (kW)
Stray
losses
1 – 125 1 – 92 1.8%
125 – 500 93 – 370 1.5%
501 – 2499 371 – 1840 1.2%
2500 and
above
Above 1840 0.9%

16. Full load efficiency
■ Motor input = 𝑃𝑅,𝑖𝑛𝑝𝑢𝑡 + 𝑃𝑓𝑖𝑥 + 𝑃𝑠 + 𝑃𝑠𝑡𝑟𝑎𝑦
■ Efficiency =
𝑀𝑜𝑡𝑜𝑟 𝑜𝑢𝑡𝑝𝑢𝑡
𝑀𝑜𝑡𝑜𝑟 𝑖𝑛𝑝𝑢𝑡

17. Reference
■ Continuous Energy Improvement in Motor-Driven Systems, A Guidebook for
Industry. US Department of Energy, 2014.
(https://www.energy.gov/sites/prod/files/2014/04/f15/amo_motors_guidebo
ok_web.pdf)
■ Energy Efficiency in Electrical Utilities. India Bureau of Energy Efficiency.
(https://drive.google.com/file/d/1PEhQ_ZMAzDVhnBdUWdrak5O88S29Mjbi/vi
ew)