Part 1: Direct on-line start
This series talks about solutions to start larger medium voltage motors. Aspects such as effectiveness, cost, impact on the supplying grid and driven load or effects on the motor design are taken into account. Article at hand describes the simplest solution: motor direct on-line start.
Let’s start with the explanation why to bother about motor start?
The simplest method to start a classical electric motor is direct on-line (across the line) start. The disadvantage is significant starting current in the range 5-7 times the nominal motor current (special motor designs with 3.5 times the nominal current) and at the same time relatively low starting torque. For strong grids with high short circuit capacity and up to certain motor power the direct on-line start might be acceptable. For less strong grids or for larger motors the stress becomes simply too high and voltage drop gets too large. The latter one may negatively impact other equipment connected to the same grid.
Motor types and direct on-line start
In our considerations regarding direct on-line starting we focus on main industrial motor types: squirrel cage induction machine and electrically excited synchronous machine.
Permanent magnet machines need a VFD anyway so they are exempted from the comparison. Wound rotor induction machines (WRIM) offer possibilities for a controlled start and acceptable starting current. In fact, starting is one of the strengths of WRIM. However, it is not a universal solution. One has to do with sliprings and brushes. Moreover, in some applications, particularly motors in hazardous area, WRIM is generally not accepted.
Advantages of direct on-line start
♦ Simplest possible method
Direct on-line start is the simplest method to start an industrial electric motor.
♦ No extra equipment needed
No extra equipment is required for direct on-line start. However, the switchgear must be able to handle the starting current (relevant for large motors).
♦ Lowest initial investment (CapEex)
From above it is obvious that this method has the lowest investment cost.
♦ Easy to operate (no deep expertise required)
Due to the inherent simplicity no special expertise is required.
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Drawbacks of direct on-line start
♦ High starting current
During direct online start the machine draws high current from the grid that is multiple of the nominal current. Special designs lead to somewhat reduced starting current. However, it is a big challenge to design a machine with low starting current, reasonable starting torque and high efficiency.
– 5 to 7 times nominal current for standard asynchronous motors
– specially designed motors with 3.5 to 4 times nominal current as best
– stress for the supply grid (dynamics, voltage drop, high reactive power)
♦ Limited number of starts
We have just described the phenomenon of high starting current. Apart from the impact on the grid the high current also influences thermal behavior of the machine. Standard motors are limited by number of consecutive starts, e.g. 3 starts from cold / 2 starts from warm. After the limit is reached, the machine needs to cool down. Cooling period is specified by the motor manufacturer.
♦ Voltage drop
The voltage drop might be an issue for the motor itself as well as for other equipment:
– reduced motor starting torque (90% voltage means 81% torque, i.e. torque ~voltage²)
– potential issue for other consumers on the same bus or nearby
♦ Reduced starting torque
Initial starting torque (at zero speed) in case of a direct on-line start is typically ranging 0.4 – 0.7 pu. It is a design challenge to achieve high starting torque without compromising the motor efficiency in normal operation. When the value of starting torque is critical, a special type of squirrel cage might be necessary.
– uncritical for quadratic load torque type of loads [1]
– issue for constant torque type of loads [1]
– even more critical for applications with overload at low speed
♦ High transient torques
Large torque pulsation during direct on-line start may affect the mechanical components such as couplings, gearbox or load machine.
– starting torque in datasheet typically shown just as average torque
– in reality large pulsating torque components are present
– torsional excitation –> risk of mechanical damages
We have simulated the real machine torque at standstill or close to zero speed. Transient torque is clearly visible in figure 2. Although the mean value of starting torque at standstill is approx. 0.4 pu (red line) the actual value fluctuates between -4.24 pu and + 5.09 pu. Of course, such behavior has severe on the mechanical system.
♦ Thermal overdimensioning of motor
Restriction in number of consecutive starts was already mentioned. The longer the start-up the more significant is the thermal impact. Loads with larger inertia will therefore considerably stress the machine from thermal point of view.
– especially for longer starts (systems with larger inertia)
– challenging for motors in hazardous area
♦ Uncontrolled start-up
– possible issue for driven load or (especially) for the process – no control over acceleration rate
Simulation of direct on-line start
After explaining all the advantages and drawbacks of a direct on-line start we would like to illustrate the behavior by using few figures from a simulation.
In our case we have simulated a squirrel cage asynchronous motor with following:
Fundamental data
– Rated power: 7’400 kW
– Stator voltage: 6’600 V
– Grid frequency: 50 Hz
– Nominal speed: 1’492 rpm
– Rated torque: 47.1 kNm
– Inertia: 200 kg-m2 (motor + load)
– Load torque: quadratic [1]
Simulation results
(i) Torque and speed in time domain
The entire start-up from standstill up to nominal speed is depicted in figure 3.
– torque in time domain
– speed in time domain
When looking at the motor torque during start up we can recognize the shape of the mean value as shown in simplified figure 1. However, we also notice large torque pulsations at the beginning of start up corresponding to what was explained earlier and depicted in figure 2.
The motor speed is a result of motor torque, load torque and total inertia of the system.
(ii) Torque versus speed
Actually comparing figure 1 and figure 3 is technically not correct as one is showing motor torque as function of speed (or slip) while the figure from simulation depicts torque over time. Of course, we can plot simulated torque versus speed as well – see figure 4.
Principally the shape looks similar, but not exactly the same. Explanation is very simple – the acceleration during direct on-line start is not constant as we could already notice. In addition, there is certain oscillation in motor speed due to large torque pulsations in the initial part of start up. As we plot torque versus speed, it will be seen as elliptic shape rather than sinusoidal oscillation observed in time domain.
(iii) Grid voltage and motor current
Finally, let’s look at the grid voltage and motor starting current. Grid was modeled as voltage source with internal impedance corresponding to 100 MVA short circuit power. That might be a fairly realistic value for 6.6 kV grid. Figures are scaled in per unit so that it is easier to follow and make own conclusion.
At the beginning the grid voltage drops to approx. 71% of its nominal value. That is a significant drop and would not be accepted in most cases (typically the grid voltage shall remain above 80%). After 5 seconds, when the motor already reached over 70% of its nominal speed, the grid voltage is still only 74%. That is because motor starting current remains high during major part of the start-up.
Such significant voltage drop has a negative impact on motor torque as already explained. In our example we have assumed a quadratic load torque meaning very small load at low and medium speeds.
Summary
Direct on-line start (across the line) is the simplest method how to start an electric motor. It does not require any extra equipment and has therefore the lowest investment cost. However, it comes with multiple drawbacks, such as high starting current, associated voltage drop in the grid, torque pulsations, thermal stress of the machine etc.
In next articles we will introduce other starting methods and compare them. Direct on-line start will often be used as a reference for benchmarking.
References
[1] Load types and characteristics, https://mb-drive-services.com/load-types-and-characteristics/
[2] Motor start with a VFD, https://mb-drive-services.com/vfd-motor-start/
[3] Medium voltage AC drives, https://new.abb.com/drives/medium-voltage-ac-drives
[4] Motors and generators, https://new.abb.com/motors-generators
2 Comments
Motor starting methods: Overview - MB Drive Services · December 10, 2020 at 9:48 pm
[…] time in the introduction the main advantages and drawbacks of direct on-line (across the line) start [1] were described. And the list of drawbacks was the longer […]
VFD as soft starter - MB Drive Services · December 12, 2020 at 10:41 pm
[…] is not just about the mean value of starting torque. We have shown in [1] that direct on-line start creates large torque pulsations right after starting from standstill. […]
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