What is the starting time of a VFD-fed motor?
This question sounds rather trivial: What is the starting time of a VFD-fed motor? The answer is not complicated either. We just need to be specific what we mean:
- Time from energization of the system?
- Time from run command of the VFD?
- What are the system parameters?
- How is the VFD control set?
Let’s look at those aspects one by one. We start with the state of the VFD.
A. VFD already charged and ready (hot standby)
In this case the dc bus of the VFD is already charged and the incoming circuit breaker is closed. As soon as the run command is received, the VFD starts magnetizing the motor. After reaching the desired magnetic flux, the speed controller is released and the motor accelerates along the speed ramp.
- Magnetization of motor 2…5 seconds (approx. 3 x rotor time constant)
- Acceleration time – this time depends on the system characteristics, read further below
- Synchronization and transfer to grid supply 20…30 seconds ¹
¹ Applicable only for VFD soft starters
Figure 1 depicts an exemplary motor start-up in VFD duty. The magnetization takes approx. 3 seconds. After reaching the flux setpoint (~ 100%), the speed controller is released and the VFD control follows the speed ramp. The entire start-up is depicted: from standstill over nominal speed through field weakening range up to the maximum speed.
B. VFD in cold standby
In this state the process takes longer time. The VFD needs to be charged first (most voltage source inverters use auxiliary charging to pre-charge the dc link), incoming circuit breaker shall close (optional transformer pre-magnetization). The next steps are the same as in A: Magnetization of the motor and acceleration to the speed setpoint.
- Pre-charging of the dc bus 20…60s
- Pre-magnetization of input transformer 5..10s ²
- Closing of the incoming breaker 0.5s
- Magnetization of motor 2…5s
- Acceleration time
- Synchronisation and transfer to grid 20..30s ¹
¹ Applicable only for VFD soft starters
² Optional, purpose to limit the inrush current of the transformer (see e.g. [3])
System parameters influencing the acceleration time
The actual acceleration time of the motor depends on several factors. One of them is the total inertia of the shaft string, i.e. the sum of the inertia of the electric motor, inertia of the driven load and inertia of the other shaft components (couplings, gear etc). Other factor is the load torque. The larger the load torque the longer the acceleration time as the net acceleration torque is the difference between the motor torque and load torque. Finally, the acceleration is affected by the speed ramp of the speed controller. Very slow speed ramp means that the VFD applies much slower motor torque than what is the hardware capability. That is in fact quite often the case: From process perspective it is not desired to accelerate too fast.
Full torque from zero speed
VFD supplied motor can principally provide full torque across the entire speed range (from zero speed). Thus, the entire acceleration time can be shorter than starting time of direct on-line motor. Especially in heavy duty starts the VFD can significantly outperform the DOL motor.
Example 1
- 1’800 kW pump
- 4-pole motor with 1’560 rpm rated speed
- Total inertia 141 kg-m2
- Start up time:
- approx. 3 seconds from hot standby
- approx. 5 seconds from cold standby
Example 2
- 8’700 kW shaft power
- 4-pole motor with 1’386 rpm rated speed
- Driven load: centrifugal turbo-compressor
- Load torque profile: quadratic
- Total inertia 2’263 kg-m2
- Start up time:
- approx. 11.7 seconds (excluding charging and magnetization)
Example 3
- 22 MW motor drive
- 4-pole motor with 1’800 rpm rated speed
- Driven load: boiler feedwater pump
- Load torque profile: quadratic
- Total inertia 2’307 kg-m2
- Start up time:
- approx. 5.8 seconds (excluding charging and magnetization)
- approx. 8.3 seconds (including closing of upstream breaker and motor magnetization)
Example 4
- 6’600 kW shaft power
- 6-pole motor with 995 rpm rated speed
- Driven load: mixer
- Load torque profile: constant torque
- Total inertia 744 kg-m2
- Start up time:
- approx. 8.6 seconds assuming 75% load torque (excluding charging and magnetization)
- approx. 42.1 seconds assuming 100% load torque (excluding charging and magnetization)
Summary of system factors:
- Selected ramp time
- Load torque profile
- Total inertia (motor plus driven load)
Take away
- Initial state (e.g. cold standby versus hot standby)
- Time to pre-charge the dc link (if applicable)
- Motor time constant for magnetization
- Speed ramp / acceleration rate
- VFD (inverter) capability when using max. possible torque
- Load torque profile (quadratic torque, constant torque etc)
- Total inertia of the shaft line (motor + driven load)
References
[1] Motor start with VFD, MB Drive Services, March 2020, available online, https://mb-drive-services.com/vfd-motor-start/
[2] VFD as soft starter, MB Drive Services, December 2020, available online, https://mb-drive-services.com/vfd-as-soft-starter/#google_vignette
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