Download Energy savings with AC VSDs and motors on pumps

D R I V E S
&
S W I T C H G E A R
A
ny initiative that can contribute to a significant national energy
savings would be very advantageous in South Africa currently.
Energy savings with AC VSDs
and motors on pumps
This paper presents one aspect of energy
savings that can be shown to realistically
save an amount of power roughly equal
to the output of a single coal fired power
station.
Electric motors account for 50 – 65% of
the total national electrical consumption
of South Africa. In turn, a significant
percentage of these motors are used
on pumps. In most industries this
percentage varies from 20 – 80% of
all motors. Many of these may be VSD
driven giving significant energy savings
and reduced life-cycle costs.
This paper deals with the following
factors that are part of correct application
of AC VSDs for effective life cycle cost
reduction:
l Pump energy savings calculation
l Practical considerations of VSD
applications:
- VSD compatible motor selection
- Voltage peaks, dv/dt, harmonics
and other VSD effects on motors
- Motor speed range
- Harmonics
l High efficiency motors:
- The real facts on motor efficiency
requirements
- VSDs and high efficiency motors
This paper will assist with giving a clearer
understanding of the above factors,
many of which have been a cause of
concern within industry.
Pump energy savings
calculation
It is relatively common knowledge that
there is a large potential for energy
savings when a pump motor is VSD
driven. Though the general concept is
by Johan van Niekerk, Zest Electric Motors & Drives
Fig.1: Torque load.
Fig. 2: Comparison.
Zest pump VSD energy saving calculation
Input data - yellow blocks only
Customer: Test
Application absorbed power (kW):
180
Application: Test
Installed motor power (kW):
200
Flow %
Time %
Days of operation per year:
360
100%
10%
Hours operation per day:
kWhr cost in Rands:
Estimated cost of VSD installation:
Duty
24
90%
R0,15
80%
20%
R150 000,00
70%
35%
60%
20%
543745
40%
0%
R81 561,69
30%
0%
20%
0%
10%
0%
Throttling KkWhr
VSD kWhr cost
Throtting
kWh cost
Calculation results
15%
50%
Annual savings in kW hour
Annual savings in Rands
Payback period
22
0%
Calculation
Total hours per year
8640
Total duty percentage
100%
Flow %
Running
hours
VSD absorbed
power (kW)
Throttled
abbsorbed power
(kW)
Difference
between
absorbed power
values
VSD kWhr
Annual saving
kWhr
Annual savings
Rands
100
864
180
180
0
155520
155520
R23 328,00
R23 328,00
0,00
R0,00
90
1296
131
172,8
42
17061
223949
R25 509,17
R33 592,32
53 887,68
R8 083,15
97
165,6
69
167215
R42 923,52
118 841,70
R17 841,25
70
3024
81
158,4
77
244944
479002
R36 741,60
R71,850,24
234 057,60
R35 108,64
60
80
1728
1728
72
151,2
79
124416
261274
286157
R18 662,40
R25 082,27
R39 191.04
136,857,60
R20 528,64
50
0
63
144
81
0
0
R0,00
R0,00
0,00
R0,00
Totals:
862156
1405901
R129 323,43
R210
885,12
543745
R81 561,69
All flow below %0% absorbs power equivalent to 50% flow
Estimated cost of VSD installation
R150 000,00
Payback period - months
22
Important notes: In a centrifugal pump, the outflow varies along with the speed, and the pressure varies with the square of the speed. As the pressure varies along with the square of speed, at
70% of speed the pressure has a theoretical value of 50% of the rated pump pressure.
Fig. 3: VSD energy saving.
accepted the actual facts when quantified
in terms of kW/hours and Rands are
often unclear. Also, unrealistically
September 2008 - Vector - Page 37
optimistic scenarios are often given. We
present here a straightforward method to
calculated energy savings. The benefit is
that the end user is empowered to make
an informed decision regarding the
cost effectiveness of a VSD installation.
Centrifugal pumps use power that is
proportional to the speed cubed.[P∝N3].
This means that at reduced speed, the
power consumption is greatly reduced.
When 100% flow is not required, speed
can be reduced in order to reduce
flow, resulting in power savings as well.
Another method would be to throttle the
pump, thereby reducing flow. Throttling
however also uses much more power
than speed control. Where possible,
the best method of controlling flow and
power is by variable speed drive. The
power savings can be simply calculated
using an Excel based software from
Zest. The possible power savings are
illustrated. (Fig. 1 to 3)
It is common for several pumps to be
used together in order to maintain a
required flow rate or pressure. In such
an application it is possible to do PID
control on one such pump by VSD. The
VSD may be used to start and stop the
other pumps as and when required.
Using this method the VSD energy
savings benefit is also realised.
Practical considerations of VSD
applications
Not all standard AC motors are truly
suitable for VSD applications. There are
two main issues to consider:
l Motor heat rise – a motor on a VSD
will run at higher temperature than
a DOL motor. It is important for this
additional heat rise to be minimal
and to be precisely known.
l Motor insulation – VSD output
pulses have peak values (Vpeak) and
sharp rise times (dv/dt) far above
the normal grid 50 Hz sine wave.
These values as produced by the
VSD and the corresponding motor
insulation must be known and must
be compatible.
It is well known that motors in VSD
applications will have a higher heat
rise than normal and must therefore
be de-rated for VSD applications. The
reasons and details of this are however
not well known. There are three reasons
for increased heat rise:
l Current distortion on the VSD
output.
l Reduced cooling at reduced speeds.
(N < Nnominal) < LI>)
l Reduced torque at increased speeds.
(N > Nnominal)
The basic operating principle of
all standard VSDs is to convert the
50 Hz AC supply voltage to DC and
then convert the DC back to AC. For this
the voltage cannot increase but the
frequency does. This results in reduced
flux and consequently reduced torque.
Fig. 5 illustrates this.
Harmonics
Fig. 4.
reason VSDs are sometimes referred to
as frequency converters. The conversion
from DC back to AC uses a system
of calculated and controlled pulses
referred to as pulse width modulation
(PWM). This system allows the VSD
output frequency and voltage to be
controlled, thereby controlling motor
speed. However the PWM results in
current distortion. This can be seen in
Fig. 4. The current distortion causes
additional heat in the motor.
A motor depends on a shaft mounted
cooling fan on its non drive end for
cooling air flow. When the motor runs
at speeds slower than the normal
50 Hz speed, this fan turns slower and
the motor has reduced cooling. Due
to this the motor thermal capacity and
hence its allowable output torque is
reduced. Motors torque is dependant
on magnetic flux. Flux is a part of motor
design and is directly proportional
to the ratio of voltage to frequency.
For a 400 V 50 Hz motor this ratio is
400/50 = 8. For a 525 V 50 Hz motor
this ratio is 525/50 = 10,5. At full
speed, e.g. 1460 rpm on a 4 pole
motor, the VSD output is full voltage
and nominal frequency. This gives full
flux and torque. Above this speed,
To an ever increasing extent, users
are becoming aware of the reality
and negative impact of VSD induced
harmonics. These harmonics are
inherently a result of the VSD method
of operation. The initial rectification of
the AC supply voltage to DC causes an
irregular or distorted current to be drawn.
This causes harmonics. All standard
VSDs produce very similar levels of
harmonics. Harmonic current are
currents at multiples of the fundamental
frequency. For example the 5th harmonic
is at 5 x 50 Hz = 250 Hz. These
harmonics cause additional heating in
the supply transformer and at sufficiently
high levels may cause interference with
other equipment. Most large end users
find a harmonic level that causes 5%
distortion on the voltage acceptable,
i.e. 5% voltage THD. Fortunately this
level of distortion is seldom reached.
It is important to note that vurrent THD
and voltage THD are at very different
values. Also it is possible to accurately
predict the harmonic distortion prior to
proceeding with an installation. Fig. 6
shows an example of such a calculation.
Once this calculation has been made
the user can determine if the harmonic
distortion level will be acceptable or
not. The simplest method to reduce the
harmonic distortion is by adding line
reactors.
High efficiency motors
In a relatively short period the use of
high efficiency motors has become a
matter of much increased consideration
within industry. There is generally not a
Fig. 5.
September 2008 - Vector - Page 38
Fig. 6: Calculation of harmonics.
clear understanding as to what constitutes
a “high efficiency” motor. Also, what are
the international standards specifying
efficiencies? What is the effect of using
a high efficiency motor when it is VSD
driven?
We will also deal with ground breaking
technology regarding the use of VSDs
and high efficiency motors.
There are several international standards
dealing with high efficiency motors. There
is however no international standard
that is complete and in line with local
standards.
l Europe – CEMEP standard covers from
1,1 kW to 90 kW only (EFF1,2&3)
l
Australia & New Zealand – MEPS
covers 0,75 to 185 kW only
USA – NEMA Premium – frame sizes
and other aspects that are not in
accordance with SABS
l
IEC & SABS 60034-30 – workgroup
drafts in progress – the drafts will only
be tabled in 1 to 3 years.
These standards are not mandatory in
South Africa, nor are they complete.
This means that many motors are being
marketed as high efficiency, when in fact
they are only higher than similar motors
from the same manufacturer. Though
the above standards are not complete,
the CEMEP and MEPS do correspond on
the motors covered by both standards.
This means that the MEPS standard can
reasonably be applied as a definition
of high efficiency until such time as the
l
September2008 - Vector - Page 39
IEC and SABS standards are complete.
There is only one motor supplier locally
that complies with both the current SABS
motor standards and the MEPS high
efficiency standard. When VSD driven,
both normal and high efficiency motors
suffer a reduced efficiency. However, the
high efficiency motor retains its efficiency
advantage. This means that the use of
high efficiency motors in VSD applications
is still advantageous. Furthermore, when
Optimal Flux VSD technology is used the
high efficiency motor and VSD combination
offers thermal characteristics far better than
currently possible with a standard motor.
Contact Johan van Niekerk, Zest Electric
Motors & Drives, Tel 011 723-6157,
[email protected] D