How to Select VSDs and AC motor converters (part 1)

INTRO:

Although manufacturers' catalogues try to make it as easy as possible, there are many variables associated with the selection and rating of the optimum electric motor and AC converter for a variable speed drive (VSD) application. In many cases, it requires considerable experience to get the selection right. The reason why it’s difficult is because there is always an engineering trade-off between the following:

• The need to build in a margin of safety into the selection procedure

• The need to keep the initial cost to a minimum, by selecting the optimum type and size of motor and converter for each application.

This section covers many of the principles for the correct selection procedure for AC variable speed drives, which use PWM-type variable voltage variable frequency (VVVF) converters to control the speed of standard AC squirrel cage induction motors.

The following checklist covers most of the factors that need to be considered:

• The nature of the application

• Maximum torque and power requirements and how these change with speed

• Starting torque requirements

• The speed range - minimum and maximum speed

• Acceleration and deceleration requirements (Is braking necessary?)

• Compatibility with the mains supply voltage

• Environmental conditions where the converter and motor are required to operate, ambient temperature, altitude, humidity, water, chemicals, dust, etc.

• Ventilation and cooling for the converter and motor

• Direction (uni- or bi-directional)

• Accuracy of the speed control

• Dynamic response (speed and torque response requirements)

• Speed regulation requirements with changes in load, temperature, supply voltage

• The duty cycle, including the number of starts and stops per hour

• Overall power factor of the drive system and its effect on the mains supply

• EMI and harmonics in the mains power supply and in the motor and motor cable

• Are EMI filters required?

• Grounding, shielding and surge protection requirements

• Torque pulsations in the rotor shaft

• Control method - manual, automatic, analog, digital, communications

• Control and communications interfaces required for the plant control system

• Indications required

• Reliability requirements, is a dedicated standby unit required

• Protection features, in-built and external features required

• Power and control cable requirements

• Parameter settings, local or remote programming

• Maintenance, spares and repair considerations

• Cost of the alternative systems, taking into consideration the capital cost, performance advantages, energy savings, efficiency or process improvements.

• Noise due to the harmonics in the motor

• Mechanical resonance at certain motor speeds

This section covers many of the technical issues that need to be considered, but won’t be able to address all the above factors in detail.

BASICS/PROCEDURE OF SELECTION:

Experience has shown that most of the problems experienced with AC VSD applications can usually be attributed to human error, mainly.

• The incorrect selection and rating of the AC induction motor

• The incorrect selection and rating of the AC converter

• The incorrect parameter settings installed in the VSD control system

As with all other electrical equipment, it’s essential that the drive be correctly rated to do the job under all anticipated circumstances. The AC variable speed drive system is correctly selected and rated when:

• Motor specification is correct

The correct type and size of electric motor has been selected, whose output torque, speed and accuracy are adequate for all load and environmental conditions.

• AC converter specification is correct

The correct type and size of AC converter has been selected, whose output (voltage, current, frequency) meets the motor requirements for all load and environmental conditions.

Usually, too much emphasis is placed on the selection of the converter, which is the expensive part, while too little emphasis is placed on the selection of the motor.

The correct procedure is:

• The first step is to select a correctly rated electric motor

• Only when this is completed, a suitable AC converter is chosen to match the requirements of the motor

From the motor point of view, the main factors which need to be considered are the motor power rating (kW), the number of poles (speed) and the frame size so that the load torque on the motor shaft remains within the continuous torque capability of the motor at all speeds within the speed range. High torques of short duration, such as starting torque, can usually be easily accommodated within certain limits outlined below.

Converter-based Squirrel-cage Motor

When selecting an AC motor for any drive application, the most important requirement is to ensure that the motor does not become overloaded or stall under all circumstances of speed and load, i.e. over the entire speed range.

To stay within the temperature rise limits of the motor, the torque required by the load for starting, acceleration and for continuous running must be within the rated output torque capacity of the motor.

For AC motors connected to the power supply direct-on-line (DOL), it’s usually sufficient to ensure that load torque is sufficiently below motor torque at the rated speed of the motor, E.g. the torque at 1450 rev/m on a 4-pole motor. These fixed speed drives operate only at one speed. It may also be necessary to ensure that the starting torque of the motor is higher than the breakaway torque of the load.

In the case of a variable speed drive, the load torque usually changes with speed, so it’s essential to check that the motor torque exceeds the load torque at all speeds in the speed range. E.g., a centrifugal pump has a variable torque characteristic, where the starting torque is low and the torque increases as the square of the speed. Other loads, such as a conveyor, may have a constant torque characteristic, where the load torque remains constant for all speeds.

The continuous load torque capacity (loadability) of a standard TEFC squirrel cage induction motor used with VVVF converters is always lower than the rated torque of the motor itself for the following reasons:

• At all speeds, the load capacity is reduced as a result of additional heating in the motor caused by harmonic currents, however small. These occur because the output current waveform of the converter is not completely sinusoidal, even with modern PWM inverters with switching frequencies around 10 kHz.

Traditionally, a de-rating of between 5% and 10% was used, depending on the type of motor (number of poles) and the type of converter. But, it has become common practice with modern PWM inverters to make provision for no de rating at all. This relies on the fact that modern IEC motors always have a built-in thermal reserve, which will accommodate any additional heating. Also, the mechanical load is seldom exactly equal to the motor rating and is often lower by as much as 20%. It’s considered good engineering practice to allow a small margin of safety, so a de-rating of up to 5% is usually provided.

Consequently, the overall output torque of the VSD, running at its base speed of 50 Hz, is taken to be about 95% of the motor catalogue's rated torque at 50 Hz and at rated DOL supply voltage.

• At speeds below base speed, in the speed range 0-50 Hz, the motor's continuous load capacity is reduced because of decreased fan cooling of both the stator and the rotor. The reduction in continuous torque output depends on the type and size of the motor, but in the absence of other de-rating tables, can be assumed to reduce to about 40% of rated torque at standstill. Some natural radiated and convectional cooling from the motor frame takes place even when stopped.

For some constant torque applications, a separately powered auxiliary cooling fan mounted onto the motor can be used to improve stator cooling and increase load capacity at low speeds. These are usually designed to provide a motor with a volume of air equal to that flowing at rated speed for a motor of equal frame size. This supplementary cooling does not entirely overcome the load capacity problem. In a squirrel cage motor, the rotor losses are usually higher than the stator losses and the rotor losses become difficult to dissipate at low speeds even with supplementary cooling. Separate cooling is more effective with open motors (IC01).

• At speeds above base speed, the output torque capability of the motor is reduced because of reduced air-gap flux (lower magnetic field). The output torque reduces in direct proportion with the motor speed above 50 Hz. (Refer to next Section.)

The AC VSD loadability curve summarizes the factors above and the solid line marks the maximum limits of continuous load torque. Motors fed from VVVF converters can be loaded continuously at torques below the loadability limit line for the speed range.

However, motors can tolerate load torques greater than the level permitted by the loadability curve for short periods of time. High torques are usually required during starting and acceleration up to the preset speed range. The duration of the allowed overload depends on several factors such as the size of the motor, the magnitude of the overload and the speed. Many AC converters have an over-current capacity of up to 150% for 60 secs to cover starting and transient operation.

++++ The speed range and load torque capacity (loadability) of a TEFC squirrel cage motor when controlled by a PWM-type VVVF converter

The above curve shows the thermal load capacity of an AC VSD that is typical of curves used by many drives specialists. They are based on standard IEC-type squirrel cage motors running with PWM-type VVVF converters. The curves are given in per unit values, so they can be applied to motors of any voltage and size. Small motors below 5.5 kW have a slightly higher load capacity at low speeds.

The equivalent load power (kW) capacity curve is shown in the figure below. In the region below base speed, known as the constant torque region, the power capability increases linearly from zero at zero speed to full power at the base speed.

Above base speed, the power output capability cannot increase further and remains constant for further increases in speed. This is known as the constant power region.

++++ Load power capacity of a TEFC squirrel cage motor when controlled by a PWM-type VVVF converter.