IEEE 1584 · Engineering
Motor Start Analysis
Motor Start Analysis. Starting current for most AC motors runs several times their normal full-load current, and that surge can pull system voltage down enough to dim lights, trip breakers that were never at risk, or stall the motor before it reaches running speed. Carelabs calculates the voltage dip and acceleration time your motor will cause and recommends the starting method — across-the-line, autotransformer, reactor, or variable frequency drive — that avoids it.
Free ConsultationStarting on a supply that cannot absorb it
On a strong network a large start is a brief dip. On a weak feeder, or an island supply running on local generation, the same start can disturb the whole system.
What We Deliver
Dip calculated against real source strength
Voltage dip depends directly on source impedance, so the study uses the fault level at your actual point of supply rather than a typical value. On a long feeder or a small network, that difference decides whether the start is viable.
WHY IT MATTERS
It starts on mains and fails on generator
The classic symptom is a plant that operates normally for years, then cannot restart during an outage. Nothing has changed except the source, and the standby supply was never assessed against the starting duty.
- Source impedance established from the network operator's fault level for the actual point of supply, since dip depth follows it directly.
- Generator parameters modelled from the machine's own data, as subtransient reactance and prime mover response govern both voltage and frequency behaviour.
- Restart sequencing assessed where several large motors must recover together after an interruption.
- Load torque characteristics taken from the driven machines rather than assumed, because a deeper dip reduces available torque and lengthens acceleration.
- Undervoltage protection settings checked against the calculated dip, so protection does not trip the plant during a start it was designed to survive.
DELIVERABLES
What you receive
Issued against ELOT HD 384, with the supply basis, generator data and load characteristics stated explicitly.
- 1Voltage dip profile at every bus through acceleration, on utility and on standby supply
- 2Frequency behaviour during starting where generator supply is involved
- 3Acceleration time with torque margin shown against the driven load
- 4A restart sequence where simultaneous starting is not achievable
- 5Protection and undervoltage settings that permit the start without leaving equipment unprotected
How the study runs
Establish both supplies
Source impedance is determined for the utility connection and for any standby generation, since the two produce very different starting conditions.
Collect motor and load data
Nameplate values, starting characteristics and the driven machine's torque and inertia data are gathered, as the load governs acceleration as much as the motor.
Model the network
The network is built in ETAP with motors represented dynamically, which is what allows acceleration to be simulated rather than approximated.
Simulate on each source
Starting is simulated on utility and on generator supply, with voltage, frequency, current, torque and speed tracked throughout.
Resolve the failures
Where a start is not viable, starting methods and restart sequencing are tested on the same model until a workable arrangement is demonstrated.
Report and settings
Findings are issued with the recommended arrangement, any required sequencing, and protection settings that accommodate the start.