26789_Top 10 Pitfalls in Dimensioning AC Drives

7/30/2019 26789_Top 10 Pitfalls in Dimensioning AC Drives

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ABB Feature article AC motor and drive selection Drives dimensioning

Top 10 pitfalls in dimensioning AC drives

Most electrical engineers feel confident that they can quickly assess a particular application and choose a

drive to suit. There are, however, a number of pitfalls in dimensioning drives, and the same errors seemto be made again and again.

Very often these mistakes can lead to over dimensioning, leading to unnecessary expense and poorerperformance. For instance, the harmonic current content of a lightly loaded rectifier will be inherentlyworse than for a fully loaded system.

Most pitfalls can be avoided by bearing in mind a few simple points.

Here Geoff Brown, Principal Engineer at ABB Industrial Systems presents the top 10 dimensioningpitfalls, and how to avoid them.

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Remember the motor is a source of TORQUE.Any AC motor is a source of torque, and yet it is power rating which is most often used when looking atthe catalogue ratings for motors. These ratings are based on the fixed speed output when connected tothe network.

Torque is related to both power and speed, such that T (Nm) = P (kW) x (9550)/ n (r/min). Whendimensioning any variable speed drive it is important to think in terms of torque.

2 - Correct motor selection based on load typeThe motor is an inherent part of an AC drive system: the motor and converter cannot be considered inisolation.

It is important to know the type of load to be controlled before making any selections.

There are basically three different load groups:

i) Quadratic load

This is the most common load type and covers centrifugal pumps, fans and the majority of mixers.

ii) Constant torque

This occurs when fixed volumes are being handled, such as in screw compressors, extruders, or instraight line web handling.

iii) Constant power

This is typically used when a material is being rolled up and the diameter changes.

There may also be combinations of these load types. However, selecting the wrong load characteristiccould easily lead to oversizing.


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The figure below shows the load curves for the different types of applications.

3 Know the operating speed rangeIt is essential to know the real operating speed range. This is not normally too critical with a centrifugalload, but for a constant torque load it is quite possible to over-frame the motor if the real speed range issay 10% to 100% (say 5 50 Hz) and the motor is specified for 0 100% (0 50 Hz).

The critical factor here is the ventilation capability of the motor fan at low speed the cooling capability isclearly reduced, but this reduction is not linear. This is shown in the figure below.


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4 Remember, motor speed is not limited by network frequencyChoosing a motor to operate over a speed range of 0 1500 rpm can equate to a frequency range of 050 Hz for a 4-pole motor and 0 - 75 Hz for a 6-pole motor.

This can give some advantages:-

A specific motor frame is capable of a specific output torque not power.

IEC frame 315 M with a shaft mounted cooling fan (Cooling form IC 411) is capable of:

1028 Nm/160 kW as a 4-pole motor

1062 Nm/110 kW as a 6-pole motor1100 Nm/90 kW as an 8-pole motor

As an example, if a constant torque of 750 Nm from 150 1500 rpm is needed, which is basically 115 kWat 1500 rpm, then a 160 kW 4-pole motor with shaft mounted fan could be used. Alternatively, a 110 kW6-pole motor could be used.


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If a higher starting torque than nominal is needed for these combinations say at 1000 Nm the currentequivalent to 160 kW for the 4-pole option is needed, and 110 kW from the 6-pole option this could givea saving of one frame size for the converter.

5 - Forced ventilationChoosing a fixed speed fan on the motor (cooling form IC 06 or IC416) can sometimes give an advantagfor constant torque loads. With a fixed speed cooling fan we need extra connections to the motor.

However, it should be capable of producing constant torque output from standstill or a very low speed upto the base speed.

The addition of a fixed speed fan, which was the standard solution with a DC motor, can result in asmaller frame size, but often also results in lower ingress protection than off the shelf TEFC motors.

6 - Ensure the duty cycle is correctEven with a relatively simple drive application such as a pump or fan there will be a duty cycle. Initial starup may require a high torque to achieve a specific rate of acceleration.

The thermal time constant for a drive converter is relatively short possibly 4 6 minutes, and it will heatup and cool down quickly, while the motor thermal time constant can range from 15 minutes for a small

motor to a couple of hours for a large motor.

This means that the overload requirements are generally far more critical for converter sizing than formotor sizing.

Taking an extreme example, a cement kiln is a constant torque load. However, to start this load fromstandstill it is necessary to know what load is distributed inside the drum. There is a spiral inside thedrum, which can support material and therefore a very high starting torque is generally needed. But oncethe system is rotating, the load settles back to its nominal value quite quickly.

This leads us back to the optimum solution of a motor run beyond base speed.

If an overload is defined at maximum speed, the possibility of running beyond base speed is reduced dueto the inherent characteristics of the motor.

7 - Motor thermal capabilityMost motors are designed with Class F insulation materials, and are offered with Class B temperaturerise. This means that the insulation materials are capable of sustaining a 20C temperature increase, andperhaps 10% torque increase.

Drives do however tend to cause other loading effects in motors due to the non-sinusoidal waveform.The insulation system in an ABB motor, for example, is designed to operate satisfactorily with a Class Brise, and inverter operation for a period of 20 years. Utilising Class F rise will shorten this time period.

In addition, as motors become larger, the thermally critical area transfers from stator to rotor. Thishappens with motors in 315 L (200 kW 4 pole/1285 Nm) and 355 frames and larger.

With these motors, the advice of the motor designer is needed.

8 Do not dimension the converter according to the motor nominal currentMotor nominal current is NOT ALWAYS motor load current.


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When dimensioning a motor there may well be some de-rating, for factors such as ventilation, ambienttemperature, use in a hazardous area, etc. Also motors have built in standardised preferred powerratings.

This means that the real maximum current needed for continuous and overload duty could be different. Itis important when designing to this parameter to ensure that the converter control philosophy permits thispractice.

9 Do not design the inverter supply components to the overload requirements

V1

C

+

-

U, 3 ~

MotorFrequency converter

V3 V5

V2 V4 V6

Rectifier DC- circuit Inverter unit

Illustration 3: The frequency converter consists of rectifier, DC link and inverter

Always remember that the AC Drive is a frequency converter, which consists of a rectifier, a DC link andan inverter.

When there is an overload demand from the driven load, this current is seen in the inverter part of the

circuit, because the current has to be fed to the motor. However, the rectifier part will only pass thePOWER associated with the shaft output and the system losses. If the high torque is delivered at lowspeed, then the power will also be relatively low.

10 - Do not underestimate the effect of supply variations on the outputA frequency converter is designed to pass a specific current. If the converter catalogue shows a designcovering a wide supply range (say 380 460 V to be suitable for both European and US installations), thoutput power available from a connected motor will vary. A 20 A converter at 380 V will give 11 kW motoshaft power, while at 460 V in the US it will give 13.5 kW (17.5 hp).

This problem becomes more apparent when making selections for less usual supplies such as 550 V inSouth Africa or 575 V in Canada.

Following some of these basic points should help in correctly sizing a drive system to exactly meet theusers needs. It is important to remember that correct dimensioning will keep capital and operating coststo a minimum.


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Boxed itemWhat direction is the power flow?

A conventional diode rectifier has a unidirectional power flow it supports the driven load being motored.

There are instances when the driven load needs to be considered as a generator, and therefore usesome means of absorbing this generated power.

If the generation is continuous, and a single drive is involved, a true 4-quadrant rectifier will be needed.If there are several drives it is possible to connect the DC bus together and absorb the generated energyin a motoring section.

If the generation is intermittent, it is possible to dump this generated energy through a resistor.

As a typical situation where regeneration is often overlooked we can consider a fan. Here the fan is a verlarge rotating mass, with a high inertia. When rotating, it has a high level of kinetic energy.

If the fan can rotate in reverse when unpowered (which is quite common for a boiler ID fan), to slow itdown and drive it to its desired speed, the stored energy needs to be dissipated.

Even if the speed needs to be reduced quickly, the same problem can be encountered and will needsome means of energy dissipation.


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More info: ABB Industry OyDrives

Eero SaarinenPO Box 184FIN-00381 Helsinki, Finland

Tel. + 358 10 222 2527Fax. + 358 10 222 2287Internet: http://www.abb.com/automationEmail: [email protected]

26789_Top 10 Pitfalls in Dimensioning AC Drives

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