How to read a guarantee sheet before you agree to it
No-load loss, load loss, impedance and temperature rise are four promises with very different tolerances and very different consequences when you miss them.
· 9 min read · Design
The four numbers that decide the design
Every guarantee sheet contains dozens of entries, but four of them determine whether the design closes at all: no-load loss, load loss, percentage impedance and temperature rise. Everything else is either derived from these or is a constraint on how you are allowed to reach them.
They pull against each other. Lowering no-load loss means a larger core or a better grade of steel, which raises the core weight, which raises the tank, which raises the cost. Lowering load loss means more copper area, which raises the radial build, which moves the impedance. You cannot optimise them one at a time.
Tolerances are not decoration
IEC 60076 allows a tolerance on each loss component and on the total, and a separate tolerance on impedance. Those numbers are the difference between a design that passes and one that is rejected at works test, and they are frequently narrowed in the tender document without the designer noticing.
Read the tolerance clause before the loss values. A guaranteed no-load loss with zero positive tolerance is a fundamentally different design problem from the same figure with the standard allowance, because it removes the margin you would normally hold against building factor and joint variation.
Impedance is a contract with the network, not with you
Loss guarantees cost you money if you miss them. Impedance can make the unit unusable. It sets the fault level the network sees and the voltage regulation the customer experiences, and it is frequently specified with a tight band because parallel operation with an existing unit depends on it.
Check which tap the impedance is guaranteed at, and whether the guarantee applies across the whole tap range or only at principal tap. A design that meets impedance at principal tap and drifts outside the band at the extreme tap is a design that will be argued about after delivery.
Temperature rise is where optimism gets punished
Top-oil and winding rise limits look like simple ceilings, but they are computed from a gradient that depends on the winding geometry, the duct arrangement, the oil flow and the radiator surface. Small changes in the winding build move the gradient more than most designers expect.
If you are working near the limit, the honest position is to say so in the offer. A design at 59 K guaranteed rise against a 60 K limit is not a margin, it is a rounding error, and the works test will find it.
A short checklist
Before agreeing to a guarantee sheet: confirm the tolerance on each loss component and on the total; confirm the tap at which impedance is guaranteed and the band across the range; confirm the ambient basis for the temperature rise; confirm the reference temperature for load loss; and confirm which standard's test method applies where two standards are both cited.
Every one of those has ended in a dispute at some works, and every one of them is cheap to settle at tender stage and expensive to settle after the unit is on the test floor.