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Energy Efficient (Hight Efficiency) Motors-What all can go wrong?

After being made conscious of energy bills/energy conservation, you wish to buy an energy efficient motor to replace an existing motor. What all can go wrong and make you feel that the energy efficient motor that you bought did not give you the savings you expected out of it.

Listed below are a few clauses present in specifications asking for Energy Efficient/High Efficiency Motors that can adversely affect the sole purpose of going in for an energy efficient motor.

Starting Current Restriction

The major chunk of a motor's losses (more than 78%) comes from copper losses (in stator and rotor). To reduce these losses the resistance in the stator and rotor needs to be reduced.

Reduced resistance results in the net impedance of the motor reducing, which causes an increased inrush (starting) current when the same rated voltage is impressed across motor terminals. Therefore all the premium/high efficiency motors worldwide have a high starting current. We end up imposing a restriction on the starting currnet, which has the following adverse effect:

  • A motor with a lower rotor resistance has a lower starting torque by default. Further imposing a restriction on starting current causes the starting torque to go down further. To ensure suitability of motors to start against the driven equipment, a higher rated motor needs to be selected (to ensure sufficient torque availability) and this causes the motor being under utilised. Efficiency will be adversely affected.
  • A motor with a lower rotor resistance has a higher rated speed. When such a motor replaces an earlier motor (which had a lower rated speed), it drives the driven equipment at a higher speed. this, in case of centrifugal pumps or blowers causes the load demand to increase in proportion to the cube of the speed (characteristics of pumps and blowers). Also the flow increases. But due to increased load demand, the input to the motor increases, which is interpreted as the motor not showing any saving. If we were to measure the increased flow, we would be able to see the saving.

Reduced Voltage Starting Suitability

As already indicated above, a premium/high efficiency motor has lower starting torque because of lower rotor resistance. When we impose reduced voltage starting suitability condition, to ensure that the motor does develop sufficient torque to overcome the load, a higher rated motor is selected. This results in the motor being under utilised during service and this under-utilization affects the efficiency adversely.

Wide Supply Variation

The clause 4.3 of IS:12615-2004 explicitly specifies the following:

When motors are operated at extreme voltage limits, temperature rise can be 10K higher for motors rated less than or equal to 1000kw.

Motors when operated at extreme conditions of voltage and frequency, may not necessarily have their performance in accordance with this standard. Even is (or for that matter any other standard) does make provisions for the fact that efficiency cannot remain at its peak with motor running at supply conditions other than rated.

All that is meant by suitability of a motor for supply variation is that,

When Operating with terminal voltage differing from the rated value by not more than ±% specified, OR,

When operating with frequency differing from the rated value by not more that ±% specified, OR,

combination of V&F variation % specified,
the motor should be able to deliver the rated ouput, without injurious heating, at the specified design ambient.

The following chart also shows the effect of voltage variation:

Therefore do not assume that a motor suitable for a 415V?10%, 50Hz?5% will give the same efficiency irrespective of the voltage and frequency within the variation range.

 

Motor Efficiency & Health Check

Pre-Audit information- Furnished by MDS Consultants, Chennai
"Motor Efficiency and Health Check" Audit on LT induction motors can be carried out for EE motor replacement programme or for reliability centered maintenance.

Any client introducing policies for first-time-purchase or Replacement-purchase should be conversant with efficiency values given by motor manufacturers at various load points.

Since induction motors have poor efficiency below 50% load Factor, lightly loaded motors should be relocated under "Sizing" exercise, after careful understanding of the motor dimensioning for high inertia loads.

Field Audit on motors is a support programme to assist clients interested in Replacing motors to gain on energy efficiency or eliminating unreliable motors (failed many times). The audit is conducted using advanced instruments and methods duplicating a test bench.

This programme can be used for assessing the efficiency value at the operating load point, operating load factor, specific energy for the process to implement “Energy Efficient” and “Reliable Maintenance” of the motor driven system .