Everyone is a gangster (Rajdhani, Shatabdi, Tejas) until the real Gangsta (Vande Bharat) walks in 😂 - Mushfique Khalid

Yes, they are restricted to 105 kmph. But it's not simply because of the alternators.
A coach's MPS depends mainly on the bogie, and if any additions are made to the bogie, it will also depend on how well the additions are integrated into the bogie. Just adding alternators does not necessarily reduce the MPS.
For example, a normal ICF coach can comfortably hit 120kmph, but it will require a much higher level of maintenance. This is more because the normal ICF bogie is...
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not designed for high speeds. However, the normal ICF bogies have a very good arrangement to include alternators as well, so for a normal ICF coach, the speed limit of 110kmph is more due to the bogie design rather than the addition of alternators.
In this case however, the FIAT bogies in LHB coaches were not designed for including alternators. IR had to do some Jugaad to add the alternators to these coaches, and even there they had to try multiple configurations. Some configurations did support 130+ speeds, but weren't durable enough. So, in this case even though the bogie was fit for high speeds, since it was not designed to accommodate alternators, improper addition of those components reduced the MPS.