Measuring Mains Asymmetry: Derating Bosch 400 V Motors Without Guesswork

Voltage unbalance is one of those supply conditions that rarely announces itself. A motor may run for months with a slight asymmetry, then fail in a summer heatwave or after a production increase pushes it towards its thermal limit. On UK 400 V three-phase supplies, the cause is often mundane: single-phase loads distributed unevenly across a distribution board, a weak incoming supply, or a long cable with unequal phase impedances. For engineers specifying or maintaining Bosch motors, the useful question is not whether the supply is perfect. It is whether the motor has enough thermal margin for the asymmetry that actually exists.

Start at the supply, not the drive

A drive parameter is not a substitute for a supply measurement. Before changing current limits, carrier frequency or the thermal model, establish the line-to-line voltages at the motor terminals under a representative load. Measure at the MCC and at the motor where practicable; a poor connection or long cable run can create a different picture at the machine. Use a calibrated true-RMS meter rated for the installation category, and record phase sequence as well as magnitude. Average-responding instruments can mislead when the waveform is distorted.

Take readings over a full production cycle, not a single snapshot. Unbalance often changes when large single-phase loads switch, when an adjacent drive runs, or when the site generator is paralleled. A ten-minute log at fixed intervals is usually enough for a first pass; a full shift is better for a critical asset. Capture minimum, maximum and typical values rather than one convenient number.

A calculation you can defend

For a three-phase motor, calculate unbalance from the line-to-line voltages:

1. Add the three readings and divide by three to obtain the average.
2. Find the largest deviation from that average, ignoring sign.
3. Divide the largest deviation by the average and multiply by 100.

For example, readings of 402 V, 396 V and 405 V give an average of 401 V. The largest deviation is 5 V, so the unbalance is 1.25%. That is already above the 1% threshold at which full rated output should be questioned. The current unbalance will typically be several times larger than the voltage unbalance, which is why a supply that looks only slightly uneven can produce a noticeable temperature rise.

Logging template

A simple record prevents the same argument being repeated at every site visit.

Add columns for drive input voltage, DC bus ripple where available, and winding or bearing temperature if the motor is instrumented. The notes column matters: record whether a large press, chiller or oven was running, because the unbalance may be a site load pattern rather than a DNO characteristic.

From unbalance to thermal margin

The negative-sequence component created by unbalance produces a reverse rotating field. It does not contribute useful torque, but it does produce rotor and stator losses. The result is extra heating in the winding and, on servo motors, additional current ripple and DC bus disturbance. A typical NEMA-style derating curve gives the shape of the problem: around 0.98 of rated output at 1% unbalance, 0.95 at 2%, 0.88 at 3% and progressively steeper penalties beyond that. Treat those figures as a starting point, not a universal law. Bosch motor data, insulation class, ambient, altitude, enclosure and duty cycle all affect the real margin.

For an induction motor running continuously near its rating, a sustained unbalance above 1% should trigger a thermal review. Above 2%, investigate the supply and the load profile before accepting full output. Above 3%, continuous operation at rated load is difficult to justify without manufacturer guidance. For servo motors, the drive's current limit may mask the effect dynamically, but the motor and drive thermal models still accumulate heat. Check the drive's diagnostics for current ripple, DC bus variation and thermal warnings rather than relying on the absence of a trip.

Thermal margin is the deciding variable. Calculate the actual continuous torque or power demand, add the site ambient, check the enclosure and altitude corrections, and compare with the motor's rated data at the expected winding temperature. If the resulting margin is below roughly 15% on a critical continuous duty, the supply unbalance is consuming margin that the application may need for starts, overloads or a warmer than expected summer. A thermistor or KTY sensor, where fitted, is valuable evidence; it is not a licence to run closer to the limit.

When a larger frame is the right answer

Specify a larger frame when the unbalance is persistent and the duty is continuous, when the motor already runs hot at normal load, when ambient is above the standard rating conditions, or when the application cannot tolerate nuisance trips. The same applies where a servo axis needs high dynamic torque and the drive is already close to its current limit. Upsizing is not inelegant; it is a controlled way to restore thermal margin without pretending the supply is better than it is.

Do not chase drive parameters as a substitute. Raising current limits or altering thermal models can delay a trip while increasing insulation stress. Reducing carrier frequency may change losses, but it does not remove negative-sequence heating. The defensible sequence is measure, calculate, check thermal margin, then decide between supply correction, load reduction, duty change or a larger motor.

Field checklist

• Measure line-to-line voltages at the motor terminals under representative load.
• Log for at least ten minutes; a full shift for critical assets.
• Calculate unbalance from the average, not from the highest phase.
• Record load, ambient and nearby single-phase loads.
• Compare the result with Bosch motor data and the actual thermal margin.
• If unbalance exceeds 1%, review; above 2%, investigate; above 3%, seek manufacturer guidance before continuous rated duty.
• Upsize when margin is thin and the duty is unforgiving.

The aim is not to eliminate every fraction of a volt. It is to make the derating decision visible, repeatable and defensible, so that a Bosch motor is selected for the supply it will actually see.