E x e r c i s e nr 12

Measurement of machine tool power loss with load

and total efficiency of machine tool

Aim of classes is learning reasons of machine tool power losses in multistage power transmis-

sion system and measurement methods of power losses at work without load and total efficiency of

machine tool for example lathe.

1. Introduction

Either of machine tool diagnostic signals with many information about machine tool condition is

power loss of kinematic chain. Value of power loss depends first of all on kinematic chain length,

types and chain elements quantity. For every specimen of machine tool value of power loss depends

on assembly quality, work conditions and wear degree of chain elements. Periodic control of power

losses are necessary when machine tool is new, after repairs and during inspections. Increase of pow-

er losses in kinematical chain can be a symptom of incorrect assembly quality, abnormal lubricant

conditions and wear degree.

2. Sources of power loss in machine tools – energy balance

Watt power which is lost during work without a load is a sum of losses in kinematic chain of en-

gine drive. This power is necessary to support movement without cutting forces. During machine tool

work with load, spindle useful power is lower than electric energy consumed from the network. Pow-

er losses in electric engine of machine tool are represented by total efficiency coefficient.

Sources of the main power losses in machine tool are stages in the kinematic chain from electric

engine to spindle. These are mostly: engine, belt transmission, toothed gear, clutch, brake, bearing

and all rotate elements. In another machine tools it could be for example toggling mechanism (slot-

ting machine - slotter), feed mechanism: screw – nut, toothed wheel – toothed bar, shears etc. Be-

sides, electric power that is also consumed for supportive devices like oil pump, cooling pump, chips

conveyor, store tools, devices for automatically tool change, palette changer, workplace lighting,

drive of rate of feed, auxiliary drive (rotation of tool head), power of control devices, lightening of

service elements etc.

Sources of machine tool principal motion are usually electric engines, mainly: squirrel-cage mo-

tor, asynchronous three-phases motor or direct-current shunt motor. Old machine tool types have

usually use asynchronous motors. CNC machine tools are equipped in direct-current motors because

of possibility to easy rotation speed change. Recently, more and more asynchronous drives are used

again, but with modern supply control, that gives possibility to easy modify parameters of alternating

current (AC) – voltage and frequency.

In case of asynchronous motor power, the main sources of power losses are: induction losses in

stator and rotor winding, mechanical losses in motor bearings and drive of cooling fan. Mechanical

and induction losses are independent of load. For example, asynchronous motors power losses can be

counted:

(

)

























−

+

+

−

=

∆

2

.

.

100

1

25

75

,

0

0075

,

0

75

,

0

2

k

P

P

nom

mot

η

η

,

(1)

where:

∆P

mot.

[W] – total power losses in asynchronous motor,

P

nom.

[W] – nominal power of motor,

η

[%] – motor efficiency measured with nominal load

k – coefficient of motor load.

.

.

nom

absorb

P

P

k

=

P

absorb.

[W] – motor absorbed power

Nominal load (P

nom.

) of machine tool engine is defined as a maximum power that theoretically

can be supplied in endless time without causing any damage (overheating dangerous). Asynchronous

motor efficiency changes as coefficient „k” changes. Coefficient of motor load characteristic is

shown at figure 1. Motor maximal efficiency is achieved with k = 0,75. Efficiency goes to zero when

motor is overloaded, which results in stopping the rotor rotations. Lined section on figure 1 shows

range values of coefficient k. This value depends on type and on size of a motor.

Pic. 1. Coefficient of asynchronous motor efficiency dependence for load coefficient.

Power losses of DC shunt motor can be estimated by equation:

load

i

Fe

m

c

P

P

P

P

P

∆

+

∆

+

∆

+

∆

=

∆

,

(2)

where: -

∆

P

m

– mechanical losses,

-

∆

P

Fe

– losses in iron sheets,

-

∆

P

i

– induction losses,

-

∆

P

load

– losses with load.

Sum of mechanical losses (

∆

P

m

) and losses in iron sheets (

∆

P

Fe

) is called loss without a load.

Dielectric losses are omitted it this formula, since their value is negligibly small if comparing other

loss components. Mechanical losses result from bearings friction, commutator and bush friction, rotor

and air friction, and cooling losses.

Losses which are dependent on load are losses in stator

∆

P

t

and losses on bushes

∆

P

bu

, where:

∆

∆

,

– electric power in armature and armature resistance,

– bushes voltage drop.

Conventional machine tools usually use V-belt transmissions, while CNC machine tools are

equipped in toothed belts more often.

Losses in belt transmission are caused by bending on pulley or by belt slip.

Belt slip is unavoidable and it equals about 1-2%. If machine tool is overloaded then belt slips

more and it is cause of fast wear.

To minimize energy losses in belt transmission, the most favorable belts ratio is 1:1. Additional-

ly, a belt should not be too thick. It is better to use several thin belts then to use a thick one. Energy

losses depend also on pulleys diameter and their preload. In multi belt transmission systems, addi-

tional losses are caused by little differences between every subsequent groove.

Besides, other geometrical features of system elements have additional influence on power loss-

es, e.g. axis distance, mechanical properties of belts and assemble quality.

Example of efficiency coefficient value for most important parameters in belt transmission with

V-belt (proportion of pulley diameter to thickness of V-belt is shown in picture 2. Average value of

efficiency coefficient is represented by a dashed line. Area surrounded by dashed line shows efficien-

cy for different belt pulls power and other parameters of belt transmission.

AF 21/05/2013 – beginning

Gear transmissions can be found nearly in every complex machine drives. The efficiency of

these mechanisms is high, which means that they cause small power losses. Energy losses of these

kind transmission systems are caused mostly by tooth sliding friction, lateral surfaces deformation

and by hydrodynamic losses (caused by presence of lubricants). The value of the losses results from

material friction coefficient, surface roughness, lubricant viscosity and its distribution method, angu-

lar velocity of the wheels and their geometrical parameters, slip velocity and, generally, from the type

of the transmission system. Another important factor to mention is the quality of assembly.

The most common clutches and brakes in the principal motion drive chain are constant and re-

leasable couplings. In case of constant clutches, energy losses are caused only by friction between

clutch material and the surrounding medium. In case of releasable mechanisms, energy losses prob-

lem is much more complex and depends, in first place, of clutch state (engaged/disengaged). For ex-

ample, engaged multi-plate clutches with mechanical activation mechanism are strongly affected by

friction in activation mechanisms (thrust bearings and fork guides), when in case of disengaged

clutch (no load) energy can be lost as well due to random contact between clutch plates. The latter

can be a result of the incorrect balance of the clutch, defects in manufacturing or hydrodynamic fric-

tion (lubrication). Mechanisms that use electromagnetic activation systems instead of mechanical

ones can also differ in the context of energy loss. Some losses can be caused by losses in winding

(i.e. during coupling engaging), some by unexpected plates contact (as in the previous example). In

electromagnetic devices a residual magnetism phenomenon can occur, which can demonstrate by ex-

cess tightening clutch plates even though the voltage from the windings is removed (magnetical

histeresis). Very often, in case of such situation, outer and inner part of a clutch have to rotate in re-

verse direction, which additionally increases energy losses. Lubrication can have a significant influ-

ence on energy losses too, especially for mulit-plate clutches working with no load. One should avoid

centrifugal lubrication of a clutch – losses in this case can be even 20 times bigger then losses in a

case of glazing lubrication. Obviously, some additional losses appear during engaging/disengaging

coupling mechanism, however, these are only temporary and short-lasting and will not be taken into

consideration in our analysis.

Energy losses in rolling-element bearings are caused by rolling friction and sliding friction be-

tween rolling elements and races or by hydrodynamic friction. The latter has the most significant im-

pact in case of large bearings working with high velocities, i.e. spindle bearings.

The resultant value of losses in bearings is affected by bearing type and its construction, geometrical

dimensions, angular velocity, load, amount and viscosity of used lubricant. This relation can be ap-

proximated by the following equation:

W

,

)

(M

N

9,55

n

1

h

³

M

+

=

∆

(5)

Where:

M

h

– hydrodynamic frictional moment, in [Nm]; equals M

10,66 f (10

n)

d

h

0

6

2/3

m

3

=

⋅

⋅ ⋅

⋅

ν

,

M

1

– frictional moment, in [Nm], originating from a load; equals: M

1

=f

p

·P

0

·d

m

,

n – angular velocity in [RPM],

f

0

– coefficient depended from bearing type and lubrication,

ν

- kinematic viscosity of used oil, depends from temperature, in [m

2

/s],

d

m

– bearing diameter, in [m],

f

p

– coefficient depended from bearing capacity and applied load,

P

0

– bearing load, in [N].

Moreover, any rotating elements in general can cause energy losses in a machine tool. Value of

these losses depends on geometrical dimensions of spinning elements, their shape and velocity. Ex-

cept from drive chain elements described above, there can be more parts affecting machine efficien-

cy. One should remember that besides principal motion drive, there may be several auxiliary devices

mounted on the machine tool, which consume supplied energy as well. Obviously, they do not have

to participate in motion transfer from an engine to a tool, but can be used to perform additional activi-

ties, not related with tooling directly.

Basically, energy losses in principal motion chain depend from many factors like i.e. number and

type of chain elements, their angular velocity, precision of manufacturing, engineering fit type, as-

sembly precision, temperature, method of lubrication, etc. In case of simple kinematic chains losses

are usually increased with the increase of spinning velocity. This dependence is, however, not appli-

cable to complex systems made of many simple elements. Analysis of such systems is much more

difficult, since velocity of given parts usually differs, depending on their location in the chain.

As a result of described losses, some of supplied electrical power is diffused in the kinematic

chain and only the remaining part can be used effectively for tooling. In order to simplify the analysis

of the problem, usually more general approach to machine tools efficiency is used. It is assumed that

supplied electrical power is divided into effective power, power of losses in an engine and power of

losses in drive system. Subsequently, for purpose of analysis, losses are then divided into two groups:

power losses while working with a load and power losses while working without a load. The first

group is, obviously, independent from a load. The second group can be usually characterized by line-

ar relation between energy losses and a load. Electrical power can be therefore described by the fol-

lowing equation:

N

el

= (N

ls

+ N

ld

) + (N

l

+ N

d

) + N

ef

,

Where:

- N

ls

– power consumed by engine with no load,

- N

ld

– additional losses in engine if load is applied (mostly electrical losses),

- N

l

– power consumed to compensate forces in transmission system with no load,

- N

d

– additional losses in transmission system if load is applied (increase linearly with a load),

- N

ef

– effective power of the machine tool.

Energy losses in the elements of a drive

chain can be easily represented on a Sankey di-

agram (energy flow diagram). An example of

such a diagram for TUR-50 lathe drive (spindle

velocity = 1800 rpm) is shown in pic. 3. The

lathe’s drive consists of bipolar asynchronous

motor of nominal power = 6 or 11 kW, nine-step

reducer with reversing switch, belt drive, gear

transmission (two on three shafts) in a headstock

and a spindle. The diagram shows quantitive and proportional flow of power consumed from the

network and distributed to each part of the chain during while working without a load. Attention

should be paid especially on a spindle bearing due to its huge power consumption (almost 50%).

3. Conditions of measurement

Power loss determination with no-load and the overall efficiency of the machine are the sub-

ject of Polish standard no. PN-66/M-55606, which is still valid. This document gives a general
rules of measurement, it says that: it is required to use class 0.5 or 0.1 wattmeters, class 0.5 volt-
age transformers and class 0.5 or 0.1 voltmeters and ampremeters as well. The measurement
ranges of these instruments should be selected in the way that during measurement the pointer
deflection was in between half to full-scale range. The standard gives also the directions of prep-
aration the machine tool for measurement. The measurements should be done on the machine
which is fully assembled, placed on the solid basis, connected to the network and has enough of
lubricants in places that need to be lubricate. On the spindle, permanently mounted equipment
only should be left. In the protocol it is required to provide what kind of additional mechanisms
are driven by the engine of kinematic spindle system such as lubrication pump, cooling pump,
feed box (if it is not possible to disconnect) and others.

The measurement needs to be made in specific heat conditions, according to the normal work

environment of the machine. The ways to achieve thermal equilibrium are determined by the
machine manufacturer. It is believed that the machine has reached thermal equilibrium, when the
change of the oil temperature in the characteristic point (usually the front bearing node of the
spindle), or a change of the power consumption does not exceed 2% in the last 15 minutes of op-
eration. The standard gives also the dependences which are required to determine the moment of
the spindle load, its effective, electric power and efficiency. The document shows the sequence
of measurement, examples of the protocols from the measurement in form of tables and charts
that help to analyze and compare the results.

To measure the consumption power of the asynchronous engine with the spindle load and

with no load as well, the one, two (Aron’s systems) or three wattmeters can be used. Using of
one wattmeter assumes the equal load of all the phases, then the one of the phases power is
measured and the result needs to multiplied by 3. In case of non equal load of phases the meas-
urement result can be incorrect. The diagram of measurement system is presented on Picture 4a.
The principal is to follow the rule that the wattmeters need to be switched on between "B" fus-
es and "ST" contractors of electric power machine system. The coil of wattmeter current (indi-
cated by thick broken line) needs to be switched in series into the selected phase such as "R", and
the voltage coil (fine broken line) needs to be switched between this phase and neutral conduc-
tor. The standard mentioned above does not recommend this method of power measurement. It
can be used only for the initial measurements and
landmarks.

The Aron’s system uses two wattmeters to meas-

ure the power consumed by the engine. The diagram
of the connection is presented on Picture 4b. W1 and
W2 coils of wattmeter current need to be switch in
series into two selected phases of the power (e.g. R
and S on the Picture 4b), while the voltage coils ac-
cordingly between these two phases (R and S) and
the third phase (T), which is not connected to the
current coil. In case of this measurement the neutral
conductor is not used. Then the total power con-
sumed by the engine is the sum of the power indicat-
ed by both wattmeters. If one of the wattmeters indi-
cates the negative strength, you should change the di-
rection of current flow through the coil, read indica-
tions of the wattmeter and add to the indications of
the second wattmeter, but with the negative sign.
The system with three wattmeters (Pic. 4c) requires
an access to the neutral conductor. It is a three times
replicated system, which is presented on Picture 4a.
Then each of wattmeters measures the power con-
sumption from the one phase and the total power is
the sum of the results of each device.

To read the power of the wattmeters, it is required to determine the “constant” of each of

them. The constant is the ratio of the current and voltage coil range and the number of plots on
the scale of the wattmeter. Measuring ranges of the coils need to be read from the wattmeters.

It is crucial to remember that during the start of machine the engine takes more of current

than during determined working time. To prevent the damage of wattmeters, during acceleration
and deceleration, the current coils of wattmeters need to be bypassed, to protect them against the
overload. The wattmeters situated on the desktop, usually have the switches or plugs used to by-
pass the current coils.

AF 21/05/2013 - end

Pic.4. The diagrams of wattmeters connections
for the measurement of power: a) with one watt-
meter, b) in Aron’s system, c) with three wattme-
ters

Wrocław University of Technology

Name and surname

. . . . . . . . . . . . . . .

I n s t y t u t

. . . . . . . . . . . . . . . . . . . . . . . . . .

Technologii Maszyn i Automatyzacji

. . . . . . . . . . . . . . . . . . . . .

Year

. . . . . . .

Lab group

. . . . . . . . . .

Exercise nr 4

Exercise data

. . . . . . . . . . . . . . . .

Measurement of machine tool power loss with load

and total efficiency of machine tool

1. Testing equipment:

1.1. Wattmeter

1.2. Tachometr

1.3. Lathe TUD-50

4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2. Tables with measurement results

2.1. Measurement of machine tool power loss without load

Ord.

number

First gear of engine (n

nominal

=710 rpm)

Second gear of engine (n

nominal

=1440 rpm)

n

spindle

∆P [W]

n

spindle

∆P [W]

Constant

of wattme-

ter

c

Graduations

i

3×c×i

Constant

of wattme-

ter

c

Graduations

i

3×c×i

[rpm]

[W/unit]

[units]

[W]

[rpm]

[W/unit]

[units]

[W]

1.

2.

3.

4.

5.

6.

7.

8.

9.

10.

11.

12.

13.

14.

15.

16.

17.

18.

18

22,4

28

35,5

45

56

71

90

112

140

180

224

280

355

450

560

710

900

35,5

45

56

71

90

112

140

180

224

280

355

450

560

710

900

1120

1400

1800

2.2 Measurement of lathe total efficiency.

On.

n

spind.

Measurement of electric power P

el

[kW]

Effective power P

eff

[kW]

Efficien-

cy [η

0

]

P

1

[W]

P

2

[W]

1000

P

Σ

Breake

Con-
stant

Current

intensi-

ty

Out-

put

torque

P

eff

el

eff

P

P

[rpm

]

Wattm.

const

unit qu-

antity

c×i

wattme-

ter con-

stant

Quanti-

ty unit

c×i

c

I

M=c×I

9555

WR

n

M

⋅

c

i

c

i

[Nm/A]

A

Nm

[kW]

[%]

[W/unit

]

[units]

[W

]

[W/unit]

[units]

[W

]

[kW

]

1.

2.

3.

4.

5.

6.

7.

8.

9.

10.

11.

12.

13.

14.

15.

16.

17.

18.

19.

20.

21.

1. Type of machine tool ……………………………, 2. Symbol of machine tool ……………,

3. Fabrical number and production year …………………………, 4. Producer …………,

5. Main engine: type ………………………., fabrical number ………………………………,

power rating ……………………………, rated rotational speed ……………………………,

6. Machine tool working hours quantity ………………………., from last remount ………,

7. Electrical measurement devices: wattmeter (type/firm) …………………………………,

8. Ambient temperature ………………………, 9. Date of measurement: …………………,

10. measurement did: …………………………

3. Power losses of machine tool versus spindle rotation speed without load

4. Total efficiency of machine tool versus effective load

5. Conclusions

18 28 45 71 112 180 280 450 710 1120 1800 [rpm]

2
kW


1,5



1



0,5

Spindle rotation speed n

spind.

P

o

w

er

l

o

ss

es

∆

P

75

%



50




25

1 2 3 4 [kW] 5

Effective power of machine tool

T

o

ta

l

ef

ic

ie

n

cy