Asynchronous motors draw a starting current far higher than their nominal current at the moment of start-up, approximately 8xIn. This sudden inrush current creates unwanted electrical and mechanical stress on the motor.
Over time, this stress can damage the motor or shorten its service life. It can also lead to unexpected failures and unplanned downtime at the facility. To prevent this type of failure, several starting methods have been developed as alternatives to direct-on-line (DOL) starting. In this article, we will cover three of these methods, auto-transformer starting, thyristor-based soft starting, and MV VFD (VSD, MV frequency converter) starting. Each method is explained separately below.
Method 1: Auto-Transformer Starting:
For medium voltage motors, an auto-transformer is used instead of DOL starting to limit the starting current or reduce the mechanical torque on the motor. Auto-transformers are a type of transformer that regulates voltage. In this starting method, the grid voltage is connected to the auto-transformer. The voltage taken from the tapped secondary winding of the auto-transformer is then applied to the motor. With this method, the starting current can be reduced to as low as 65% of the nominal operating current. However, at each tap transition, although less severe than in direct-on-line starting, the motor is still exposed to electrical and mechanical stress.
Method 2: MV Soft Starter Starting:
Thyristors are semiconductors that, due to their structure and design, allow current-controlled starting. By adjusting the nominal voltage applied to the motor at the moment of start-up, the motor can be started with a lower current value. For example, in fan and pump applications, starting with 3 to 4 times the nominal current becomes possible.
Thyristor-controlled soft starters provide both soft starting and soft stopping for motors. This allows starting without the electrical and mechanical stresses described above. This is an especially valuable option in applications where the motor does not require speed adjustment once started, and it is widely used wherever speed control is not needed.
Thyristor-controlled soft starters are connected between the grid and the motor. The soft starter limits the voltage and current applied to the motor to carry out the starting process.
Modern soft starters are microprocessor-controlled and equipped with additional control systems that adjust the torque-current relationship of the motor over time, allowing the motor to start and stop without the shock seen in auto-transformer starting. With a soft starter, the voltage and current limitation applied to the motor makes it possible to set start and stop durations by selecting the appropriate ramp type for the load. Since the soft starter has a built-in bypass contactor, no additional contactor is required. Once the motor has started and the bypass is engaged, the soft starter’s advanced protection parameters remain active and continue to protect the motor. Operation can also be monitored through the digital display, and fault information can be reviewed from the statistical data stored in its memory.
Method 3: Starting with MV Frequency Converter (VFD, VSD):
This is the appropriate starting and control method for any medium voltage motor that requires speed adjustment. Depending on the application, it can generate energy savings that pay for itself in a short period. Medium voltage electric motors are used in most medium and large scale factories and industrial facilities. They appear in a wide range of applications, from fan and pump systems to demanding applications such as mills. To keep production uninterrupted, these motors need to operate under proper conditions. Over time, mechanical and electrical stress on the motor shortens its service life. To reduce these effects, frequency converters (MV drives or MV VFDs) should be used instead of older starting methods wherever speed adjustment is required.
The required medium voltage level is achieved by increasing the number of low voltage power cells. For example, at the 6 kV level, this is done by connecting 5 low voltage power cells in series per phase. Similarly, at the 11 kV level, 9 low voltage power cells are connected in series per phase.
The table below shows the total number of power cells used for each kV level.
The basic structure of a low voltage power cell is as follows:
Frequency converters have a harmonic-reducing effect at medium voltage. For example, using a 36-pulse power cell configuration at the 6.3 kV level reduces the impact of harmonics on the grid. To offer a more competitive price, some competing suppliers may propose 24-pulse or 30-pulse products for the 6.3 kV level. This is a point where customers need to make an informed choice and should favor products with a higher pulse output.
Bypass Structure (Optional):
Medium voltage frequency converters are microprocessor-controlled electronic products. This brings advanced protection parameters for both the motor and the drive, preventing possible damage during operation. Preventing faults before they grow allows the maintenance team to carry out more planned maintenance at lower cost. If the MV frequency converter needs to be taken out of service for maintenance, a manual or automatic bypass cabinet is required. This allows the motor to be started directly across the line (DOL) when necessary.
Medium voltage frequency converters (MV VFDs) are available in both air-cooled and water-cooled models, depending on the sector. Our Hiconics brand MV frequency converters offer both air-cooled and water-cooled solutions. For example, sectors such as iron and steel, water treatment, power generation, and timber tend to prefer air-cooled MV VFDs, while sectors such as mining, which involve explosive (Exproof) environments, may prefer water-cooled MV VFDs. Regenerative products may also be needed when the motor operates across all four quadrants. Hiconics MV VFD products are also available in regenerative frequency converter models. These products can draw energy from the grid or feed energy back to the grid, depending on the motor’s operating quadrant. As with low voltage drives, medium voltage frequency converters are available in different models capable of starting both asynchronous and synchronous motor types.
MV frequency converters simplify plant operations through advanced protection parameters. Medium voltage electric motors are large and heavy by design. Even a minor fault can lead to significant maintenance costs. Electric motors are exposed to overcurrent, overtemperature, and overvoltage, all of which can cause unwanted effects. To extend the motor’s service life, these three values need to be continuously monitored.
Operation at Different Voltage Levels (Depending on Site Conditions):
Medium voltage frequency converters include a dedicated multi-output MV transformer. Each power cell has its own separate secondary output on the transformer. For example, if the plant operates at a 10 kV level but the motor to be started with the MV drive operates at 6.3 kV, this conversion is achieved by selecting the appropriate transformer winding within the MV VFD during manufacturing. This means no separate power transformer is needed between the point of supply and the MV frequency converter, which represents additional cost savings for the facility. Hiconics medium voltage frequency converters are manufactured in accordance with IEEE 519 standards. The MV transformer used inside the product is dry type, with 8% impedance and copper windings.
Cell Bypass Structure (Optional):
There may be situations where a fault in one of the power cells while the MV frequency converter is in operation must not be allowed to interrupt production. For this reason, when the optional cell bypass feature is included in the frequency converter, the drive detects the faulty power cell and bypasses it. The motor then continues running, even if at a speed lower than nominal. To prevent unbalanced loading, the system recalculates the neutral point so that each phase produces the same output power. Once the faulty power cell is replaced, the MV frequency converter ramps back up to the previous set speed. This way, production is not interrupted. Power cells are designed to be easily replaced. The cell bypass feature is limited to one cell per phase, for a total of up to three cells (three phases). If more than one power cell fails on the same phase, this indicates a serious fault and the MV frequency converter shuts down.
Key Points to Consider:
- The product should be selected based on the motor type. (For example, according to whether the motor is asynchronous or synchronous)
- What is the MV VFD input voltage and the application voltage to be applied to the motor? (The transformer inside the MV VFD may have custom windings, for example 10 kV input, 6.3 kV output)
- Selection should be based on the application type. (For example, fan, pump, mixer, and so on)
- The drive’s operating temperature should be selected to ensure loss-free power delivery.
- The loading ratio required by the application type. (For example, 120% loading for 60 seconds, or 150% loading for 60 seconds)
- Is a specific communication protocol required? (For example, Modbus RTU, Profibus DP, Ethernet)
- The required IP protection class (some special applications may require a higher IP rating)
- Product selection should be made according to the motor structure. (Example: according to asynchronous or synchronous motors)
- What is the MV VFD input voltage and the application voltage to be applied to the motor? (The transformer inside the MV VFD may have a special winding. Input: 10 kV Output: 6.3 kV, etc.)
- It should be selected according to the application type. (Example: Fan, Pump, Mixer, etc.)
- The operating temperature of the drive should be selected to provide power without losses.
- Loading rate according to the application type (Example: 120% load in 60 seconds or 150% load in 60 seconds)
- Is a special communication structure required? (Example: Modbus RTU, Profibus DP, Ethernet)
- IP protection class (In some special applications, a high IP protection may be required.)
In Summary;
Although classic direct-on-line starting is still used today, the slightly improved auto-transformer stepped starting method is becoming less common over time. Instead of this older generation approach, starting motors with microprocessor-controlled power electronics products (thyristor or IGBT based) offers safer, more advanced solutions with higher level protection parameters. The electrical and mechanical stress on the motor at the moment of starting is significantly reduced, and the facility’s supply transformer operates under safer conditions.
Depending on the facility’s requirements, MV VFDs (frequency converters) are preferred where speed adjustment is needed, while MV soft starters are preferred in projects where soft starting and stopping alone is sufficient.
Advantages of MV Soft Starters Over Auto-Transformers:
- Extends the service life of mechanical equipment by preventing sudden motor starts and stops
- MV and LV sections communicate via fiber optics and are fully isolated from each other
- Advanced motor protection features (overcurrent protection, high and low voltage protection, ground fault protection, phase protection, and more) extend motor life
- Selectable start and stop ramps
- RS485 Modbus and Profibus communication options (optional)
- Designed for harsh operating conditions
- Available in protection classes ranging from IP30 to IP54
- Low maintenance requirements
- Smaller footprint compared to MV VFDs
- Adjustable motor stop time
- More cost effective than MV VFDs
Advantages of MV VFDs Over MV Soft Starters:
- Suitable starting and control method for any medium voltage motor that requires speed adjustment, with the potential to generate energy savings that pay for itself in a short period
- Ability to start the motor at its nominal current
- Can operate with scalar or vector control
- Design that does not require a dedicated transformer or supply
- Cell bypass and/or redundant cell capability
- Modular structure for easy operation and servicing
- Low harmonic values compliant with IEEE 519-1992 without requiring input or output filters (THD < 5%, output THDI < 3%)
- Can be used with standard or existing motors (retrofit)
- Extends the service life of mechanical equipment by preventing sudden motor starts and stops
- High efficiency
- Can operate at 50°C
- Advanced motor protection features (overcurrent protection, high and low voltage protection, ground fault protection, phase protection, and more) extend motor life
- RS485 Modbus, Ethernet, Profibus, and DeviceNet communication options
- Durable, long service life
- More expensive than MV soft starters
For all your medium voltage soft starter and frequency converter (VFD, VSD) needs, we can support you with technical guidance and proposals. We can develop solutions tailored to your project and requirements. Reach us anytime through our website at www.cedetas.com.tr or by phone at +90 216 311 3069.
