Motor start with VFD
Our experience shows that the VFD motor start is sometimes not fully understood. This post shall explain how motor start with VFD works, what are the main benefits and how does it compare to a direct on-line start.
Abbreviations
AFE = Active Front End
DFE = Diode Front End
DOL = Direct on-line
VFD = Variable Frequency Drive
VVVF = Variable Voltage Variable Frequency (drive)
DOL motor start
Direct on-line start is the simplest method of starting an electric motor. It basically just requires an input circuit breaker that closes when the motor shall be started. This simple method has one significant drawback – the motor starting current. Why is it a challenge?
The starting current typically ranges 4 to 8 times the nominal current. It has negative impact on the supplying grid, on the motor itself as well as on the mechanical shaft line.
Impact on the grid
Starting current causes a voltage drop. Besides the high magnitude of current it is also the low power factor during starting that is affecting the grid. Motor draws a considerable amount of reactive power that pushes down the grid voltage. Other consumers connected to locally to the same grid might be impacted or might even trip.
Impact on the motor
Large starting current causes a significant heating of the motor. Remember that resistivity heat losses are proportional to the square of the current. Although the overall thermal time constant is rather long, some components might reach high temperature within a short time. Also, there is a risk of hot spots.
Impact on the shaft string
Mechanical components of the shaft string get affected, too. DOL starting is associated with transient torques that are by far not negligible. Applications with frequent starting might see accelerated aging of shaft components, such as couplings or gears.
Why is actually starting current in DOL operation so high?
DOL start is a transient process. At zero speed the rotor is at standstill and the nominal frequency is directly applied to the stator. The slip (relative movement etween stator field and rotor) is 1.0 (100%). Fairly large voltage is induced into the rotor. The rotor current is high; limited only by the stray impedance of the motor. As the rotor accelerates the slip proportionally reduces and the induced voltage drops.
The starting current is easily up to 5÷7 x In. At the same time the starting torque is often below the nominal torque. Why do we get relatively low torque with high current? The reason is the low power factor during starting (as poor as 0.1 pu).
The frequency of the voltage induced into the rotor is the difference between stator frequency and mechanical frequency multiplied by the number of pole pairs npp. At standstill the rotor frequency is same as the stator frequency, i.e. 50 Hz or 60 Hz for normal grids. The rotor impedance has large reactive part causing low power factor. As the rotor accelerates, the frequency induced into the rotor drops. At nominal speed the rotor frequency of asynchronous machine is in the range 0.2÷0.5 Hz. For such very low frequency the reactance of the rotor becomes very small resulting in high power factor (up to 0.94 for high efficient medium/large asynchronous motors).
Let’s illustrate it on an example of 8’000 kW, 4-pole machine:
Shaft power = 8’000 kW
Stator voltage = 6’600 V
Rated frequency = 50 Hz
Rated speed = 1’493 rpm
Number of pole pairs = 2 (4-pole)
At 10% speed the induced rotor frequency is 45 Hz. At 100% speed the induced rotor frequency is as low as 0.23 Hz. This is almost factor 200!
The rotor impedance is basically R-L combination (resistive-inductive).
At low speed (high rotor frequency) the reactive part of the impedance is much more dominant than the resistive part –> low power factor.
At speed around nominal speed the rotor frequency is fraction of Hz and the reactive part is very small. The resistive part is dominant –> high power factor.
VFD motor start
Starting the motor with a VFD mitigates all above problems. The starting (inrush) current is basically completely eliminated. But how is that possible? In order to answer that question we need to refresh the principle of VFD a bit.
1. Smooth starting, resp. ramping up the motor speed with VFD
There are no transients unlike the DOL start. In case of induction (asynchronous) machine the stator frequency is kept slightly above the mechanical frequency multiplied by number of pole pairs. Therefore, the slip is small. Consequently, the rotor frequency is low and the motor has high power factor across the entire speed range. The motor is always operated on the stable side of torque-speed characteristic. Nominal slip of a high power medium voltage motor is about 0.3 – 0.8%. The VFD control keeps the slip in that range throughout the entire speed range. This is the main difference to a DOL start where the initial slip is 100% (the rotor is at standstill and the stator frequency is equal to the grid frequency, i.e. 50 Hz or 60 Hz).
2. VFD decouples the motor from the grid
Most VFDs are indirect frequency converters, i.e. the input power of fix frequency is first rectified (AC/DC) and then inverted to variable output frequency (DC/AC). The intermediate dc link either consists of reactor (current source inverters) or capacitor (voltage source inverter).
Due to this topology the motor is decoupled from the grid. Possible small transients, e.g. due to control method or due to load changes, will be handled by the inverter. They will not be seen on the grid side of the VFD.
3. Input power factor is independent on motor power factor
Due to the same reason – decoupling of motor and grid through the VFD – the motor power factor has no direct relevance to the input power factor of the VFD or the input power factor at the point of connection, respectively.
The reactive power required by asynchronous motor is provided from the dc link capacitors (VSI drives) or by the capacitors installed between inverter and motor (CSI drives). LCI drives are slightly different as there the reactive power is supplied from the grid and from the overexcited synchronous motor.
Note that many VFDs just have a diode rectifier (DFE). The diodes are uncontrolled, i.e. the rectifier is passive. There is no way that motor parameters could impact the input power factor. It only depends on the impedance of the grid and impedance of input transformers. Practically all VFD manufacturers state the diode front end VFDs have input PF > 0.95.
As visualized in figure above, the input power factor of the VFD is independent on the power factor of the motor. The reactive power of the motor is supplied from the capacitors of dc link. The input power factor is given by grid and transformer impedance and actual load (DFE drives) or can be actively controlled to any value – typically 1.0 (AFE drives).
Power flow consideration
The low input current of VFD motor start can be easily explained (or proven) by following power considerations:
1. Load torque as function of speed is provided. Many applications are of quadratic torque type.
2. Motor torque is equal to load torque plus acceleration torque for start-up. Normally the application is started with a speed ramp, i.e. constant rate of change of acceleration –> acceleration torque is constant value and motor torque is equal to the load torque plus fix offset.
3. Shaft power is the product of motor torque and speed.
4. VFD input active power is the shaft power plus system losses (3-7%).
5. VFD input apparent power is the input active power divided by power factor. DFE VSI drives have input power factor > 0.95, AFE VSI drives usually control input power factor to 1.0.
The input power increases with the motor speed. The relation is roughly quadratic for quadratic torque applications (as depicted in Figure 4 and Figure 5) or linear for constant torque applications. The start-up time depends on inertia of the rotating system. The x-axis in Figure 5 is just an example from a specific project. A constant acceleration torque was selected (fix offset between motor torque and load torque). In such case the speed increases linearly with constant acceleration. Close to nominal speed the acceleration margin drops and the rate of change of speed reduces (“S” curve). VFD might have short time overload capability to further reduce start-up time if necessary. However, many applications prefer a smooth start-up.
Since the grid voltage is approximately constant (fluctuations of few %), the increasing input power corresponds to increasing input current. Large input current during start-up is physically not possible 🙂
VFD starter versus soft starter with fix frequency
VFD motor start shall not be confused with starters that only adjust the voltage, but do not change the frequency. Such soft starters are much cheaper. However, they don’t provide the same functionality. Their aim is to reduce the stator voltage at the beginning of the start-up. Through that the starting current can be limited. Anyway, the starting current still exceeds the nominal motor current.
Finally the main disadvantage: reduced starting torque. Asynchronous machine started in DOL mode already has low starting torque. This torque further drops when using a soft starter – quadratic with the voltage! So when the soft starter reduces the input voltage down to 80% you would get only 64% of starting torque of the DOL machine!
VFD soft starter does not have this issue. In opposite, VFD soft start can provide full torque from zero speed (if needed) which is superior to the DOL start and necessary for constant torque loads that are started in loaded condition.
To differentiate between proper VFD starter and soft starter only adjusting the voltage, the term VVVF (variable voltage – variable frequency) is sometimes used as a synonym for VFD starter. See [3] for more details on soft starter and VFD soft starter.
References
[1] Input power factor and reactive power consumption, https://mb-drive-services.com/energy-efficiency-part-6-power-factor-and-reactive-power-consumption/
[2] Medium voltage AC drives, https://new.abb.com/drives/medium-voltage-ac-drives
[3] What is the difference between soft starter and VFD soft starter? https://mb-drive-services.com/what-is-the-difference-btw-soft-starter-and-vfd-soft-starter/
