Precautions to yield high efficiencies in Inverter duty motors during the life cycle

CII’s 10th Energy Efficiency Summit , Hyderabad 27th Aug. 2011

A.V.Bhalerao, Chief Technology Officer – Motors & Drives

National Standard for Energy Efficient Motors

§ IS 12615: 2004 (First Revision)Energy Efficient Induction Motors — Three Phase Squirrel Cage

§ IS 12615 covers Energy Efficient motors from 0.37kW to 160kW (up to Fr. 315L)

§ IS 12615 specifies two efficiency levels : - eff2 and eff1

§ IS 12615: 2011 (Second Revision) ( Under printing now)The new revision is in line with IEC 60034-30 ““Efficiency classes of single speed 3phase cage induction motors” .

Defines efficiency classes IE1, IE2 and IE3.

Range covered now upto 375 kW

Testing as per IEC 60034-2-1

IE1 and IE2 : Slightly lower than eff2 and eff1 (to compensate for new testing method in which stray losses are higher)

Comparison of eff & IE classes

Output eff2 IE1 eff1 IE2 IE3 IE4

5.5kW4P 85.7 84.7 89.2 88.2 89.9 91.3

11kW4P

88.4 87.6 91.0 90.2 91.7 92.9

Why is the Energy Efficient motor more expensive?

§ eff1 motors are generally priced 5 - 15 % higher than eff2 motors

§ More laminations - longer core length and/or

§ High Grade (low loss) laminations

§ More Copper

Rs

Energy Efficient Motors- Speed and Starting current

§ Due to reduced rotor losses, speed is higher.(5-15rpm)

§ The driven equipment ( fan / pump ) should be designed accordingly

§ Due to reduced rotor and stator resistance, starting current is higher. Thiscan go to 700 % in eff1 motors.

§ When used with a VVVF drive, this does not matter as the drive takes care.§ In other cases, this should be taken care during system design.

Bharat Bijlee Range of eff1 & eff2 motors.

§ eff2 and eff1 motors are offered by Bharat Bijlee ex-stock

§ Bharat Bijlee also offers a complete range of eff1 customised motors orders with a

lead time of 8 weeks

§ Bharat Bijlee does not manufacture motors of efficiency class less than eff2

Energy Efficient Motors for Hazardous Areas

BBL has standard catalogued energy efficient series motors for use in hazardous areas

§ Type MD – Flameproof Ex(d) eff2§ Type MJ – Flameproof Ex(d) eff1§ Type ME – Increased Safety Ex(e) eff2§ Type MI – Increased Safety Ex(e) eff1§ Type MN – Non Sparking Ex(n) eff2§ Type MS – Non Sparking Ex(n) eff1

Growth in Eff1 motors business

% Eff1 to stdYear kW Value

2007 - 08 4.6 4.32008 - 09 14 12.52009 -10 21 182010 -11 25.3 21

§ Availability of the complete range of Energy Efficient motors , popular ratings

available Ex stock.

§ Customer awareness and interest.

Advantages of VFD fed motors

Speed Control§ Good & precise speed control in required speed range§ Adjust the speed of the motor and keep it constant under variable

system parameters

Limiting starting current

§ Controls starting current of motor without affecting the torques of the motor & reduces starting losses

Power Saving§ Less power consumed for fans/ pump applications by operating at

low speeds instead of throttling / damper use.

Adverse effects of VFD supply

Additional heat generation

§ Since the waveform is not a sine wave and contains harmonics, additional heat is generated in the windings

High stresses on insulation

§ PWM, IGBT devices operate at very high frequencies (2kHz to 15kHz)and have very short rise times leading to high dv/dt. This stresses the insulation, especially the first coil

Shaft induced voltages and bearing failures

§ High frequencies of the stator induce shaft voltages in the rotor . This can cause current through the bearing if the voltage is high ( > 500 mV)

§ This current passes through bearings and causes pitting and subsequent early failure of the bearings

Increase in noise level

§ Motor noise levels are slightly higher due to the high frequencies and rapid voltage changes

• Stress on Phase insulation:– Higher peak voltages between phases ( V ph to ph) due to reflection of

the voltage at the motor terminals. These can be up to twice the DC link

voltage, depending on the cable length and rise time.

• Stress on slot insulation:– Higher peak voltages between the winding and the stator laminated

core( V ph to earth).

• Stress on the turn insulation :– Considerably higher electrical stress between the turns ( V turns) due to

the rise time and steep voltage change at the entry coils ( which are connected to the motor terminals).

Types of additional voltage stresses

VoltagesV ph tp ph

V ph to ph upto 2x due to long cable

Effects of rise time and voltages on the stresses in the insulation• Evaluation of suitability of the motor insulation for VFD operation:

– Determine phase to phase voltage at motor terminals. – Determine the rise time ( from VFD manufacturer) .

• Short rise time : ( 0.01 to 0.5•s ):– The sharp voltage edge causes over proportionate voltage drop in the

first coil sections of the winding. The maximum voltage value is based on the winding design and the turn insulation.

• Longer rise time : (more than 0.5•s ):– The maximum voltage value is constant and does not depend on the

rise time. The limit is based on the phase and slot insulation.

Limiting values

Permissible Voltages

• Graph showing permissible voltage vrs rise time for BBL motorsPeak Phase to phase voltage = 2 x 1.35 x 1.1 x Vn( 2 for voltage doubling due to long cables, 1.35 for DC conversion and 1.1 for voltage variation)

Determine the Rise time and voltage at motor terminals.• This should be obtained from the VFD manufacturer along with the

estimated Peak phase to phase voltage which will appear at the motor terminals. (This also depends on the cable length.)

Order motor with suitable insulation scheme to match VFD.

• Inform the above details to BBL while ordering the motors. We will select Insulation scheme I,II or III from the graph based on rise time and peak voltage.

In case motor is already available and voltages are higher thanpermissible.

• Use special circuit components between the VFD and the motor. eg.– Sine filters– Smoothing reactors– dv/dt .filter circuits

Precautions to be taken by Users

BBL Forced cooled motor

§ Separate motor drives the cooling fan.

§ Fan cowl is modified to mount separate fan motor

§ At all speeds of main motor, fan motor runs at fixed speed and gives constant cooling

§ No de-ration for main motor is required. It is offered in standard frame size

§ Encoder mounting arrangement is optional

§ Power source of fan motor is separate so interlocking of main motor and fan motor contactors is necessary.

BBL Forced cooled motor

A. Classical mechanism of bearing currents.

These are caused by asymmetries in the magnetic circuit of the motor due to,1. Manufacturing tolerances.2. The size of the machine.3. Design parameters eg. N1, N24. Number of poles.5. Material anistropies.

• These assymetries cause alternating flux around the shaft which induces a voltage in the shaft.

• This can be measured between shaft ends.• Problem only in larger machines. ( Voltage >= 500mV)

Types of bearing currents.

Paths of bearing currents.

B. Currents due to VFD's

1. The Circular current.2. The rotor shaft current.3. The EDM current.( Electrostatic Discharge Machining)

Bearing currents in VFD's are a problem of the whole drive configuration and not only the motor.

High frequency components ( 10 kHz up to several 100kHz) are important.

Types of bearing currents

The following are currently available bearing current reduction methods:

• Improve the high-frequency grounding connections at each equipment, right from the transformer to the motor including the drive and the driven equipment. Use high frequency earthing conductorsmade of finely stranded braided copper wires with a large cross section.

• Install one insulated bearing on the non drive end of the motor.

• Install Sine filters / dv/dt filters /smoothing reactors between drive and motor.

• Install two insulated bearings on the motor.

• Install a shaft grounding brush across one motor bearing.

Latest IEC draft standard : IEC 60034-25 : A.C. Motors when used in power drive systems – Application guide

Important clauses:4.3 : Torque speed4.3.3 : Limiting factors for torque speed4.3.5 : Cooling4.4 : Motor requirements5.1 : Increase in losses, Fig 37.3 : Winding electrical stresses, Fig 137.5 : Methods to reduce voltage stresses.8 : Bearing currents8.6 : Preventing bearing damage, Table 59.1.2 : Earthing

Optional Accessories for VFD motors

§ Temperature sensing devices like thermistors, thermostats etc.

§ Encoder mounting arrangement

§ Forced cooling.

§ Insulated bearing on NDE side of the motor.

§ Special insulation if system voltage is higher than 500V.

• When an electric motor has been properly installed, It requires little attention later on to keep it working properly. If the motor is kept clean and dry, and properly lubricated periodically, it will give trouble free service for a long time.

• The operation of cleaning, lubrication and inspection at regular intervals should be carried out according to a schedule, and proper records should be kept of what has been done on each occasion.

• Motors should be cleaned by blowing air at regular intervals to keep their ventilating passages clear, particularly when operating in dusty atmospheres.

Maintenance ( Ref IS 900)

• Terminals and screw connections shall be kept clean and tight. If they become dirty or corroded, they should be disconnected and all contact surfaces made clean and smooth. Bad contact leads to sparking and ultimate breakdown.

• Only the required quantity of grease should be put into the bearings as too much grease will also cause overheating in the same ways as too little grease.

Precautions to be taken during maintenance ( Ref IS 900)

Determine the location of the failure. If it is in the entry coil , then high dV/dt could be the problem

Never burn the winding for easy removal.

Use dual coat winding wire.

Increase slot and phase to phase insulation if possible.

Use VPI impregnation with solventless resin.

Recommend insulated bearing on NDE if bearing has failed due to bearing currents.

If complete overheating is observed go for forced cooling arrangement.

Precautions to be taken by Rewinders

Summary and Conclusions

§ There exists a national standard IS 12615: 2004

(Revision 1) for energy efficient three phase induction motors from 0.37kW to 160 kW.

(Revision 2) will bring it in line with IEC stds and extends range to 375 kW

§ Payback period of eff1 motor over eff2 motor is approx 2500 working hrs.

§ EE motors when used with VFD’s give much more savings.

§ Proper selection of EE motors to be used with VFD is a must w.r.t. insulation and bearings

§ Installation methods affect the life of the motor when used with VFD’s.

§ Maintenance of running motors is very important

§ Extra care should be taken when rewinding a EE motor

§ Use of Energy efficient motors is on the rise.

Thank You